- t represents the thickness of the first and second substrates cemented together as measured along the optical axis of the laser light; and
- NA represents the numerical aperture of the incident laser light.

With this method, the refractive indices of the first and second substrates for laser light of wavelengths of 420 nm or shorter are measured, for example, on all candidates for the first and second substrates. Next, the refractive indices of the first and second substrates are compared so that such candidates for them of which the difference ΔN1 in refractive index fulfills the relationship defined by formula (1) above are combined together. The thus combined first and second substrates are bonded together with adhesive or the like. The refractive indices need not be measured on all candidates for the first and second substrates; instead, random inspection may be performed on a lot-by-lot basis. In a case where the refractive indices of the first and second substrates involve only small variations, their measurement may be omitted.

In the above-described method for fabricating a prism, in a case where the prism further comprises a third substrate that is bonded to the second substrate with an optical thin film interposed at the interface in between, the method may further include the step of: bonding together the second and third substrates of which the difference ΔN2 in refractive index at the wavelength of the laser light fulfills the following condition:
ΔN2≦|1/(0.3×10⁴×NA×t)| (2)

With this method, the refractive indices of the second and third substrates for laser light of wavelengths of 420 nm or shorter are measured, for example, on all candidates for the second and third substrates. Next, the refractive indices of the second and third substrates are compared so that such candidates for them of which the difference ΔN2 in refractive index fulfills the relationship defined by formula (2) above are combined together. The thus combined second and third substrates are bonded together with adhesive or the like.

According to another aspect of the present invention, a method for fabricating a prism that comprises a first and a second substrate, both translucent, bonded together with an optical thin film interposed at the interface in between and that is used with the interface inclined relative to the optical axis of incident laser light of a wavelength of 420 nm or shorter, includes the step of: bonding together the first and second substrates of which the difference in refractive index at the wavelength of the laser light is 1/300 or smaller.

With this method, the refractive indices of the first and second substrates for laser light of wavelengths of 420 nm or shorter are measured, for example, on all candidates for the first and second substrates. Next, the refractive indices of the first and second substrates are compared so that such candidates for them of which the difference in refractive index is 1/300 or smaller are combined together. The thus combined first and second substrates are bonded together with adhesive or the like.

In the above-described method for fabricating a prism, in a case where the prism further comprises a third substrate that is bonded to the second substrate with an optical thin film interposed at the interface in between, the method may further include the step of: bonding together the second and third substrates of which the difference in refractive index at the wavelength of the laser light is 1/300or smaller.

With this method, the refractive indices of the second and third substrates for laser light of wavelengths of 420 nm or shorter are measured, for example, on all candidates for the second and third substrates. Next, the refractive indices of the second and third substrates are compared so that such candidates for them of which the difference in refractive index is 1/300 or smaller are combined together. The thus combined second and third substrates are bonded together with adhesive or the like.

According to another aspect of the present invention, a method for fabricating a prism that comprises a first and a second substrate, both translucent, bonded together with an optical thin film interposed at the interface in between and that is used with the interface inclined relative to the optical axis of incident laser light of a wavelength of 420 nm or shorter, includes the step of: bonding together the first and second substrates of which the difference in refractive index at the wavelength of the laser light is 1/1500 or smaller.

With this method, the refractive indices of the first and second substrates for laser light of wavelengths of 420 nm or shorter are measured, for example, on all candidates for the first and second substrates. Next, the refractive indices of the first and second substrates are compared so that such candidates for them of which the difference in refractive index is 1/1500 or smaller are combined together. The thus combined first and second substrates are bonded together with adhesive or the like.

In the above-described method for fabricating a prism, in a case where the prism further comprises a third substrate that is bonded to the second substrate with an optical thin film interposed at the interface in between, the method may further include the step of: bonding together the second and third substrates of which the difference in refractive index at the wavelength of the laser light is 1/1500 or smaller.

With this method, the refractive indices of the second and third substrates for laser light of wavelengths of 420 nm or shorter are measured, for example, on all candidates for the second and third substrates. Next, the refractive indices of the second and third substrates are compared so that such candidates for them of which the difference in refractive index is 1/1500 or smaller are combined together. The thus combined second and third substrates are bonded together with adhesive or the like.

