USRE41455E1 - Objective lens for optical disks having different thicknesses - Google Patents

Abstract

A compound objective lens is composed of a hologram lens or transmitting a part of incident light without any diffraction to form a beam of transmitted light and diffracting a remaining part of the incident light to form a beam of first-order diffracted light, and an objective lens for converging the transmitted light to form a first converging spot on a front surface of a thin type of first information medium and converging the diffracted light to form a second converging spot on a front surface of a thick type of second information medium. Because the hologram selectively functions as a concave lens for the diffracted light, a curvature of the transmitted light differs from that of the diffracted light. Therefore, even though the first and second information mediums have different thicknesses, the transmitted light incident on rear surface of the first information medium is converged on the its front surface, and the diffracted light incident on a rear surface of the second information medium is converged on the its front surface. That is, the compound objective lens has two focal points.

Description

Notice: More than one reissue application has been filed for the reissue of U.S. Pat. No.5,815,293. The reissue applications are the present application having application Ser. No.11/076,588, a child application from the present application having application Ser. No.11/332,813, filed Jan.17,2006, and parent application Ser. No.09/671,674 filed Sep.27,2000, now Reissue Pat. No. RE38,943, all of which are reissues of U.S. Pat. No.5,815,293.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of reissue application Ser. No.09/671,674 filed on Sep.27,2000, (now U.S. Pat. No. RE38,943), which is a reissue of U.S. Pat. No.5,815,293, which is acontinuation-in-part of U.S. Pat. application Ser. No. 08/190,520, filed Feb. 1, 1994 now U.S. Pat. No. 5,446,565.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a compound objective lens composed of an objective lens and a hologram lens which has two focal points, an imaging optical system for converging light on two converging spots placed at different depths of an information medium with the compound objective lens, an optical head apparatus for recording, reproducing or erasing information on or from an information medium such as an optical medium or a magneto-optical medium like an optical disk or an optical card with the imaging optical system, an optical disk in which a series of high density recording pits and a series of comparatively low density recording pits are provided, an optical disk apparatus for recording or reproducing information on or from the optical disk with the compound objective lens, a binary focus microscope having two focal points in which two types of images drawn at different depths are simultaneously observed, and an alignment apparatus for aligning two types of images drawn at different depths with the binary focus microscope.

2. Description of the Related Art

An optical memory technique has been put to practical use to manufacture an optical disk in which a pit pattern formed of a series of pits is drawn to record information. The optical disk is utilized as a high density and large capacity of information medium. For example, the optical disk is utilized for a digital audio disk, a video disk, a document file disk, and a data file disk. To record information on the optical disk and to reproduce the information from the optical disk, a light beam radiated from a light source is minutely converged in an imaging optical system, and the light beam minutely converged is radiated to the optical disk through the imaging optical system. Therefore, the light beam is required to be reliably controlled in the imaging optical system with high accuracy.

The imaging optical system is utilized for an optical head apparatus in which a detecting system is additionally provided to detect the intensity of the light beam reflected from the optical disk. Fundamental functions of the optical head apparatus are classified into a converging performance for minutely converging a light beam to form a diffraction-limited micro-spot of the light beam radiated on the optical disk, a focus control in a focus servo system, a tracking control in a tracking serve system, and the detection of pit signals (or information signals) obtained by radiating the light beam on a pit pattern of the optical disk. The fundamental function of the optical head apparatus is determined by the combination of optical sub-systems and a photoelectric transfer detecting process according to a purpose and a use. Specifically, an optical head apparatus in which a holographic optical element (or hologram) is utilized to minimize and thin the optical head apparatus has been recently proposed.

2.1. Previously Proposed Art

FIG. 1 is a constitutional view of a conventional optical head apparatus proposed in Japanese Patent Application No. 46630 of 1991 which is applied by inventors of the present invention.

As shown inFIG. 1, a conventionaloptical head apparatus11 for recording or reproducing information on or from aninformation medium12 such as an optical disk is provided with alight beam source13 such as a semiconductor laser, a transmission type ofblazed hologram14 for transmitting a light beam L1 radiated from thelight beam source12 without any diffraction in an outgoing optical path and diffracting a light beam L2 reflected on theinformation medium12 in a returning optical path, anobjective lens15 for converging the light beam L1 transmitting through thehologram13 on theinformation medium14 to read the information, anactuator16 for integrally moving theobjective lens15 with theblazed hologram13 to focus the light beam L1 on theinformation medium12 with theobjective lens15, and aphoto detector17 for detecting the intensity of the light beam L2 reflected on theinformation medium12 to reproduce the information.

As shown inFIG. 2A, a relative position between theblazed hologram14 and theobjective lens15 is fixed by afixing means18. Or, as shown inFIG. 2B, a blazed pattern is formed on a side of theobjective lens15 to integrally form theblazed hologram14 with theobjective lens15.

In the above configuration, a light beam L1 (or a laser beam) radiated from thelight beam source13 is radiated to theblazed hologram14, and the light beam L1 mainly transmits through theblazed hologram14 without any diffraction in an outgoing optical path. The light beam L1 transmitting through theblazed hologram14 is called zero-order diffracted light. Thereafter, the zero-order diffracted light L1 is converged on theinformation medium12 by theobjective lens15. In theinformation medium12, information indicated by a series of patterned pits is recorded and read by the zero-order diffracted light L1. Thereafter, a beam light L2 having the information is reflected toward theobjective lens15 in a returning optical path and is incident to theblazed hologram14. In theblazed hologram14, the light L2 is mainly diffracted. The light L2 diffracted is called first-order diffracted light. Thereafter, the first-order diffracted light L2 is received in thephoto detector17.

In thephoto detector17, the intensity distribution of the first-order diffracted light L2 is detected. Therefore, a servo signal for adjusting the position of theobjective lens15 by the action of theactuator16 is obtained. Also, the intensity of the first-order diffracted light L2 is detected in thephoto detector17. Because theinformation medium12 is rotated at high speed, the patterned pits radiated by thelight17 are changed so that the intensity of the first-order diffracted light L2 detected is changed. Therefore, an information signal indicating the information recorded in theinformation medium12 is obtained by detecting the change in intensity of the first-order diffracted light L2.

In the above operation, a part of the light beam L1 is necessarily diffracted in theblazed hologram14 when the light beam L1 is radiated to theblazed hologram14 in the outgoing optical path. Therefore, unnecessary diffracted light such as first-order diffracted light and minus first-order diffracted light necessarily occurs. In cases where thehologram14 is not blazed, the unnecessary diffracted light in the outgoing optical path also reads the information recorded in theinformation medium12, and the unnecessary light is undesirably received in thephoto detector17. To prevent the unnecessary light from transmitting to theinformation medium12, theblazed hologram14 is manufactured to form a blazed hologram pattern on the surface thereof, so that the intensity of the unnecessary light received in thephoto detector17 is decreased.

Also, because an objective lens of a conventional microscope has only a focal point, images placed within a focal depth of the objective lens can be only observed with the conventional microscope.

Also, a minute circuit is formed on a semiconductor such as a group III-V compound semiconductor to form a microwave circuit, an opto-electronic detector or a solid state laser. In this case, a photo-sensitive material is coated on a sample made of the semiconductor. Thereafter, a relative position between the sample and a photo mask covering the sample is adjusted by utilizing an alignment apparatus, and the photo-sensitive material is exposed by a beam of exposure light through the photo mask to transfer a circuit pattern drawn on the photo mask to the photo-sensitive material in an exposure process by utilizing an exposure apparatus. For example, an alignment pattern is drawn on a reverse side of the sample, and a relative position between the sample and the photo mask is adjusted with high accuracy while simultaneously observing the alignment pattern of the sample and the circuit pattern of the photo mask with the conventional microscope. Thereafter, the circuit pattern of the photo mask is transferred to a front side of the sample.

In this case, because images placed within a focal depth of an objective lens utilized in the conventional microscope can be only observed with the conventional microscope, it is required to utilize the conventional microscope having a deep focal depth in the alignment apparatus in cases where the alignment pattern and the circuit pattern are simultaneously observed with the conventional microscope. Therefore, the magnification of the conventional microscope having a deep focal depth is lowered.

2.2. Problems to be Solved by the Invention

An optical disk having a high density memory capacity has been recently developed because of the improvement in a design technique of an optical system and the shortening of the wavelength of light radiated from a semiconductor laser. For example, a numerical aperture at an optical disk side of an imaging optical system in which a light beam converged on an optical disk is minutely narrowed in diameter is enlarged to obtain the optical disk having a high density memory capacity. In this case, the degree of aberration occurring in the imaging optical system is increased because an optical axis of the system tilts from a normal line of the optical disk. As the numerical aperture is increased, the degree of the aberration is enlarged. To prevent the increase of the numerical aperture, it is effective to thin the thickness of the optical disk. The thickness of the optical disk denotes a distance from a surface of the optical disk (or an information medium) radiated by a light beam to an information recording plane on which a series of patterned pits are formed.

FIG. 3 shows a relationship between the thickness of the optical disk and the numerical aperture on condition that the tilt of the optical axis is constant.

As shown inFIG. 3, because the numerical aperture is 0.5 when the thickness of the optical disk is 1.2 mm, it is effective to thin the optical disk to 0.6 mm in thickness when the numerical aperture is increased to 0.6. In this case, even though the numerical aperture is increased on condition that the tilt of the optical axis is not changed, the degree of the aberration is not increased. Therefore, it is preferred that the thickness of the optical disk be thinned to obtain the optical disk having a high density memory capacity.

Accordingly, it is expected that the thickness of a prospective optical disk having a high density memory capacity becomes thinner than that of a present optical disk such as a compact disk appearing on the market now. For example, the thickness of the compact disk is about 1.2 mm, and the thickness of the prospective optical disk is expected to range from 0.4 mm to 0.8 mm. In this case, it is required to record or reproduce information on or from an optical disk with an optical head system regardless of whether the optical disk is the present optical disk or the prospective optical disk having a high density memory capacity. That is, an optical head apparatus having an imaging optical system in which a light beam is converged on an optical disk within the diffraction limit regardless of whether the optical disk is thick or thin is required.

However, in a conventional optical head apparatus, a piece of information is only recorded or reproduced on or from an optical disk having a fixed thickness. For example, in cases where the thickness of theinformation medium12 is off a regular range by about ±0.1 mm or more, an aberration such as a spherical aberration occurs when theoptical head apparatus11 is operated. Therefore, the recording or the reproduction of the information is impossible. Accordingly, there is a drawback that an optical head apparatus in which a piece of information is recorded or reproduced on or from an optical disk regardless of whether the optical disk is the present optical disk or the prospective optical disk having a high density memory capacity cannot be manufactured in a conventional technique.

Also, there is a problem in the conventional microscope. That is, because an objective lens of the conventional microscope has only a focal point and images placed within a focal depth of the objective lens can be only observed with the conventional microscope, the magnification of the conventional microscope and an observed range in an optical axis direction are in a trade-off relationship. Therefore, there is a drawback that it is impossible to observe the images over a wide observed range in the optical axis direction at high magnification

Also, there is a problem in the alignment apparatus. That is, when a circuit pattern drawn on the photo mask is transferred to the front side of the sample after an alignment pattern is drawn on the reverse side of the sample, the alignment of the photomask and the sample is performed by simultaneously observing the circuit pattern of the photo mask and the alignment pattern of the sample with the conventional microscope having a deep focal depth and a low magnification. Therefore, because the conventional microscope has a low magnification, there is a drawback that it is impossible to align the photo mask with the sample at a high accuracy ranging within 5 μm.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide, with due consideration to the drawbacks of such a conventional objective lens having a focal point, a compound objective lens having two focal points.

A second object of the present invention is to provide an imaging optical system having the compound objective lens in which light transmitting through the compound objective lens is converged at a diffraction limit on two converging spots placed at different depths of an information medium.

A third object of the present invention is to provide an optical head apparatus having the imaging optical system in which information is recorded, reproduced or erased on or from one of the converging spots of the information medium at which light is converged by the action of the imaging optical system.

A fourth object of the present invention is to provide a high density optical disk in which a series of first recording pits is formed to record pieces of information at high density on a thin substrate.

A fifth object of the present invention is to provide an optical disk apparatus in which information is recorded or reproduced on or from the optical disk with the compound objective lens regardless of whether a series of recording pits expressing pieces of information is recorded or reproduced on or from the high density optical disk having a thin thickness or a conventional compact disk having an ordinary thickness.

A sixth object of the present invention is to provide a binary focus microscope having two focal points in which two types of images drawn at different depths are simultaneously observed.

A seventh object of the present invention is to provide an alignment apparatus in which two types of images drawn at different depths are aligned with the binary focus microscope.

An eighth object of the present invention is to provide a focusing method for focusing light on an information medium with the optical head apparatus.

A ninth embodiment of the present invention is to provide an information reproducing method for reproducing a piece of recording information recorded on a high density optical disk having a thin thickness.

The first object is achieved by the provision of a compound objective lens having two focal points, comprising:

• • hologram means for transmitting a part of incident light without any diffraction to form a beam of transmitted light and diffracting a remaining part of the incident light to form a beam of diffracted light, the hologram means functioning as a lens for the diffracted light to diverge the diffracted light from the hologram means or converge the diffracted light; and
  • lens means for converging the transmitted light formed in the hologram means to form a first converging spot at a first focal point and converging the diffracted light formed in the hologram means to form a second converging spot at a second focal point, the second focal point differing from the first focal point.

In the above configuration, a part of incident light transmits through the hologram means without any diffraction. Therefore, a beam of transmitted light not diverged from the hologram means or not converging is formed. Thereafter, the transmitted light is refracted and converged by the lens means, so that the transmitted light is focused on a first converging spot positioned at a first focal point.

In contrast, a remaining part of incident light is diffracted by the hologram means. Therefore, a beam of diffracted light such as a beam of first-order diffracted light which is diverged from the hologram lens or converges is formed. Thereafter, the diffracted light is refracted and converged by the lens means, so that the diffracted light is focused on a second converging spot positioned at a second focal point.

In this case, because a propagation direction of the transmitted light differs from that of the diffracted light, the first focal point of the compound objective lens for the transmitted light differs from the second focal point of the compound objective lens for the diffracted light. Therefore, the compound objective lens has two focal points, and the incident light transmitting through the compound objective lens are converged on two converging points.

Accordingly, the incident light transmitting through the compound objective lens can be reliably converged on an information medium regardless whether the information medium has a first thickness or a second thickness.

It is preferred that a grating pattern be drawn in the hologram means in a concentric circle shape, the grating pattern of the hologram means be formed in relief to concentrically form alternating rows of bottom portions and top portions, a height H of relief in the grating pattern be set to H <λ/(n(λ)−1) where a symbol λ denotes a wavelength of the incident light and a symbol n(λ) denotes a refractive index of the hologram means made of a glass material for the incident light having the wavelength λ, and a difference in phase modulation degree between the incident light transmitting through a bottom portion of the grating pattern and the incident light transmitting through a top portion of the grating pattern be lower than 2π radians to set a diffraction efficiency of the hologram means to a value lower than 100%.

In the above configuration, because the height of the relief in the grating pattern is lower than a value λ/(n(λ)−1), the difference in phase modulation degree is induced to be lower than 2π radians. Therefore, the diffraction efficiency of the hologram means is set to a value lower than 100% over the entire grating pattern, so that the transmitted light and the diffracted light are simultaneously formed in the hologram lens.

The first object is also achieved by the provision of a compound objective lens, comprising:

• • lens means for converging a beam of first incident light on a front surface of a first information medium having a first thickness T1 and converging a beam of second incident light on a front surface of a second information medium having a second thickness T2 (T2<T1), the first incident light passing through the first information medium from its rear surface, and the second incident light passing through the second information medium from its rear surface; and
  • aperture limiting means for selectively limiting an aperture of the lens means for the second incident light which is incident on the lens means, a second numerical aperture of the lens means for the second incident light which is incident on the lens means being lower than a first numerical aperture of the lens means for the first incident light which is incident on the lens means.

In the above configuration, an aperture of the lens means for the second incident light is limited by the aperture limiting means, and another aperture of the lens means for the first incident light is limited. Thereafter, the first incident light is converged by the lens means on the first information medium at a high numerical aperture, and the second incident light is converged by the lens means on the second information medium at a low numerical aperture. Accordingly, the compound objective lens has two focal points. Also, the intensity of the first incident light can be larger than that of the second incident light.

The first object is also achieved by the provision of a compound objective lens, comprising:

• • lens means for converging a beam of first incident light on a front surface of a first information medium having a first thickness and converging a beam of second incident light on a front surface of a second information medium having a second thickness, the first incident light passing through the first information medium from its rear surface, and the second incident light passing through the second information medium from its rear surface; and
  • curvature changing means for changing a curvature of spherical waves of a part of incident light to form the first incident light which is incident on the lens means and not changing a curvature of spherical waves of a remaining part of incident light to form the second incident light which is incident on the lens means.

In the above configuration, a curvature of spherical waves of a part of incident light is changed by the curvature changing means to form a beam of first incident light, and a curvature of spherical waves of a remaining part of incident light is not changed to form a beam of second incident light. Thereafter, the first incident light is converged on the first information medium, and the second incident light is converged on the second information medium.

Accordingly, the compound objective lens has two focal points.

The second object is achieved by the provision of an imaging optical system, comprising:

• • a light source for radiating a beam of incident light of which a far field pattern is distributed to decrease intensity of the incident light toward a peripheral portion of the beam;
  • hologram means for transmitting a part of the incident light radiated from the light source without any diffraction to form a beam of transmitted light and diffracting a remaining part of the incident light to form a beam of diffracted light, a grating pattern being formed in relief in the hologram means to concentrically form alternating rows of bottom portions and top portions on condition that a height H of relief in the grating pattern is set to H <λ/(n(λ)−1) where a symbol λ denotes a wavelength of the incident light and a symbol n(λ) denotes a refractive index of the hologram means made of a glass material for the incident light having the wavelength λ, a difference in phase modulation degree between the incident light transmitting through the bottom portions of the grating pattern and the incident light transmitting through the top portions of the grating pattern being lower than 2π radians to set a diffraction efficiency of the hologram means to a value lower than 100%, the height H of the relief in the grating pattern being gradually lowered toward an outer direction of a pattern region in which the grating pattern is drawn, and the diffraction efficiency of the hologram means for the incident light being gradually lowered toward the outer direction of the pattern region to distribute intensity of the transmitted light in a gently-sloping shape; and
  • lens means for converging the transmitted light formed in the hologram means to form a first converging spot at a first focal point and converging the diffracted light formed in the hologram means to form a second converging spot at a second focal point.

In the above configuration, a beam of incident light is radiated from a light source. A far field pattern of the incident light is distributed to decrease intensity of the incident light toward a peripheral portion of the beam. For example, the light source is a semiconductor laser, the far field pattern of the incident light is distributed in the Gaussian distribution. Thereafter, the incident light transmits through the hologram means. In this case, because a grating pattern is drawn in relief in the hologram means and because the height of the relief is lower than a value λ/(n(λ)−1), the difference in phase modulation degree between the incident light transmitting through a bottom portion of the grating pattern and the incident light transmitting through a top portion of the grating pattern is induced to be lower than 2π radians. Therefore, the diffraction efficiency of the hologram means for the incident light is set to a value lower than 100% over the entire grating pattern, so that the transmitted light and the diffracted light are simultaneously formed in the hologram lens. In addition, because the height H of the relief in the grating pattern is gradually lowered toward the outer direction of the pattern region, the diffraction efficiency of the hologram means for the incident light is gradually lowered toward the outer direction of the pattern region. Therefore, the incident light positioned at a center portion of its beam are mainly changed to the diffracted light, and the incident light positioned at a peripheral portion of its beam are mainly changed to the transmitted light.

Thereafter, the transmitted light is refracted and converged by the lens means, so that the transmitted light is focused on a first converging spot positioned at a first focal point. Also, the diffracted light is refracted and converged by the lens means, so that the diffracted light is focused on a second converging spot positioned at a second focal point. In this case, because a propagation direction of the transmitted light differs from that of the diffracted light, the first focal point for the transmitted light differs from the second focal point for the diffracted light.

Accordingly, even though the far field pattern of the incident light is distributed in the Gaussian distribution, the far field pattern of the transmitted light is distributed in a gently-sloping shape. Therefore, secondary maxima (or side lobes) of the transmitted light can be prevented from occurring at the first converging spot.

Also, because the height H of the relief in the grating pattern is gradually lowered over the entire pattern region, a numerical aperture of the lens means for the diffracted light can be sufficiently heightened.

The second object is also achieved by the provision of an imaging optical system, comprising:

• • a light source for radiating a beam of incident light of which a far field pattern is distributed to decrease intensity of the incident light toward a peripheral portion of the beam;
  • hologram means for transmitting a part of the incident light radiated from the light source without any diffraction to form a beam of transmitted light and diffracting a remaining part of the incident light to form a beam of diffracted light, a grating pattern being formed in relief in the hologram means to concentrically form alternating rows of bottom portions and top portions on condition that a height H of relief in the grating pattern is set to H <λ/(n(λ)−1) where a symbol λ denotes a wavelength of the incident light and a symbol n(λ) denotes a refractive index of the hologram means made of a glass material for the incident light having the wavelength λ, a difference in phase modulation degree between the incident light passing through the bottom portions of the grating pattern and the incident light passing through the top portions of the grating pattern being lower than 2π radians to set a diffraction efficiency of the hologram means to a value lower than 100%, the height H of the relief in the grating pattern being gradually lowered toward an inner direction of a pattern region in which the grating pattern is drawn, and the diffraction efficiency of the hologram means for the incident light being gradually lowered toward the inner direction of the pattern region to distribute intensity of the diffracted light in a gently-sloping shape; and
  • lens means for converging the transmitted light formed in the hologram means to form a first converging spot at a first focal point and converging the diffracted light formed in the hologram means to form a second converging spot at a second focal point.

In the above configuration, the transmitted light and the diffracted light are simultaneously formed in the hologram lens in the same manner. Also, because the height H of the relief in the grating pattern is gradually lowered toward the inner direction of the pattern region, the diffraction efficiency of the hologram means for the incident light is gradually lowered toward the inner direction of the pattern region. Therefore, the incident light positioned at a center portion of its beam are mainly changed to the transmitted light, and the incident light positioned at a peripheral portion of its beam are mainly changed to the diffracted light.

Accordingly, even though the far field pattern of the incident light is distributed in the Gaussian distribution, the far field pattern of the diffracted light is distributed in a gently-sloping shape. Therefore, secondary maxima (or side lobes) of the diffracted light can be prevented from occurring at the second converging spot.

Also, because the height H of the relief in the grating pattern is gradually lowered over the entire pattern region, a numerical aperture of the lens means for the transmitted light can be sufficiently heightened.

The third object is achieved by the provision of an optical head apparatus for recording or reproducing a piece of information on or from a thin type of first information medium having a first thickness or a thick type of second information medium having a second thickness larger than the first thickness, comprising:

• • a light source for radiating a beam of incident light;
  • hologram means for transmitting a part of the incident light radiated from the light source without any diffraction on an outgoing path to form a beam of transmitted light and diffracting a remaining part of the incident light radiated from the light source on the outgoing path to form a beam of diffracted light, the hologram means functioning as a lens for the diffracted light to diverge the diffracted light from the hologram means or converge the diffracted light;
  • lens means for converging the transmitted light formed in the hologram means at a first focal length on the outgoing path to form a first converging spot at a front surface of the first information medium or converging the diffracted light formed in the hologram means at a second focal length on the outgoing path to form a second converging spot at a front surface of the second information medium, the transmitted light being incident on a rear surface of the first information medium and being converged at the front surface of the first information medium, the transmitted light being reflected at the rear surface of the first information medium and again passing through the lens means and the hologram means on an incoming path, the diffracted light being incident on a rear surface of the second information medium and being converged at the front surface of the second information medium, and the diffracted light being reflected at the rear surface of the first information medium and again passing through the lens means and the hologram means on the incoming path;
  • wavefront changing means for changing a wavefront of the transmitted light or the diffracted light passing through the lens means and the hologram means on the incoming path to form one or more beams of information light; and
  • detecting means for detecting intensities of the information light formed by the wavefront changing means and generating an information signal according to the intensities of the information light, the information signal expressing a piece of information recorded on the first information medium or the second information medium.

In the above configuration, a beam of incident light is radiated from the light source, and a part of the incident light transmits through the hologram lens to form a beam of transmitted light. Also, a remaining part of the incident light is diffracted by the hologram lens to form a beam of diffracted light. Thereafter, the transmitted light and the diffracted light are converged by the converging means. In cases where a piece of information is recorded or reproduced on or from the first information medium, the transmitted light is incident on the rear surface of the first information medium and is converged at the front surface of the first information medium to form the first converging spot. Thereafter, the transmitted light is reflected at the rear surface of the first information medium and again passes through the lens means and the hologram means without any diffraction. In contrast, in cases where a piece of information is recorded or reproduced on or from the second information medium, the diffracted light is incident on the rear surface of the second information medium and is converged at the front surface of the second information medium to form the second converging spot. Thereafter, the diffracted light is reflected at the rear surface of the second information medium and again passes through the lens means. Thereafter, the diffracted light is again diffracted by the hologram means.

Thereafter, a wavefront of the transmitted light or the diffracted light is changed by the wavefront changing means to form a plurality of beams of reflected light, and the intensities of the reflected light is detected by the detecting means. Therefore, an information signal expressing the information recorded on the first or second information medium is generated according to the intensities of the reflected light.

Accordingly, because the first converging spot of the transmitted light differs from the second converging spot of the diffracted light. a compound objective lens composed of the hologram lens and the lens means has two focal points. Therefore, a piece of information can be recorded or reproduced on or from an information medium regardless of whether the information medium has the first thickness or the second thickness.

The third object is also achieved by the provision of an optical head apparatus for recording or reproducing a piece of information on or from a first information medium having a first thickness T1 or a second information medium having a second thickness T2 (T1<T2), comprising:

• • lens means for converging a beam of first incident light on a front surface of a first information medium having a first thickness T1 or converging a beam of second incident light on a front surface of a second information medium having a second thickness T2 (T2<T1), the first incident light passing through the first information medium from its rear surface, and the second incident light passing through the second information medium from its rear surface;
  • aperture limiting means for selectively limiting an aperture of the lens means for the second incident light which is incident on the lens means, a second numerical aperture of the lens means for the second incident light which is incident on the lens means being lower than a first numerical aperture of the lens means for the first incident light which is incident on the lens means, the first incident light being reflected at the front surface of the first information medium and again passing through the lens means and the aperture limiting means on an incoming path, and the second incident light being reflected at the front surface of the second information medium and again passing through the lens means and the aperture limiting means on the incoming path;
  • a light source for radiating the first incident light and the second incident light to the aperture limiting means;
  • wavefront changing means for changing a wavefront of the first or second incident light passing through the lens means and the aperture limiting means on the incoming path to form one or more beams of information light; and
  • detecting means for detecting intensities of the information light formed by the wavefront changing means and generating an information signal according to the intensities of the information light, the information signal expressing a piece of information recorded on the first information medium or the second information medium.

In the above configuration, a compound objective lens composed of the lens means and the aperture limiting means has two focal points, and the intensity of the first incident light is larger than that of the second incident light.

Accordingly, a piece of information can be recorded or reproduced on or from an information medium regardless of whether the information medium has the first thickness or the second thickness. Also, a piece of information can be efficiently recorded on the first information medium with the first incident light having a high intensity, and a piece of information can be efficiently reproduced from the second information medium with the second incident light having a comparatively low intensity.

The third object is also achieved by the provision of an optical head apparatus for recording or reproducing a piece of information on or from a first information medium having a first thickness or a second information medium having a second thickness, comprising:

• • lens means for converging a beam of first incident light on a front surface of the first information medium and converging a beam of second incident light on a front surface of the second information medium, the first incident light passing through the first information medium from its rear surface, and the second incident light passing through the second information medium from its rear surface; and
  • curvature changing means for changing a curvature of spherical waves of a part of incident light to form the first incident light which is incident on the lens means and not changing a curvature of spherical waves of a remaining part of incident light to form the second incident light which is incident on the lens means, the first incident light being reflected at the front surface of the first information medium and again passing through the lens means and the curvature changing means on an incoming path, and the second incident light being reflected at the front surface of the second information medium and again passing through the lens means and the curvature changing means on the incoming path;
  • a light source for radiating the incident light to the curvature changing means;
  • wavefront changing means for changing a wavefront of the first or second incident light passing through the lens means and the curvature changing means on the incoming path to form one or more beams of information light; and
  • detecting means for detecting intensities of the information light formed by the wavefront changing means and generating an information signal according to the intensities of the information light, the information signal expressing a piece of information recorded on the first information medium or the second information medium.

In the above configuration, because a compound objective lens is composed of the lens means and the curvature changing means, the compound objective lens has two focal points. Accordingly, a piece of information can be recorded or reproduced on or from an information medium regardless of whether the information medium has the first thickness or the second thickness.

The fourth object is achieved by the provision of an optical disk, comprising:

• • an information recording substrate partitioned into a first region and a second region, the first region having a first thickness, and the second region having a second thickness smaller than the first thickness;
  • a plurality of first recording pits placed at the first region of the information recording substrate for recording pieces of recording information at a high recording density, the first recording pits being formed at narrow intervals; and
  • a plurality of second recording pits placed at the second region of the information recording substrate for recording pieces of distinguishing information at an ordinary recording density of a compact disk, the distinguishing information informing that the recording information are recorded on the information recording substrate having the first thickness, and the recording density of the recording information being higher than that of the distinguishing information.

In the above configuration, a substrate of a conventional compact disk has the same second thickness as that of the second region of the information recording substrate in the optical disk according to the present invention. Therefore, in cases where a beam of reproducing light is incident on a prescribed region of an unknown disk selected from a group of the conventional compact disk and the optical disk, the reproducing light is focused on a recording pit of the conventional compact disk or one of the second recording pits of the optical disk regardless of whether the unknown disk is the conventional compact disk or the optical disk.

In cases where the unknown disk is the optical disk, a piece of distinguishing information is read by the reproducing light. Because the distinguishing information informs that pieces of recording information are recorded on the information recording substrate having the first thickness, a curvature of the reproducing light is automatically changed to focus the reproducing light on the information recording substrate having the first thickness, and the reproducing light is automatically focused on one of the first recording pits. Therefore, a piece of recording information is reproduced.