According to another aspect of the present invention, a method for fabricating an optical system comprising a light source that emits laser light of a wavelength of 420 nm or shorter and a prism that comprises a first and a second substrate, both translucent, bonded together with an optical thin film interposed at the interface in between and that is used with the interface inclined relative to the optical axis of incident laser light of a wavelength of 420 nm or shorter includes the step of: bonding together the first and second substrates of which the difference ΔN1 in refractive index at the wavelength of the laser light fulfills the following condition:
ΔN1≦|1/(0.3×10⁴×NA×t)|
where

In the above-described method for fabricating an optical system, in a case where the prism further comprises a third substrate that is bonded to the second substrate with an optical thin film interposed at the interface in between, the method may further comprise the step of: bonding together the second and third substrates of which the difference ΔN2 in refractive index at the wavelength of the laser light fulfills the following condition:
ΔN2≦|1/(0.3×10⁴×NA×t)| (2)

According to another aspect of the present invention, a method for fabricating an optical system comprising a light source that emits laser light of a wavelength of 420 nm or shorter and a prism that comprises a first and a second substrate, both translucent, bonded together with an optical thin film interposed at the interface in between and that is used with the interface inclined relative to the optical axis of incident laser light of a wavelength of 420 nm or shorter includes the step of: bonding together the first and second substrates of which the difference in refractive index at the wavelength of the laser light is 1/300 or smaller.

In the above-described method for fabricating an optical system, in a case where the prism further comprises a third substrate that is bonded to the second substrate with an optical thin film interposed at the interface in between, the method may further include the step of: bonding together the second and third substrates of which the difference in refractive index at the wavelength of the laser light is 1/300 or smaller.

According to another aspect of the present invention, a method for fabricating an optical system comprising a light source that emits laser light of a wavelength of 420 nm or shorter and a prism that comprises a first and a second substrate, both translucent, bonded together with an optical thin film interposed at the interface in between and that is used with the interface inclined relative to the optical axis of incident laser light of a wavelength of 420 nm or shorter includes the step of: bonding together the first and second substrates of which the difference in refractive index at the wavelength of the laser light is 1/1500 or smaller.

In the above-described method for fabricating an optical system, in a case where the prism further comprises a third substrate that is bonded to the second substrate with an optical thin film interposed at the interface in between, the method may further include the step of: bonding together the second and third substrates of which the difference in refractive index at the wavelength of the laser light is 1/1500 or smaller.

In the above-described method for fabricating a prism or optical system, the optical thin film may be one of a polarizing beam splitter film, a beam splitter film, a dichroic film, an anti-reflection film, and a total-reflection film.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of the present invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanying drawings in which:

FIG. 1 is a diagram showing the construction of an optical pickup;

FIG. 2 is a diagram showing a process of fabricating a prism according to the present invention;

FIG. 3 is a diagram showing the characteristics of a prism fabricated by a process of fabricating a prism according to the present invention;

FIG. 4 is a diagram illustrating the conditions by which the substrates are sorted out in a process of fabricating a prism according to the present invention;

FIG. 5 is a diagram showing another prism used in an optical pickup; and

FIG. 6 is a diagram showing still another prism used in an optical pickup.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In this embodiment, an optical pickup constructed as shown inFIG. 1 described earlier is used. The wavelength (first wavelength) of the light emitted from the firstlight source2 is in the 405 nm band, and the wavelength (second wavelength) of the light emitted from the secondlight source3 is in the 650 nm band.

Thus, when the disk D is a next-generation DVD, blu-ray disk, or the like, reading and writing of signals are achieved by the use of the light emitted from the firstlight source2, and, when the disk D is a DVD-ROM, reading and writing of signals are achieved by the use of the light emitted from the secondlight source3. A third light source may additionally be provided to permit writing and reading of signals to and from a CD-ROM or the like.

FIG. 2 is a process diagram showing the process for fabricating theprism8 of theoptical pickup1. As described earlier, theprism8 is composed of

substrates

8a,8b, and8c (inFIG. 2, referred to as substrates A, B, and C, respectively) bonded together. The substrates are each made of a translucent material, for example glass such as SK10 or resin.