In contrast, in cases where the unknown disk is the conventional compact disk, a piece of recording information is read by the reproducing light in the same manner as in a prior art.

Accordingly, a piece of recording information formed on an information recording substrate can be reliably reproduced even though the thickness of the information recording substrate is unknown.

The fourth object is also achieved by the provision of an optical disk, comprising:

• • an information recording substrate having a thin thickness, the thin thickness of the information recording substrate being thinner than that of a compact disk;
  • a plurality of first recording pits placed at a first region of the information recording substrate for recording pieces of recording information at a high recording density, the first recording pits being formed at narrow intervals; and
  • a plurality of second recording pits placed at a second region of the information recording substrate for recording pieces of distinguishing information at a low recording density, the distinguishing information informing that the recording information are recorded on the information recording substrate having the thin thickness, the recording density of the recording information being higher than that of the distinguishing information, each of the second recording pits being larger than that of a recording pit in the compact disk, and a converging spot of a beam of reproducing light, which is converged to focus on an ordinary recording pit formed on a substrate having an ordinary thickness of the compact disk, being formed in one of the second recording pits to read the distinguishing information.

In the above configuration, a beam of reproducing light, of which a curvature is adjusted to focus the reproducing light on a recording pit formed on an information recording substrate of the compact disk, is incident on a prescribed region of an unknown disk selected from a group of the compact disk having an ordinary thickness and the optical disk according to the present invention. In cases where the unknown disk is the optical disk, the reproducing light is converged on one of the second recording pits in defocus because the information recording substrate of the optical disk has the thin thickness. However, because each of the second recording pits is large in size, a converging spot of the reproducing light is formed in the second recording pit. Therefore, a piece of distinguishing information is read by the reproducing light. Because the distinguishing information informs that pieces of recording information are recorded on the information recording substrate having the thin thickness, a curvature of the reproducing light is automatically changed to focus the reproducing light on the information recording substrate having the thin thickness, and the reproducing light is automatically focused on one of the first recording pits. Therefore, a piece of recording information is reproduced.

In contrast, in cases where the unknown disk is the compact disk, a piece of recording information is read by the reproducing light in the same manner as in a prior art.

Accordingly, a piece of recording information formed on an information recording substrate can be reliably reproduced even though the thickness of the information recording substrate is unknown.

The fifth object is achieved by the provision of an optical disk apparatus for recording or reproducing pieces of recording information on or from an optical disk in which the recording information are recorded or reproduced at a high density on or from a first substrate having a first thickness and a piece of distinguishing information informing that the recording information are recorded or reproduced on or from the first substrate having the first thickness is recorded at an ordinary density on a second substrate having a second thickness larger than the first thickness, comprising:

• • rotating means for rotating the optical disk at a regular speed;
  • a light source for radiating a beam of incident light;
  • hologram means for transmitting a part of the incident light radiated from the light source without any diffraction on an outgoing path to form a beam of transmitted light and diffracting a remaining part of the incident light radiated from the light source on the outgoing path to form a beam of diffracted light, the hologram means functioning as a lens for the diffracted light to diverge the diffracted light from the hologram means;
  • lens means for converging the transmitted light formed in the hologram means on the first substrate of the optical disk rotated by the rotating means to record or reproduce a piece of recording information on or from the optical disk and converging the diffracted light formed in the hologram means on the second substrate of the optical disk rotated by the rotating means to reproduce the distinguishing information from the optical disk, the transmitted light being reflected by the first substrate of the optical disk and again passing through the lens means and the hologram means on an incoming path, and the diffracted light being reflected by the second substrate of the optical disk and again passing through the lens means and the hologram means on the incoming path;
  • wavefront changing means for changing a wavefront of the transmitted light passing through the lens means and the hologram means on the incoming path to form one or more beams of recording information light and changing a wavefront of the diffracted light passing through the lens means and the hologram means on the incoming path to form one or more beams of distinguishing information light;
  • detecting means for detecting intensities of the recording information light formed by the wavefront changing means to generate a recording information signal according to the intensities of the recording information light and detecting intensities of the distinguishing information light formed by the wavefront changing means to generate a distinguishing information signal according to the intensities of the distinguishing information light, the distinguishing information signal expressing the distinguishing information recorded on the second substrate of the optical disk, and the recording information signal expressing the recording information recorded on the first substrate of the optical disk; and
  • moving means for moving an optical head apparatus comprising the light source, the hologram means, the lens means and the detecting means to converge the diffracted light formed in the hologram means on the second substrate of the optical disk and moving the optical disk, in which the diffracted light formed in the hologram means is converged on the second substrate of the optical disk, to converge the transmitted light formed in the hologram means on the first substrate of the optical disk in cases where the distinguishing information is detected in the detecting means.

In the above configuration, an optical head apparatus comprising the light source, the hologram means, the lens means and the detecting means has the same configuration as that described before. Initially, the optical head apparatus is moved by the moving means to converge in focus the diffracted light formed in the hologram means on the second substrate of the optical disk rotated by the rotating means. Therefore, the distinguishing information recorded on the second substrate is reproduced in the detecting means, and it is informed that pieces of recording information are recorded or reproduced on or from the first substrate having the first thickness. Thereafter, the optical head apparatus is moved by the moving means to converge in focus the transmitted light formed in the hologram means on the first substrate of the optical disk rotated by the rotating means. Therefore, a piece of recording information is recorded or reproduced on or from the first substrate of the optical disk.

Accordingly, even though a high density type of optical disk, in which pieces of recording information is recorded or reproduced on or from the substrate having the first thickness smaller than the second thickness of a conventional optical disk, is utilized, the recording information can be reliably recorded or reproduced.

The fifth object is also achieved by the provision of an optical disk apparatus for recording or reproducing pieces of recording information on or from an optical disk in which the recording information are recorded or reproduced at a high density on or from a first substrate having a thin thickness thinner than that of a compact disk and a piece of distinguishing information informing that the recording information are recorded or reproduced on or from the first substrate having the thin thickness is recorded at a low density on a second substrate having the thin thickness, comprising:

• • rotating means for rotating the optical disk at a regular speed;
  • a light source for radiating a beam of incident light;
  • hologram means for transmitting a part of the incident light radiated from the light source without any diffraction on an outgoing path to form a beam of transmitted light and diffracting a remaining part of the incident light radiated from the light source on the outgoing path to form a beam of diffracted light, the hologram means functioning as a lens for the diffracted light to diverge the diffracted light from the hologram means;
  • lens means for converging in focus the transmitted light formed in the hologram means on the first substrate of the optical disk rotated by the rotating means to record or reproduce a piece of recording information on or from the optical disk and converging in defocus the diffracted light formed in the hologram means on the second substrate of the optical disk rotated by the rotating means to reproduce the distinguishing information from the optical disk, the transmitted light being reflected by the first substrate of the optical disk and again passing through the lens means and the hologram means on an incoming path, and the diffracted light being reflected by the second substrate of the optical disk and again passing through the lens means and the hologram means on the incoming path;
  • wavefront changing means for changing a wavefront of the transmitted light passing through the lens means and the hologram means on the incoming path to form one or more beams of recording information light and changing a wavefront of the diffracted light passing through the lens means and the hologram means on the incoming path to form one or more beams of distinguishing information light;
  • detecting means for detecting intensities of the recording information light formed by the wavefront changing means to generate a recording information signal according to the intensities of the recording information light and detecting intensities of the distinguishing information light formed by the wavefront changing means to generate a distinguishing information signal according to the intensities of the distinguishing information light, the distinguishing information signal expressing the distinguishing information recorded on the second substrate of the optical disk, and the recording information signal expressing the recording information recorded on the first substrate of the optical disk; and
  • moving means for moving an optical head apparatus comprising the light source, the hologram means, the lens means and the detecting means, to converge the diffracted light formed in the hologram means in defocus on the second substrate of the optical disk and moving the optical disk, in which the diffracted light formed in the hologram means is converged on the second substrate of the optical disk in defocus, to converge the transmitted light formed in the hologram means on the first substrate of the optical disk in focus in cases where an intensity of the distinguishing information signal generated in the detecting means is larger than a threshold value.

In the above configuration, an optical head apparatus comprising the light source, the hologram means, the lens means and the detecting means has the same configuration as that described before. Initially, the optical head apparatus is moved by the moving means to converge in defocus the diffracted light formed in the hologram means on the second substrate of the optical disk rotated by the rotating means. In this case, because the distinguishing information is recorded at a low density, a plurality of recording pits expressing the distinguishing information are respectively large in size. Therefore, even though the diffracted light is converged on each of the recording pits in defocus, a converging spot of the diffracted light is formed in each of the recording pits. Therefore, the distinguishing information recorded on the second substrate is reproduced in the detecting means, and it is informed that pieces of recording information are recorded or reproduced on or from the first substrate having the thin thickness. Thereafter, the optical head apparatus is moved by the moving means to converge in focus the transmitted light formed in the hologram means on the first substrate of the optical disk rotated by the rotating means. Therefore, a piece of recording information is recorded or reproduced on or from the first substrate of the optical disk.

Accordingly, even though a high density type of optical disk, in which pieces of recording information are recorded or reproduced on or from the substrate having the thin thickness smaller than an ordinary thickness of a conventional optical disk, is utilized, the recording information can be reliably recorded or reproduced.

The sixth object is achieved by the provision of a binary focus microscope for simultaneously observing a first image put on a first image plane and a second image put on a second image plane, comprising:

• • an objective lens for refracting a beam of first light diverging from the first image and a beam of second light diverging from the second image, a first distance between the objective lens and the first image of the first image plane differing from a second distance between the objective lens and the second image of the second image plane;
  • a hologram lens for transmitting the first light refracted by the objective lens without any diffraction to form a beam of transmitted light and diffracting the second light refracted by the objective lens to form a beam of diffracted light, the hologram lens functioning as a lens for the second light to pass the diffracted light through the same optical path as the transmitted light passes through, and a beam of superposed light being formed of the transmitted light and the diffracted light;
  • an inner lens for converging the superposed light formed by the hologram lens at an image point of a third image plane to simultaneously form the first image and the second image enlarged on the third image plane; and
  • an ocular lens for converging the superposed light which is converged by the inner lens and diverges from the image point to simultaneously form the first image and the second image moreover enlarged.

In the above configuration, a beam of first light diverging from the first image and a beam of second light diverging from the second image are refracted together by the objective lens. In this case, because a first distance between the objective lens and the first image of the first image plane differs from a second distance between the objective lens and the second image of the second image plane, a curvature of the first light refracted differs from another curvature of the second light refracted. Thereafter, the first light refracted transmits through the hologram lens without any diffraction to form a beam of transmitted light, and the second light refracted is diffracted by the hologram lens to form a beam of diffracted light. In this case, because the hologram lens functions as a lens for the diffracted light, a curvature of the diffracted light agrees with that of the transmitted light. In other words, the diffracted light passes through the same optical path as the transmitted light passes through. Therefore, a beam of superposed light is formed of the transmitted light and the diffracted light. Thereafter, the superposed light is converged by the inner lens at an image point of a third image plane, so that the first image and the second image enlarged are simultaneously formed on the third image plane. Thereafter, the superposed light diverging from the image point is converged by the ocular lens, so that the first image and the second image moreover enlarged are simultaneously formed.

Accordingly, an operator can observe the first image and the second image sufficiently enlarged.

The sixth object is also achieved by the provision of a binary focus microscope for simultaneously observing a first image put on a first image plane and a second image put on a second image plane, comprising:

• • an objective lens for refracting a beam of first light diverging from the first image and a beam of second light diverging from the second image, a first distance between the objective lens and the first image of the first image plane differing from a second distance between the objective lens and the second image of the second image plane;
  • a hologram lens for transmitting the first light refracted by the objective lens without any diffraction to form a beam of transmitted light and diffracting the second light refracted by the objective lens to form a beam of diffracted light, the hologram lens functioning as a lens for the second light to pass the diffracted light through the same optical path as the transmitted light passes through, and a beam of superposed light being formed of the transmitted light and the diffracted light;
  • an inner lens for converging the superposed light formed by the hologram lens at an image point of a third image plane to simultaneously form the first image and the second image enlarged on the third image plane; and
  • photographing means for photographing a superposed image formed of the first and second images enlarged on the third image plane by converging the superposed light in the inner lens.

In the above configuration, the first image and the second image enlarged are simultaneously formed on the third image plane in the same manner. Thereafter, the first image and the second image enlarged are photographed by the photographing means as a superposed image.

Accordingly, the first image and the second image enlarged can be observed.

The seventh embodiment is achieved by the provision of an alignment apparatus for aligning a first reference image drawn on a photomask with a second reference image drawn on a sample, comprising:

• • a light source for radiating beams of alignment light to illuminate the first and second reference images;
  • an objective lens for refracting both a beam of first alignment light diverging from the first reference image and a beam of second alignment light diverging from the second reference image which are illuminated with the alignment light radiated from the light source, a first distance between the objective lens and the first reference image of the photomask differing from a second distance between the objective lens and the second reference image of the sample;
  • a hologram lens for transmitting the first alignment light refracted by the objective lens without any diffraction to form a beam of transmitted light and diffracting the second alignment light refracted by the objective lens to form a beam of diffracted light, the hologram lens functioning as a lens for the second alignment light to pass the diffracted light through the same optical path as the transmitted light passes through, and a beam of superposed light being formed of the transmitted light and the diffracted light;
  • an inner lens for converging the superposed light formed by the hologram lens at an image point of an image plane to simultaneously form the first and second reference images enlarged on the image plane, an optical axis passing through centers of the objective lens, the hologram lens and the inner lens;
  • photographing means for photographing a superposed image formed of the first and second images enlarged on the image plane by converging the superposed light in the inner lens; and
  • moving means for moving the photomask or the sample according to the superposed image photographed by the photographing means to align the first reference image with the second reference image along the optical axis.

In the above configuration, the objective lens, the hologram lens and the inner lens are the same as those in the binary focus microscope. Therefore, the first and second images enlarged on the image plane is photographed by the photographing means as a superposed image. Thereafter, the photomask or the sample is moved in a direction perpendicular to the optical axis by the moving means to align the first reference image with the second reference image along the optical axis.

Accordingly, because the superposed image formed of the first and second reference images enlarged is photographed by the photographing means, a relative position between the first and second reference images can be accurately observed. Therefore, the first reference image can be accurately aligned with the second reference image.

The eighth embodiment is achieved by the provision of a focusing method for focusing light on a first information medium having a first thickness or a second information medium having a second thickness to record or reproduce a piece of information on or from the first information medium or the second information medium, comprising the steps of:

• • moving an optical head apparatus in a direction to decrease or increase the distance between the optical head apparatus and the first or second information medium, the optical head apparatus comprising
  • a light source for radiating a beam of incident light,
  • hologram means for transmitting a part of the incident light radiated from the light source without any diffraction on an outgoing path to form a beam of transmitted light and diffracting a remaining part of the incident light radiated from the light source on the outgoing path to form a beam of diffracted light, the hologram means functioning as a lens for the diffracted light to diverge the diffracted light from the hologram means or converge the diffracted light,
  • lens means for converging the transmitted light formed in the hologram means at a first focal length on the outgoing path to form a first converging spot at a front surface of the first information medium or converging the diffracted light formed in the hologram means at a second focal length on the outgoing path to form a second converging spot at a front surface of the second information medium, the transmitted light being incident on a rear surface of the first information medium and being converged at the front surface of the first information medium, the transmitted light being reflected at the rear surface of the first information medium and again passing through the lens means and the hologram means on an incoming path, the diffracted light being incident on a rear surface of the second information medium and being converged at the front surface of the second information medium, and the diffracted light being reflected at the rear surface of the first information medium and again passing through the lens means and the hologram means on the incoming path,
  • wavefront changing means for changing a wavefront of the transmitted light or the diffracted light passing through the lens means and the hologram means on the incoming path to form one or more beams of information light, and
  • detecting means for detecting intensities of the information light formed by the wavefront changing means and generating an information signal and a focus error signal according to the intensities of the information light, the information signal expressing a piece of information recorded on the first information medium or the second information medium;
  • judging whether or not an intensity of the focus error signal generated in the detecting means is larger than a threshold value; and
  • adjusting the position of the optical head apparatus to decrease the intensity of the focus error signal to zero when the intensity of the focus error signal becomes larger than the threshold value.

In the above steps, the focusing method is performed by utilizing the optical head apparatus described above. The intensity of the focus error signal is largely increased when the distance between the lens means and the first or second information medium is near to a focal length of the lens means. Therefore, when the intensity of the focus error signal becomes larger than a threshold value, the lens means is placed near to a just-focus point in which the transmitted light or the diffracted light is converged on the first or second information medium in focus.

Accordingly, in cases where the position of the optical head apparatus is adjusted to decrease the intensity of the focus error signal to zero when the intensity of the focus error signal becomes larger than the threshold value, the transmitted light or the diffracted light can be focused on the first or second information medium.

The ninth embodiment is achieved by the provision of an information reproducing method for reproducing a piece of recording information from an optical disk in which the recording information are recorded at a high density on a first substrate having a first thickness and a piece of distinguishing information informing that the recording information are recorded on the first substrate is recorded at an ordinary density on a second substrate having a second thickness larger than the first thickness, comprising the step of:

• • moving an optical disk apparatus under the second substrate of the optical disk, the optical disk comprising
  • rotating means for rotating the optical disk at a regular speed,
  • a light source for radiating a beam of incident light;
  • hologram means for transmitting a part of the incident light radiated from the light source without any diffraction on an outgoing path to form a beam of transmitted light and diffracting a remaining part of the incident light radiated from the light source on the outgoing path to form a beam of diffracted light, the hologram means functioning as a lens for the diffracted light to diverge the diffracted light from the hologram means,
  • lens means for converging the transmitted light formed in the hologram means on the first substrate of the optical disk rotated by the rotating means to record or reproduce a piece of recording information on or from the optical disk and converging the diffracted light formed in the hologram means on the second substrate of the optical disk rotated by the rotating means to reproduce the distinguishing information from the optical disk, the transmitted light being reflected by the first substrate of the optical disk and again passing through the lens means and the hologram means on an incoming path, and the diffracted light being reflected by the second substrate of the optical disk and again passing through the lens means and the hologram means on the incoming path,
  • wavefront changing means for changing a wavefront of the transmitted light passing through the lens means and the hologram means on the incoming path to form one or more beams of recording information light and changing a wavefront of the diffracted light passing through the lens means and the hologram means on the incoming path to form one or more beams of distinguishing information light, and
  • detecting means for detecting intensities of the recording information light formed by the wavefront changing means to generate a recording information signal according to the intensities of the recording information light and detecting intensities of the distinguishing information light formed by the wavefront changing means to generate a distinguishing information signal according to the intensities of the distinguishing information light, the distinguishing information signal expressing the distinguishing information recorded on the second substrate of the optical disk, and the recording information signal expressing the recording information recorded on the first substrate of the optical disk; and
  • converging the diffracted light on the second substrate of the optical disk to reproduce the distinguishing information;
  • moving the optical disk apparatus to a position under the first substrate of the optical disk to converge the transmitted light on the first substrate of the optical disk when the distinguishing information is detected in the detecting means; and
  • reproducing the recording information by generating the recording information signal in the detecting means.

In the above steps, the information reproducing method is performed by utilizing the optical disk apparatus described above. The distinguishing information placed on the second substrate of the optical disk is reproduce with the diffracted light. In this case, because the second substrate has the second thickness, the diffracted light is just focused on the second substrate. Thereafter, when the distinguishing information is detected, the optical disk apparatus is moved to a position under the first substrate of the optical disk, and the transmitted light is converged on the first substrate of the optical disk. In this case, because the first substrate has the first thickness, the diffracted light is just focused on the first substrate.

Accordingly, the recording information can be reliably reproduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a constitutional view of a conventional optical head apparatus proposed in Japanese Patent Application No. 46630 of 1991;

FIGS. 2A,2B are respectively a cross sectional view of a set of an objective lens and a blazed hologram shown inFIG. 1;

FIG. 3 shows a relationship between the thickness of an optical disk and a numerical aperture of an objective lens on condition that the tilt of the optical axis is constant;

FIG. 4A is a constitutional view of an imaging optical system having a compound objective lens according to a first embodiment of the present invention, a beam of transmitted light not diffracted being converged on a thin type of information medium;

FIG. 4B is a constitutional view of the imaging optical system shown inFIG. 4A, a beam of first-order diffracted light being converged on a thick type of information medium;

FIG. 5 is a plan view of a hologram lens shown inFIGS. 4A,4B, a grating pattern of the hologram lens being depicted;

FIG. 6 is a cross sectional view of the hologram lens shown inFIG. 5, the grating pattern formed in relief on the hologram lens being shown;

FIG. 7 is an explanatory diagram showing an intensity distribution of transmitted light L4 converged on a converging spot S1 of a first information medium, a primary maximum and secondary maxima suppressed occurring in the converging spot S1;

FIG. 8A is a cross sectional view of the hologram lens shown inFIG. 5, the grating pattern approximating to a stepwise shape composed of four stairs being shown;

FIG. 8B is a cross sectional view of the hologram lens shown inFIG. 5, the grating pattern approximating to a stepwise shape composed of a plurality of stairs being shown;

FIG. 9A is a constitutional view of an imaging optical system having a compound objective lens according to a modification of the first embodiment, a beam of first-order diffracted light being converged on a thin type of information medium;

FIG. 9B is a constitutional view of the imaging optical system shown inFIG. 9A, a beam of transmitted light not diffracted being converged on a thick type of information medium;

FIG. 10A is a constitutional view of an imaging optical system having a compound objective lens according to a second embodiment of the present invention, a beam of transmitted light not diffracted being converged on a thin type of information medium;

FIG. 10B is a constitutional view of the imaging optical system shown inFIG. 10A, a beam of first-order diffracted light being converged on a thick type of information medium;

FIG. 11 shows a change of a diffraction efficiency of a hologram lens shown inFIGS. 10A,10B;

FIGS. 12A to12E are respectively a cross sectional view of the hologram lens shown inFIGS. 10A,10B, the grating pattern of the hologram lens approximating to a step-wise shape;

FIG. 13A shows an intensity distribution of incident light utilized in the second embodiment, a far field pattern of the incident light being distributed in a Gaussian distribution;

FIG. 13B shows an intensity distribution of transmitted light transmitting through a hologram lens shown inFIGS. 10A,10B, a far field pattern of the incident light being distributed in a gently-sloping shape;

FIGS. 14A to14C show intensity distributions of transmitted light and diffracted light transmitting through a hologram lens shown inFIGS. 10A,10B;

FIG. 15A is a plan view of a hologram lens according to a modification of the second embodiment, a grating pattern of the hologram lens being depicted;

FIGS. 15B,15C are respectively a constitutional view of an imaging optical system having a compound objective lens according to another modification of the second embodiment;

FIG. 16A is a constitutional view of an imaging optical system having a compound objective lens according to a third embodiment of the present invention, a beam of first-order diffracted light being converged on a thin type of information medium;

FIG. 16B is a constitutional view of the imaging optical system shown inFIG. 16A, a beam of transmitted light not diffracted being converged on a thick type of information medium;

FIG. 17 shows a change of a diffraction efficiency of a hologram lens shown inFIGS. 16A,16B;

FIGS. 18A to18C show intensity distributions of transmitted light and diffracted light transmitting through a hologram lens shown inFIGS. 16A,16B;

FIG. 19A is a cross sectional view of a compound objective lens according to a fourth embodiment of the present invention;

FIG. 19B is a cross sectional view of a compound objective lens according to a modification of the fourth embodiment of the present invention;

FIG. 20 is a cross sectional view of a compound objective lens according to a fifth embodiment of the present invention;

FIG. 21 is a constitutional view of an optical head apparatus according to a sixth embodiment of the present invention;

FIG. 22 is a plan view of a wavefront changing device utilized in the six, ninth and twelfth embodiments, a grating pattern of a hologram lens utilized as the wavefront changing device being depicted;

FIG. 23 shows a positional relation between focal points of diffracted light occurring in the wavefront changing device shown inFIG. 22 and a photo detector;

FIG. 24 is a plan view of a photo detector utilized in the six, ninth, tenth, twelfth, thirteenth and seventeenth embodiments;

FIG. 25A and 25C respectively show a converging spot of first-order diffracted light radiated to detecting sections SE1, SE2 and SE3 of a sextant photo-detector shown in FIG.24 and another converging spot of minus first-order diffracted light radiated to detecting sections SE4, SE5 and SE6 of the sextant photo-detector on condition that an objective lens shown inFIG. 21 is defocused on an information medium;

FIG. 25B shows a converging spot of first-order diffracted light radiated to the detecting sections SE1, SE2 and SE3 of the sextant photo-detector and another converging spot of minus first-order diffracted light radiated to the detecting sections SE4, SE5 and SE6 of the sextant photo-detector on condition that the objective lens is just focused on the information medium;

FIG. 26 shows a relationship between beams of diffracted light occurring in the wavefront changing device shown in FIG.22 and the photo detector shown inFIG. 24;

FIG. 27 is a constitutional view of an optical head apparatus according to a seventh embodiment;

FIG. 28 is a plan view of a photo detector utilized in the seven, ninth, tenth, twelfth and thirteenth embodiments;

FIGS. 29A,29B,29C show various shapes of converging spots converged on the photo detector shown inFIG. 28;

FIG. 29D shows a radial direction Dr and a tangential direction Dt;

FIG. 30 is a constitutional view of an optical head apparatus according to a first modification of the seventh embodiment;

FIG. 31 is a constitutional view of an optical head apparatus according to a second modification of the seventh embodiment;

FIG. 32 is a constitutional view of an optical head apparatus according to a third modification of the seventh embodiment;

FIG. 33 is a constitutional view of an optical head apparatus according to a forth modification of the seventh embodiment;

FIG. 34 shows a beam of transmitted light not diffracted on an incoming optical path and a beam of transmitted light diffracted on the incoming optical path, the beams being utilized to detect an information signal;

FIG. 35A graphically shows a change of a focus error signal obtained by detecting the intensity of transmitted light, the strength of the focus error signal depending on a distance between an objective lens and a first information medium;

FIG. 35B graphically shows a change of a focus error signal obtained by detecting the intensity of diffracted light, the strength of the focus error signal depending on a distance between an objective lens and a second information medium.

FIG. 36A graphically shows a change of a focus error signal obtained by detecting the intensity of diffracted light, the strength of the focus error signal depending on a distance between an objective lens and a first information medium;

FIG. 36B graphically shows a change of a focus error signal obtained by detecting the intensity of transmitted light, the strength of the focus error signal depending on a distance between an objective lens and a second information medium;

FIG. 37 is a constitutional view of an optical head apparatus according to a ninth embodiment;

FIG. 38 is a constitutional view of an optical head apparatus according to a tenth embodiment;

FIG. 39 is a plan view of a beam splitter having a reflection type of hologram utilized in the optical head apparatus shown inFIG. 38;

FIGS. 40A,40B are respectively a constitutional view of an optical head apparatus according an eleventh embodiment;

FIG. 41 is a plan view of a beam splitter having a reflection type of hologram utilized in the optical head apparatus shown inFIG. 38;

FIG. 42A and 42C respectively show a converging spot of first-order diffracted light radiated to detecting sections SE1, SE2 and SE3 of a sextant photo-detector shown in FIG.24 and another converging spot of minus first-order diffracted light radiated to detecting sections SE4, SE5 and SE6 of the sextant photo-detector on condition that diffracted light is converged in defocus on a second information medium;

FIG. 42B shows a converging spot of first-order diffracted light radiated to the detecting sections SE1, SE2 and SE3 of a sextant photo-detector shown in FIG.24 and another converging spot of minus first-order diffracted light radiated to the detecting sections SE4, SE5 and SE6 of the sextant photo-detector on condition that diffracted light is converged in focus on a second information medium;

FIG. 43 is a constitutional view of an optical head apparatus according to a twelfth embodiment;

FIG. 44 is a constitutional view of an optical head apparatus according to a thirteenth embodiment;

FIG. 45 is a constitutional view of an optical head apparatus according to a fourteenth embodiment;

FIG. 46 is a plan view of a hologram lens utilized in the optical head apparatus shown inFIG. 45;

FIG. 47 is a constitutional view of an optical head apparatus according to a fifteenth embodiment;

FIG. 48 is a plan view of a hologram lens utilized in the optical head apparatus shown inFIG. 47;

FIGS. 49A,49B respectively show a positional relation between unnecessary light occurring in the hologram lens shown inFIG. 48 and a photo detector shown inFIG. 47;

FIG. 50 is a constitutional view of an optical head apparatus according to a sixteenth embodiment;

FIG. 51 is a diagonal view of a light source and photo detectors utilized in the optical head apparatus shown inFIG. 50;

FIG. 52 is a constitutional view of an optical head apparatus according to a seventeenth embodiment;

FIG. 53 is a diagonal view of a high density optical disk according to an eighteenth embodiment, a cross sectional view of the disk being partially shown;

FIG. 54 is a diagonal view of a high density optical disk according to a nineteenth embodiment, a cross sectional view of the disk being partially shown;

FIG. 55 is a block diagram of an optical disk apparatus with one of the optical head apparatuses shown inFIGS. 21,27,30,31,32,33,37,38,40A,43,44,50 and52 according to a twentieth embodiment;

FIG. 56 is a flow chart showing the operation of the optical disk apparatus shown inFIG. 55;

FIG. 57 is a block diagram of an optical disk apparatus with one of the optical head apparatuses shown inFIGS. 21,27,30,31,32,33,37,38,40A,43,44,50 and52 according to a twenty-first embodiment;

FIG. 58 is a flow chart showing the operation of the optical disk apparatus shown inFIG. 57;

FIG. 59 is a constitutional view of a binary focus microscope according to a twenty-second embodiment;

FIG. 60 is a partial view of the binary focus microscope shown inFIG. 59 in cases where first and second samples are located at bottoms of sample holders;

FIG. 61 is a constitutional view of a binary focus microscope according to a modification of the twenty-second embodiment;

FIG. 62 is a constitutional view of an alignment apparatus according to a twenty-third embodiment;

Fig. 63 is a constitutional view of the optical disk apparatus of which the block diagram is shown inFIG. 55;

FIG. 64 is a constitutional view of an exposure apparatus in which the alignment apparatus is included;

FIG. 65 is a diagonal view of an image reproducing apparatus for which theoptical disk apparatus176 is provided;

FIG. 66 is a diagonal view of a voice reproducing apparatus for which theoptical disk apparatus176 is provided;

FIG. 67 is a diagonal view of an information processing apparatus for which theoptical disk apparatus176 is provided; and

FIG. 68 is a diagonal view of another information processing apparatus for which theoptical disk apparatus176 is provided.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of a compound objective lens, an imaging optical system, an optical head apparatus, an optical disk, an optical disk apparatus, a binary focus microscope and an alignment apparatus according to the present invention are described with reference to drawings.