The

substrates

8a and8c are formed by similar processes, specifically each in the following manner. First, in the step of polishing the inclined surface, one face of a plate-shaped member which will be used as the inclined surface is polished to a predetermined surface roughness and flatness by lapping or polishing. Next, in the step of cutting, the plate-shaped member is cut with a slicer or the like so as to have a trapezoidal sectional shape as shown in FIG.1. Next, in the step of measuring the refractive index, the

substrate

8a or8c is irradiated with blue light of a wavelength of 405 nm to measure its refractive index.

On the other hand, thesubstrate8b is formed in the following manner. First, in the step of polishing the inclined surface, two opposite faces of a plate-shaped member which will be used as the inclined surfaces are polished to a predetermined surface roughness and flatness. Next, in the step of forming films, aPBS film8d is formed on one of those faces, and aBS film8e is formed on the other. Next, in the step of cutting, the plate-shaped member is cut with a slicer or the like so as to have a parallelogrammatic sectional shape as shown in FIG.1. Next, in the step of measuring the refractive index, thesubstrate8b is irradiated with blue light of a wavelength of 405 nm to measure its refractive index.

After the measurement of the refractive indices of the

substrates

8a,8b, and8c, next, in the step of combining appropriate refractive indices, thesubstrate8b is combined with such candidates for the

substrates

8a and8c of which the difference in refractive index from that of thesubstrate8b is 1/1500 or smaller. Next, in the step of bonding, the

substrates

8a,8b, and8c thus paired together are bonded together at their respective inclined surfaces with ultraviolet-curing resin or the like.

Next, in the step of polishing the external shape, of all the faces of the

substrates

8a,8b, and8c thus bonded together, the two that are parallel to the disk D and the one that faces the light-receivingdevice13 are polished to a predetermined surface roughness and flatness by lapping and polishing. Next, in the step of forming anti-reflection films, anti-reflection films are formed on the two faces that are parallel to the disk D. In this way, theprism8 is fabricated.

FIG. 3 is a diagram showing the results of measurement of the wavefront aberration observed with theprism8 fabricated by the fabrication process described above. Along the vertical axis is taken the wavefront aberration (in mλ rms), and along the horizontal axis is taken the difference in refractive index of the

substrates

8a,8b, and8c. The thickness t (seeFIG. 1) of theprism8 is 2.67 mm. The

substrates

8a,8b, and8c are all made of SK10. The numerical aperture AN of the light incident on theprism8 is 0.15.

In the figure, white circles indicate the wavefront aberration occurring in the light that passes through the

substrates

8a and8b in the direction of the thickness t (seeFIG. 1) by way of thePBS film8d, and black circles indicate the wavefront aberration occurring in the light that passes through the

substrates

8b and8c in the direction of the thickness t by way of theBS film8e. For comparison, line B (a broken line) indicates the approximated values for cases where the differences in refractive index are 1/500 and 1/830. Moreover, curve A indicates the theoretical values of the simulated wavefront aberration.

This figure shows that, by combining together and bonding together

substrates

8a,8b, and8c of which the difference in refractive index is 1/1500 or smaller, it is possible to restrict the wavefront aberration to 22 mλ rms or smaller on a theoretical value basis, and to 25 mλ rms or smaller on an actually measured (approximated) value basis. Thus, it is possible to reduce the number of defective prisms with wavefront aberration larger than 25 mλ rms and thereby improve the yield of theprism8.

Moreover, by limiting the difference in refractive index of the

substrates

8a,8b, and8c to 1/3000 or smaller, it is possible to restrict the wavefront aberration to 12 mλ rms or smaller on a theoretical value basis, and to 15 mλ rms or smaller on an actually measured value basis. Thus, in the step of combining appropriate refractive indices, by combining thesubstrate8b with such candidates for the

substrates

8a and8c as are sorted out as having a difference of 1/3000 or smaller in refractive index, it is possible to further reduce the number of defective prisms with wavefront aberration larger than 25 mλ rms and thereby improve the yields of theprism8 and theoptical pickup1.

Furthermore, by limiting the difference in refractive index of the

substrates

8a,8b, and8c to 1/10000 or smaller, it is possible to restrict the wavefront aberration to 10 mλ rms or smaller both on a theoretical value basis and on an actually measured value basis. Therefore, in the step of combining appropriate refractive indices, it is more preferable to combine thesubstrate8b with such candidates for the

substrates

8a and8c as are sorted out as having a difference of 1/10000 or smaller in refractive index.