(First Embodiment)

FIG. 4A is a constitutional view of an imaging optical system having a compound objective lens according to a first embodiment of the present invention, a beam of transmitted light not diffracted being converged on a thin type of information medium.FIG. 4B is a constitutional view of the imaging optical system shown inFIG. 4A, a beam of first-order diffracted light being converged on a thick type of information medium.FIG. 5 is a plan view of a hologram lens shown inFIGS. 4A,4B, a grating pattern of the hologram lens being depicted.

As shown inFIGS. 4A,4B, an imagingoptical system21 for converging light on afirst substrate22 of a thin type of first information medium23 (a thickness T1) or asecond substrate24 of a thick type of second information medium25 (a thickness T2) to form a diffraction-limited converging spot comprises a blazedhologram lens26 for transmitting a part of incident light L3 radiated from a light source without any diffraction to form a beam of transmitted light L4 and diffracting a remaining part of the incident light L3 to form a beam of first-order diffracted light L5, and anobjective lens27 for converging the transmitted light L4 on thefirst information medium23 or converging the first-order diffracted light L5 on thesecond information medium25.

Thefirst information medium23 represents a prospective optical disk having a high density memory capacity, and the thickness T1 of thefirst information medium23 ranges from 0.4 mm to 0.8 mm. Thesecond information medium25 represents a compact disk or a laser disk appearing on the market now, and the thickness T2 of thesecond information medium25 is about 1.2 mm.

The term “convergence” denotes in this specification that divergent light or collimated light is focused to form a diffraction-limited micro spot.

In the above configuration, a part of incident light L3 collimated transmits through thehologram lens26 without any diffraction, and a beam of transmitted light L4 (that is, a beam of zero-order diffracted light L4) is formed. Thereafter, the transmitted light L4 is converged by theobjective lens27. Also, a remaining part of the incident light L3 is diffracted and refracted by thehologram lens26, and a beam of first-order diffracted light L5 is formed. In this case, thehologram lens26 selectively functions as a concave lens for the first-order diffracted light L5, so that the first-order diffracted light L5 diverges from thehologram lens26. Thereafter, the first-order diffracted light L5 is converged by theobjective lens27.

In cases where the thin type offirst information medium23 is utilized to record or reproduce pieces of information on or from a front surface of the medium23, as shown inFIG. 4A, the transmitted light L4 is incident on a rear surface of thefirst information medium23 and is focused on its front surface by theobjective lens27 to form a diffraction-limited converging spot S1 on thefirst information medium23. In contrast, in cases where the thick type ofsecond information medium25 is utilized to record or reproduce pieces of information on or from a front surface of the medium25, the diffracted light L5 is incident on a rear surface of thesecond information medium25 and is focused on its front surface to form a diffraction-limited converging spot S2 on thesecond information medium25. Because thehologram lens26 functions as a concave lens to diverge the first-order diffracted light L5, the diffraction-limited converging spots S1, S2 are formed even though the thickness T1 of thefirst information medium23 differs from the thickness T2 of thesecond information medium25. Therefore, a compoundobjective lens29 composed of thehologram lens26 and theobjective lens27 has substantially two focal points.

As shown inFIG. 5, thehologram lens26 is formed by drawing a grating pattern P1 in apattern region26A of atransparent substrate28 in a concentric circle shape. Thepattern region26A is positioned in a center portion of thetransparent substrate28, and a no-pattern region26B is positioned in a peripheral portion of thetransparent substrate28 to surround thepattern region26A. An optical axis of the imagingoptical system21 passes through a central point of the grating pattern P1 and a central axis of theobjective lens27.

In addition, as shown inFIG. 6, the grating pattern P1 of thehologram lens26 is formed in relief to produce a phase modulation type of hologram lens. That is, blocks which each are composed of a bottom portion and a top portion are concentrically formed in the grating pattern P1. The height H of the relief in the grating pattern P1 is set to:
H <λ/(n(λ)−1),  (1)
where the symbol λdenotes a wavelength of the incident light L3 and the symbol n(λ) denotes a refractive index of thetransparent substrate28 for the incident light L3. In this case, a difference in phase modulation degree between the incident light L3 transmitting through a bottom portion of the grating pattern P1 and the incident light L3 transmitting through a top portion of the grating pattern P1 is lower than 2π radians. Therefore, a diffraction efficiency of thehologram lens26 for the incident light L3 transmitting through the grating pattern P1 is less than 100% to generate the light L4 transmitting through the grating pattern P1. Also, the incident light L3 transmitting through the no-pattern region26B is not diffracted. As a result, the intensity of the transmitted light L4 can be sufficient to record or reproduce pieces of information on or from thefirst information medium23.

Also, because the intensity of the transmitted light L4 is sufficient over the entire surface of thehologram lens26, secondary maxima (side lobes) of the transmitted light L4 undesirably occurring in the converging spot S1 can be suppressed. In detail, as an intensity distribution of the transmitted light L4 converged on the converging spot S1 is shown inFIG. 7, a primary maximum (a main lobe) of the transmitted light L4 positioned in a center of the converging spot S1 is utilized to record or reproduce a piece of information on or from thefirst information medium23, and secondary maxima positioned around the primary maximum are unnecessary because the secondary maxima deteriorate a recording pit or a reproducing signal formed by the primary maximum.

The grating pattern P1 ofhologram lens26 formed in relief is blazed as shown inFIG. 6, so that the occurrence of minus first-order diffracted light is considerably suppressed. Therefore, the intensity sum of the transmitted light L4 and the first-order diffracted light L5 is maximized. In other words, a utilization efficiency of the incident light L3 is enhanced.

The numerical aperture NA of theobjective lens27 is equal to or more than 0.6. Also, when the transmitted light L4 is converged by theobjective lens27, the diffraction-limited converging spot S1 is formed on thefirst information medium23 having a thickness T1.

A diameter of thehologram lens26 is almost the same as an aperture of theobjective lens27, so that a diameter of thepattern region26A is smaller than the aperture of theobjective lens27. Because the incident light L3 transmitting through the no-pattern region26B is not diffracted, not only the light L4 transmitting through thepattern region26A but also the light L4 transmitting through the no-pattern region26B are converged on thefirst information medium23 by theobjective lens27 having a high numerical aperture. Therefore, the intensity of the transmitted light L4 converged at the converging point S1 can be increased. In contrast to the transmitted light L4, only the incident light L3 transmitting through thepattern region26A of thehologram lens26 is changed to the first-order diffracted light L5, and the first-order diffracted light L5 is converged on thesecond information medium25 by theobjective lens27 having substantially a low numerical aperture.

The phase of the light L4 transmitting through the grating pattern P1 of thepattern region26A is determined by an average value of the phase modulation degrees in the light L4 transmitting through the bottom and top portions of the grating pattern P1. In contrast, because the height of the no-pattern region26B is constant, the phase of the light L4 transmitting through the no-pattern region26B is modulated at a phase modulation degree. Therefore, as shown inFIG. 6, the height of the no-pattern region26B is set even with an average height of the grating pattern P1 to enhance the convergence function of theobjective lens27.

For example, as shown inFIG. 8A, in cases where each block of the grating pattern P1 in thehologram lens26 shown inFIG. 6 approximates to a step-wise shape composed of four stairs, a first step is etched at a depth h1+h2 and a width W1, a second step is etched at a depth hi and a width W2, a third step is etched at a depth h2 and a width W2, and a fourth step is etched at a width W1. Therefore, the grating pattern P1 approximating to the step-wise shape is formed in thepattern region26A. Thereafter, a peripheral portion of thetransparent substrate28 is etched by a depth h1 or h2 to form the no-pattern region26B. Therefore, the height of the no-pattern region26B is almost the same as an average height of thepattern region26A, so that the phase of the light L4 transmitting through thepattern region26A is almost the same as that of the light L4 transmitting through the no-pattern region26B.

In addition, as shown inFIG. 8B, an ideal blazed shape of thehologram lens26 shown inFIG. 6 can approximate to a step-wise shape which is obtained by etching a center portion of thetransparent substrate28 many times. In this case, the height H0 of the step-wise shape is set to satisfy an equation H0<λ/(n(λ)−1) so that the difference in phase modulation degree is set to a value lower than 2π radians. Specifically, in cases where the step-wise shape of thehologram lens26 is composed of a flight of N stairs having the same difference n_oin level, the difference n_oin level is set to satisfy an equation n_o<λ/{(n(λ)−1)*N} to set the difference in phase modulation degree of each stairs to a value lower than 2π/N radians. A peripheral portion of thetransparent substrate28 is etched to set the thickness of the no-pattern region26B to a thickness of thepattern region26A at one of the N stairs which is not the top stair or the bottom stair. Therefore, the height of the no-pattern region26B is almost the same as an average height of thepattern region26A, so that the phase of the light L4 transmitting through thepattern region26A is almost the same as that of the light L4 transmitting through the no-pattern region26B.

The grating pattern P1 of thehologram lens26 is designed to correct any aberration occurring in theobjective lens27 and thesecond information medium25, so that the first-order diffracted light L5 transmits through thesecond information medium25 having a thickness T2 and is converged on the medium25 to form the diffraction-limited converging spot S2 without any aberration. A method for designing thehologram lens26 having an aberration correcting function is described.

After the first-order diffracted light L5 is converged on thesecond information medium25, spherical waves diverge from the converging spot S2 and transmit through thesecond substrate24 and theobjective lens27. Thereafter, the spherical waves transmit through thetransparent substrate28 and optically interfere with the incident light L3. Therefore, an interference pattern is formed by the interference between the spherical waves and the incident light L3. The interference pattern can be calculated by subtracting the phase of the spherical waves from an inverted phase obtained by inverting the phase of the incident light L3. Accordingly the grating pattern P1 of thehologram lens26 which agrees with the interference pattern calculated can be easily formed according to a computer generated hologram technique.

Accordingly, because the compoundobjective lens29 is composed of theobjective lens27 and thehologram lens26 in which a part of the incident light L3 is diffracted and refracted, a diffraction-limited converging spot can be reliably formed on an information medium regardless of whether the information medium has a thickness T1 or a thickness T2. Also, two diffraction-limited converging spots can be simultaneously formed on an information medium at difference depths. In other words, the compound objective lens has substantially two focal points.

Also, because the diffraction efficiency of thehologram lens26 is less than 100% and the intensity of the light L4 transmitting through thehologram lens26 is sufficient to record or reproduce information on or from thefirst information medium23, the secondary maxima of the transmitted light L4 converged on the converging spot S1 can be suppressed.

Also, because thehologram lens26 is blazed, the occurrence of minus first-order diffracted light can be considerably suppressed. Therefore, the intensity sum of the transmitted light L4 and the first-order diffracted light L5 can be maximized, and a utilization efficiency of the incident light L3 can be enhanced.

Also, because thehologram lens26 functions as a lens only for the first-order diffracted light, the position of the converging point S1 formed by the transmitted light L4 differs from that of the converging point S2 formed by the first-order diffracted light L5 in an optical axis direction. Therefore, when the transmitted light L4 is converged in focus on an information recording plane of the information medium23 to record or read a piece of information, the first-order diffracted light L5 converged on theinformation medium23 is out of focus at the information recording plane. In the same manner, when the first-order diffracted light L5 is converged in focus on an information recording plane of theinformation medium25, the transmitted light L4 converged on theinformation medium25 is out of focus at the information recording plane. Accordingly, when the light L4 (or L5) is converged on the converging spot S1 (or S2) in focus to record or read the information, the light L5 (or L4) not converged on the converging spot S1 (or S2) in focus does not adversely influence on the recording or reading of the information. To reliably prevent the adverse influence on the recording or reading of the information, a difference in the optical axis direction between the converging spots S1, S2 is required to be equal to or more than 50 μm. That is, when the difference is equal to or more than 50 μm, the light L5 (or L4) largely diverges to reduce the intensity of the light L5 (or L4) at an information recording plane when the light L4 (or L5) is converged on the converging spot S1 (or S2) of the information recording plane at a high intensity.

Also, because the thickness T2 of thesecond information medium25 representing the compact disk or the laser disk is about 1.2 mm and because the thickness T1 of thefirst information medium23 representing a prospective optical disk ranges from 0.4 mm to 0.8 mm, the difference in the optical axis direction between the converging points S1, S2 is required to be equal to or less than 1.0 mm by considering a moving range of an actuator with which the position of the compoundobjective lens29 composed of theobjective lens27 and thehologram lens26 is adjusted according to a focus servo signal. Because thehologram lens26 functions as a concave lens for the first-order diffracted light, the difference between the converging points S1, S2 can be increased to about 1 mm.

Accordingly, even though the transmitted light L4 and the first-order diffracted light L5 are simultaneously converged by theobjective lens27, no adverse influence is exerted on the recording or reproduction of the information on condition that the difference between the converging points S1, S2 ranges from 50 μm to 1 mm.

Examples of the utilization of the imagingoptical system21 for various types of optical disks are described.

In cases where the imageoptical system21 is utilized for an optical disk device in which pieces of information recorded in a thin type of high density optical disk and a thick type of compact disk are exclusively reproduced, the diffraction efficiency of thehologram lens26 for changing the incident light L3 to the diffracted light L5 is set in a range from about 20% to 70%. In this case, the intensity of the transmitted light L4 converged on the high density optical disk is almost the same as that of the first-order diffracted light L5 converged on the compact disk. Therefore, the output power of the incident light L3 can be minimized.

Also, in cases where the imageoptical system21 is utilized for an optical disk device in which pieces of information recorded in a thin type of high density optical disk are recorded or reproduced and pieces of information recorded in a thick type of optical disk are exclusively reproduced, the diffraction efficiency of thehologram lens26 for changing the incident light L3 to the first-order diffracted light L5 is set to a value equal to or lower than 30%. In this case, even though a high intensity of transmitted light L4 is required to record a piece of information on the high density optical disk, the recording of the information can be reliably performed without increasing the intensity of the incident light L3 because a transmission efficiency of thehologram lens26 for the incident light L3 is high. In other words, a utilization efficiency of the incident light L3 can be enhanced when a piece of information is recorded on the high density optical disk, so that the output power of the incident light L3 can be minimized.

In the first embodiment, thehologram lens26 functions as a concave lens for the first-order diffracted light L5. However, it is applicable that ahologram lens26M functioning as a convex lens for the first-order diffracted light L5 be utilized in place of thehologram lens26. That is, as shown inFIGS. 9A,9B, the diffracted light L5 is converged on thefirst information medium23 by theobjective lens27 to form the diffraction limited converging spot S1, and the transmitted light L4 is converged on thesecond information medium25 by theobjective lens27 to form the diffraction limited converging spot S2. In this case, the difference between the converging points S1, S2 is required to be equal to or less than 0.5 mm by considering the moving range of the actuator. However, the occurrence of a chromatic aberration can be prevented in an imagingoptical system21M in which thehologram lens26M functioning as a concave lens for the diffracted light L5 is utilized. The achromatization function in the imaging optical system is described in detail.

When a focal length of thehologram lens26M for the incident light L3 having a wavelength λ_ois represented by f_Hoand another focal length of thehologram lens26M for the incident light L3 having a wavelength λ_lis represented by f_Hl, an equation (2) is satisfied.
f_H1=f_Ho×λ_o/λ_l  (2)
The focal length f_Hof thehologram lens22 is shortened as the wavelength λ of the incident light L3 becomes longer. Also, when a refractive index of theobjective lens27 for the incident light L3 having a wavelength λ_ois represented by n(λ_o) and another refractive index of theobjective lens27 for the incident light L3 having a wavelength λ_lis represented by n(λ_l), a focal length f_D(λ) of theobjective lens27 for the incident light L3 having a wavelength λ is formulated by an equation (3).
f_D(λ_l)=f_D(λ_o)×(n(λ_o)−1)/(n(λ_l)−1)  (3)
The focal length f_D(λ) of theobjective lens27 is lengthened as the wavelength λ of the incident light L3 becomes longer. That is, the dependence of the focal length f_D(λ) on the wavelength λ in theobjective lens27 is opposite to that of the focal length f_Hon the wavelength λ in thehologram lens26M. Therefore, a condition that the compoundobjective lens29M composed of theobjective lens27 and thehologram lens26M functions as an achromatic lens is formulated by an equation (4).
1/f_Ho+1/f_D(λ_o)=1/f_Hl+1/f_D(λ_l)=1/(f_Ho×λ_o/λ_l)+(n(λ_l)−1)/{f_D(λ_o)×(n(λ_o)−1)}  (4)

Accordingly, because the dependence of the focal length f_D(λ) on the wavelength λ in theobjective lens27 is opposite to that in thehologram lens22, the compoundobjective lens29M having an achromatic function can be formed by the combination of thelenses26M,27, and the occurrence of the chromatic aberration can be prevented. Also, even though the equation (4) is not strictly satisfied, the occurrence of the chromatic aberration can be largely suppressed.

Also, a curvature of theobjective lens27 can be small because thehologram lens26M functions as a convex lens for the first-order diffracted light L5. Also, because thehologram lens26M is a plane type of element, a lightweight type of compound objective lens having an achromatic function can be made in large scale manufacture. A principal of the achromatization has been proposed in a first literature (D. Faklis and M. Morris, Photonics Spectra(1991), November p.205 & December p.131), a second literature (M. A. Gan et al., S.P.I.E.(1991), Vol.1507, p.116), and a third literature (P. Twardowski and P. Meirueis, S.P.I.E.(1991), Vol.1507, p.55).

(Second Embodiment)

FIG. 10A is a constitutional view of an imaging optical system having a compound objective lens according to a second embodiment of the present invention, a beam of transmitted light not diffracted being converged on a thin type of information medium.FIG. 10B is a constitutional view of the imaging optical system shown inFIG. 10A, a beam of first-order diffracted light being converged on a thick type of information medium.

As shown inFIGS. 10A,10B, an imagingoptical system31 for converging light on thefirst substrate22 of the first information medium23 (the thickness T1) or thesecond substrate24 of the second information medium25 (the thickness T2) to form a diffraction-limited converging spot, comprises a blazedhologram lens32 for transmitting a part of incident light L3 without any diffraction to form a beam of transmitted light L4 and diffracting a remaining part of incident light L3 to form a beam of first-order diffracted light L5, and theobjective lens27 for converging the transmitted light L4 on thefirst information medium23 or converging the first-order diffracted light L5 on thesecond information medium25.

Thehologram lens32 is formed by drawing a grating pattern P2 in apattern region32A of thetransparent substrate28 in a concentric circle shape. Thepattern region32A is positioned in a center portion of thetransparent substrate28. An diameter of the grating pattern P2 is equal to or larger than an aperture of theobjective lens27. Also, a diffraction efficiency of thehologram lens32 for the incident light L3 transmitting through the grating pattern P2 is less than 100% in the same manner as in the first embodiment, so that the intensity of the transmitted light L4 is sufficient to record or reproduce a piece of information on or from thefirst information medium23.

In addition, as shown inFIG. 11, the diffraction efficiency in a central portion of thepattern region32A is high, and the diffraction efficiency is gradually decreased toward an outer direction of thepattern region32A. In other words, in cases where the grating pattern P2 of thehologram lens32 is formed in relief, the height H of the relief in the grating pattern P2 is gradually lowered toward the outer direction of thepattern region32A. Or, in cases where an ideal blazed shape of thehologram lens26 approximates to a step-wise shape, each block of the grating pattern P2 positioned in the central portion of thetransparent substrate28 is formed in a step-wise shape shown inFIG. 12A in which an inclined angle θ₁of stairs is large and a relationship W1 >W2 between a first etching width W1 and a second etching width W2 is satisfied, and the grating pattern P2 formed in the step-wise shape shown inFIG. 12A is gradually changed by decreasing the first etching width W1 and increasing the second etching width W2 toward the outer direction of thepattern region32A while the height H of the grating pattern P2 is gradually decreased. Therefore, each block of the grating pattern P2 positioned in a peripheral portion of thetransparent substrate28 is formed in a step-wise shape shown inFIG. 12B in which an inclined angle θ₂of stairs is small and a relationship W1<W1 between the etching widths is satisfied. Also, each block of the grating pattern P2 positioned in a middle portion between the central and peripheral portions is formed in a step-wise shape shown inFIG. 12C in which the etching widths W1, W2 is the same.

In the above configuration of the imagingoptical system31, a part of the incident light L3 transmits through thehologram lens32 without any diffraction to form a beam of transmitted light L4, and the transmitted light L4 is converged by theobjective lens27. Also, a remaining part of the incident light L3 is diffracted and refracted by thehologram lens32. In this case, thehologram lens32 functions as a concave lens for the incident light L3, so that a first-order diffracted light L5 diverges from thehologram lens32. Thereafter, the first-order diffracted light L5 is converged by theobjective lens27.

In cases where the thin type offirst information medium23 is utilized to record or reproduce pieces of information on or from a front surface of the medium23, as shown inFIG. 10A, the transmitted light L4 is incident on a rear surface of thefirst information medium23 and is focused on its front surface by theobjective lens27 to form a diffraction-limited converging spot S3 on thefirst information medium23. In this case, because the diffraction efficiency in the central portion of the grating pattern P2 is high and because the diffraction efficiency is gradually decreased toward the outer direction of the grating pattern P2, a diffraction probability of the incident light L3 is lowered in the peripheral portion of the grating pattern P2. Therefore, the light L4 transmits through theobjective lens27 on condition that the numerical aperture NA of theobjective lens27 is high.

In contrast, in cases where the thick type ofsecond information medium25 is utilized to record or reproduce pieces of information on or from a front surface of the medium25, the diffracted light L5 is incident on a rear surface of thesecond information medium25 and is focused on its front surface to form a diffraction-limited converging spot S5 on thesecond information medium25. In this case, because thehologram lens32 functions as a concave lens to diverge the first-order diffracted light L5, the diffraction-limited converging spots S3, S4 are formed even though the thickness T1 of thefirst information medium23 differs from the thickness T2 of thesecond information medium25. Therefore, a compoundobjective lens34 composed of thehologram lens32 and theobjective lens27 has substantially two focal points.

Accordingly, because the light L4 transmits through theobjective lens27 on condition that the numerical aperture NA of theobjective lens27 is high, the intensity of the transmitted light L4 converged on thefirst information medium23 can be high.

Also, in cases where the incident light L3 is radiated from a semiconductor laser, a far field pattern of the incident light L3 is distributed in a Gaussian distribution as shown in FIG.13A. Therefore, because the diffraction efficiency is gradually decreased toward the outer direction of the grating pattern P2, a far field pattern of the transmitted light L4 is distributed in a gently-sloping shape as shown in FIG.13B. In contrast to the second embodiment, because the incident light L3 is not diffracted in the no-pattern region26b of thehologram lens26 in the first embodiment, the intensity of the transmitted light L4 is suddenly increased at the peripheral portion of thehologram lens26.

Accordingly, secondary maxima of the transmitted light L4 converged on the converging spot S3 can be moreover suppressed in the second embodiment as compared with in the first embodiment. That is, the recording and reproducing of the information can be performed without any deterioration of the information by utilizing the imagingoptical system31.

In addition, in cases where the first-order diffracted light L5 is converged on thesecond information medium25 to form the diffraction-limited converging spot S4, a numerical aperture of theobjective lens27 for the first-order diffracted light L5 is low because the diffraction efficiency of thehologram lens32 is decreased toward an outer direction of thepattern region32A. As a result, the intensity of the first-order diffracted light L5 becomes lowered. In cases where the diffraction efficiency of thehologram lens32 is heightened to increase the intensity of the first-order diffracted light L5, the intensity of the transmitted light L4 at its inner beam portion is largely decreased, and secondary maxima (or side lobes) of the transmitted light L4 at the converging spot S3 is undesirably increased. Therefore, the incident light L3 of which the far field pattern is distributed in the Gaussian distribution is radiated to thehologram lens32 to increase the intensity of the first-order diffracted light L5 without any increase of the second maxima. In detail, as shown inFIG. 14A, the incident light L3 distributed not only in a central portion of the Gaussian distribution but also in a peripheral portion of the Gaussian distribution transmits through thehologram lens32 and is refracted by theobjective lens27 because the diameter of the grating pattern P2 is equal to or larger than the aperture of theobjective lens27. Therefore, a numerical aperture NA of theobjective lens27 at a light source side for the incident light L3 becomes higher than that in the first embodiment, and the diffraction efficiency of thehologram lens32 is heightened. As a result, the intensity of the first-order diffracted light L5 converged on thesecond information medium25 can be increased, as shown in FIG.14B. Also, because the intensity of the incident light L3 at the peripheral portion of the Gaussian distribution is low and because the diffraction efficiency of thehologram lens32 is increased toward the inner direction of thegrating pattern region32A, intensity of the transmitted light L4 is distributed in a gently-sloping shape as shown in FIG.14C. Accordingly, secondary maxima of the transmitted light L4 at the converging spot S3 can be suppressed.

Examples of the utilization of the imagingoptical system31 for various types of optical disks are described.

In cases where the imageoptical system31 is utilized for an optical disk device in which pieces of information recorded in a thin type of high density optical disk and a thick type of compact disk are exclusively reproduced, the diffraction efficiency of thehologram lens32 for the incident light L3 is set in a range from about 20% to 70%. In this case, the intensity of the transmitted light L4 converged on the high density optical disk is almost the same as that of the first-order diffracted light L5 converged on the compact disk. Therefore, the output power of the incident light L3 can be minimized.

Also, in cases where the imageoptical system31 is utilized for an optical disk device in which pieces of information recorded in a thin type of high density optical disk are recorded or reproduced and pieces of information recorded in a thick type of optical disk are exclusively reproduced, the diffraction efficiency of thehologram lens32 for the incident light L3 is set to a value equal to or lower than 30%. In this case, even though a high intensity of transmitted light L4 is required to record a piece of information on the high density optical disk, the recording of the information can be reliably performed without increasing the intensity of the incident light L3 because a transmission efficiency of thehologram lens32 for the incident light L3 is high. In other words, a utilization efficiency of the incident light L3 can be enhanced when a piece of information is recorded on the high density optical disk, so that the output power of the incident light L3 can be minimized.

In the second embodiment, the grating pattern P2 positioned in the central portion of thetransparent substrate28 is gradually changed toward the outer direction of thepattern region32A from the step-wise shape shown inFIG. 12A to the step-wise shape shown in FIG.12B through the step-wise shape shown in FIG.12C. However, because the occurrence of unnecessary diffracted light such as minus first-order diffracted light can be effectively prevented in the middle portion of thetransparent substrate28 in which the grating pattern P2 is formed in the step-wise shape shown inFIG. 12C, it is preferred that the middle portion occupy a large part of thepattern region32A of thehologram lens32. In this case, the intensity sum of the transmitted light L4 and the first-order diffracted light L5 can be maximized, so that a utilization efficiency of the incident light L3 can be enhanced.

Also, because the first etching width W1 of the grating pattern P2 is gradually decreased toward the outer direction of thepattern region32A, it is applicable that the grating pattern P2 formed in the step-wise shape shown inFIG. 12B be changed to a step-wise shape shown inFIG. 12D when the first width W1 is decreased to a value lower than about 1 μm. That is, a flight of four stairs shown inFIG. 12B is changed to a flight of two stairs. In this case, the grating pattern P2 formed in the step-wise shape shown inFIG. 12D can be easily made. In addition, in cases where a height H4 of the grating pattern P2 formed in the step-wise shape shown inFIG. 12D is moreover decreased toward the outer direction of thepattern region32A, it is preferred that the grating pattern P2 be formed in a step-wise shape shown in FIG.12E. That is, a third etching width W3 is gradually decreased toward the outer direction of thepattern region32A while decreasing a height H5 of the grating pattern P2. Therefore, the diffraction efficiency of thehologram lens32 can be gradually decreased toward the outer direction of thepattern region32A without any manufacturing difficulty of the grating pattern P2.

In addition, as shown inFIG. 15A, it is applicable that ahologram lens33 be formed, in place of thehologram lens32, by placing the grating pattern P1 of thepattern region32A in a central portion of thetransparent substrate28 and placing four types ofdiffraction regions33A,33B,33C and33D which surround the grating pattern P1. A part of the incident light L3 transmitting through each of thediffraction regions33A to33D is diffracted to control a transmission efficiency of thehologram lens33. In this case, the intensity of the transmitted light L4 at its peripheral portion is decreased, so that secondary maxima occurring in the converging spot S3 can be suppressed. Also, it is applicable that the grating pattern P1 of the hologram be replaced with the grating pattern P2. Also, it is applicable that grating directions of thediffraction regions33A to33D differ from each other. In this case, even though the first-order diffracted light L5 diffracted in thediffraction region33A is, for example, incident on the diffraction region33c after the diffracted light L5 is reflected by thesecond information medium25, the diffracted light L5 again diffracted in the diffraction region33c does not pass in parallel to the optical axis. Therefore, in cases where a piece of information read from thesecond information medium25 is detected in a detector to reproduce the information, the first-order diffracted light L5 diffracted in thediffraction regions33A to33D is not detected by the detector as stray light. Accordingly, the reproduction of the information does not deteriorate.

Also, as shown inFIG. 15B, it is applicable that thehologram lens32 function as a convex lens. In this case, the diffracted light L5 is converged on thefirst information medium23, and the transmitted light L4 is converged on thesecond information medium25, as shown in FIG.15C.

(Third Embodiment)

FIG. 16A is a constitutional view of an imaging optical system having a compound objective lens according to a third embodiment of the present invention, a beam of first-order diffracted light being converged on a thin type of information medium.FIG. 16B is a constitutional view of the imaging optical system shown inFIG. 16A, a beam of transmitted light not diffracted being converged on a thick type of information medium.