In this embodiment, in the step of measuring the refractive indices, the refractive indices of the

substrates

8a,8b, and8c are measured on all candidates for them. However, it is not absolutely necessary to measure the refractive indices of all candidates so long as, in the step of bonding, the

substrates

8a and8b are combined together so as to have a difference of 1/1500 in refractive index and the

substrates

8b and8c are combined together so as to have a difference of 1/1500 in refractive index. For example, in a case where variations in refractive index are small within each individual plate-shaped member, random inspection may be performed instead on a lot-by-lot basis by subjecting, of all the substrates obtained from a single plate-shaped member, only a predetermined number to refractive index measurement. In a case where substrates obtained from a single plate-shaped member within which variations in refractive index are small are combined together, or in a case where variations in refractive index are small in all plate-shaped members, it is possible to omit the measurement of the refractive indices of the substrates.

InFIG. 3 described above, the theoretical values obtained through simulation contain spherical aberration that occurs when rays are incident on theprism8. This makes the theoretical values coincident with the actually measured values. As shown in the figure, the spherical aberration prevents the wavefront aberration from becoming zero even when the difference in refractive index of the

substrates

8a,8b, and8c is made zero.

However, with an optical system free from spherical aberration, it is possible to further reduce the wavefront aberration by reducing the difference in refractive index of the

substrates

8a,8b, and8c.FIG. 4 shows the theoretical values of the wavefront aberration occurring in the light that passes through the

substrates

8a and8b as simulated in an optical system free from spherical aberration. Along the vertical axis is taken the wavefront aberration (in mλ rms), and along the horizontal axis is taking the difference in refractive index of the substrates.

The thickness t of the

substrates

8a and8b along the optical axis of the light incident thereon and the numerical aperture NA of the light incident on the

substrates

8a and8b are t=1 mm and NA=0.1 in the case indicated by line C1, t=1 mm and NA=0.3 in the case indicated by line C2, and t=4 mm and NA=0.1 in the case indicated by line C3. For comparison, the simulated values (A) shown inFIG. 3 described above are shown together.

The wavefront aberration AS (mλ rms) is proportional to the difference ΔN1 in refractive index of the

substrates

8a and8b, the numerical aperture NA of the incident light, and the thickness t (mm) of the

substrates

8a and8b along the optical axis of the incident light, and is thus given by formula (3) below. Accordingly, to restrict the wavefront aberration AS to 25 mλ rms or smaller, the relationship defined by formula (4) below needs to be fulfilled. Thus, in the step of combining appropriate refractive indices (see FIG.2), by combining the

substrates

8a and8b so that they fulfill the relationship defined by formula (4), it is possible to restrict the wavefront aberration AS to 25 mλ rms or smaller. Incidentally, the light incident on theprism8 may have its numerical aperture NA converted by an NA conversion lens or the like.
AS=7.5×10⁴×ΔN1×NA×t) (3)
ΔN1≦|1/(0.3×10⁴×NA×t)| (4)

The difference ΔN2 in refractive index of the

substrates

8b and8c can be defined in a similar manner. Here, the light that passes through the

substrates

8b and8c is perpendicular to the optical axis of the light incident on the prism8 (see FIG.1). The smaller the effective diameter of light passing through a given medium, the smaller the wavefront aberration, and therefore the wavefront aberration in the light passing through the

substrates

8b and8c is smaller than that in the light passing through the

substrates

8a and8b. On the other hand, making the

substrates

8a and8c common components helps reduce the number of fabrication steps.

For these reasons, by defining the difference ΔN2 in refractive index of the

substrates

8b and8c by the use of the thickness t of the

substrates

8b and8c along the optical axis of the light incident thereon as it appears in formula (4), it is possible to define the difference ΔN2 under stricter conditions and to make the

substrates

8a and8c common components. Thus, in the step of combining appropriate refractive indices (see FIG.2), by combining the

substrates

8b and8c so that they fulfill the relationship defined by formula (5), it is possible to restrict the wavefront aberration AS occurring in the light passing through the

substrates

8b and8c to 25 mλ rms or smaller.
ΔN2≦|1/(0.3×10⁴×NA×t)| (5)

In the optical system of a pickup optical system designed for use in the 405 nm band as currently assumed, the numerical aperture NA of the light incident on theprism8 is about 0.1 to 0.4, and the thickness t of theprism8 is about 1.0 to 4.0 mm. The upper limit of wavefront aberration permitted in such an optical system is in the range from 5 to 25 mλ rms, though it depends on the detection system configuration, the targeted system design, and other factors.