As shown inFIGS. 16A,16B, an imagingoptical system41 for converging light on thefirst substrate22 of the first information medium23 (the thickness T1) or thesecond substrate24 of the second information medium25 (the thickness T2) to form a diffraction-limited converging spot comprises a blazedhologram lens42 for transmitting a part of incident light L3 without any diffraction to form a beam of transmitted light L4 and diffracting a remaining part of incident light L3 to form a beam of first-order diffracted light L6, and theobjective lens27 for converging the first-order diffracted light L6 on thefirst information medium23 or converging the transmitted light L4 on thesecond information medium25.

Thehologram lens42 is formed by drawing a grating pattern P3 in apattern region42A of thetransparent substrate28 in a concentric circle shape. Thepattern region42A is positioned in a center portion of thetransparent substrate28. An diameter of the grating pattern P3 is equal to or larger than an aperture of theobjective lens27. Also, a diffraction efficiency of thehologram lens42 for the incident light L3 transmitting through the grating pattern P3 is less than 100% in the same manner as in the first embodiment, so that the intensity of the transmitted light L4 is sufficient to record or reproduce a piece of information on or from thesecond information medium25.

In addition, as shown inFIG. 17, the diffraction efficiency of thehologram lens42 is high in a peripheral portion of thepattern region42A, and the diffraction efficiency is gradually decreased toward an inner direction of thepattern region42A. In other words, in cases where the grating pattern P3 of thehologram lens42 is formed in relief, the height H of the relief in the grating pattern P3 is gradually lowered toward the inner direction of thepattern region42A. Or, in cases where an ideal blazed shape of thehologram lens26 approximates to a step-wise shape, each pitch of the grating pattern P3 positioned in the peripheral portion of thetransparent substrate28 is formed in a step-wise shape shown inFIG. 12A in which the inclined angle θ₁of stairs is large and the relationship W1>W2 between the first and second etching widths W1, W2 is satisfied, and the grating pattern P3 formed in the step-wise shape shown inFIG. 12A is gradually changed by decreasing the first etching width W1 and increasing the second etching width W2 toward the inner direction of thepattern region42A while the height H of the grating pattern P3 is gradually decreased. Therefore, each pitch of the grating pattern P3 positioned in a central portion of thetransparent substrate28 is formed in a stepwise shape shown inFIG. 12B in which the inclined angle θ₂of stairs is small and the relationship W1<W2 is satisfied. Also, each pitch of the grating pattern P3 positioned in a middle portion between the central and peripheral portions is formed in a step-wise shape shown inFIG. 12C in which the etching widths W1, W2 is the same.

In the above configuration of the imagingoptical system41, as shown inFIG. 16B, a part of the incident light L3 transmits through thehologram lens42 without any diffraction to form a beam of transmitted light L4, and the transmitted light L4 is converged by theobjective lens27. Also, a remaining part of the incident light L3 is diffracted by thehologram lens42 to form a beam of first-order diffracted light L6. In this case, thehologram lens42 functions as a convex lens for the incident light L3, so that a first-order diffracted light L6 formed in thehologram lens42 converges. Thereafter, the diffracted light L6 is converged by theobjective lens27.

In cases where the thin type offirst information medium23 is utilized to record or reproduce pieces of information on or from a front surface of the medium23, as shown inFIG. 16A, the diffracted light L6 is incident on a rear surface of thefirst information medium23 and is focused on its front surface to form a diffraction-limited converging spot S5 on thefirst information medium23. In contrast, in cases where the thick type ofsecond information medium25 is utilized to record or reproduce pieces of information on or from a front surface of the medium25, the transmitted light L4 is incident on a rear surface of thesecond information medium25 and is focused on its front surface to form a diffraction-limited converging spot S6 on thesecond information medium25.

In this case, because thehologram lens42 functions as a convex lens to converge the diffracted light L6, the diffraction-limited converging spots S5, S6 are formed even though the thickness T1 of thefirst information medium23 differs from the thickness T2 of thesecond information medium25. Therefore, a compoundobjective lens43 composed of thehologram lens42 and theobjective lens27 has substantially two focal points.

Also, because thehologram lens42 functions as a convex lens for the diffracted light L6, the diffracted light L6 transmits through theobjective lens27 on condition that the numerical aperture NA of theobjective lens27 is substantially high.

In addition, because the diffraction efficiency in the peripheral portion of the grating pattern P3 is high and because the diffraction efficiency is gradually decreased toward the inner direction of the grating pattern P3, a diffraction probability of the incident light L3 is higher in the peripheral portion of the grating pattern P3.

The grating pattern P3 of thehologram lens42 is designed to correct any aberration occurring in theobjective lens27 and thefirst information medium23, so that the diffracted light L6 transmits through thefirst information medium23 having the thickness T1 and is converged on the medium23 to form the diffraction-limited converging spot S5 without any aberration. A method for designing thehologram lens42 having an aberration correcting function is described.

After the diffracted light L6 is converged on thefirst information medium23, spherical waves diverge from the converging spot S5 and transmit through thefirst substrate22 and theobjective lens27. Thereafter, the spherical waves transmit through thetransparent substrate28 and optically interfere with the incident light L3. Therefore, an interference pattern is formed by the interference between the spherical waves and the incident light L3. The interference pattern can be calculated by adding the phase of the spherical waves to an inverted phase obtained by inverting the phase of the incident light L3. Accordingly, the grating pattern P3 of thehologram lens42 which agrees with the interference pattern calculated can be easily formed according to a computer generated hologram technique.

Accordingly, because thehologram lens42 functions as a convex lens for the first-order diffracted light L6, a curvature of theobjective lens27 can be lowered. Also, a glass material having a high refractive index is not required to produce theobjective lens27.

Also, because the first-order diffracted light L6 formed in thehologram lens42 converges before the diffracted light L6 is incident on theobjective lens27, the distance in an optical axis direction between the converging spots S5, S6 can be lengthened to about 1 mm. Therefore, even though the transmitted light L4 (or the first-order diffracted light L6) is converged on the converging spot S6 (or S5) in focus to record or read a piece of information, the light L6 (or L4) is not converged on the converging spot S6 (or S5) in focus to reduce the intensity of the light L6 (or L4) at the converging spot S6 (or S5). Accordingly, no adverse influence is exerted on the recording or reproduction of the information

Also, because thehologram lens42 functions as a convex lens for the first-order diffracted light L6, the occurrence of a chromatic aberration can be prevented in the imagingoptical system41. In detail, the focal length of thehologram lens42 is shortened as the wavelength of the incident light L3 becomes longer. In contrast, the focal length of theobjective lens27 is lengthened as the wavelength of the incident light L3 becomes longer. That is, the dependence of the focal length on the wavelength in theobjective lens27 is opposite to that of the focal length on the wavelength in thehologram lens42. Therefore, the compoundobjective lens43 having an achromatic function can be formed by the combination of thelenses27,42, and the occurrence of the chromatic aberration can be prevented.

Also, because thehologram lens42 is a plane type of element, a lightweight type of compound objective lens can be made in large scale manufacture.

Also, because the diffraction efficiency of thehologram lens42 is gradually decreased toward an inner direction of thepattern region42A, the numerical aperture of theobjective lens27 for the first-order diffracted light L6 becomes substantially enlarged. Therefore, the intensity of the first-order diffracted light L6 can be enlarged to record or reproduce a piece of information on or from thefirst information medium23.

Also, in cases where the incident light L3 is radiated from a semiconductor laser, a far field pattern of the incident light L3 is distributed in a Gaussian distribution as shown in FIG.13A. Therefore, because the diffraction efficiency of thehologram lens42 is gradually decreased toward the inner direction of the grating pattern P2, a far field pattern of the first-order diffracted light L6 is distributed in a gently-sloping shape. Accordingly, secondary maxima of the first-order diffracted light L6 converged on the converging spot S5 can be moreover suppressed in the third embodiment as compared with in the first embodiment. That is, the recording and reproducing of the information can be performed without any deterioration of the information by utilizing the imagingoptical system41.

In addition, in cases where the transmitted light L4 is converged on thesecond information medium25 to form the diffraction-limited converging spot S6, a numerical aperture of theobjective lens27 for the transmitted light L4 is low because the diffraction efficiency of thehologram lens42 is increased toward an outer direction of thegrating pattern42A. As a result, the intensity of the transmitted light L4 becomes lowered. In cases where a transmission efficiency of thehologram lens42 is heightened to increase the intensity of the transmitted light L4, the intensity of the first-order diffracted light L6 at its inner beam portion is largely decreased, and secondary maxima (or side lobes) of the first-order diffracted light L6 at the converging spot S6 is undesirably increased. Therefore, the incident light L3 of which the far field pattern is distributed in the Gaussian distribution is radiated to thehologram lens42 to increase the intensity of the transmitted light L4 without any increase of the second maxima. In detail, as shown inFIG. 18A, the incident light L3 distributed in not only a central portion of the Gaussian distribution but also a peripheral portion of the Gaussian distribution transmits through thehologram lens42 and is refracted by theobjective lens27 because the diameter of the grating pattern P3 is equal to or larger than the aperture of theobjective lens27. Therefore, a numerical aperture NA of theobjective lens27 at a light source side for the incident light L3 becomes higher than that in the first embodiment, and a transmission efficiency of thehologram lens42 is heightened. As a result, the intensity of the transmitted light L4 converged on thesecond information medium25 can be increased, as shown in FIG.18B. Also, because the intensity of the incident light L3 at the peripheral portion of the Gaussian distribution is low and because the diffraction efficiency of thehologram lens42 is decreased toward the inner direction of thegrating pattern42A, the first-order diffracted light L6 is distributed in a gently-sloping shape as shown in FIG.18C. Accordingly, secondary maxima of the first-order diffracted light L6 at the converging spot S5 can be suppressed.

Examples of the utilization of the imagingoptical system41 for various types of optical disks are described.

In cases where the imageoptical system41 is utilized for an optical disk device in which pieces of information recorded in a thin type of high density optical disk and a thick type of compact disk are exclusively reproduced. the diffraction efficiency of thehologram lens42 for the incident light L3 is set in a range from about 20% to 70%. In this case, the intensity of the transmitted light L4 converged on the compact disk is almost the same as that of the first-order diffracted light L6 converged on the high density optical disk. Therefore, the output power of the incident light L3 can be minimized.

Also, in cases where the imageoptical system41 is utilized for an optical disk device in which pieces of information recorded in a thin type of high density optical disk are recorded or reproduced and pieces of information recorded in a thick type of optical disk are exclusively reproduced, the diffraction efficiency of thehologram lens42 for the incident light L3 is set to a value equal to or higher than 55%. In this case, even though a high intensity of the first-order diffracted light L6 is required to record a piece of information on the high density optical disk, the recording of the information can be reliably performed without increasing the intensity of the incident light L3 because the diffraction efficiency of thehologram lens42 for changing the incident light L3 to the first-order diffracted light L6 is high. In other words, a utilization efficiency of the incident light L3 can be enhanced when a piece of information is recorded on the high density optical disk, so that the output power of the incident light L3 can be minimized. Also, because the diffraction efficiency of thehologram lens42 is gradually decreased toward an inner direction of thepattern region42A, the numerical aperture of theobjective lens27 for the first-order diffracted light L6 becomes substantially enlarged. Therefore, the intensity of the first-order diffracted light L6 can be enlarged to record or reproduce a piece of information on or from the high density optical disk.

In the third embodiment, the grating pattern P3 positioned in thepattern region42A of thetransparent substrate28 is gradually changed toward the outer direction of thepattern region42A from the step-wise shape shown inFIG. 12B to the step-wise shape shown in FIG.12A through the step-wise shape shown inFIG. 12C while increasing the height H of the grating pattern P3. However, because the occurrence of unnecessary diffracted light such as minus first-order diffracted light can be effectively prevented in the middle portion of thetransparent substrate28 in which the grating pattern P3 is formed in the step-wise shape shown inFIG. 12C, it is preferred that the middle portion occupy a large part of thepattern region42A of thehologram lens42. In this case, the intensity sum of the transmitted light L4 and the first-order diffracted light L6 can be maximized, so that a utilization efficiency of the incident light L3 can be enhanced.

Also, because the first etching width W1 of the grating pattern P3 is gradually decreased toward the inner direction of thepattern region42A, it is applicable that the grating pattern P3 formed in the step-wise shape shown inFIG. 12B be changed to a step-wise shape shown inFIG. 12D when the first width W1 is decreased to a value lower than about 1 μm. In this case, the grating pattern P3 formed in the step-wise shape shown inFIG. 12D can be easily made. In addition, in cases where a height H4 of the grating pattern P3 formed in the step-wise shape shown inFIG. 12D is moreover decreased toward the inner direction of thepattern region42A, it is preferred that the grating pattern P3 be formed in a step-wise shape shown in FIG.12E. In this case, a third etching width W3 is gradually decreased toward the inner direction of thepattern region42A while decreasing a height H5 of the grating pattern P3. Therefore, the diffraction efficiency of thehologram lens42 can be gradually decreased toward the inner direction of thepattern region42A without any manufacturing difficulty of the grating pattern P3.

In the first to third embodiments of the imageoptical systems21,31 and41, the grating patterns P1, P2 and P3 of thehologram lenses26,32 and42 are respectively formed on a front side of thetransparent substrate28 not facing theobjective lens27. Therefore, a beam of light reflected at the front side of thetransparent substrate28 does not adversely influence as stray light on the recording or reproduction of the information. In detail, because the reflected light is diffracted by the hologram lens, the reflected light is scattered. Also, even though the first-order diffracted light L5 or L6 is reflected at a reverse side of thetransparent substrate28, the diffracted light reflected is again diffracted by the hologram lens and is scattered. Therefore, the light reflected at the front or reverse side of the hologram lens does not adversely influence on the recording or reproduction of the information.

However, in cases where an anti-reflection film is coated on a front side of thehologram lens28 at which the grating pattern is not formed, it is applicable that the grating patterns P1, P2 and P3 of thehologram lenses26,32 and42 be respectively formed on a reverse side of thetransparent substrate28 facing theobjective lens27. In this case, because the first-order diffraction light L5, L6 is not refracted at the front side of thehologram lens28, the design of the imageoptical systems21,31 and41 can be simplified.

Also, in the first to third embodiments, the grating patterns P1, P2 and P3 of thehologram lenses26,32 and42 are respectively formed in relief to produce a phase modulation type of hologram lens. However, as is described in Provisional Publication No. 189504/86 (S61-189504) and Provisional Publication No. 241735/88 (S63-241735), the phase modulation type of hologram lens can be produced by utilizing a liquid crystal cell. Also, the phase modulation type of hologram lens can be produced by utilizing a birefringece material such as lithium niobate. For example, the phase modulation type of hologram lens can be produced by proton-exchanging a surface part of a lithium niobate substrate.

(Fourth Embodiment)

Also, in the first to third embodiments, the compoundobjective lens29,34 or43 having two focal points is composed of theobjective lens27 and thehologram lens26,32 or42. However, as a compound objective lens according to a fourth embodiment is shown inFIG. 19A, it is preferred that each of thehologram lenses26,32 and42 and theobjective lens27 be unified with a packaging means44 to form a compoundobjective lens45 in which a relative position between each of thehologram lenses26,32 and42 and theobjective lens27 is fixed. In this case, the transmitted light L4 and the first-order diffracted light L5, L6 can be easily converged on the first orsecond information medium23,25 by adjusting the position of the packing means44 with an actuator. Also, as another compound objective lens according to a modified fourth embodiment is shown inFIG. 19B, it is preferred that each of the grating patterns P1, P2 and P3 be directly drawn on a curved side of theobjective lens27 facing a light source side to form a compoundobjective lens46 in which each of thehologram lenses26,32 and42 is integrally formed with theobjective lens27.

Accordingly, the central axis of theobjective lens27 can always agree with that of each of thehologram lenses26,32 and42, so that abaxial aberrations of each of thehologram lenses26,32 and42 such as a coma aberration and an astigmatic aberration occurring in the first-order diffracted light can be prevented in the fourth embodiment. Also, because thehologram lens26,32 or42 is placed on a lens surface of theobjective lens27 of which a curvature is higher than those of other lens surfaces of theobjective lens27, a sine condition for the hologram lens treated as a lens can be easily satisfied. Therefore, the degree of aberrations resulting from a constitutional error of an optical head apparatus can be sufficiently reduced.

(Fifth Embodiment)

Also, as a compound objective lens according to a fifth embodiment is shown inFIG. 20, it is preferred that each of the grating patterns P1, P2 and P3 be directly drawn on a side of theobjective lens27 facing the information medium23 or25 to form a compoundobjective lens47 in which each of thehologram lenses26,32 and42 is integrally formed with theobjective lens27. In this case, a curvature at the side of theobjective lens27 can be small or in a plane shape. Therefore, each of the grating patterns P1, P2 and P3 can be made at a low cost. Also, in cases where an aberration is caused by tilting the hologram lens from the optical axis, the aberration can be prevented by fixing the hologram lens and a light source of the incident light L3 on the same base.

(Sixth Embodiment)

An optical head apparatus with one of the compoundobjective lenses29,29M,34,43,45,46 and47 shown in the first to fifth embodiments is described with reference toFIGS. 21 to26 according to a sixth embodiment of the present invention. X, Y and Z co-ordinates shown inFIGS. 21 to26 are utilized in common.

FIG. 21 is a constitutional view of an optical head apparatus according to a sixth embodiment.

As shown inFIG. 21, an optical head apparatus51 for recording or reproducing pieces of information on or from the information medium23 or25, comprises a light source52 such as a semiconductor laser for radiating the incident light L3, a collimator lens53 for collimating the incident light L3, a beam splitter54 for transmitting the incident light L3 on an outgoing optical path and reflecting a beam of transmitted light L4 R formed by reflecting the transmitted light L4 on the information medium23 or25 or a beam of diffracted light L5R (or L6R) formed by reflecting the diffracted light L5 (or L6) on the information medium23 or25 on an incoming optical path, the compound objective lens29 (or29M,34,43,45,46 or47) composed of the hologram lens26 (or26M,32,33 or42) and the objective lens27, a converging lens55 for converging the transmitted light L4R or the diffracted light L5R reflected by the beam splitter54, a wavefront changing device56 such as a hologram for changing a wavefront of the transmitted light L4R or the diffracted light L5R to form a plurality of converging spots of the transmitted light L4R or the diffracted light L5R, a photo detector57 for detecting intensities of the converging spots of the transmitted light L4R or the diffracted light L5R of which the wavefront is changed by the wavefront changing device56 to obtain an information signal recorded on the information medium23 or25 and servo signals such as a focus error signal and a tracking error signal, and an actuating unit58 for moving the compound objective lens composed of the hologram lens26 and the objective lens27 according to the servo signals.

In the above configuration, a beam of incident light L3 radiated from thelight source52 is collimated in thecollimator lens53 and transmits through thebeam splitter54. Thereafter, a part of the incident light L3 transmits through the compoundobjective lens29 without any diffraction, and a remaining part of the incident light L3 is diffracted.

Thereafter, in cases where a piece of information is recorded or reproduced on or from thefirst information medium23, the transmitted light L4 is converged on thefirst information medium23 to form the first converging spot S1. That is, the transmitted light L4 is incident on a rear surface of thefirst information medium23, and the first converging spot S1 is formed on a front surface of thefirst information medium23. Thereafter, a beam of transmitted light L4R reflected at the front surface of the first information medium23 passes through the same optical path in the reverse direction. That is, a part of the transmitted light L4R again transmits through the compoundobjective lens29 without any diffraction and is reflected by thebeam splitter54. In this case, the transmitted light L4R is collimated. Thereafter, the transmitted light L4R is converged by the converginglens55, and the wavefront of a large part of the transmitted light L4R is changed to form a plurality of converging spots on thephoto detector57. Thereafter, the intensities of the converging spots of the transmitted light L4R are detected in thephoto detector57. Therefore, an information signal and servo signals such as a focus error signal and a tracking error signal are obtained. The actuatingunit58 are operated according to the servo signals to move the compoundobjective lens29 at high speed, so that the transmitted light L4 is converged on thefirst information medium23 in focus.

Also, in cases where a piece of information is recorded or reproduced on or from thesecond information medium25, the diffracted light L5 is converged on thesecond information medium25 to form the second converging spot S2. That is, the diffracted light L5 is incident on a rear surface of thesecond information medium25, and the second converging spot S2 is formed on a front surface of thesecond information medium25. Thereafter, a beam of diffracted light L5R reflected at the front surface of the second information medium25 passes through the same optical path in the reverse direction. That is, a part of the diffracted light L5R is again diffracted by thehologram lens26 and is reflected by thebeam splitter54. In this case, the diffracted light L5R is collimated. Thereafter, the diffracted light L5R is converged by the converginglens55, and the wavefront of a large part of the diffracted light L5R is changed to form a plurality of converging spots on thephoto detector57. In this case, the diffracted light L5R incident on the converginglens55 is collimated in the same manner as the transmitted light L4R incident on the converginglens55, the converging spots of the diffracted light L5R are formed at the same positions as those of the transmitted light L4R. Thereafter, the intensities of the converging spots of the diffracted light L5R are detected in thephoto detector57. Therefore, an information signal and servo signals such as a focus error signal and a tracking error signal are obtained. The actuatingunit58 are operated according to the servo signals to move the compoundobjective lens29 at high speed, so that the diffracted light L5 is converged on thesecond information medium25 in focus.

In this case, because the transmitted light L4R again transmits through the compound objective lens without any diffraction and the diffracted light L5R is again diffracted by thehologram lens26, the outgoing optical path agrees with the incoming optical path in a range between the information medium23 or25 and thebeam splitter54 even though the converging spot S1 differs from the converging spot S2. Therefore, a converging spot S7 on thephoto detector57 at which the light L4R or L5R not diffracted by thewavefront changing device56 is converged relates to a radiation point of thelight source52 in a mirror image, so that the light L4R and L5R not diffracted by thewavefront changing device56 are converged at the same converging point S7. In the same manner, the light L4R and L5R diffracted by thewavefront changing device56 are converged at the same other converging points.

Accordingly, even though the compound objective lens has two focal points, thewavefront changing unit56 and thephoto detector57 required to detect the intensity of the transmitted light L4R can be utilized to detect the intensity of the diffracted light L5R. Therefore, the number of parts required to manufacture theoptical head apparatus51 can be reduced, and a small sized optical head apparatus can be manufactured at a low cost and in light weight even though pieces of information are recorded or reproduced on or from an information medium by utilizing theoptical head apparatus51 regardless of whether the information medium is thick or thin.

In cases where the hologram lens26 (or32,33,42) is integrally formed with theobjective lens27 as shown inFIG. 19A,19B or20, each of the compoundobjective lenses45,46 and47 can be manufactured in light weight because the hologram lens26 (or32,33,42) is a plane type of optical device. For example, the hologram lens26 (or32,33,42) is less than several tens mg in weight. Therefore, thehologram lens26 integrally formed with theobjective lens27 can be easily moved by the actuatingunit58.

Next, a detecting method of the servo signals is described.FIG. 22 is a plan view of thewavefront changing unit56.FIG. 23 is an enlarged view of first-order diffracted light and transmitted light detected in thephoto detector57. As shown inFIG. 22, thewavefront changing unit56 is partitioned into as diffractedlight generating region56a in which a grating pattern P4 is drawn and a pair of diffractedlight generating regions55b,56c in which a pair of grating patterns P5, P6 are drawn. The light L4R or L5R incident on the diffractedlight generating region56a is diffracted to obtain a focus error signal. The light L4R or L5R incident on each of the diffractedlight generating regions56b,56c is diffracted to obtain a tracking error signal.

Initially, a spot size detection method utilized to detect a focus error signal is described as an example of a detecting method of a focus error signal. The method is proposed in Japanese Patent Application No. 185722 of 1990. In short, in cases where the method is adopted, an allowable assembly error in an optical head apparatus can be remarkably enlarged, and the servo signal such as a focus error signal can be stably obtained to adjust the position of the compound objective lens even though the wavelength of the incident light L3 varies.

In detail, as shown inFIG. 23, the grating pattern P4 is designed to change the transmitted light L4R (or the diffracted light L5R) transmitting through the diffractedlight generating region56a of thewavefront changing unit56 to a beam of first-order diffracted light L7 and a beam of minus first-order diffracted light L8. The diffracted light L7, L8 are expressed by two types of spherical waves having different curvatures. That is, interference fringes are produced by actually interfering a spherical wave having a focal point FP1 in the front of thephoto detector57 with another spherical wave diverging from the converging spot S7 according to a two-beam interferometric process, so that the grating pattern P4 agreeing with the interference fringes is formed. In other case, the interference fringes are calculated according to a computer generated hologram method. As a result, the transmitted light L4R (or the diffracted light L5R) transmitting through the diffractedlight generating region56a of thewavefront changing unit56 is diffracted and changed to beams of conjugate diffracted light such as a beam of first-order diffracted light L7 and a beam of minus first-order diffracted light L8. The beam of first-order diffracted light L7 has the focal point FP1 at the front surface of thephoto detector57, and the beam of minus first-order diffracted light L8 has a focal point FP2 in the rear of thephoto detector57.

As shown inFIG. 24, thephoto detector57 comprises a sextant photo-detector59 (or a six-division photo detector) in which six detecting sections SE1, SE2, SE3, SE4, SE5 and SE6 are provided. The intensity of the first-order diffracted light L7 is detected by each of the detecting sections SE1, SE2 and SE3 of the sextant photo-detector59 and is changed to electric current signals SC1, SC2 and SC3. Also, the intensity of the minus first-order diffracted light L8 is detected by each of the detecting sections SE4, SE5 and SE6 of the sextant photo-detector59 and is changed to electric current signals SC4, SC5 and SC6.

FIG. 25A and 25C respectively show a converging spot of the first-order diffracted light L7 radiated to the detecting sections SE1, SE2 and SE3 of the sextant photo-detector59 and another converging spot of the minus first-order diffracted light L8 radiated to the detecting sections SE4, SE5 and SE6 of the sextant photo-detector59 on condition that theobjective lens27 is defocused on the information medium23 or25.FIG. 25B shows a converging spot of the first-order diffracted light L7 radiated to the detecting sections SE1,' SE2 and SE3 of the sextant photo-detector59 and another converging spot of the minus first-order diffracted light L8 radiated to the detecting sections SE4, SE5 and SE6 of the sextant photo-detector59 on condition that theobjective lens27 is just focused on the information medium23 or25.

As shown inFIGS. 25A to25C, in cases where the transmitted light L4 (or the diffracted light L5) is converged on the information medium23 (or25) on condition that theobjective lens27 is defocused on the information medium23 (or25), a converging spot S8 of the diffracted light L7 shown at the left side ofFIGS. 25A,25C is formed on the sextant photo-detector59, and another converging spot S9 of the diffracted light L8 shown at the right side ofFIG. 25A or25C is formed on the sextant photo-detector59. In contrast, in cases where the transmitted light L4 (or the diffracted light L5) is converged on the information medium23 (or25) on condition that theobjective lens27 is just focused on the information medium23 (or25), a converging spot S8 of the diffracted light L7 shown at the left side ofFIG. 25B is formed on the sextant photo-detector59, and another converging spot S9 of the diffracted light L8 shown at the right side ofFIG. 25B is formed on the sextant photo-detector59. The intensity of the diffracted light L7 is detected in each of the detecting sections SE1, SE2 and SE3 of the sextant photo-detector59 and is changed to electric current signals SC1, SC2, SC3. Also, the intensity of the diffracted light L8 is detected in the detecting sections SE4, SE5 and SE6 of the sextant photo-detector59 and is changed to electric current signals SC4, SC5 and SC6. Thereafter, a focus error signal S_fθ. is obtained according to the spot size detection method by calculating an equation (5).
S_fθ=(SC1+SC3−SC2)−(SC4+SC6−SC5)  (5)
Thereafter, the position of the compound objective lens is moved in a direction along an optical axis at high speed so as to minimize the absolute value of the focus error signal S_fθ.

In the spot size detection method, the diffracted light L7, L8 are expressed by two types of spherical waves having different curvatures to detect the focus error signal S_fθ. However, two beams of diffracted light L7, L8 radiated to thephoto detector57 are not limited to the spherical waves. That is, because the change of the diffracted light L7, L8 in a Y-direction is detected by thephoto detector57 according to the spot size detection method, it is required that a one-dimensional focal point of the diffracted light L7 is positioned in the front of thephoto detector57 and a one-dimensional focal point of the diffracted light L8 is positioned in the rear of thephoto detector57. Therefore, it is applicable that diffracted light including astigmatic aberration be radiated to thephoto detector57.

In addition, an information signal S_inis obtained by adding all of the electric current signals according to an equation (6).
S_in=SC1+SC2+SC3+SC4+SC5+SC6  (6)

Because the information medium23 or25 is rotated at high speed, a patterned track pit radiated by the converging spots S8, S9 of the diffracted light L7,L8 is rapidly changed one after another, so that the intensity of the information signal S_inis changed. Therefore, the information stored in the information medium23 or25 can be reproduced according to the information signal S_in.

Next, the detection of a tracking error signal depending on a relative position between a converging spot and a patterned track pit on the information medium23 or25 is described.

The grating pattern P5 drawn in the diffractedlight generating region56b shown inFIG. 22 is designed to change the transmitted light LR (or the diffracted light L5R) transmitting through the diffractedlight generating region56b of thewavefront changing unit56 to a beam of first-order diffracted light L9 and a beam of minus first-order diffracted light L10. Also, the grating pattern P6 drawn in the diffractedlight generating region56c shown inFIG. 22 is designed to change the transmitted light L4R (or the diffracted light L5R) transmitting through the diffractedlight generating region56c of thewavefront changing unit56 to a beam of first-order diffracted light L11 and a beam of minus first-order diffracted light L12.

As shown inFIG. 24, thephoto detector57 further comprises four tracking photo-detectors60a to60d for detecting intensities of the diffracted light L9 to L12. As shown inFIG. 26, the intensity of the diffracted light L9 is detected by the tracking photo-detector60a and is changed to an electric current signal SC7, the intensity of the diffracted light L10 is detected by the tracking photo-detector60d and is changed to an electric current signal SC10, the intensity of the diffracted light L11 is detected by the tracking photo-detector60b and is changed to an electric current signal SC8, and the intensity of the diffracted light L12 is detected by the tracking photo-detector60c and is changed to an electriccurrent signal SC9. A tracking error signal S_tθ is calculated according to an equation (7).
S_tθ=SC7−SC8−SC9+SC10  (10)

Therefore, the asymmetry of the intensity distribution of the transmitted light L4R (or the diffracted light L5R) incident on thewavefront changing unit56, which changes in dependence on the positional relation between the converging spot S1 (or S2) and a patterned track pit radiated by the light L4 or L5, is expressed by the tracking error signal S_tθ.

Thereafter, theobjective lens27 is moved in a radial direction so as to reduce a tracking error indicated by the tracking error signal S_tθ. The radial direction is defined as a direction perpendicular to both the optical axis and a series of patterned track pits. Therefore, the converging spot S1 (or S2) of the transmitted light L4 (or the diffracted light L5) on the information medium23 (or25) can be formed in the middle of the patterned track pit, so that the tracking error becomes zero.

Accordingly, focus and tracking servo characteristics can be stably obtained in theoptical head apparatus51. That is, because thewavefront changing unit56 has a wavefront changing function, a focus error signal can be easily obtained. Also, because the diffractedlight generating regions56b,56c are provided in thewavefront changing unit56, a tracking error signal can be easily obtained. Therefore, the number of parts required to manufacture theoptical head apparatus51 can be reduced, and the number of manufacturing steps can be reduced. In addition, the optical head apparatus can be manufactured at a low cost and in light weight.

Also, because the compound objective lens having two focal points is utilized in theoptical head apparatus51, pieces of information can be reliably recorded or reproduced from an information medium by utilizing theoptical head apparatus51 regardless of whether the information medium is thick or thin.

(Seventh Embodiment)

Next, an optical head apparatus in which servo signals such as a focus error signal and a tracking error signal are detected according to an astigmatic aberration method is described according to a seventh embodiment of the present invention.

FIG. 27 is a constitutional view of an optical head apparatus according to a seventh embodiment.

As shown inFIG. 27, anoptical head apparatus61 for recording or reproducing pieces of information on or from the information medium23 or25, comprises thelight source52, thecollimator lens53, thebeam splitter54, the compound objective lens29 (or29M,34,43,45,46 or47) composed of the hologram lens26 (or26M,32,33 or42) and theobjective lens27, the actuatingunit58, the converginglens55, an astigmaticaberration generating unit62 such as a plane parallel plate for generating an astigmatic aberration in the transmitted light L4R or the diffracted light L5 R converged by the converginglens55, and aphoto detector63 for detecting the intensity of the transmitted light L4R or the diffracted light L5R in which the astigmatic aberration is generated to obtain an information signal and servo signals such as a focus error signal and a tracking error signal.

The astigmaticaberration generating unit62 is classified into one of thewavefront changing unit56 because a wavefront of the transmitted light L4R or the diffracted light L5R is changed by the generatingunit62 to generate the astigmatic aberration in the light L4R or L5R. Also, a normal line of theunit62 is tilted from an optical axis.

As shown inFIG. 28, thephoto detector63 comprises a quadrant photo-detector64 in which four detecting sections SE7, SE8, SE9 and SE10 are provided.

In the above configuration, the transmitted light L4R (or the diffracted light L5R) reflected by the information medium23 (or25) is converged by the converginglens55 in the same manner as in the sixth embodiment. Thereafter, the transmitted light L4R (or the diffracted light L5R) transmits through the astigmaticaberration generating unit62 and is converged on thephoto detector57 to form a converging spot S10 on the detecting sections SE7, SE8, SE9 and SE10 of the quadrant photo-detector64. In this case, because the transmitted light L4R (or the diffracted light L5R) converged by the converginglens55 is a spherical wave, an astigmatic aberration is generated in the transmitted light L4R (or the diffracted light L5R) by the astigmaticaberration generating unit62. Therefore, as shown inFIGS. 29A to29C, the shape of the converging spot S10 considerably changes depending on a distance between the compoundobjective lens29 and the information medium23 (or25).

For example, in cases where the transmitted light L4 (or the diffracted light L5) is converged on the information medium23 (or25) on condition that theobjective lens27 is defocused on the information medium23 (or25), the converging spot S10 of the transmitted light L4R (or the diffracted light L5R) is formed on the quadrant photo-detector64 as shown inFIGS. 29A,29C. In contrast, in cases where the transmitted light L4 (or the diffracted light L5) is converged on the information medium23 (or25) on condition that theobjective lens27 is just focused on the information medium23 (or25), the converging spot S10 of the transmitted light L4R (or the diffracted light L5R) is formed on the quadrant photo-detector64 as shown in FIG.25B.

The intensity of the transmitted light L4R (or the diffracted light L5R) is detected in the detecting sections SE7, SE8, SE9 and SE10 of the quadrant photo-detector64 and is changed to electric current signals SC11, SC12, SC13 and SC14. Thereafter, a focus error signal S_fθ is obtained according to an astigmatic aberration method by calculating an equation (8).
S_fθ=(SC11+SC14)−(SC12+SC13)  (8)
Thereafter, the position of the compoundobjective lens29 is moved in a direction parallel to an optical axis at high speed so as to minimize the absolute value of the focus error signal S_fθ.

Also, a tangential direction Dt agreeing with an extending direction of patterned recording pits and a radial direction Dr perpendicular to both the optical axis and the patterned recording pits are defined as shown in FIG.29D. In this case, when the quadrant photo-detector64 is directed as shown inFIGS. 29A to29C, a tracking error signal S_tθ is calculated according to an equation (9) by utilizing an intensity distribution change of the transmitted light L4R (or the diffracted light L5R) which depends on a positional relation between the converging spot S10 and a recording pit radiated by the light L4 or L5.
S_tθ=SC11+SC13−(SC12+SC14)  (9)

Thereafter, theobjective lens27 is moved in the radial direction so as to reduce a tracking error indicated by the tracking error signal S_tθ. Therefore, the converging spot S1, (or S2) of the transmitted light L4 (or the diffracted light L5) on the information medium23 (or25) can be formed in the middle of the recording pit, so that the tracking error becomes zero.

In other case, the tracking error signal S_tθ is obtained according to a phase difference method by utilizing the result calculated in the equation (8).

In addition, an information signal S_inis obtained by adding all of the electric current signals according to an equation (10).
S_in=SC11+SC12+SC13+SC14  (10)

Accordingly, focus and tracking servo characteristics can be stably obtained in theoptical head apparatus61. That is, because an astigmatic aberration is generated in the transmitted light L4R (or the diffracted light L5R) by the astigmaticaberration generating unit62 made of a plane parallel plate, the servo signals such as a focus error signal and a tracking error signal can be easily obtained. Therefore, the number of parts required to manufacture theoptical head apparatus61 can be reduced, and the number of manufacturing steps can be reduced. In addition, theoptical head apparatus61 can be manufactured at a low cost and in light weight.

Also, because the compound objective lens having two focal points is utilized in theoptical head apparatus61, pieces of information can be reliably recorded or reproduced from an information medium by utilizing theoptical head apparatus61 regardless of whether the information medium is thick or thin.

In the seventh embodiment, the astigmaticaberration generating unit62 formed out of the plane parallel plate is arranged between the converginglens55 and thephoto detector63. However, as anoptical head apparatus65 is shown inFIG. 30, it is applicable that acylindrical lens66 integrally formed with the converginglens55 be arranged in place of the plane parallel plate to generate an astigmatic aberration in the transmitted light L4R (or the diffracted light L5R). In this case, because thecylindrical lens66 is integrally formed with the converginglens55, the optical head apparatus can be moreover manufactured at low cost. In addition, as shown inFIG. 30, it is applicable that a normal line of the hologram lens26 (or32,33,42) be tilted from an optical axis passing through the center of theobjective lens27 by about one degree to prevent stray light reflected in a surface of thehologram lens26 from being incident on thephoto detector57 or63. Also, it is applicable that the hologram lens26 (or32,33,42) be coated with an anti-reflection coating to prevent the occurrence of stray light.

Also, as anoptical head apparatus67 is shown inFIG. 31, it is applicable that apolarized beam splitter68 be arranged in place of thebeam splitter54 to perfectly transmit the incident light L3 and a ¼−λplate69 be additionally placed between the hologram lens26 (or32,33,42) and thepolarized beam splitter68. In this case, because the incident light L3 transmits through the ¼−λplate69 in an outgoing optical path and because the transmitted light L4R (or the diffracted light L5) again transmits through the ¼−λplate69 in an incoming optical path, the transmitted light L4R (or the diffracted light L5R) is perfectly reflected by thepolarized beam splitter68. Accordingly, a utilization efficiency of the incident light L3 can be enhanced. Also, a signal-noise ratio of each of the servo signals and the information signal can be enhanced.

Also, as anoptical head apparatus70 is shown inFIG. 32, it is applicable that thepolarized beam splitter68 be arranged in place of thebeam splitter54 to perfectly transmit the incident light L3 and the ¼−λplate69 be additionally placed between the hologram lens26 (or32,33,42) and theobjective lens27. In this case, the transmitted light L4R (or the diffracted light L5R) is perfectly reflected by thepolarized beam splitter68 in the same manner as the optical head apparatus shown in FIG.31. In addition, because stray light reflected from the hologram lens26 (or32,33,42) transmits through thepolarized beam splitter68, the stray light is not incident on thephoto detector63. Accordingly, a signal-noise ratio of each of the servo signals and the information signal can be moreover enhanced.

Also, as an optical head apparatus71 is shown inFIG. 33, it is applicable that a wedge-like prism72 for reshaping the incident light L3 radiated from thelight source52 be additionally placed between thecollimator lens53 and thepolarized beam splitter68. In this case, an elliptic wavefront of the incident light L3 is reshaped to a circular wavefront by the wedge-like prism72. Accordingly, a utilization efficiency of the incident light L3 can be enhanced.

In the sixth and seventh embodiments, when the transmitted light L4 (that is, zero-order diffracted light L4) converged on thefirst information medium23 is reflected toward the compound objective lens to reproduce a piece of information recorded on thefirst information medium23, a part of the transmitted light L4R is diffracted in the hologram lens26 (or32,33,42) on the incoming optical path, so that the part of the transmitted light L4R is changed to a beam of first-order diffracted light L13. Therefore, the first-order diffracted light L13 diverges from thehologram lens26, and a converging spot S11 of the diffracted light L13 is formed on thephoto detector57 or63 in a relatively large size, as shown in FIG.34. The size of the converging spot S11 is larger than those of the sextant photo-detector59 and the quadrant photo-detector64. Therefore, there is a drawback that a signal-noise ratio in the information signal deteriorates.

To solve the drawback, it is preferred that the photo detector57 (or63) further comprise an information photo-detector73 surrounding the sextant photo-detector59 (or the quadrant photo-detector64). The size of the information photo-detector73 is equal to or larger than a 1 mm square. Therefore, in cases where the information signal is determined by the sum of the intensity of the transmitted light L4 detected in the sextant photo-detector59 (or the quadrant photo-detector64) and the intensity of the diffracted light L13 detected in the information photo-detector73, the signal-noise ratio in the information signal can be enhanced, and frequency characteristics of the information signal can be enhanced.

(Eighth Embodiment)

Next, a method of focusing performed in theoptical head apparatuses51,61,65,67,70 and71 is described according to an eighth embodiment of the present invention.

FIG. 35A graphically shows a change of the focus error signal obtained by detecting the intensity of the transmitted light L4 formed in thehologram lens26,32 or33, the strength of the focus error signal depending on a distance between theobjective lens27 and thefirst information medium23.FIG. 35B graphically shows a change of the focus error signal obtained by detecting the intensity of the diffracted light L5 formed in thehologram lens26,32 or33, the strength of the focus error signal depending on a distance between theobjective lens27 and thesecond information medium25.

The intensity of the transmitted light L4 is high because the numerical aperture of theobjective lens27 for the transmitted light L4 is large. Therefore, as shown inFIG. 35A, a change of a focus error signal FE1 obtained in cases where theobjective lens27 is almost focused on thefirst information medium23 is considerably large as compared with a change of an unnecessary focus error signal FE2 obtained in cases where theobjective lens27 is defocused on thefirst information medium23. In addition, in cases where thehologram lens26,32 or33 is utilized in each of theoptical head apparatuses51,61,65,67,70 and71, the unnecessary focus error signal FE2 is generated when the distance between theobjective lens27 and thefirst information medium23 is larger than the focal length of theobjective lens27 for the transmitted light L4.

In contrast, the intensity of the diffracted light L5 is comparatively low because the numerical aperture of theobjective lens27 for the diffracted light L5 is comparatively small. Therefore, as shown inFIG. 35B, a change of a focus error signal FE3 obtained in cases where theobjective lens27 is almost focused on thesecond information medium25 is almost the same as that of an unnecessary focus error signal FE4 obtained in cases where theobjective lens27 is defocused on thesecond information medium25. In addition, in cases where thehologram lens26,32 or33 is utilized in each of theoptical head apparatuses51,61,65,67,70 and71, the unnecessary focus error signal FE4 is generated when the distance between theobjective lens27 and thesecond information medium25 is smaller than the focal length of theobjective lens27 for the diffracted light L5.

Therefore, in cases where the focusing of the transmitted light L4 on thefirst information medium23 is performed, theobjective lens27 placed far from thefirst information medium23 is gradually brought near to thefirst information medium23. Thereafter, when the strength of the focus error signal reaches a threshold value, a focus servo loop provided in thephoto detector57 or63 is set to an operation condition, so that theobjective lens27 is set to be focused on thefirst information medium23. Also, in cases where the focusing of the diffracted light L5 on thesecond information medium25 is performed, theobjective lens27 placed far from thesecond information medium25 is gradually brought near to thesecond information medium25 in the same manner. Thereafter, when the strength of the focus error signal reaches a threshold value, a focus servo loop provided in thephoto detector57 or63 is set to an operation condition, so that theobjective lens27 is set to be focused on thesecond information medium25.

Accordingly, the inverse influence of the unnecessary focus error signal FE4 on the focusing of the diffracted light L5 can be prevented. Also, because theobjective lens27 placed far from the information medium23 or25 is gradually brought near to the information medium23 or25 regardless of whether the information medium is T1 or T2 in thickness, a focusing operation in each of theoptical head apparatuses51,61,65,67,70 and71 with thehologram lens26,32 or33 can be performed according to a common procedure by changing the threshold value or performing an auto gain control in which the focus error signal is normalized by detecting the total intensity of the transmitted light L4R or the diffracted light L5R. Therefore, a control circuit required to perform the focusing operation can be made at a low cost.

FIG. 36A graphically shows a change of the focus error signal obtained by detecting the intensity of the diffracted light L6 formed in thehologram lens42, the strength of the focus error signal depending on a distance between theobjective lens27 and thefirst information medium23.FIG. 36B graphically shows a change of the focus error signal obtained by detecting the intensity of the transmitted light L4 formed in thehologram lens42, the strength of the focus error signal depending on a distance between theobjective lens27 and thesecond information medium25.

As shown inFIG. 36A, a change of a focus error signal FE5 obtained in cases where theobjective lens27 is almost focused on thefirst information medium23 is considerably large as compared with a change of an unnecessary focus error signal FE6 obtained in cases where theobjective lens27 is defocused on thefirst information medium23. In addition, in cases where thehologram lens42 is utilized in each of theoptical head apparatuses51,61,65,67,70 and71, the unnecessary focus error signal FE6 is generated when the distance between theobjective lens27 and thefirst information medium23 is smaller than the focal length of theobjective lens27 for the diffracted light L6.

In contrast, as shown inFIG. 36B, a change of a focus error signal FE7 obtained in cases where theobjective lens27 is almost focused on thesecond information medium25 is almost the same as that of an unnecessary focus error signal FE8 obtained in cases where theobjective lens27 is defocused on thesecond information medium25. In addition, in cases where thehologram lens42 is utilized in each of theoptical head apparatuses51,61,65,67,70 and71, the unnecessary focus error signal FE8 is generated when the distance between theobjective lens27 and thesecond information medium25 is larger than the focal length of theobjective lens27 for the transmitted light L4.

Therefore, in cases where the focusing of the diffracted light L6 on thefirst information medium23 is performed, theobjective lens27 placed near to thefirst information medium23 is gradually moved away from thefirst information medium23. Thereafter, when the strength of the focus error signal reaches a threshold value, a focus servo loop provided in thephoto detector57 or63 is set to an operation condition, so that theobjective lens27 is set to be focused on thefirst information medium23. Also, in cases where the focusing of the transmitted light L4 on thesecond information medium25 is performed, theobjective lens27 placed near to thesecond information medium25 is gradually moved away from thesecond information medium25 in the same manner. Thereafter, when the strength of the focus error signal reaches a threshold value, a focus servo loop provided in thephoto detector57 or63 is set to an operation condition, so that theobjective lens27 is set to be focused on thesecond information medium25.

Accordingly, the inverse influence of the unnecessary focus error signal FE8 on the focusing of the transmitted light L4 can be prevented. Also, because theobjective lens27 placed near to the information medium23 or25 is gradually moved away from the information medium23 or25 regardless of whether the information medium is T1 or T2 in thickness, a focusing operation in each of theoptical head apparatuses51,61,65,67,70 and71 with thehologram lens42 can be performed according to a common procedure by changing the threshold value or performing the auto gain control. Therefore, a control circuit required to perform the focusing operation can be made at a low cost.

(Ninth Embodiment)

An optical head apparatus with the compoundobjective lens29,34,45,46 or47 in which the incident light L3 is efficiently utilized to obtain an information signal and servo signals is described with reference toFIGS. 29,37 according to a ninth embodiment of the present invention.

FIG. 37 is a constitutional view of an optical head apparatus according to a ninth embodiment.

As shown inFIG. 37, anoptical head apparatus81 for recording or reproducing pieces of information on or from the information medium23 or25, comprises thelight source52, thecollimator lens53, thebeam splitter54, the compound objective lens the compound objective lens29 (34,45,46 or47) composed of the hologram lens26 (or32 or33) and theobjective lens27, the actuatingunit58, the converginglens55, abeam splitter82 for transmitting a beam of diffracted light L5R or reflecting a beam of transmitted light L4R, thephoto detector63 for detecting the intensity of the diffracted light L5R transmitting through thebeam splitter82 to obtain servo signals and an information signal recorded on thesecond information medium25, thewavefront changing device56 such as a hologram for changing a wavefront of the transmitted light L4R reflected by thebeam splitter82, and thephoto detector57 for detecting the intensity of the transmitted light L4R to obtain servo signals and an information signal recorded on thefirst information medium23. Thebeam splitter82 is made of a plane parallel plate of which a normal line is tilted from an optical path, so that an astigmatic aberration is generated in the diffracted light L5R passing through thebeam splitter82. Also, a coating is applied on a surface of the plane parallel plate.

In the above configuration, the transmitted light L4 (or the diffracted light L5) are converged by the converginglens27 in the same manner as in the sixth embodiment. Thereafter, in cases where a piece of information is recorded or reproduced on or from thefirst information medium23, the transmitted light L4 is converged on thefirst information medium23 to form the first converging spot S1. Thereafter, a beam of transmitted light L4R reflected by the first information medium23 passes through the same optical path in the reverse direction. That is, a great part of the transmitted light L4R again transmits through the compound objective lens without any diffraction and is reflected by thebeam splitter54. Thereafter, the transmitted light L4R is converged by the converginglens55, and a part of the transmitted light L4R is reflected by thebeam splitter82. Thereafter, the wavefront of a great part of the transmitted light L4R is changed by thewavefront changing unit56, and the great part of the transmitted light L4R is converged on thephoto detector57 to form the converging spots S8, S9. Therefore, an information signal and servo signals such as a focus error signal and a tracking error signal are obtained in the same manner as in the sixth embodiment. Also, a remaining part of the transmitted light L4R not changed its wavefront is converged on thephoto detector57 to form the converging spot S7.

In contrast, in cases where a piece of information is recorded or reproduced on or from thesecond information medium25, the diffracted light L5 is converged on thesecond information medium25 to form the second converging spot S2. Thereafter, a beam of diffracted light L5R reflected by the second information medium25 passes through the same optical path in the reverse direction, and a great part of the diffracted light L5R transmits through thehologram lens26 without any diffraction. Therefore, the diffracted light L5R passes through the incoming optical path differing from the outgoing optical path. Thereafter, the diffracted light L5R is reflected by thebeam splitter54 and is converged by the converginglens55. Thereafter, a part of the diffracted light L5R transmits through thebeam splitter82. In this case, an astigmatic aberration is generated in the diffracted light L5R. Thereafter, the diffracted light L5R is converged on thephoto detector63 to form a converging spot S12 of which the shape is the same as the converging spot S10 shown inFIGS. 29A to29C, and the intensity of the diffracted light L5R is detected in thephoto detector63. Therefore, an information signal and servo signals such as a focus error signal and a tracking error signal are obtained in the same manner as in the seventh embodiment.

In this case, though a remaining part of the transmitted light L4R transmits through thebeam splitter82, the remaining part of the transmitted light L4R is not converged at the converging spot S12 because the transmitted light L4R passes through the same optical path. Also, though a remaining part of the diffracted light L5R is reflected thebeam splitter82, the remaining part of the diffracted light L5R is not converged at the converging spot S7, S8 or S9 because the diffracted light L5R passes through the incoming optical path differing from the outgoing optical path.

In the ninth embodiment, because the diffracted light L5R transmits through thehologram lens26 without any diffraction, the converging spot S12 formed on thephoto detector63 does not relate to a radiating point of thelight source52 in a mirror image, while the converging spot S7 formed on thephoto detector57 relates to the radiating point of thelight source52 in the mirror image. In other words, a focal point of the diffracted light L5R converged by the converginglens55 differs from that of the transmitted light L4R converged by the converginglens55. Therefore, thephoto detector57 for detecting the intensity of the transmitted light L4R and thephoto detector63 for detecting the intensity of the diffracted light L5R are required.

Accordingly, because the compound objective lens having two focal points is utilized in theoptical head apparatus81, pieces of information can be reliably recorded or reproduced on or from an information medium regardless of whether the information medium is thick or thin.

An example of the utilization of theoptical head apparatus81 for various types of optical disks is described.

In cases where theoptical head apparatus81 is utilized for an optical disk device in which pieces of information recorded in a thin type of high densityoptical disk23 are recorded or reproduced and pieces of information recorded in a thick type ofoptical disk25 are exclusively reproduced, the diffraction efficiency of thehologram lens26,32 or33 in the compoundobjective lens29,34,45,46 or47 for changing a beam of light to a beam of first-order diffracted light is set to a value equal to or lower than 30%. Therefore, in cases where a piece of information recorded on the thick type ofoptical disk25 is reproduced in thephoto detector63, a signal-noise ratio of each of the servo signals and the information signal obtained in thephoto detector63 can be enhanced because the diffracted light L5R transmitting through thehologram lens26,32 or33 at a high transmission efficiency is utilized to obtain the servo signals and the information signal. In other words, a utilization efficiency of the incident light L3 can be enhanced when a piece of information recorded on the thick type ofoptical disk25 is reproduced, so that the output power of the incident light L3 can be minimized. Also, even though a high intensity of the transmitted light L4 is required to record a piece of information on the high densityoptical disk23, the recording of the information can be reliably performed without increasing the intensity of the incident light L3 because a transmission efficiency of thehologram lens26,32 or33 for the incident light L3 is high. Also, in cases where a piece of information recorded on the high densityoptical disk23 is reproduced in thephoto detector57, a signal-noise ratio of each signal obtained in thephoto detector57 can be enhanced because the transmission efficiency of thehologram lens26,32 or33 for the light L3, L4R is high.

(Tenth Embodiment)

An optical head apparatus with the compoundobjective lens29,34,45,46 or47 in which the incident light L3 is efficiently utilized to obtain an information signal and servo signals is described with reference toFIGS. 38,39 according to a tenth embodiment of the present invention. X1 and Y1 coordinates shown inFIGS. 38,39 are utilized in common.

FIG. 38 is a constitutional view of an optical head apparatus according to a tenth embodiment.FIG. 39 is a plan view of a beam splitter having a reflection type of hologram utilized in the optical head apparatus shown in FIG.38.

As shown inFIG. 38, anoptical head apparatus91 for recording or reproducing pieces of information on or from the information medium23 or25, comprises thelight source52, thecollimator lens53, thebeam splitter54, the compound objective lens29 (or34,45,46 or47) composed of the hologram lens26 (or32 or33) and theobjective lens27, the actuatingunit58, the converginglens55, abeam splitter92 having a reflection type ofhologram93 for transmitting a large part of the transmitted light L4R or reflecting all of the diffracted light L5R incident on thehologram93, thephoto detector63 for detecting the intensity of the transmitted light L4R transmitting through thebeam splitter92 to obtain servo signals and an information signal recorded in thefirst information medium23, and thephoto detector57 for detecting the intensity of the diffracted light L5R to obtain servo signals and an information signal recorded in thesecond information medium25.

Thebeam splitter92 is made of a plane parallel plate inclined to an optical path, so that an astigmatic aberration is generated in the transmitted light L4R passing through thebeam splitter92. Also, as shown inFIG. 39, the reflection type ofhologram93 is arranged at a center portion of thebeam splitter92, and a light transmitting region92a is arranged at a peripheral portion of thebeam splitter92 to surround thehologram93. Light incident on the light transmitting region92a transmits without any diffraction. Thehologram92 is partitioned into a diffractedlight generating region93a in which a grating pattern P7 is drawn and a pair of diffractedlight generating regions93b,93c in which a pair of grating patterns P8, P9 are drawn. The diffracted light L5R incident on the diffractedlight generating region93a is diffracted to obtain a focus error signal in thephoto detector57. The diffracted light L5R incident on each of the diffractedlight generating regions93b,93c is diffracted to obtain a tracking error signal in thephoto detector57.

In the above configuration, the transmitted light L4 and the diffracted light L5 are converged by the converginglens27 in the same manner as in the sixth embodiment. Thereafter, in cases where a piece of information is recorded or reproduced on or from thefirst information medium23, the transmitted light L4 is converged on thefirst information medium23 to form the first converging spot S1. Thereafter, a beam of transmitted light L4R reflected by the first information medium23 passes through the same optical path in the reverse direction. That is, a large part of the transmitted light L4R again transmits through the compoundobjective lens29 without any diffraction and is reflected by thebeam splitter54. Thereafter, the transmitted light L4R is converged by the converginglens55, and a large part of the transmitted light L4R transmits through thebeam splitter92. In this case, an astigmatic aberration is generated in the transmitted light L4R. Thereafter, the transmitted light L4R is converged on thephoto detector63 to form a converging spot S13 of which the shape is the same as the converging spot S10 shown inFIGS. 29A to29C, and the intensity of the transmitted light L4R is detected in thephoto detector63. Therefore, an information signal and servo signals such as a focus error signal and a tracking error signal are obtained in the same manner as in the seventh embodiment.

In contrast, in cases where a piece of information is recorded or reproduced on or from thesecond information medium25, the diffracted light L5 is converged on thesecond information medium25 to form the second converging spot S2. Thereafter, a beam of diffracted light L5R reflected by the second information medium25 passes through the same optical path in the reverse direction, and a large part of the diffracted light L5R transmits through thehologram lens26 without any diffraction. Therefore, the diffracted light L5R transmits on the incoming optical path differing from the outgoing optical path in the same manner as in the ninth embodiment. Thereafter, the diffracted light L5R is reflected by thebeam splitter54 and is converged by the converginglens55 on thebeam splitter92 to form a converging spot on the reflection type ofhologram93 of thebeam splitter92. Therefore, all of the diffracted light L5R is diffracted and reflected by thehologram93 to be converged on thephoto detector57. That is, the diffracted light L5R diffracted and reflected in the diffractedlight generating region93a of thehologram93 is splitted into two beams and is converged on the detecting sections SE1 to SE6 of the sextant photo-detector59 in thephoto detector57 in the same manner as in the sixth embodiment. Also, the diffracted light L5R diffracted and reflected in the diffractedlight generating region93b of thehologram93 is splitted into two beams, and the intensity of the diffracted light L5R is detected in the tracking photo-detectors60a and60d. Also, the diffracted light L5R diffracted and reflected in the diffractedlight generating region93c of thehologram93 is splitted into two beams, and the intensity of the diffracted light L5R is detected in the tracking photo-detectors60b and60c. Therefore, an information signal and servo signals such as a focus error signal and a tracking error signal are obtained in the same manner as in the sixth embodiment.

In the tenth embodiment, because the transmitted light L4R transmits through thehologram lens26 without any diffraction, the converging spot S13 formed on thephoto detector63 does not relate to a radiating point of thelight source52 in a mirror image. Therefore, thephoto detector57 for detecting the intensity of the diffracted light L5R and thephoto detector63 for detecting the intensity of the transmitted light L4R are required.

Accordingly, because the compound objective lens having two focal points is utilized in theoptical head apparatus91, pieces of information can be reliably recorded or reproduced on or from an information medium regardless of whether the information medium is thick or thin.

Also, because all of the diffracted light L5R is completely diffracted and reflected by thehologram93 of thebeam splitter92, the diffracted light L5R can be utilized at high efficiency. Therefore, a signal-noise ratio of the signals obtained in thephoto detector57 can be enhanced.

An example of the utilization of theoptical head apparatus91 for various types of optical disks is described.

In cases where theoptical head apparatus91 is utilized for an optical disk device in which pieces of information recorded in a thin type of high densityoptical disk23 are recorded or reproduced and pieces of information recorded in a thick type ofoptical disk25 are exclusively reproduced, the diffraction efficiency of thehologram lens26,32 or33 in the compoundobjective lens29,34,45,46 or47 for changing a beam of light to a beam of first-order diffracted light is set to a value equal to or lower than 30%. Therefore, in cases where a piece of information recorded on the thick type ofoptical disk25 is reproduced in thephoto detector57, a signal-noise ratio of each of the servo signals and the information signal obtained in thephoto detector57 can be enhanced because the diffracted light L5R transmitting through thehologram lens26,32 or33 at a high transmission efficiency is utilized to obtain the servo signals and the information signal. In other words, a utilization efficiency of the incident light L3 can be enhanced when a piece of information recorded on the thick type ofoptical disk25 is reproduced, so that the output power of the incident light L3 can be minimized. Also, even though a high intensity of the transmitted light L4 is required to record a piece of information on the high densityoptical disk23, the recording of the information can be reliably performed without increasing the intensity of the incident light L3 because a transmission efficiency of thehologram lens26,32 or33 for the incident light L3 is high. Also, in cases where a piece of information recorded on the high densityoptical disk23 is reproduced in thephoto detector63, a signal-noise ratio of each signal obtained in thephoto detector63 can be enhanced because the transmission efficiency of thehologram lens26,32 or33 for the light L3, L4R is high.