Suppose, for example, that theprism8 is fabricated so that the difference ΔN1 in refractive index of the

substrates

8a and8b and the difference ΔN2 in refractive index of the

substrates

8b and8c are both 1/300 or smaller. When thisprism8 is incorporated in an optical system where NA=0.3 and t=1 mm, the maximum value of the produced wavefront aberration is 75 mλ rms on a theoretical value basis. Thus, here, theprism8 cannot be used.

By contrast, when thesame prism8 is incorporated in an optical system where NA=0.1 and t=1 mm, the produced wavefront aberration is 25 mλ rms or smaller on a theoretical value basis. Thus, here, theprism8 can be used. In this way, by limiting the difference ΔN1 in refractive index of the

substrates

8a and8b and the difference ΔN2 in refractive index of the

substrates

8b and8c both to 1/300 or smaller, it is possible to obtain an optical system for a pickup optical system that produces wavefront aberration of 25 mλ rms or smaller. If either of the differences ΔN1 and ΔN2 in refractive index is larger than 1/300, the produced wavefront aberration is larger than 25 mλ rms so long as the numerical aperture NA of the light incident on the prism and the thickness t of the prism are in the ranges currently assumed.

When theprism8 is fabricated so that the difference ΔN1 in refractive index of the

substrates

8a and8b and the difference ΔN2 in refractive index of the

substrates

8b and8c are both 1/400 or smaller, and thisprism8 is incorporated in an optical system where NA=0.1 and t=1 mm, it is possible to reduce the produced wavefront aberration to 20 mλ rms or smaller on a theoretical value basis. In this way, it is possible to obtain an optical system for a pickup optical system that produces wavefront aberration of 20 mλ rms or smaller. Incidentally, in an optical system where NA=0.13 and t=1 mm, or in an optical system where NA=0.1 and t=1.3 mm, it is possible to reduce the produced wavefront aberration to 25 mλ rms or smaller on a theoretical value basis.

substrates

8a and8b and the difference ΔN2 in refractive index of the

substrates

8b and8c are both 1/300 or smaller, and thisprism8 is incorporated in an optical system where NA=0.1 and t=1 mm, it is possible to reduce the produced wavefront aberration to 15 mλ rms or smaller on a theoretical value basis. In this way, it is possible to obtain an optical system for a pickup optical system that produces wavefront aberration of 15 mλ rms or smaller. Incidentally, in an optical system where NA=0.17 and t=1 mm, or in an optical system where NA=0.1 and t=1.7 mm, it is possible to reduce the produced wavefront aberration to 25 mλ rms or smaller on a theoretical value basis.

substrates

8a and8b and the difference ΔN2 in refractive index of the

substrates

8b and8c are both 1/1500 or smaller, and thisprism8 is incorporated in an optical system where NA=0.1 and t=1 mm, it is possible to reduce the produced wavefront aberration to 5 mλ rms or smaller on a theoretical value basis. In this way, it is possible to obtain an optical system for a pickup optical system that produces wavefront aberration of 5 mλ rms or smaller. Incidentally, in an optical system where NA=0.25 and t=2 mm, or in an optical system where NA=0.2 and t=2.5 mm, it is possible to reduce the produced wavefront aberration to 25 mλ rms or smaller on a theoretical value basis.

As described above, the difference in refractive index of the substrates is selected appropriately from among the results of simulation according to the permitted wavefront aberration on the basis of the actually used system configuration and the required accuracy.

Theprism8 may be given any other shape than the one shown in FIG.1. For example, as shown inFIG. 5, it is possible to form it by bonding together only the

substrates

8a and8b, with thesubstrate8c (seeFIG. 1) omitted. Alternatively, as shown inFIG. 6, it is possible to increase the thickness of the

substrates

8a and8b in the direction perpendicular to the disk D so that the light of the first and second wavelengths is emitted in the same direction so as to be received by the light-receivingdevices12 and13 (see FIG.1).

The embodiment described above deals with an optical pickup that employs a prism designed for use in the 405 nm wavelength band. However, it is also possible to achieve the same advantages in an optical system incorporating an optical pickup that employs a prism designed for use with light beam of wavelengths of 420 nm or shorter with variations in the wavelength of the light source taken into consideration.