(Eleventh Embodiment)

An optical head apparatus with the compoundobjective lens29,34,45,46 or47 in which the incident light L3 is efficiently utilized to obtain an information signal and servo signals is described with reference toFIGS. 40 to42 according to an eleventh embodiment of the present invention. X1 and Y1 co-ordinates shown inFIGS. 40,41 are utilized in common, and X, Y and Z co-ordinates shown inFIGS. 40,42 are utilized in common.

FIGS. 40A,40B are respectively a constitutional view of an optical head apparatus according an eleventh embodiment.FIG. 41 is a plan view of a beam splitter having a reflection type of hologram utilized in the optical head apparatus shown in FIG.38.

As shown inFIGS. 40A,40B, anoptical head apparatus101 for recording or reproducing pieces of information on or from the information medium23 or25, comprises thelight source52, thecollimator lens53, thebeam splitter54, the compound objective lens29 (or34,45,46 or47) composed of the hologram lens26 (or32 or33) and theobjective lens27, the actuatingunit58, the converginglens55, abeam splitter102 having a transmission type ofhologram103 for transmitting the transmitted light L4R converged on thefirst information medium23 and the diffracted light L5R converged on thesecond information medium25 and diffracting the transmitted light L4R which is converged on thesecond information medium25 in defocus, aphoto detector104 for detecting the intensity of the transmitted light L4R converged on thefirst information medium23 to obtain servo signals and an information signal recorded in thefirst information medium23, detecting in defocus the intensity of the diffracted light L5R to obtain an information signal recorded in thesecond information medium25, and detecting the intensity of the transmitted light L4R converged on thesecond information medium25 in defocus to obtain a focus error signal.

Thebeam splitter102 is made of a plane parallel plate inclined to an optical path, so that an astigmatic aberration is generated in the light L4R, L5R passing through thebeam splitter102. Also, as shown inFIG. 41, the transmission type ofhologram103 is arranged at a center portion of thebeam splitter102, and alight transmitting region102a is arranged at a peripheral portion of thebeam splitter102 to surround thehologram103. The transmitted light L4R incident on thelight transmitting region102a transmits without any diffraction. Thehologram102 is partitioned into diffractedlight generating regions103a,103b alternately arranged to detect a focus error signal according to the spot size detection method described in the sixth embodiment. That is, a grating pattern P10 is drawn in each of the diffractedlight generating regions103a, and a converging spot is formed by the transmitted light L4R diffracted in theregions103a. Also, a grating pattern P11 is drawn in each of the diffractedlight generating regions103b, and another converging spot is formed by the transmitted light L4R diffracted in theregions103b.

Thephoto detector104 comprises the sextant photo-detector59 in which the detecting sections SE1, SE2, SE3, SE4, SE5 and SE6 are provided in the same manner as thephoto detector57.

In the above configuration, the transmitted light L4 and the diffracted light L5 are converged by the converginglens27 in the same manner as in the sixth embodiment. Thereafter, in cases where a piece of information is recorded or reproduced on or from thefirst information medium23, as shown inFIG. 40A, the transmitted light L4 is converged on thefirst information medium23 to form the first converging spot S1. Thereafter, a beam of transmitted light L4R reflected by the first information medium23 passes through the same optical path in the reverse direction. That is, the transmitted light L4R again transmits through the compound objective lens without any diffraction and is reflected by thebeam splitter54. Thereafter, the transmitted light L4R is converged by the converginglens55, and a major part of the transmitted light L4R transmits through thebeam splitter103. In this case, an astigmatic aberration is generated in the transmitted light L4R. Thereafter, the transmitted light L4R is converged on thephoto detector104 to form a converging spot S14 of which the shape is the same as the converging spot S10 shown inFIGS. 29A to29C, and the intensity of the transmitted light L4R is detected in thephoto detector104. Therefore, an information signal and servo signals such as a focus error signal and a tracking error signal are obtained in the same manner as in the seventh embodiment. In this case, because the position of thephoto detector104 detecting the transmitted light L4R relates to a radiation point of thelight source52 in a mirror image, the transmitted light L4R is converged on thephoto detector104 just in focus.

In contrast, in cases where a piece of information is recorded or reproduced on or from thesecond information medium25, as shown inFIG. 40B, the diffracted light L5 is converged on thesecond information medium25 to form the second converging spot S2. Thereafter, a beam of diffracted light L5R reflected by the second information medium25 passes through the same optical path in the reverse direction and transmits through thehologram lens26 without any diffraction. Therefore, the diffracted light L5R transmits on the incoming optical path differing from the outgoing optical path in the same manner as in the ninth embodiment. Thereafter, the diffracted light L5R is reflected by thebeam splitter54 and is converged by the converginglens55. Thereafter, a major part of the diffracted light L5R transmits through thebeam splitter102, and the diffracted light L5R is converged on thephoto detector57. In this case, an astigmatic aberration is generated in the diffracted light L5R. Also, because the diffracted light L5R is not diffracted by thehologram lens26 on the incoming optical path, the position of thephoto detector104 detecting the diffracted light L5R does not relate to the radiation point of thelight source52 in the mirror image. Therefore, the diffracted light L5R is converged on thephoto detector104 in defocus. However, because the entire intensity of the diffracted light L5R converged in defocus is detected in thephoto detector104, an information signal is obtained in the same manner as in the seventh embodiment.

Also, the transmitted light L4 is converged on thesecond information medium25 in defocus as shown in FIG.40B. That is, the transmitted light L4 incident on the rear surface of thesecond information medium25 is converged at the front surface of thesecond information medium25. Thereafter, a beam of transmitted light L4R reflected at the front surface of thesecond information medium25 again transmits through the compound objective lens without any diffraction and is reflected by thebeam splitter54. Thereafter, the transmitted light L4R is converged by the converginglens55 on thebeam splitter102 to form a converging spot on the ofhologram103 of thebeam splitter102. Therefore, all of the transmitted light L4R is diffracted by thehologram103 and is converged on thephoto detector104. That is, the transmitted light L4R diffracted in the diffractedlight generating regions103a of thehologram103 is changed to a first spherical wave SW1 of which a focal point is placed at the front of thephoto detector104, and the transmitted light L4R diffracted in the diffractedlight generating regions103b of thehologram103 is changed to a second spherical wave SW2 of which a focal point is placed at the rear of thephoto detector104. Thereafter, as shown inFIGS. 42A to42C, the first spherical wave SW1 is converged on the detecting sections SE1 to SE3 of the sextant photo-detector59 in thephoto detector104 to form a converging spot S15A, and the second spherical wave SW2 is converged on the detecting sections SE4 to SE6 of the sextant photo-detector59 to form a converging spot S15B. Because theregions103a,103b are divided into many pieces, the converging spots S15A, S15B are respectively divided into many pieces.

In cases where the diffracted light L5 is converged on the information medium25 in defocus, the converging spots S15A, S15B of the transmitted light L4R shown inFIGS. 42A,42C are formed on the sextant photo-detector59. In contrast, in cases where the diffracted light L5 is converged on the information medium25 in focus, the converging spots S15A, S15B of the transmitted light L4R shown inFIG. 42B are formed on the sextant photo-detector59. The intensity of the transmitted light L4R is detected in each of the detecting sections SE1 to SE6 of the sextant photo-detector59 and is changed to electric current signals SC15 to SC20. Thereafter, a focus error signal S_fθ is obtained according to the spot size detection method by calculating an equation (11).
S_fθ=(SC15+SC17−SC16)−(SC18+SC20−SC19)  (11)
Thereafter, the position of the compound objective lens is moved in a direction along an optical axis at high speed so as to minimize the absolute value of the focus error signal S_fθ. Therefore, the focus error signal is obtained in the same manner as in the sixth embodiment.

Accordingly, because the compound objective lens having two focal points is utilized in theoptical head apparatus101, pieces of information can be reliably recorded or reproduced on or from an information medium regardless of whether the information medium is thick or thin.

Also, because all of the transmitted light L4R reflected by thesecond information medium25 is completely diffracted by thehologram103 of thebeam splitter102 to detect the focus error signal, the transmitted light L4R can be utilized at high efficiency. Therefore, a signal-noise ratio of the focus error signal obtained in thephoto detector104 can be enhanced.

Also, the information signal and the servo signals can be obtained in thephoto detector104 regardless of whether the information medium23 or25 is thin or thick. Therefore, the number of parts required to manufacture theoptical head apparatus101 can be reduced, and a small sized optical head apparatus can be manufactured at a low cost and in light weight even though pieces of information are recorded or reproduced on or from an information medium by utilizing theoptical head apparatus101 regardless of whether the information medium is thick or thin.

An example of the utilization of theoptical head apparatus101 for various types of optical disks is described.

In cases where theoptical head apparatus101 is utilized for an optical disk device in which pieces of information recorded in a thin type of high densityoptical disk23 are recorded or reproduced and pieces of information recorded in a thick type ofoptical disk25 are exclusively reproduced, the diffraction efficiency of thehologram lens26,32 or33 in the compoundobjective lens29,34,45,46 or47 is set to a value equal to or lower than 30%. Therefore, in cases where a piece of information recorded on the thick type ofoptical disk25 is reproduced in thephoto detector104, a signal-noise ratio of each of the servo signals and the information signal obtained in thephoto detector104 can be enhanced because the diffracted light L5R transmitting through thehologram lens26,32 or33 at a high transmission efficiency is utilized to obtain the information signal. In other words, a utilization efficiency of the incident light L3 can be enhanced when a piece of information recorded on the thick type ofoptical disk25 is reproduced, so that the output power of the incident light L3 can be minimized. Also, even though a high intensity of the transmitted light L4 is required to record a piece of information on the high densityoptical disk23, the recording of the information can be reliably performed without increasing the intensity of the incident light L3 because a transmission efficiency of thehologram lens26,32 or33 for the incident light L3 is high. Also, in cases where a piece of information recorded on the high densityoptical disk23 is reproduced in thephoto detector63, a signal-noise ratio of each signal obtained in thephoto detector63 can be enhanced because the transmission efficiency of thehologram lens26,32 or33 for the light L3, L4R is high.

(Twelfth Embodiment)

An optical head apparatus with the compoundobjective lens29M,43,45,46 or47 in which the incident light L3 is efficiently utilized to obtain an information signal and servo signals is described with reference toFIG. 43 according to a twelfth embodiment of the present invention.

FIG. 43 is a constitutional view of an optical head apparatus according to a twelfth embodiment.

As shown inFIG. 43, anoptical head apparatus111 for recording or reproducing pieces of information on or from the information medium23 or25, comprises thelight source52, thecollimator lens53, thebeam splitter54, the compoundobjective lens29M (or43,45,46 or47) composed of the hologram lens42 (or26M or32) and theobjective lens27, the actuatingunit58, the converginglens55, thebeam splitter82, thephoto detector63, thewavefront changing device56, and thephoto detector57.

In the above configuration, a beam of incident light L3 radiated from thelight source52 is collimated in thecollimator lens53 and transmits through thebeam splitter54. Thereafter, a part of the incident light L3 transmits through the compoundobjective lens29 without any diffraction, and a remaining part of the incident light L3 is diffracted.

Thereafter, in cases where a piece of information is recorded or reproduced on or from thefirst information medium23, the diffracted light L6 is converged on thefirst information medium23 to form the converging spot S5. That is, the diffracted light L6 is incident on the rear surface of thefirst information medium23, and the converging spot S5 is formed on the front surface of thefirst information medium23. Thereafter, a beam of diffracted light L6R reflected at the front surface of the first information medium23 passes through the same optical path in the reverse direction, and a great part of the diffracted light L6R is again diffracted by thehologram lens42. Therefore, the diffracted light L6R transmits on the incoming optical path agreeing with the outgoing optical path. Thereafter, the diffracted light L6R is reflected by thebeam splitter54 and is converged by the converginglens55. Thereafter, a part of the diffracted light L6R transmits through thebeam splitter82. In this case, an astigmatic aberration is generated in the diffracted light L6R. Thereafter, the diffracted light L6R is converged on thephoto detector63 to form the converging spot S10 of which the shape is shown inFIGS. 29A to29C, and the intensity of the diffracted light L6R is detected in thephoto detector63. Therefore, an information signal and servo signals such as a focus error signal and a tracking error signal are obtained in the same manner as in the seventh embodiment.

In contrast, in cases where a piece of information is recorded or reproduced on or from thesecond information medium25, the transmitted light L4 is converged on thesecond information medium25 to form the converging spot S6. That is, the transmitted light L4 is incident on the rear surface of thesecond information medium25, and the converging spot S6 is formed on the front surface of thesecond information medium25. Thereafter, a beam of transmitted light L4R reflected at the front surface of the second information medium25 passes through the same optical path in the reverse direction. That is, the transmitted light L4R is collimated by theobjective lens27 on the incoming optical path. Thereafter, a great part of the transmitted light L4R is diffracted by thehologram lens42. Therefore, the transmitted light L4R transmits on the incoming optical path differing from the outgoing optical path. Thereafter, the transmitted light L4R is reflected by thebeam splitter54 and is converged by the converginglens55. Thereafter, a part of the transmitted light L4R is reflected by thebeam splitter82. Thereafter, the wavefront of a great part of the transmitted light L4R is changed by thewavefront changing unit56, and the great part of the transmitted light L4R is converged on thephoto detector57 to form converging spots S16, S17. Therefore, an information signal and servo signals such as a focus error signal and a tracking error signal are obtained in the same manner as in the sixth embodiment. Also, a remaining part of the transmitted light L4R not changed its wavefront by thewavefront changing unit56 is converged on thephoto detector57 to form the converging spot S18.

In the twelfth embodiment, because the transmitted light L4R is diffracted by thehologram lens42 on the incoming optical path, the converging spot S18 formed on thephoto detector57 does not relate to a radiation point of thelight source52 in a mirror image, while the converging spot S10 formed on thephoto detector63 relates to the radiation point of thelight source52 in the mirror image. In other words, a focal point of the transmitted light L4R converged by the converginglens55 differs from that of the diffracted light L6R converged by the converginglens55. Therefore, thephoto detector57 for detecting the intensity of the transmitted light L4R and thephoto detector63 for detecting the intensity of the diffracted light L6R are required.

Accordingly, even though pieces of information are recorded or reproduced on or from an information medium, the information can be reliably recorded or reproduced on or from the information medium regardless of whether the information medium is thick or thin.

Also, because the diffracted light L6 formed in thehologram lens42 converges before the diffracted light L6 is incident on theobjective lens27, the distance in an optical axis direction between the converging spots S5, S6 can be lengthened to about 1 mm. Therefore, even though the transmitted light L4 (or the diffracted light L6) is converged on the converging spot S6 (or S5) in focus to record or read a piece of information, the light L6 (or L4) is not converged on the converging spot S6 (or S5) in focus to reduce the intensity of the light L6 (or L4) at the converging spot S6 (or S5). Accordingly, no adverse influence is exerted on the recording or reproduction of the information

Also, because thehologram lens42 functions as a convex lens for the first-order diffracted light L6, the occurrence of a chromatic aberration can be prevented in theoptical head apparatus111.

An example of the utilization of theoptical head apparatus111 for various types of optical disks is described.

In cases where theoptical head apparatus111 is utilized for an optical disk device in which pieces of information recorded in a thin type of high densityoptical disk23 are recorded or reproduced and pieces of information recorded in a thick type ofoptical disk25 are exclusively reproduced, the diffraction efficiency of thehologram lens26M or42 in the compoundobjective lens29M,43,45,46 or47 for changing a beam of light to a beam of first-order diffracted light is set to a value equal to or higher than 55%. Therefore. in cases where a piece of information recorded on the thick type ofoptical disk25 is reproduced in thephoto detector57, a signal-noise ratio of each of the servo signals and the information signal obtained in thephoto detector57 can be enhanced because the transmitted light L4R diffracted by thehologram lens26M or42 at a high diffraction efficiency is utilized to obtain the servo signals and the information signal. In other words, a utilization efficiency of the incident light L3 can be enhanced when a piece of information recorded on the thick type ofoptical disk25 is reproduced, so that the output power of the incident light L3 can be minimized. Also, even though a high intensity of the diffracted light L6 is required to record a piece of information on the high densityoptical disk23, the recording of the information can be reliably performed without increasing the intensity of the incident light L3 because the diffraction efficiency of thehologram lens26M or42 for the incident light L3 and the diffracted light L6R is high. Also, in cases where a piece of information recorded on the high densityoptical disk23 is reproduced in thephoto detector63, a signal-noise ratio of each signal obtained in thephoto detector63 can be enhanced because the diffraction efficiency of thehologram lens26M or42 for the light L3, L6R is high.

(Thirteenth Embodiment)

An optical head apparatus with the compoundobjective lens29M,43,45,46 or47 in which the incident light L3 is efficiently utilized to obtain an information signal and servo signals is described with reference toFIG. 44 according to a thirteenth embodiment of the present invention.

FIG. 44 is a constitutional view of an optical head apparatus according to a thirteenth embodiment.

As shown inFIG. 44, anoptical head apparatus121 for recording or reproducing pieces of information on or from the information medium23 or25, comprises thelight source52, thecollimator lens53, thebeam splitter54, the compound objective lens43 (or29M,45,46 or47) composed of the hologram lens42 (or26M or32) and theobjective lens27, the actuatingunit58, the converginglens55, thebeam splitter92 having the reflection type ofhologram93, thephoto detector63, and thephoto detector57.

In the above configuration, the transmitted light L4 and the diffracted light L6 are converged by the converginglens27 in the same manner as in the twelfth embodiment. Thereafter, in cases where a piece of information is recorded or reproduced on or from thefirst information medium23, the diffracted light L6 is converged on thefirst information medium23 to form the converging spot S5. Thereafter, a beam of diffracted light L5R reflected by the first information medium23 passes through the same optical path in the reverse direction, and a large part of the diffracted light L6R is diffracted by thehologram lens42. Therefore, the diffracted light L6R transmits on the incoming optical path agreeing with the outgoing optical path in the same manner as in the twelfth embodiment. Thereafter, the diffracted light L6R is reflected by thebeam splitter54 and is converged by the converginglens55 on thebeam splitter92 to form a converging spot on the reflection type ofhologram93 of thebeam splitter92. Therefore, all of the diffracted light L6R is diffracted and reflected by thehologram93 to be converged on thephoto detector57 in the same manner as in the tenth embodiment. Therefore, an information signal and servo signals such as a focus error signal and a tracking error signal are obtained in the same manner as in the sixth embodiment.

In contrast, in cases where a piece of information is recorded or reproduced on or from thesecond information medium25, the transmitted light L4 is converged on thesecond information medium25 to form the converging spot S6. Thereafter, a beam of transmitted light L4R reflected by the second information medium25 passes through the same optical path in the reverse direction. That is, a large part of the transmitted light L4R is collimated by theobjective lens27 on the incoming optical path. Thereafter, a great part of the transmitted light L4R is diffracted by thehologram Lens42. Therefore, the transmitted light L4R transmits on the incoming optical path differing from the outgoing optical path in the same manner as in the twelfth embodiment. Thereafter, the transmitted light L4R is reflected by thebeam splitter54 and is converged by the converginglens55. Thereafter, a large part of the transmitted light L4R transmits through thebeam splitter92. In this case, an astigmatic aberration is generated in the transmitted light L4R. Thereafter, the transmitted light L4R is converged on thephoto detector63 to form a converging spot S19 of which the shape is the same as the converging spot S10 shown inFIGS. 29A to29C, and the intensity of the transmitted light L4R is detected in thephoto detector63. Therefore, an information signal and servo signals such as a focus error signal and a tracking error signal are obtained in the same manner as in the seventh embodiment.

In the thirteenth embodiment, because the transmitted light L4R is diffracted by thehologram lens42, the converging spot S19 formed on thephoto detector63 does not relate to a radiation point of thelight source52 in a mirror image. Therefore, thephoto detector57 for detecting the intensity of the diffracted light L6R and thephoto detector63 for detecting the intensity of the transmitted light L4R are required.

Accordingly, because the compound objective lens having two focal points is utilized in theoptical head apparatus121, pieces of information can be reliably recorded or reproduced on or from an information medium regardless of whether the information medium is thick or thin.

Also, because the diffracted light L6 formed in thehologram lens42 converges before the diffracted light L6 is incident on theobjective lens27, the distance in an optical axis direction between the converging spots S5, S6 can be lengthened to about 1 mm. Therefore, even though the transmitted light L4 (or the diffracted light L6) is converged on the converging spot S6 (or S5) in focus to record or read a piece of information, the light L6 (or L4) is not converged on the converging spot S6 (or S5) in focus to reduce the intensity of the light L6 (or L4) at the converging spot S6 (or S5). Accordingly, no adverse influence is exerted on the recording or reproduction of the information

Also, because thehologram lens42 functions as a convex lens for the first-order diffracted light L6, the occurrence of a chromatic aberration can be prevented in theoptical head apparatus121.

An example of the utilization of theoptical head apparatus121 for various types of optical disks is described.

In cases where theoptical head apparatus121 is utilized for an optical disk device in which pieces of information recorded in a thin type of high density optical disk,23 are recorded or reproduced and pieces of information recorded in a thick type ofoptical disk25 are exclusively reproduced, the diffraction efficiency of thehologram lens26M or42 in the compoundobjective lens29M,43,45,46 or47 for changing a beam of light to a beam of first-order diffracted light is set to a value equal to or higher than 70%. Therefore, in cases where a piece of information recorded on the thick type ofoptical disk25 is reproduced in thephoto detector57, a signal-noise ratio of each of the servo signals and the information signal obtained in thephoto detector57 can be enhanced because the transmitted light L4R diffracted by thehologram lens26M or42 at a high diffraction efficiency is utilized to obtain the servo signals and the information signal. In other words, a utilization efficiency of the incident light L3 can be enhanced when a piece of information recorded on the thick type ofoptical disk25 is reproduced, so that the output power of the incident light L3 can be minimized. Also, even though a high intensity of the diffracted light L6 is required to record a piece of information on the high densityoptical disk23, the recording of the information can be reliably performed without increasing the intensity of the incident light L3 because the diffraction efficiency of thehologram lens26M or42 for the incident light L3 and the diffracted light L6R is high. Also, in cases where a piece of information recorded on the high densityoptical disk23 is reproduced in thephoto detector63, a signal-noise ratio of each signal obtained in thephoto detector63 can be enhanced because the diffraction efficiency of thehologram lens26M or42 for the light L3, L6R is high.

(Fourteenth Embodiment)

An optical head apparatus in which noises included in an information signal are reduced is described with reference toFIGS. 45,46 according to a fourteenth embodiment of the present invention.

FIG. 45 is a constitutional view of an optical head apparatus according to a fourteenth embodiment.FIG. 46 is a plan view of a hologram lens utilized in the optical head apparatus shown in FIG.45.

As shown inFIG. 45, anoptical head apparatus131 for recording or reproducing pieces of information on or from the information medium23 or25, comprises thelight source52, abeam splitter132 having apolarizing separation film133 on its surface for reflecting the incident light L3 radiated from thelight source52 on an outgoing optical path and transmitting through the light L4R or L5R reflected on the information medium23 or25 on an incoming optical path, acollimator lens134 for collimating the incident light L3 on the outgoing optical path and converging the light L4R or L5R on the incoming optical path, ahologram lens135 for transmitting a part of the incident light L3 without any diffraction and diffracting a remaining part of the incident light L3, the ¼−λplate69, theobjective lens27, the actuatingunit58, and aphoto detector136 for detecting the light transmitting through or diffracted by thehologram lens135 on the incoming optical path.

As shown inFIG. 46, thehologram lens135 is formed by drawing the grating pattern P1 in acentral region135a of thetransparent substrate28 and a grating pattern P12 in aperipheral region135b surrounding thecentral region135a. The grating pattern P12 is drawn in a non-concentric shape. Because the grating pattern P1 are drawn in thehologram lens135, a compoundobjective lens137 having two focal points is composed of thehologram lens135 and theobjective lens27. Light passing through theperipheral region135b of thehologram lens135 is detected by thephoto detector136 to cancel noises included in an information signal. An optical axis of theoptical head apparatus131 passes through a central point of the grating pattern P1 and a central axis of theobjective lens27.

Thephoto detector136 comprises the quadrant photo-detector64 having the detecting sections SE7 to SE10 and a noise cancellingphoto detector138 for detecting the intensity of light passing through theperipheral region135b of thehologram lens135. Because the grating pattern P12 of theperipheral region135b is drawn in the non-concentric shape, light diffracted in theperipheral region135b is not converged on the detecting sections SE7 to SE10.

In the above configuration, the incident light L3 linearly polarized in a first direction is radiated from thelight source52 and is reflected by thebeam splitter132 because thepolarizing separation film133 functions as a mirror for the incident light L3 linearly polarized in the first direction. Therefore, the incident light L3 is directed in an upper direction and is collimated by thecollimator lens134. Thereafter, a part of the incident light L3 incident on thecentral region135a of thehologram lens135 transmits through thecentral region135a without any diffraction to form the transmitted light L4, and a remaining part of the incident light L3 incident on thecentral region135a of thehologram lens135 is diffracted in thecentral region135a to form the diffracted light L5. Also, a part of the incident light L3 incident on theperipheral region135b of thehologram lens135 transmits through theperipheral region135b without any diffraction to form a beam of noise cancelling light L14. Thereafter, the light L4, L5 and L14 pass through the ¼−λ plates so that the light L4, L5 and L14 linearly polarized in the first direction is changed to the light L4, L5 and L14 circularly polarized. Thereafter, the light L4, L5 and L14 are converged by the converginglens27.

Thereafter, in cases where a piece of information is recorded or reproduced on or from the first information medium23 (or the second information medium25), the transmitted light L4 (or the diffracted light L5) is converged on the information medium23 (or25) to form the converging spot S1 (or S2). Thereafter, a beam of transmitted light L4R (or a beam of diffracted light L5R) reflected by the information medium23 (or25) passes through the same optical path in the reverse direction. That is, the transmitted light L4R (or the diffracted light L5R) is circularly polarized in reverse and again passes through the converginglens27 and the ¼−λplate69. Therefore, the light L4R (or L5R) is linearly polarized in a second direction perpendicular to the first direction. Thereafter, a part of the transmitted light L4R transmits through thecentral region135a of thehologram lens135 without any diffraction, or a part of the diffracted light L5R is again diffracted in thecentral region135a. Thereafter, the transmitted light L4R (or the diffracted light L5R) is converged by thecollimator lens134 and passes through thebeam splitter132 without any reflection because thepolarizing separation film133 functions as a transparent plate for the light L4R (or L5R) linearly polarized in the second direction. In this case, an astigmatic aberration is generated in the transmitted light L4R (or the diffracted light L5R) in the same manner as in the seventh embodiment. Thereafter, the transmitted light L4R (or the diffracted light L5R) is incident on the detecting sections SE7 to SE10 of thephoto detector136 to form a converging spot S20 of which the shape is the same as the converging spot S10 shown inFIGS. 29A to29C. The intensity of the transmitted light L4R (or the diffracted light L5R) is changed to electric current signals SC21 to SC24 in the detecting sections SE7 to SE10. Therefore, servo signals such as a focus error signal and a tracking error signal are obtained in the same manner as in the seventh embodiment, so that the position of the compoundobjective lens137 is adjusted to converge the transmitted light L4 (or the diffracted light L5) on the information medium23 (or25) in focus. Also, an information signal expressing a piece of information recorded on the information medium23 (or25) is obtained according to an equation (12).
S_in=SC21+SC22+SC23+SC24  (12)

Also, the noise cancelling light L14 is converged on the information medium23 to form a converging spot surrounding the converging spot S1. Thereafter, a beam of noise cancelling light L14R reflected by the first information medium23 passes through the same optical path in the reverse direction. That is, the noise cancelling light L14R again passes through the converginglens27 and the ¼−λplate69, and a part of the noise cancelling light L14R is diffracted and converged in theperipheral region135b of thehologram lens135 and is incident on the noise cancellingphoto detector138. In thephoto detector138, an output signal SC25 is generated according to the intensity of the noise cancelling light L14R. Thereafter, a noise cancelled information signal S_ncexpressing a piece of information recorded on thefirst information medium23 is obtained by adding all of the signals according to an equation (13):
S_nc=(SC21+SC22+SC23+SC24)+R ×SC25  (13),
where the symbol R is a weighting factor.

In this case, because the term R×SC25 is added to obtain the information signal S_nc, inverse influence of noises included in the term (SC21+SC22+SC23+SC24) on the noise cancelled information signal S_nccan be reduced. The reason is described.

As is well known (for example, Japanese Patent Gazette No. 22452 of 1990 laid open to public inspection on Jul. 23, 1985 under Provisional Publication No. 138748 of 1985 and Published Unexamined Patent Application No. 131245 of 1986), signals expressing pieces of information recorded on an optical disk shifts to a higher frequency as the density of the information recorded becomes high. Also, the amplitude of a signal having a high frequency becomes low as compared with that of a signal having a low frequency in cases where the signals are produced according to light passing through a central region of a hologram lens. In contrast, the amplitude of a signal having a high frequency is emphasized in cases where the signal is produced according to light passing through a peripheral region of the hologram lens. Therefore, in cases where the information signal S_ncis obtained according to the equation (13), high frequency components included in the information signal S_ncis emphasized, and low frequency noise components included in the term (SC21+SC22+SC23+SC24) are comparatively reduced. As a result, a signal-noise ratio in the information signal S_nccan be enhanced.

Accordingly, because the compound objective lens having two focal points is utilized in theoptical head apparatus131, pieces of information can be reliably recorded or reproduced or or from an information medium regardless of whether the information medium is thick or thin.