The embodiment described above deals with aprism8 having optical thin films, namely thePBS film8d and theBS film8e. However, it is also possible to achieve the same advantages with a prism having an optical thin film of any other type. For example, as an optical thin film, it is possible to use an anti-reflection film, total-reflection film, half-mirror film, dichroic film, or the like.

Claims

1. A method for fabricating a prism that comprises a first and a second substrate, both translucent, bonded together with an optical thin film interposed at an interface in between and that is used with the interface inclined relative to an optical axis of incident laser light of a wavelength of 420 nm or shorter, comprising the step of:

bonding together the first and second substrates of which a difference ΔN1 in refractive index at the wavelength of the laser light fulfills the following condition:

ΔN1≦|1/(0.3×10⁴×NA×t)|

where

t represents a thickness of the first and second substrates cemented together as measured along the optical axis of the laser light; and

NA represents a numerical aperture of the incident laser light.

2. A method for fabricating a prism as claimed inclaim 1, the prism further comprising a third substrate that is bonded to the second substrate with an optical thin film interposed at an interface in between, the method further comprising the step of:

bonding together the second and third substrates of which a difference ΔN2 in refractive index at the wavelength of the laser light fulfills the following condition:

ΔN2≦|1/(0.3×10⁴×NA×t)|.

3. A method for fabricating a prism as claimed inclaim 1,

wherein the optical thin film is one of a polarizing beam splitter film, a beam splitter film, a dichroic film, an anti-reflection film, and a total-reflection film.

4. A method for fabricating a prism that comprises a first and a second substrate, both translucent, bonded together with an optical thin film interposed at an interface in between and that is used with the interface inclined relative to an optical axis of incident laser light of a wavelength of 420 nm or shorter, comprising the step of:

bonding together the first and second substrates of which a difference in refractive index at the wavelength of the laser light is 1/300 or smaller.

5. A method for fabricating a prism as claimed inclaim 4, the prism further comprising a third substrate that is bonded to the second substrate with an optical thin film interposed at an interface in between, the method further comprising the step of:

bonding together the second and third substrates of which a difference in refractive index at the wavelength of the laser light is 1/300 or smaller.

6. A method for fabricating a prism as claimed inclaim 4,

7. A method for fabricating a prism that comprises a first and a second substrate, both translucent, bonded together with an optical thin film interposed at an interface in between and that is used with the interface inclined relative to an optical axis of incident laser light of a wavelength of 420 nm or shorter, comprising the step of:

bonding together the first and second substrates of which a difference in refractive index at the wavelength of the laser light is 1/1500 or smaller.

8. A method for fabricating a prism as claimed inclaim 7, the prism further comprising a third substrate that is bonded to the second substrate with an optical thin film interposed at an interface in between, the method further comprising the step of:

bonding together the second and third substrates of which a difference in refractive index at the wavelength of the laser light is 1/1500 or smaller.

9. A method for fabricating a prism as claimed inclaim 7,

10. A method for fabricating an optical system comprising a light source that emits laser light of a wavelength of 420 nm or shorter and a prism that comprises a first and a second substrate, both translucent, bonded together with an optical thin film interposed at an interface in between and that is used with the interface inclined relative to an optical axis of incident laser light of a wavelength of 420 nm or shorter, comprising the step of:

ΔN1≦|1/(0.3×10⁴×NA×t)|

where

NA represents a numerical aperture of the incident laser light.

11. A method for fabricating an optical system as claimed inclaim 10, the prism further comprising a third substrate that is bonded to the second substrate with an optical thin film interposed at an interface in between, the method further comprising the step of:

ΔN2≦|1/(0.3×10⁴×NA×t)|.