Also, even though pieces of information are densely recorded in a thin type of high density optical disk represented by thefirst information medium23, the information signal S_nccan be reliably reproduced at a high signal-noise ratio.

Also, because the intensity of the light L4R or L5R incident on the detecting sections SE7 to SE10 of thephoto detector136 is reduced by converging the noise cancelling light L14R on thephoto detector138, a positioning accuracy of thephoto detector136 can be coarsely lowered to 1/100.

Also, in cases where the grating pattern P12 of theperipheral region135b functions as a lens for the incident light L3 diffracted in theperipheral region135b, unnecessary diffracted light generated in theperipheral region135b on the outgoing optical path forms a comparatively large converging spot in defocus on thefirst information medium23. Therefore, pieces of information recorded on thefirst information medium23 are read by the unnecessary diffracted light, and the information are treated as a piece of averaged information in thephoto detector136 even though the unnecessary diffracted light is incident on thephoto detector136. Accordingly, the information read by the unnecessary diffracted light does not adversely influence on the information signal S_ncas a noise.

Also, in cases where a transmission efficiency of theperipheral region135b of thehologram lens135 is set to agree with another transmission efficiency of thecentral region135a, secondary maxima (or side lobes) occurring around the converging spot S1 can be lowered as compared with the first embodiment. Accordingly, a signal-noise ratio in the information signal S_nccan be enhanced.

(Fifteenth Embodiment)

An optical head apparatus in which noises included in an information signal are reduced is described with reference toFIGS. 47 to49 according to a fifteenth embodiment of the present invention.

FIG. 47 is a constitutional view of an optical head apparatus according to a fifteenth embodiment.FIG. 48 is a plan view of a hologram lens utilized in the optical head apparatus shown in FIG.47.

As shown inFIG. 47, anoptical head apparatus141 for recording or reproducing pieces of information on or from the information medium23 or25, comprises thelight source52, thebeam splitter82, thecollimator lens134, ahologram lens142 for transmitting a part of the incident light L3 without any diffraction and diffracting a remaining part of the incident light L3, theobjective lens27, the actuatingunit58, and aphoto detector143 for detecting the light transmitting through or diffracted by thehologram lens142 on the incoming optical path.

As shown inFIG. 48, thehologram lens142 is partitioned into acentral region142a in which the grating pattern P1 is drawn, a pair of sideperipheral regions142b,142c in which grating patterns P13, P14 are drawn to cancel noises included in an information signal, and a pair of no-designedregions142d,142e in which no grating pattern is drawn not to reduce the intensity of light. Because the grating pattern P1 are drawn in thehologram lens135, a compoundobjective lens144 having two focal points is composed of thehologram lens142 and theobjective lens27. An optical axis of theoptical head apparatus141 passes through a central point of the grating pattern P1 and a central axis of theobjective lens27.

Thephoto detector143 comprises the quadrant photo-detector64 having the detecting sections SE7 to SE10, a pair of noise cancellingphoto detector138a,138b for detecting the intensity of light passing through theperipheral region142b,142c of thehologram lens142.

In the above configuration, the transmitted light L4 (or the diffracted light L5) generated in thecentral region142a of thehologram lens142 is converged on the first information medium23 (or the second information medium25) in an outgoing optical path to form the converging spot S1 (or S2). Thereafter, the transmitted light L4R (or the diffracted light L5R) passes through the same optical path in the reverse direction. That is, the transmitted light L4R (or the diffracted light L5R) again passes through the converginglens27, and a part of the transmitted light L4R transmits through thecentral region142a of thehologram lens142 without any diffraction or a part of the diffracted light L5R is again diffracted in thecentral region142a. Thereafter, the transmitted light L4R (or the diffracted light L5R) is converged by thecollimator lens134 and passes through thebeam splitter82. In this case, an astigmatic aberration is generated in the transmitted light L4R (or the diffracted light L5R) in the same manner as in the seventh embodiment. Thereafter, the transmitted light L4R (or the diffracted light L5R) is incident on the detecting sections SE7 to SE10 of thephoto detector143 to form a converging spot S21 of which the shape is the same as the converging spot S10 shown inFIGS. 29A to29C. The intensity of the transmitted light L4R (or the diffracted light L5R) is changed to electric current signals SC26 to SC29 in the detecting sections SE7 to SE10. Therefore, servo signals such as a focus error signal and a tracking error signal are obtained in the same manner as in the seventh embodiment, so that the position of the compoundobjective lens144 is adjusted to converge the transmitted light L4 (or the diffracted light L5) on the information medium23 (or25) in focus. Also, an information signal recorded on thesecond information medium25 is obtained according to an equation (14).
S_in=SC26+SC27+SC28+SC29  (14)

Also, a part of the incident light L3 incident on the peripheral region142b of thehologram lens142 transmits through the peripheral region142b without any diffraction to form a beam of noise cancelling light L15, and a part of the incident light L3 incident on theperipheral region142c of thehologram lens142 transmits through theperipheral region142c without any diffraction to form a beam of noise cancelling light L16. Thereafter, the noise cancelling light L15, L16 are converged on the information medium23 to form a converging spot surrounding the converging spot S1. Thereafter, beams of noise cancelling light L15R, L16R reflected by the first information medium23 passes through the same optical path in the reverse direction. That is, the noise cancelling light L15R, L16R again passes through the converginglens27. A part of the noise cancelling light L15R is diffracted and converged in the peripheral region142b of thehologram lens142 and is incident on the noise cancellingphoto detector138a, and a part of the noise cancelling light L16R is diffracted and converged in theperipheral region142c of thehologram lens142 and is incident on the noise cancellingphoto detector138b. In thephoto detector138a, an output signal SC30 is generated according to the intensity of the noise cancelling light L15R. Also, an output signal SC31 is generated according to the intensity of the noise cancelling light L16R in thephoto detector138b. Thereafter, a noise cancelled information signal S_ncexpressing the information recorded on thefirst information medium23 is obtained by adding all of the signals according to an equation (15):
S_nc=(SC26+SC27+SC28+SC29)+R×(SC30+SC31)  (15),
where the symbol R is a weighting factor.

Accordingly, because the compound objective lens having two focal points is utilized in theoptical head apparatus141, pieces of information can be reliably recorded or reproduced from an information medium regardless of whether the information medium is thick or thin.

Also, a signal-noise ratio in the information signal S_nccan be enhanced in the same manner as in the fourteenth embodiment.

Also, even though pieces of information are densely recorded in a thin type of high density optical disk represented by thefirst information medium23, the information signal S_nccan be reliably reproduced at a high signal-noise ratio.

Also, because the intensity of the light L4R or L5R incident on the detecting sections SE7 to SE10 of thephoto detector143 is reduced by converging the noise cancelling light L15R, L16R on thephoto detectors138a,138b, a positioning accuracy of thephoto detector143 can be coarsely lowered to 1/100.

Also, in cases where the grating patterns P13, P14 of theperipheral regions142b,142c functions as a lens for the incident light L3 diffracted in theperipheral region142b,142c, unnecessary diffracted light generated by diffracting the incident light L3 in theperipheral regions142b,142c on the outgoing optical path forms a comparatively large converging spot in defocus on thefirst information medium23. Also, an numerical number of each of theperipheral regions142b,142c is lowered as compared with that of theregion135b in the fourteenth embodiment because thehologram lens142 are partitioned into many fields. Therefore, the size of the converging spot of the unnecessary diffracted light formed in defocus on thefirst information medium23 becomes larger than that in the fourteenth embodiment. As a result, more pieces of information recorded on thefirst information medium23 are read by the unnecessary diffracted light, and the information are treated as a piece of averaged information in thephoto detector143 even though the unnecessary diffracted light is incident on thephoto detector143. Accordingly, the information read by the unnecessary diffracted light is moreover averaged, and the averaged information does not adversely influence on the information signal S_ncas a noise.

Also, as shown inFIGS. 49A,49B, the unnecessary diffracted light can be prevented from being incident on thephoto detector64 in which each of the detecting sections SE7 to SE10 has SL0 square in size. In detail, in cases where light generated in theperipheral region142c on the outgoing optical path is again diffracted in the peripheral region142b to form a beam of unnecessary diffracted light Lu₁, the light Lu₁is converged on a first position PT1 spaced SP1 (SP1>SL0) from the center of thephoto detector64. In cases where light generated in the peripheral region142b on the outgoing optical path is again diffracted in theperipheral region142c to form a beam of unnecessary diffracted light Lu₂, the light Lu₂is converged on a second position PT2 spaced SP2 (SP2>SL0) from the center of thephoto detector64. In cases where light generated in theperipheral regions142b,142c on the outgoing optical path are again diffracted in the sameperipheral regions142b,142c to form beams of unnecessary diffracted light Lu₃, Lu₄, the light Lu₃, Lu₄are converged on third and fourth position PT3, PT4 spaced SP3 (SP3>SL0), SP4 (SP4>SL0) from the center of thephoto detector64. Accordingly, the adverse influence of the unnecessary diffracted light Lu₁to Lu₄can be prevented.

In cases where thelight source52 is formed of a semiconductor laser, a far field pattern of the incident light L3 incident on thehologram lens142 is distributed in the Gaussian distribution as shown inFIG. 13A, and a cross-sectional beam profile of the incident light L3 distributed in the Gaussian distribution is in an elliptic shape. That is, a beam divergent angle (or a full angle at half maximum) of the incident light L3 in a perpendicular direction is larger than that in a horizontal direction. In this embodiment, the perpendicular direction of the incident light L3 is directed in an X2 direction shown inFIG. 48, and the horizontal direction of the incident light L3 is directed in a Y2 direction shown in FIG.48. In this case, because a transmission efficiency of theregions142b,142c for the incident light L3 is smaller than that of theregions142d,142e, the intensity of the incident light L3 transmitting through thehologram lens142 without any diffraction is largely reduced in the perpendicular direction as compared with that in the horizontal direction. Therefore, the cross-sectional beam profile of the incident light L3 is corrected to a circular shape in thehologram lens142. That is, the converging spot S1 formed on thefirst information medium23 is corrected to the circular shape. Accordingly, secondary maxima (or side lobes) occurring around the converging spot S1 can be lowered, and a signal-noise ratio of the information signal S_nccan be enhanced.

In the fifteenth embodiment, the noise cancelled information signal S_ncis obtained according to the equation (15). However, it is preferred that the noise cancelled information signal S_ncbe obtained according to the equation (16):
S_nc=(SC26+SC27+SC28+SC29)+(R1×SC30+R2×SC31)  (16),
where the symbols R1, R2 are weighting factors. In this case, the noises included in the information can be moreover reduced.
(Sixteenth Embodiment)

An optical head apparatus manufactured in a small size and stably operated is described with reference toFIGS. 50,51 according to a sixteenth embodiment of the present invention.

FIG. 50 is a constitutional view of an optical head apparatus according to a sixteenth embodiment.FIG. 51 is a diagonal view of a light source and photo detectors utilized in the optical head apparatus shown in FIG.50.

As shown inFIG. 50, anoptical head apparatus151 for recording or reproducing pieces of information on or from the information medium23 or25, comprises thelight source52 for radiating the incident light L3 linearly polarized in a non-diffracting direction parallel to an X3 axis, anholographic element152 for transmitting through the incident light L3 without any diffraction on an outgoing optical path and diffracting the transmitted light L4R or the diffracted light L5R linearly polarized in a diffracting direction parallel to a Y3 axis on an incoming optical path, thecollimator lens53, the ¼−λplate69, the hologram lens26 (or26M,32,33,42,135 or142), theobjective lens27, the actuatingunit58, and aphoto detector153 for detecting the intensity of the light L4R or L5R diffracted by theholographic element152.

As shown inFIG. 51, thelight source52 and thephoto detector153 are located on asubstrate154 to precisely fix a relative position between thelight source52 and thephoto detector153. Thephoto detector153 comprises the sextant photo-detector59 having the detecting sections SE1 to SE6 and the tracking photo-detectors60a to60d. Also, amirror element155 is located on thesubstrate154 to direct the incident light L3 radiated from thelight source52 in a Z3 direction.

Theholographic element152 is produced by proton-exchange surface parts of a lithium niobate substrate or by utilizing a liquid crystal cell, as is described in Provisional Publication No. 189504/86 (S61-189504) and Provisional Publication No. 241735/88 (S63-241735). Therefore, light linearly polarized in a non-diffracting direction parallel to an X3 axis transmits through theholographic element152 without any diffraction. In contrast, light linearly polarized in a diffracting direction parallel to a Y3 axis which is perpendicular to the X3 axis is diffracted by theholographic element152.

In the above configuration, the incident light L3 linearly polarized in a non-diffracting direction parallel to an X3 axis is radiated from thelight source52 and transmits through theholographic element152 without any diffraction. Thereafter, the incident light L3 is collimated by thecollimator lens53, and the incident light L3 linearly polarized is changed to the incident light L3 circularly polarized by the ¼−λplate69. Thereafter, a part of the incident light L3 transmits through thehologram lens26 without any diffraction to form the transmitted light L4, and a remaining part of the incident light L3 is diffracted by thehologram lens26 to form the diffracted light L5. Thereafter, the light L4, L5 are converged by theobjective lens27, and the converging spot S1 of the transmitted light L4 (or the converging spot S2 of the diffracted light L5) is formed on the first information medium23 (or the second information medium25). When the light L4 or L5 is reflected by the information medium23 (or25) and is changed to the light L4R (or L5R), a rotational direction of the circular polarization in the light L4 is reversed. Therefore, the light L4R (or L5R) having the reversed circular polarization passes through the same optical path in the opposite direction. That is, the transmitted light L4R (or the diffracted light L5R) again passes through the converginglens27, and a part of the transmitted light L4R transmits through thehologram lens142 without any diffraction or a part of the diffracted light L5R is again diffracted by thehologram lens142. Thereafter, the transmitted light L4R (or the diffracted light L5R) circularly polarized in reverse is changed to the light L4R (or L5R) linearly polarized in a diffracting direction parallel to a Y3 axis by the ¼−λplate69. Thereafter, the light L4R (or L5R) is converged by thecollimator lens53 and is diffracted by theholographic element152 to form a plurality of converging spots on thephoto detectors153. Therefore, an information signal expressing a piece of information recorded on the information medium23 (or25) and servo signals such as a focus error signal and a tracking error signal are obtained in thephoto detector153 in the same manner as in the sixth embodiment.

Accordingly, because the compound objective lens having two focal points is utilized in theoptical head apparatus151, pieces of information can be reliably recorded or reproduced or or from an information medium regardless of whether the information medium is thick or thin.

Also, because all of the incident light L3 transmits through theholographic element152 on the outgoing optical path and because all of the light L4R or L5R is diffracted by theholographic element152 on the incoming optical path, a utilization efficiency of the incident light L3 can be enhanced. Therefore, even though a radiation intensity of the incident light L3 in thelight source52 is low, the information signal and the servo signals having a high signal-noise ratio can be reliably obtained.

Also, because no beam splitter is utilized in theoptical head apparatus151, theoptical head apparatus151 can be manufactured at a small size, in a light weight, and at a low cost.

Also, because optical parts of theoptical head apparatus151 are located along its optical axis, theoptical head apparatus151 stably operated can be obtained even though a circumstance temperature largely varies and the apparatus is operated for a long time.

Also, because the light L4R or L5R transmitting through theholographic element152 without any diffraction on the incoming optical path is not required, it is preferred that a diffraction efficiency of theholographic element152 be heightened to set a transmission efficiency of theholographic element152 to almost zero. In this case, a combination of theholographic element152 and the ¼−λplate69 function functions as an isolator to prevent the light L4R or L5R from returning to thelight source52. Therefore, in cases where a semiconductor laser is utilized as thelight source52, any light does not return to an active layer of the semiconductor laser. Accordingly, noises induced by the light returning to the semiconductor laser can be prevented.

Also, because thelight source52 and thephoto detector153 are located on thesame substrate154, thelight source52 and thephoto detector153 can be closely arranged each other. Therefore, a relative position between thelight source52 and thephoto detector153 can be easily set at a high accuracy. For example, the relative position can be set at an accuracy within several μm. Accordingly, a manufacturing cost of theoptical head apparatus151 can be lowered, and theoptical head apparatus151 can be moreover manufactured at a small size, in a light weight, and at a low cost.

Also, thelight source52 is electrically connected with an external circuit through first wirings, and thephoto detector153 is electrically connected with another external circuit through second wirings. In this case, because thelight source52 and thephoto detector153 are located on thesame substrate154, the first and second wirings can pass on an X3-Y3 plane in common. Therefore, thelight source52 and thephoto detector153 can be easily and automatically connected with the external circuits. In addition, because reference lines required to connect thelight source52 and thephoto detector153 with the external circuits are only drawn on the X3-Y3 plane, the relative position between thelight source52 and thephoto detector153 can be easily set at a high accuracy.

In the sixteenth embodiment, theoptical head apparatus151 with theholographic element152 is described. However, in cases where the intensity of the incident light L3 is sufficient, it is applicable that a hologram having a small grating pitch or a blazed hologram be utilized in place of theholographic element152. In this case, pieces of information can be reliably recorded or reproduced on or from an information medium regardless of whether the information medium is thick or thin. Also, because no beam splitter is utilized in theoptical head apparatus151, theoptical head apparatus151 can be manufactured at a small size, in a light weight, and at a low cost. Also, because optical parts of theoptical head apparatus151 are located along its optical axis, theoptical head apparatus151 stably operated can be obtained even though a circumstance temperature largely varies and the apparatus is operated for a long time.

(Seventeenth Embodiment)

An optical head apparatus manufactured in a small size and stably operated is described with reference toFIG. 52 according to a seventeenth embodiment of the present invention.

FIG. 52 is a constitutional view of an optical head apparatus according to a seventeenth embodiment.

As shown inFIG. 52, anoptical head apparatus161 for recording or reproducing pieces of information on or from the information medium23 or25, comprises thelight source52 for radiating the incident light L3 linearly polarized in a first direction, thecollimator lens53, apolarizing separation film162 formed on a front surface of atransparent substrate162 for reflecting the incident light L3 linearly polarized in the first direction and transmitting light linearly polarized in a second direction perpendicular to the first direction, the ¼−λplate69, the hologram lens26 (or26M,32,33,42,135 or142), theobjective lens27, the actuatingunit58, a reflection-type hologram164 formed on a rear surface of thetransparent substrate162 for diffracting and reflecting the light L4R, L5R, and thephoto detector57.

In the above configuration, the incident light L3 linearly polarized in a first direction is radiated from thelight source52 and is collimated by thecollimator lens53. Thereafter, all of the incident light L3 is reflected by thepolarizing separation film162 because the incident light L3 is linearly polarized in the first direction. Therefore, the incident light L3 is directed in an upper direction. Thereafter, the linear polarization of the incident light L3 is changed to a circular polarization in the ¼−λplate69, and a part of the incident light L3 transmits through thehologram lens26 to form the transmitted light L4. Also, a remaining part of the incident light L3 is diffracted by thehologram lens26 to form the diffracted light L5. Thereafter, the light L4, L5 are converged by theobjective lens27, and the converging spot S1 of the transmitted light L4 (or the converging spot S2 of the diffracted light L5) is formed on the first information medium23 (or the second information medium25). Thereafter, the transmitted light L4R (or the diffracted light L5R) circularly polarized in reverse again passes through the converginglens27 in the same manner as in the sixteenth embodiment, and a part of the transmitted light L4R transmits through thehologram lens26 without any diffraction or a part of the diffracted light L5R is again diffracted by thehologram lens26. Thereafter, the transmitted light L4R (or the diffracted light L5R) circularly Polarized in reverse is changed to the light L4R (or L5R) linearly polarized in a second direction perpendicular to the first direction by the ¼−λplate69. Thereafter, all of the light L4R (or L5R) is refracted by thepolarizing separation film162 and is diffracted and reflected by thehologram164. Thereafter, the light L4R (or L5R) transmits through thepolarizing separation film162 and is converged by thecollimator lens53 to form a plurality of converging spots on thephoto detector57. Therefore, an information signal expressing a piece of information recorded on the information medium23 (or25) and servo signals such as a focus error signal and a tracking error signal are obtained in thephoto detector57 in the same manner as in the sixth embodiment.

Accordingly, because the compound objective lens having two focal points is utilized in theoptical head apparatus161, pieces of information can be reliably recorded or reproduced on or from an information medium regardless of whether the information medium is thick or thin.

Also, because the incident light L3 incident on thepolarizing separation film162 is collimated, a reflectivity for the incident light L3 is uniform over theentire film162. Therefore, a diffraction-limited spot of the light L4 or L5 can be easily formed on the information medium23 or25. Also, because the light L4R, L5R incident on thepolarizing separation film162 are collimated, a transmissivity for the light L4R, L5R is uniform over theentire film162. Therefore, an offset occurring in the servo signals can be prevented.

Also, because all of the incident light L3 transmits through thehologram164 on the outgoing optical path and because all of the light L4R or L5R is diffracted by thehologram164 on the incoming optical path, a utilization efficiency of the incident light L3 can be enhanced. Therefore, even though a radiation intensity of the incident light L3 in thelight source52 is low, the information signal and the servo signals having a high signal-noise ratio can be reliably obtained.

Also, because a hybrid element composed of thefilm162, thesubstrate163 and thehologram164 functions as a beam splitter and a rising mirror, theoptical head apparatus161 can be manufactured at a small size, in a light weight, and at a low cost.

Also, because optical parts of theoptical head apparatus161 are located along its optical axis, theoptical head apparatus161 stably operated can be obtained even though a circumstance temperature largely varies and the apparatus is operated for a long time.

Also, a combination of thefilm162 and the ¼−λplate69 function functions as an isolator to prevent the light L4R or L5R from returning to thelight source52. Therefore, in cases where a semiconductor laser is utilized as thelight source52, any light does not return to an active layer of the semiconductor laser. Accordingly, noises induced by the light returning to the semiconductor laser can be prevented.

Also, it is preferred that thehologram164 be blazed. In this case, because the generation of unnecessary diffracted light such as minus first-order diffracted light in thehologram164 is prevented, a diffraction efficiency of thehologram164 for changing light to first-order diffracted light can be set to almost 100%. Therefore, the incident light L3 can be efficiently utilized to obtain the signals.

Also, because light incident on thehologram164 is diffracted to first-order diffracted light, a chromatic aberration occurring in the light L4R, L5R can be compensated in thehologram164. Therefore, the servo signals can be stably obtained.

In the seventeenth embodiment, thecollimator lens53 is located between thelight source52 and thefilm162. However, thecollimator lens53 is not necessary in theoptical head apparatus161.

Also, theoptical head apparatus161 with thefilm162 and the ¼−λplate69 is described. However, in cases where the intensity of the incident light L3 is sufficient, it is applicable that a reflection film having a reflectivity of almost ⅓ be utilized in place of thefilm162 and the ¼−λplate69 be omitted. In this case, pieces of information can be reliably recorded or reproduced on or from an information medium regardless of whether the information medium is thick or thin. Also, because a hybrid element composed of thefilm162, thesubstrate163 and thehologram164 functions as a beam splitter and a rising mirror, theoptical head apparatus161 can be manufactured at a small size, in a light weight, and at a low cost. Also, because optical parts of theoptical head apparatus161 are located along its optical axis, theoptical head apparatus161 stably operated can be obtained even though a circumstance temperature largely varies and the apparatus is operated for a long time.

In the sixth to seventeenth embodiments, pieces of information can be reliably recorded or reproduced on or from an information medium regardless of whether the information medium represents a conventional optical disk such as a compact disk having a thickness T2 of about 1.2 mm or a prospective high density optical disk having a thickness T1 ranging from 0.4 mm to 0.8 mm. However, when the information recorded or reproduced on or from the information medium, it is required to examine the thickness of the information medium in advance. Therefore, in cases where a piece of distinguishing information is recorded on the information medium in advance to distinguish the thickness of the information medium, it is convenient for a user. Because no distinguishing information is recorded on the conventional optical disk, it is preferred that the distinguishing information be recorded on the prospective high density optical disk appearing on the market in. the future. Therefore, a high density optical disk with the distinguishing information is described according to eighteenth and nineteenth embodiments.

(Eighteenth Embodiment)

FIG. 53 is a diagonal view of a high density optical disk according to an eighteenth embodiment, a cross sectional view of the disk being partially shown.

As shown inFIG. 53, a high densityoptical disk171 is partitioned into an outer region171a and an inner region171b. The outer region171a occupies a large part of theoptical disk171, and aninformation recording substrate171c of the outer region171a has the thickness T1, and theinformation recording substrate171c of the inner region171b has the thickness T2. A plurality offirst recording pits172 are formed on theinformation recording substrate171c of the outer region171a at narrow intervals in series to record pieces of information at a high density. Also, a plurality of second recording pits173 are formed on theinformation recording substrate171c of the inner region171b at ordinary intervals in series to record pieces of distinguishing information at an ordinary density of a compact disk. The distinguishing information inform that theoptical disk171 has the thickness T1. The thickness T1 of the outer region171a, for example, ranges from 0.4 mm to 0.8 mm, and the thickness T2 of the inner region is, for example, about 1.2 mm.

In the above configuration, the diffracted light L5 according to the first or second embodiment (or the transmitted light L4 according to the third embodiment) is initially converged on an inner region of theinformation medium23,25 while performing a focus control corresponding to thesecond information medium25 having the thickness T2. In cases where the information medium23 or25 is theoptical disk171, a piece of distinguishing information informing that theoptical disk171 having the thickness T1 is converged by the light L5 (or L4) is detected. Thereafter, the transmitted light L4 (or the diffracted light L6) is automatically converged on the outer region171a of theoptical disk171 while performing a focus control corresponding to thefirst information medium23 having the thickness T1.

In contrast, in cases where the information medium23 or25 is a thick type of conventional optical disk having a thickness T2, no distinguishing information is detected when the light L5 (or L4) is converged on the inner region171b of the conventional optical disk. In this case, the focus control corresponding to thesecond information medium25 is continued to detect an information signal expressing a piece of information recorded on the conventional optical disk.

Accordingly, in cases where one of the optical head apparatuses shown inFIGS. 21,27,30,31,32,33,37,38,40A,43,44,50 and52 is utilized, pieces of information can be automatically recorded or reproduced on or from an information medium regardless of whether the information medium is thin or thick.

Also, because only the distinguishing information is recorded in the inner region, the inner region can be small. Therefore, a memory capacity of theoptical disk171 is not lowered by the addition of thesecond recording pit173.

(Nineteenth Embodiment)

FIG. 54 is a diagonal view of a high density optical disk according to a nineteenth embodiment, a cross sectional view of the disk being partially shown.

As shown inFIG. 54, a high densityoptical disk174 is partitioned into anouter region174a and an inner region174b. Theouter region174a occupies a large part of theoptical disk174. Theoptical disk174 has a uniform thickness of T1. Thefirst recording pits172 are formed on an information recording substrate174c of theouter region174a to record pieces of information at a high density. Also, a plurality of second recording pits175 having a large size are formed on the information recording substrate174c of the inner region174b at wide intervals to record pieces of distinguishing information at a lower density than the ordinary density. The distinguishing information inform that the entireoptical disk174 has the thickness T1. The thickness T1 of theoptical disk174, for example, ranges from 0.4 mm to 0.8 mm.

In the above configuration, the diffracted light L5 according to the first or second embodiment (or the transmitted light L4 according to the third embodiment) is initially converged on an inner region of the information medium23 or25 while performing a focus control corresponding to thesecond information medium25 having the thickness T2. In cases where the information medium23 or25 is theoptical disk174, the light L5 (or L4) is converged on each of the second recording pits175 in defocus. However, because each of the second recording pits175 is large in size, a converging spot of the light L5 (or L4) is reliably formed in one of the second recording pits175. Therefore, a piece of distinguishing information, which informs that theoptical disk174 having the thickness T1 is converged by the light L5 (or L4), is detected. Thereafter, the transmitted light L4 (or the diffracted light L6) is automatically converged on theouter region174a of theoptical disk174 while performing a focus control corresponding to thefirst information medium23 having the thickness T1.

In contrast, in cases where the information medium23 or25 is a thick type of conventional optical disk having a thickness T2, no distinguishing information is detected when the light L5 (or L4) is converged on the inner region174b of the conventional optical disk. In this case, the focus control corresponding to thesecond information medium25 is continued to detect an information signal expressing a piece of information recorded on the conventional optical disk.

Accordingly, in cases where one of the optical head apparatuses shown inFIGS. 21,27,30,31,32,33,37,38,40A,43,44,50 and52 is utilized, pieces of information can be automatically recorded or reproduced on or from an information medium regardless of whether the information medium is thin or thick.

Also, because only the distinguishing information is recorded in the inner region of theoptical disk174, the inner region can be small. Therefore, a memory capacity of theoptical disk174 is not lowered by the addition of thesecond recording pit173.

Also, because the thickness of theoptical disk174 is uniform, theoptical disk174 can be easily manufactured at a low cost. Also, theoptical disk174 can be thinned.

(Twentieth Embodiment)

An optical disk apparatus with one of the optical head apparatuses in which it is automatically judged whether a high density optical disk having the thickness T1 or a conventional optical disk having the thickness T2 is utilized is described.

FIG. 55 is a block diagram of an optical disk apparatus with one of the optical head apparatuses shown inFIGS. 21,27,30,31,32,33,37,38,40A,43,44,50 and52 according to a twentieth embodiment.FIG. 56 is a flow chart showing the operation of the optical disk apparatus shown in FIG.55.FIG. 63 is a constitutional view of the optical disk apparatus of which the block diagram is shown in FIG.55.

As shown inFIG. 63, an optical disk apparatus176 for recording or reproducing pieces of information on or from the high density optical disk171 (or174) or the conventional optical disk25, comprises the optical head apparatus51 (or61,65,67,70,71,81,91,101,111,121,151 or161), a moving means177 such as a feed mechanism for moving the optical head apparatus51 to a prescribed position, a rotating means178 such as a spindle motor for rotating the high density optical disk171 (or174) or the conventional optical disk25, an actuating means representing the actuating unit58 (not shown in FIG.63), an electric circuit179 representing a focus control means and a tracking control means for controlling the actuating means to perform a first focus control of the optical head apparatus51 corresponding to the first thickness T1 of the optical disk171 (or174) and a second focus control of the optical head apparatus51 corresponding to the second thickness T2 of the optical disk171 (or174) or25 according to a focus error signal read by the optical head apparatus51 and controlling the actuating means to perform a first tracking error control of the optical head apparatus51 corresponding to the first thickness T1 and a second tracking error control of the optical head apparatus51 corresponding to the second thickness T2 according to a tracking error signal read by the optical head apparatus51, and a connecting means180 for electrically connecting the rotating means178, the moving means177 and the electric circuit179 with an electric source (not shown) to supply an electric power to the rotating means178, the moving means177 and the electric circuit179.