12. A method for fabricating an optical system as claimed inclaim 10,

13. A method for fabricating an optical system comprising a light source that emits laser light of a wavelength of 420 nm or shorter and a prism that comprises a first and a second substrate, both translucent, bonded together with an optical thin film interposed at an interface in between and that is used with the interface inclined relative to an optical axis of incident laser light of a wavelength of 420 nm or shorter, comprising the step of:

14. A method for fabricating an optical system as claimed inclaim 13, the prism further comprising a third substrate that is bonded to the second substrate with an optical thin film interposed at an interface in between, the method further comprising the step of:

15. A method for fabricating an optical system as claimed inclaim 13,

16. A method for fabricating an optical system comprising a light source that emits laser light of a wavelength of 420 nm or shorter and a prism that comprises a first and a second substrate, both translucent, bonded together with an optical thin film interposed at an interface in between and that is used with the interface inclined relative to an optical axis of incident laser light of a wavelength of 420 nm or shorter, comprising the step of:

17. A method for fabricating an optical system as claimed inclaim 16, the prism further comprising a third substrate that is bonded to the second substrate with an optical thin film interposed at an interface in between, the method further comprising the step of:

18. A method for fabricating an optical system as claimed inclaim 16,

19. A method for fabricating a prism for use in an optical pickup, the prism comprising a first and a second substrate, both translucent, bonded together with an optical thin film interposed at an interface in between and being used with the interface inclined relative to an optical axis of incident laser light of a wavelength of420 nm or shorter, the method comprising the steps of:

combining together the first and second substrates, which have been sorted according to refractive indices thereof, such that a difference ΔN1 between refractive indices of the first and second substrates fulfills the formula below; and

cementing together the first and second substrates thus combined together,

the formula being

ΔN1≦|1/(0.3×10⁴×NA×t)|

where:

t represents a thickness of the first and second substrates cemented together as measured along the optical axis of the laser light; and

NA represents a numerical aperture of the incident laser light.

20. A method for fabricating a prism as claimed inclaim 19, further comprising the step of sorting the first substrate and the second substrate according to refractive indices thereof.

21. A method for fabricating a prism for use in an optical pickup, the prism comprising a first and a second substrate, both translucent, bonded together with an optical thin film interposed at an interface in between and being used with the interface inclined relative to an optical axis of incident laser light of a wavelength of420 nm or shorter, the method comprising the step of:

combining together the first and second substrates, which have been sorted according to refractive indices thereof, such that a difference between refractive indices of the first and second substrates equals1/3,000 or smaller; and

cementing together the first and second substrates thus combined together.

22. A method for fabricating a prism as claimed inclaim 21, wherein the difference in refractive index between the first and second substrates at the wavelength of the laser light is1/10,000 or smaller.

23. A method for fabricating a prism as claimed inclaim 21, further comprising the step of sorting the first substrate and the second substrate according to refractive indices thereof.

24. A method for fabricating a prism as claimed inclaim 22, further comprising the step of sorting the first substrate and the second substrate according to refractive indices thereof.

25. A method for fabricating a prism as claimed inclaim 19, wherein difference ΔN1 between refractive indices of the first and second substrates at the wavelength of the laser light is1/10,000 or smaller.

26. A method for fabricating a prism for use in an optical pickup, the prism comprising a first and a second substrate, both translucent, bonded together with an optical thin film interposed at an interface in between and being used with the interface inclined relative to an optical axis of incident laser light of a wavelength of420 nm or shorter, the method comprising the step of:

combining together the first and second substrates, which have been sorted according to refractive indices thereof, such that a difference between refractive indices of the first and second substrates is1/3,000 or smaller; and

bonding together the first and second substrates thus combined together.

27. A method for fabricating a prism as claimed inclaim 26, wherein the difference in refractive index between the first and second substrates at the wavelength of the laser light is1/10,000 or smaller.

28. A method for fabricating a prism as claimed inclaim 26, further comprising the step of sorting the first and second substrates according to refractive indices thereof.

29. A method for fabricating a prism as claimed inclaim 27, further comprising the step of sorting the first and second substrates according to refractive indices thereof.

30. A method for fabricating a prism as claimed inclaim 26, the prism further comprising a third substrate bonded to the second substrate with an optical thin film interposed at an interface in between, the method further comprising the step of:

combining together the second and third substrates, which have been sorted according to refractive indices thereof, such that a difference between refractive indices of the second and third substrate is1/3,000 or smaller; and

bonding together the second and third substrates thus combined together.

31. A method for fabricating a prism as claimed inclaim 30, further comprising the step of sorting the first, second and third substrates according to refractive indices thereof.

32. A method for fabricating a prism as claimed inclaim 30, wherein the difference in refractive index between the second and third substrates at the wavelength of the laser light is1/10,000 or smaller.