In the above configuration, the high densityoptical disk171 or the conventionaloptical disk25 is set to a prescribed position of theoptical disk apparatus176, and theoptical disk171 or25 is rotated by the rotatingmeans178. Thereafter, theoptical head apparatus51 is moved to a position just under an innermost recording track of theoptical disk171 or25 by the moving means177 in astep211, and the diffracted light L5 is converged on the innermost recording track of theoptical disk171 or25 while performing a focus control corresponding to the conventionaloptical disk25 of the thickness T2 in astep212. Thereafter, a tracking control is performed, and a piece of information recorded on the innermost recording track of theoptical disk171 or25 is detected in astep213. In detail, a focus error signal and a tracking error signal corresponding to a positional relationship between theoptical head apparatus51 and theoptical disk171 or25 are sent to theelectric circuit179. In theelectric circuit179, an actuating signal is generated according to the focus error signal and the tracking error signal and is sent to theactuating unit58 to precisely move the compound objective lens29 (or29M,34,43,45,46 or47) of theoptical head apparatus51. That is, the second focus and tracking controls of theoptical head apparatus51 corresponding to the second thickness T2 are performed to read the information. Thereafter, it is judged in astep214 whether the information agrees with a piece of distinguishing information informing that theoptical disk171 having the thickness T1 is set to theoptical disk apparatus176.

In cases where the high densityoptical disk171 is set to theoptical disk apparatus176, the distinguishing information is detected. Thereafter, the transmitted light L4 is automatically converged on theoptical disk171 while performing a focus control corresponding to theoptical disk171 of the thickness T1 in astep215. In detail, a focus error signal and a tracking error signal corresponding to a positional relationship between theoptical head apparatus51 and theoptical disk171 are sent to theelectric circuit179. In theelectric circuit179, an actuating signal is generated according to the focus error signal and the tracking error signal and is sent to theactuating unit58 to precisely move the compound objective lens29 (or29M,34,43,45,46 or47) of theoptical head apparatus51. That is, the first focus and tracking controls of theoptical head apparatus51 corresponding to the first thickness T1 are performed. Therefore, pieces of information are recorded or reproduced on or from theoptical disk171.

In contrast, in cases where the conventionaloptical head25 is set to theoptical disk apparatus176, the distinguishing information is not detected. In this case, the convergence of the diffracted light L5 on the conventionaloptical disk25 is continued while performing the focus control and the tracking control corresponding to the conventionaloptical disk25 in astep216. Therefore, pieces of information are recorded or reproduced on or from the conventionaloptical disk25.

Accordingly, the thickness of the optical disk set in theoptical disk apparatus176 can be rapidly judged at a high accuracy. Also, pieces of information can be stably recorded or reproduced on or from an optical disk regardless of whether the optical disk is the high density optical disk171 (or174) or the conventionaloptical disk25.

(Twenty-first Embodiment)

An optical disk apparatus with one of the optical head apparatuses in which it is automatically judged whether a high density optical disk having the thickness T1 or a conventional optical disk having the thickness T2 is utilized is described.

FIG. 57 is a block diagram of an optical disk apparatus with one of the optical head apparatuses shown inFIGS. 21,27,30,31,32,33,37,38,40A,43,44,50 and52 according25 to a twenty-first embodiment.FIG. 58 is a flow chart showing the operation of the optical disk apparatus shown in FIG.57.

As shown inFIG. 57, anoptical disk apparatus176 for recording or reproducing pieces of information on or from a high densityoptical disk182 or the conventionaloptical disk25, comprises the optical head apparatus51 (or61,65,67,70,71,81,91,101,111,121,151 or161), the moving means177, and arotating means182 such as a spindle motor for rotating the high densityoptical disk182 or the conventionaloptical disk25. The high densityoptical disk182 has no distinguishing information and has the thickness T1.

In the above configuration, the high densityoptical disk182 or the conventionaloptical disk25 is set to a prescribed position of theoptical disk apparatus181, and theoptical disk182 or25 is rotated by the rotatingmeans182. Thereafter, theoptical head apparatus51 is moved to a position just under an innermost recording track of theoptical disk182 or25 in astep221 because a piece of information is reliably recorded on the innermost recording track, and the diffracted light L5 is converged on the innermost recording track of theoptical disk182 or25 while performing a focus control corresponding to the conventionaloptical disk25 of the thickness T2 in astep222. Thereafter, a tracking control is performed, and a piece of information recorded on the innermost recording track of theoptical disk182 or25 is detected in astep223. Thereafter, it is judged in astep224 whether the intensity of an information signal expressing the information detected is more than a threshold value. That is, the intensity of the information signal more than the threshold value denotes that the diffracted light L5 is converged in focus on theoptical disk182 or25, and the intensity of the information signal not more than the threshold value denotes that the diffracted light L5 is converged in defocus on theoptical disk182 or25.

In cases where the high densityoptical disk182 is set to theoptical disk apparatus181, the intensity of the information signal not more than the threshold value is detected. In this case, the transmitted light L4 is automatically converged on theoptical disk182 while performing a focus control corresponding to the high densityoptical disk182 of the thickness T1 in astep225. Therefore, pieces of information are recorded or reproduced on or from theoptical disk182.

In contrast, in cases where the conventionaloptical head25 is set to theoptical disk apparatus181, the intensity of the information signal more than the threshold value is detected. In this case, the convergence of the diffracted light L5 on the conventionaloptical disk25 is continued while performing the focus control and the tracking control corresponding to the conventionaloptical disk25 in astep226. Therefore, pieces of information are recorded or reproduced on or from the conventionaloptical disk25.

Accordingly, the thickness of the optical disk set in theoptical disk apparatus181 can be judged even though theoptical disk171 or174 is not utilized. Also, pieces of information can be stably recorded or reproduced on or from an optical disk regardless of whether the optical disk is the high densityoptical disk182 or the conventionaloptical disk25.

(Twenty-second Embodiment)

A binary focus microscope having two focal points in which two images formed on different planes are simultaneously observed is described according to a twenty-second embodiment.

FIG. 59 is a constitutional view of a binary focus microscope according to a twenty-second embodiment.

As shown inFIG. 59, a binary focus microscope191 for simultaneously observing a first image of a first sample SP1 put on a first sample plane PL1 and a second image of a second sample SP2 put on a second sample plane PL2, comprises an objective lens192 having a first focal length F1 for refracting a beam of first light L17 diverging from the first image and refracting a beam of second light L18 diverging from the second image, the hologram lens26 (or26M,32,33 or42) for transmitting through a part of the first light L17 without any diffraction and diffracting a part of the second light L18 to pass the second light L18 through the same optical path as the first light L17 passes through, an inner lens193 for converging a beam of superposed light L19 formed of the first and second light L17, L18 at an inner focal point Pf1, an ocular lens194 for simultaneously forming the first and second images by converging the superposed light L19 diverging from the inner focal point Pf1, an inner lens-barrel195 for moving a combination element of the inner lens193 and the hologram lens26 along an optical axis to adjust a distance between the combination element and the ocular lens194, and an outer lens-barrel196 for moving the objective lens192 along the optical axis to set the distance between the first sample plane PL1 and the objective lens192 to the focal length F1 of the objective lens192.

The optical axis passes through the centers of theobjective lens192, thehologram lens26, theinner lens195 and theocular lens194. The position of the first sample plane PL1 differs from that of the second sample plane PL2 along the optical axis.

In the above configuration, the position of theobjective lens192 is adjusted to set the distance between the first sample plane PL1 and theobjective lens192 to the first focal length F1 of theobjective lens192. In this case, the distance between the second sample plane PL2 and theobjective lens192 is set to a second focal length F2. Also, the position of the combination element is adjusted to clearly view the first and second images. That is, a beam of first light L17 diverging from the first image on the first sample plane PL1 is collimated in theobjective lens192, and a part of the first light L17 transmits through thehologram lens26. Also, a beam of second light L18 diverging from the second image on the second sample plane PL2 is refracted in theobjective lens192, and a part of the second light L18 is diffracted in thehologram lens26 to pass the second light L18 through the same optical path as the first light L17 passes through. Therefore, a beam of superposed light L19 is formed of the first and second light L17, L18. Thereafter, the superposed light L19 is converged by theinner lens193 at an inner focal point Pf1 to simultaneously form the first and second images enlarged on an image plane PL3, and the superposed light L19 diverging from the point Pf1 is converged by theocular lens194 to simultaneously form the first and second images moreover enlarged on an operator's eye.

Accordingly, because thehologram lens26,26M,32,33 or42 is utilized to form the superposed light L19, the first image of the first sample SP1 put on the first sample plane PL1 and the second image of the second sample SP2 put on the second sample plane PL2 can be simultaneously observed by simultaneously focusing thebinary focus microscope191 on the first and second samples SP1, SP2.

Also, even though the intensities of the first and second light L17, L18 are reduced when the light L17, L18 pass through thehologram lens26, the intensity of the superposed light L19 is sufficient to observe the first and second images because the intensity of the superposed light L19 is determined by adding the intensities of the first and second light L17, L18.

Also, because thehologram lens26 is blazed as shown inFIG. 6, the generation of unnecessary diffracted light such as minus first-order diffracted light is reduced when the second light L18 is diffracted in thehologram lens26. Therefore, the intensity of the superposed light L19 can be increased, so that first and second images observed can be moreover brightened.

Also, as shown inFIG. 60, in cases where the first sample SP1 is placed at a bottom of afirst sample holder197 and in cases where the second sample SP2 is placed at a bottom of asecond sample holder198 located under thefirst sample holder197, aberration such as a chromatic aberration is generated in the first and second light L17, L18 incident on theobjective lens192 because an optical path length in thefirst sample holder197 for the first light L17 differs from that in the first andsecond sample holders197,198 for the second light L18. However, the aberration can disappear in thehologram lens26 by excessively correcting the chromatic aberration in thehologram lens26 as is described in the first embodiment.

Also, the difference between the first and second focal lengths can be changed by moving the inner and outer lens-barrels195,196 because the distance between thehologram lens26 and theobjective lens192 is changed. Therefore, even though the thickness of thesecond sample holder198 is changed, the first and second images can be reliably observed.

In the twenty-second embodiment, theocular lens194 is utilized in thebinary focus microscope191. However, theocular lens194 is not necessarily required. Also, as shown inFIG. 61, it is applicable that acamera199 such as a charge-coupled device (CCD) camera be placed on the image plane PL3 in place of theocular lens194. In this case, a superposed image formed of the first and second images enlarged can be photographed with theCCD camera199, so that the first and second images can be simultaneously recorded.

(Twenty-third Embodiment)

In cases where a minute circuit is formed on a semiconductor wafer, a photosensitive material is coated on the semiconductor wafer, and the photosensitive material covering the semiconductor wafer is exposed to ultraviolet light through a photomask having a mask pattern in an exposure process. Therefore, the mask pattern of the photomask is transferred to the photosensitive material. In this case, the semiconductor wafer is required to be aligned with the photomask at a high accuracy in an alignment process performed prior to the exposure process. Therefore, a reference image drawn on the semiconductor wafer and the mask pattern drawn on the photomask are simultaneously observed with a conventional microscope having a deep focal depth. However, because the magnification of the conventional microscope having a deep focal depth is low, it is impossible to align the reference image and the mask pattern at an accuracy within 5 μm.

To solve the above drawback in the present invention, an alignment apparatus utilized in the alignment process in which the semiconductor wafer is aligned with the photomask at a high accuracy while simultaneously observing a reference image drawn on the semiconductor wafer and the mask pattern drawn on the photomask is described according to a twenty-third embodiment.

FIG. 62 is a constitutional view of an alignment apparatus according to a twenty-third embodiment.

As shown inFIG. 62, an alignment apparatus201 for aligning a first reference image RF1 of a mask pattern drawn on a bottom surface of a photomask202 with a second reference image RF2 drawn on a bottom surface of a sample such as a semiconductor substrate203, comprises a light source204 for radiating beams of alignment light having a particular wavelength to illuminate the first and second reference images RF1, RF2, the objective lens192 for receiving a beam of first alignment light L20 diverging from the first reference image RF1 illuminated with the alignment light and receiving a beam of second alignment light L21 diverging from the second reference image RF2 illuminated with the alignment light, the hologram lens26 (or26M,32,33 or42) for transmitting through a part of the first alignment light L20 without any diffraction and diffracting a part of the second alignment light L21 to pass the second alignment light L21 through the same optical path as the first alignment light L20 passes through, the inner lens193 for converging a beam of superposed alignment light L22 formed of the first and second alignment light L20, L21 at an inner focal point Pf2, a camera205 such as a charge-coupled device (CCD) camera placed at the inner focal point Pf2 for simultaneously photographing and recording the first and second reference images RF1, RF2, the inner lens-barrel195, the outer lens-barrel196, a moving means206 for moving the photomask202 in a horizontal direction perpendicular to an optical axis to align the first reference image RF1 with the second reference image RF2 along the optical axis, and a control means207 for controlling the movement of the photomask202 according to the first and second reference images RF1, RF2 recorded in the camera205.

The particular wavelength of the alignment light is determined on condition that a transmissivity of thesemiconductor substrate203 for the alignment light is sufficiently high. The optical axis passes through the centers of theobjective lens192, thehologram lens26 and theinner lens193.

Abinary microscope208 is composed of theobjective lens192, the hologram lens26 (or26M,32,33 or42), theinner lens193, the inner lens-barrel195 and the outer lens-barrel196 in the same manner as in the twenty-second embodiment.

In the above configuration, a beam of first alignment light L20 diverging from the first reference image RF1 is collimated in theobjective lens192, and a part of the first alignment light L20 transmits through thehologram lens26. Also, a beam of second alignment light L21 diverging from the second reference image RF2 is refracted in theobjective lens192, and a part of the second alignment light L21 is diffracted by thehologram lens26 to pass the second alignment light L21 through the same optical path as the first alignment light L20 passes through. Therefore, a beam of superposed alignment light L22 is formed of the first and second alignment light L20, L21. Thereafter, the superposed alignment light L22 is converged at an inner focal point Pf2 to simultaneously form the first and second reference images RF1, RF2 enlarged. Thereafter, the first and second reference images RF1, RF2 enlarged are photographed and recorded by thecamera205. Thereafter, a relative position between the first and second reference images RF1, RF2 is examined in the control means207, and thephotomask202 is moved in a horizontal direction by the moving means206 under control of the control means207 to align the first reference image RF1 with the second reference image RF2. Thereafter, as shown inFIG. 64, a photo sensitive material coated on thesemiconductor substrate203 is exposed to exposure light radiated from an exposurelight source211 through a reflectingmirror212 and ashutter213.

Accordingly, because thehologram lens26,26M,32,33 or42 is utilized to form thebinary microscope208 having two focal points, the first reference image RF1 and the second reference image RF2 can be simultaneously observed even though a focal depth of thebinary microscope208 is low. Also, because the focal depth of thebinary microscope208 can be lowered, the magnification of thebinary microscope208 can be heightened. Therefore, the first reference image RF1 of thephotomask202 can be aligned with the second reference image RF2 of thesemiconductor substrate203 at a high accuracy.

(Twenty-fourth Embodiment)

FIG. 65 is a diagonal view of an image reproducing apparatus for which theoptical disk apparatus176 is provided.

As shown inFIG. 65, animage reproducing apparatus221 comprises theoptical disk apparatus176 and a displayingunit222 such as a cathode ray tube or a liquid crystal display for displaying pieces of image information recorded in theoptical disk171,174 or25 of theoptical disk apparatus176 as an image.

In the above configuration, pieces of image information are recorded in the outer region171a of theoptical disk171 or in theoptical disk174 at a high recording density, and the thickness T1 is, for example, 0.6 mm. Also, pieces of image information are recorded in the conventionaloptical disk25 having the thickness T2 at an ordinary recording density. The conventionaloptical disk25 represents a video compact disk having a thickness of 1.2 mm. Because theoptical disk apparatus176 is provided for theimage reproducing apparatus221, regardless of whether pieces of particular image information are recorded in the optical disk171 (or174) or the conventionaloptical disk25, the particular image information are read from theoptical disk171,174 or25, and a particular image can be displayed in the displayingunit222. Therefore, a user is not required to prepare both a conventional image reproducing apparatus for reproducing the image information recorded in the conventionaloptical disk25 and an updated image reproducing apparatus for reproducing the image information recorded in an updated optical disk having a thickness of 0.6 mm.

FIG. 66 is a diagonal view of a voice reproducing apparatus for which theoptical disk apparatus176 is provided.

As shown inFIG. 66, avoice reproducing apparatus231 comprises theoptical disk apparatus176 and avoice reproducing unit232 such as a speaker for reproducing pieces of voice information recorded in theoptical disk171,174 or25 of theoptical disk apparatus176 as voices. Thevoice reproducing apparatus231 additionally comprises aradio233 and acassette tape recorder234.

In the above configuration, pieces of voice information are recorded in the outer region171a of theoptical disk171 or in theoptical disk174 at a high recording density, and the thickness T1 is, for example, 0.6 mm. Also, pieces of voice information are recorded in the conventionaloptical disk25 having the thickness T2 at an ordinary recording density. The conventionaloptical disk25 represents an audio compact disk having a thickness of 1.2 mm. Because theoptical disk apparatus176 is provided for thevoice reproducing apparatus231, regardless of whether pieces of particular voice information are recorded in the optical disk171 (or174) or the conventionaloptical disk25, the particular voice information are read from theoptical disk171,174 or25, and particular voices can be reproduced by thevoice reproducing unit232. Therefore, a user is not required to prepare both a conventional voice reproducing apparatus for reproducing the voice information recorded in the conventionaloptical disk25 and an updated voice reproducing apparatus for reproducing the voice information recorded in an updated optical disk having a thickness of 0.6 mm.

FIG. 67 is a diagonal view of an information processing apparatus for which theoptical disk apparatus176 is provided.

As shown inFIG. 67, aninformation processing apparatus241 comprises theoptical disk apparatus176, an input apparatus242 for inputting pieces of input information, acentral processing unit243 placed on acircuit substrate244 for processing pieces of recorded information recorded in theoptical disk171,174 or25 of theoptical disk apparatus176 or processing the input information to record the input information in theoptical disk171,174, and a displayingunit245 for displaying the recorded information processed in thecentral processing unit243.

In the above configuration, pieces of recorded information are recorded in the outer region171a of theoptical disk171 or in theoptical disk174 at a high recording density, and the thickness T1 is, for example, 0.6 mm. Also, pieces of recorded information are recorded in the conventionaloptical disk25 having the thickness T2 at an ordinary recording density. The conventionaloptical disk25 represents a CD-ROM having a thickness of 1.2 mm. Because theoptical disk apparatus176 is provided for thevoice reproducing apparatus231, regardless of whether pieces of particular recorded information are recorded in the optical disk171 (or174) or the conventionaloptical disk25, the particular recorded information are read from theoptical disk171,174 or25, and a particular image can be displayed in the displayingunit245 according to the recorded information. Also, when a user inputs pieces of input information to the inputting apparatus242, even though any of theoptical disks171,174 and25 is set in theoptical disk apparatus176, the input information are recorded in theoptical disk171,174 or25. Therefore, the user is not required to prepare both a conventional image reproducing apparatus for reproducing the image information recorded in the conventionaloptical disk25 and an updated image reproducing apparatus for reproducing the image information recorded in an updated optical disk having a thickness of 0.6 mm.

In cases where a game program is recorded in theoptical disk171,174 or25, the user can play a game while observing a game image displayed in the displayingunit245 and inputting pieces of operating data to the inputting apparatus242 regardless of whether the game program is recorded in any of theoptical disks171,174 or25.

Also, in cases where theinformation processing apparatus241 is utilized in a home, theinformation processing apparatus241 can be utilized as an information terminal for a domestic use.

Also, as shown inFIG. 68, in cases where akey board245 is utilized as the inputting apparatus242, theinformation processing apparatus241 can be utilized as acomputer system246.

Having illustrated and described the principles of our invention in a preferred embodiment thereof, it should be readily apparent to those skilled in the art that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications coming within the spirit and scope of the accompanying claims.

Claims (36)

1. A compound objective lens, comprising:

lens means, having a first convex surface and a second convex surface opposite to each other, for receiving a beam of incident light of one particular wavelength passing through an optical axis at the first convex surface, refracting the beam of incident light and emitting a beam of refracted light from the second convex surface; and

plural focal point generating means for receiving the beam of incident light not yet refracted by the lens means, generating from the beam of incident light a plurality of beams of divided light including a first beam of divided light and a second beam of divided light, converging the beams of divided light at a plurality of focal points which are placed on the optical axis on a side facing the second convex surface of the lens means on condition that the first beam of divided light transmits through a first substrate and is converged on an information recording plane placed at a first distance T1 from a surface of the first substrate at a diffraction limit and that the second beam of divided light transmits through a second substrate and is converged on an information recording plane placed at a second distance T2 (T1≠T2) from a surface of the second substrate at a diffraction limit.

2. A compound objective lens according toclaim 1 in which the plural focal point generating means is a hologram generating from the incident light as the beams of divided light a plurality of beams of diffracted light having different diffraction orders.

3. A compound objective lens according toclaim 2 in which the first beam of divided light is a beam of transmitted light which agrees with a beam of zero-order diffracted light generated by the hologram, and the second beam of divided light is a beam of first-order diffracted light generated by the hologram.

4. A compound objective lens according toclaim 2 in which

the hologram is a phase modulation relief type of diffraction device,

a grating pattern of the hologram has a step-wise cross section and is formed in a concentric circle shape,

the grating pattern of the hologram is concentrically partitioned into a plurality of blocks,

a phase modulation degree of the incident light passing through the grating pattern of the hologram varies in a step-wise shape of four stairs for each of the blocks, and

a ratio of an etching width of a top stair to a length of the corresponding block and another ratio of an etching width of a bottom stair to the length of the corresponding block are respectively lowered toward an outer direction of the grating pattern of the hologram.

5. A compound objective lens according toclaim 2 in which the hologram is a phase modulation relief type of diffraction device,

a grating pattern of the hologram is formed in a concentric circle shape and is concentrically partitioned into a plurality of blocks,

a phase modulation degree of the incident light passing through an inner portion of the grating pattern of the hologram varies in a step-wise shape of four inside stairs for each of the blocks,

the four inside stairs are composed of a top stair, a second stair, a third stair and a bottom stair in that order,

a ratio of an etching width of the top stair to a length of the corresponding block and another ratio of an etching width of the bottom stair to the length of the corresponding block are respectively lowered toward an outer direction of the grating pattern in the inner portion of the hologram,

another phase modulation degree of the incident light passing through an outer portion of the grating pattern of the hologram varies in a step-wise shape of two outside stairs for each of the blocks, and

a difference in width between the outside stairs is increased toward the outer direction of the grating pattern in the outer portion of the hologram.

6. A compound objective lens according toclaim 2 in which

the hologram is a phase modulation relief type of diffraction device,

a grating pattern of the hologram has a step-wise cross section and is formed in a concentric circle shape,

the grating pattern of the hologram is concentrically partitioned into a plurality of blocks,

a phase modulation degree of the incident light passing through the grating pattern of the hologram varies in a step-wise shape of four stairs for each of the blocks, and

a ratio of an etching width of a top stair to a length of the corresponding block and another ratio of an etching width of a bottom stair to the length of the corresponding block are respectively lowered toward an inner direction of the grating pattern of the hologram.

7. A compound objective lens according toclaim 2 in which

the hologram is a phase modulation relief type of diffraction device,

a grating pattern of the hologram is formed in a concentric circle shape and is concentrically partitioned into a plurality of blocks,

a phase modulation degree of the incident light passing through an outer portion of the grating pattern of the hologram varies in a step-wise shape of four inside stairs for each of the blocks,

the four inside stairs are composed of a top stair, a second stair, a third stair and a bottom stair in that order,

a ratio of an etching width of the top stair to a length of the corresponding block and another ratio of an etching width of the bottom stair to the length of the corresponding block are respectively lowered toward an inner direction of the grating pattern in the outer portion of the hologram,

another phase modulation degree of the incident light passing through an inner portion of the grating pattern of the hologram varies in a step-wise shape of two inside stairs for each of the blocks, and

a difference in width between the inside stairs is increased toward the inner direction of the grating pattern in the inner portion of the hologram.

8. A compound objective lens according toclaim 2 in which the hologram is a phase modulation type of diffraction device and is made of a liquid crystal cell.

9. A compound objective lens according toclaim 2 in which the hologram is a phase modulation type of diffraction device and is placed on a substrate of a birefringence material.

10. A compound objective lens according toclaim 2 in which a positional relationship between the lens means and the hologram is fixed.

11. A compound objective lens according toclaim 10 in which the hologram is formed on a lens surface of the lens means.

12. A compound objective lens according toclaim 11 in which the hologram is placed on a lens surface of the lens means of which a curvature is higher than those of other lens surfaces of the lens means.

13. A compound objective lens according toclaim 11 in which the hologram is placed on a lens surface of the lens means of which a curvature is lower than those of other lens surfaces of the lens means.

14. A compound objective lens according toclaim 1 in which a numerical aperture of the lens means for the incident light converged at one focal point of the focal points differs from that for the incident light converged at another focal point of the focal points.

15. A compound objective lens according toclaim 2 in which a grating pattern is formed in a first portion of a light-passing area of the hologram corresponding to an aperture of the lens means, and any grating pattern is not formed in a second portion of the light-passing area of the hologram.

16. A compound objective lens according toclaim 15 in which a phase of the incident light passing through the second portion of the light-passing area of the hologram almost agrees with an average value of phases of the incident light passing through the first portion of the light-passing area of the hologram.

17. A compound objective lens according toclaim 15 in which the grating pattern is formed in a step-wise shape having a plurality of stairs,

a surface height of the second portion of the light-passing area of the hologram in an optical direction is the same as a height of a stair selected from the stairs except a top stair and a bottom stair.

18. A compound objective lens according toclaim 1 in which a numerical aperture of the lens means is equal to or higher than 0.6.

19. An objective lens operable to lead a light beam to at least two kinds of information media having a substrate of different thicknesses T1 and T2, comprising:

a refractive lens which has a first convex surface, and a diffractive element which has a second convex surface located opposite to the first convex surface of the refractive lens;

wherein the objective lens has a numerical aperture of NA1 corresponding to a substrate thickness T1,

the diffractive element receives an approximately parallel beam not yet refracted by the refractive lens and diffracts the beam to divide the beam into divided beams in order to converge one of the divided beams corresponding to a substrate thickness of T2 with a numerical aperture of NA2(NA1>NA2, T1<T2); and

wherein focal points are placed on an optical axis on a side facing the first convex surface of the refractive lens.

20. An objective lens according toclaim 19, wherein the diffractive element is associated with the first convex surface and the second convex surface.

21. An objective lens according toclaim 19, wherein the diffractive element is an optical relief.

22. An optical head apparatus operable to perform at least one of recording and reproduction of pieces of information on and from an optical disk placed to face the optical head apparatus, the optical disk composing the information media, comprising:

(i)an optical source operable to radiate a light beam; and

(ii)the objective lens according toclaim 19.

23. An optical disk apparatus comprising:

an optical head apparatus according toclaim 22;

a rotating device operable to rotate the at least two kinds of information media; and

a moving device operable to move the optical head apparatus.

24. An objective lens operable to lead a light beam to at least two kinds of information media having a substrate of different thicknesses T1 and T2,

wherein the objective lens has a first region and a second region,

the first region is located at a position farther from an optical axis than a position of the second region;

the second region is provided with a diffractive element;

the second region has a numerical aperture NA2 to produce a focal point through the substrate of thickness T2;

the first and the second regions of the objective lens have a numerical aperture NA1 to produce a focal point through the substrate of thickness T1; and

wherein NA1>NA2 and T1<T2.

25. A focus control method to perform focus control on at least two kinds of information media having a substrate of different thicknesses T1 and T2 using an optical lens according toclaim 24, the focus control method comprising:

a step to detect a signal from the information media;

a step to decide the thickness of the substrate based on the signal; and

a step to perform the focus control corresponding to the decided thickness.

26. An optical disk apparatus to perform focus control on at least two kinds of information media having a substrate of different thicknesses T1 and T2 using an optical lens according toclaim 24, comprising:

an information detector to detect a signal from the information medium; and

a controller to decide the thickness of the substrate based on the signal and to perform the focus control corresponding to the thickness T1 or T2 based on the decided thickness.

27. An objective lens according toclaim 24, wherein the NA1 is larger than0.6.

28. An objective lens according toclaim 24, wherein the T1 is smaller than0.8.

29. An objective lens according toclaim 24, wherein the first region or the second region has at least one region having no optical relief.

30. An optical head apparatus operable to perform at least one of recording and reproduction of pieces of information on and from an optical disk placed to face the optical head apparatus, the optical disk composing the information media, comprising:

(i)an optical source operable to radiate a light beam; and

(ii)the objective lens according toclaim 24.

31. An optical head apparatus according toclaim 30, further comprising an optical detector to receive light from the information media having the substrate of different thicknesses T1 and T2.

32. An optical disk apparatus comprising:

an optical head apparatus according toclaim 31;

a rotating device operable to rotate the at least two kinds of information media; and

a moving device operable to move the optical head apparatus.

33. An optical disk apparatus comprising:

an optical head apparatus according toclaim 30;

a rotating device operable to rotate the at least two kinds of information media; and

a moving device operable to move the optical head apparatus.

34. An image reproducing apparatus comprising:

an optical disk apparatus according toclaim 33; and

a displaying unit.

35. A voice reproducing apparatus comprising:

an optical disk apparatus according toclaim 33; and

a voice reproducing unit.

36. An information processing apparatus comprising:

an optical disk apparatus according toclaim 33;

an input apparatus; and

a central processing unit.

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