US20100331829A1 - System and method for multi-beam scanning - Google Patents

Abstract

System and method of photoaltering a material. The system includes a laser source operable to produce a primary pulsed beam, a holographic optical element configured to receive the primary pulsed beam and transmit a plurality of secondary beams, and a scanner operable to direct the secondary beams to the material. The secondary beams are based on the primary pulsed beam. The method includes phase shifting a pulsed laser beam to produce an input beam, holographically altering the input beam to produce a plurality of transmission beams, and scanning a portion of the material with the transmission beams.

Description

CROSS-REFERENCE TO RELATED APPLICATION
• This application is a continuation application of U.S. patent application Ser. No. 12/325,923, filed on Dec. 1, 2008.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• The field of the present invention is generally related to photoaltering materials and more particularly, to systems and methods for scanning pulsed laser beams.
• 2. Background
• Pulsed laser beams include bursts or pulses of light, as implied by name, and have been used for photoalteration of materials, both inorganic and organic alike. Typically, a pulsed laser beam is focused onto a desired area of the material to photoalter the material in this area and, in some instances, the associated peripheral area. Examples of photoalteration of the material include, but are not necessarily limited to, chemical and physical alterations, chemical and physical breakdown, disintegration, ablation, vaporization, or the like.
• One example of photoalteration using pulsed laser beams is the photodisruption (e.g., via laser induced optical breakdown) of a material. Localized photodisruptions can be placed at or below the surface of the material to produce high-precision material processing. For example, a micro-optics scanning system may be used to scan the pulsed laser beams to produce an incision in the material and create a flap therefrom. The term “scan” or “scanning” refers to the movement of the focal point of the pulsed laser beam along a desired path or in a desired pattern. To create a flap of the material, the pulsed laser beam is typically scanned along a region within the material at a pre-determined scan rate and with a pre-determined focal spot size.
• For many applications, minimizing the scan time associated with photoalteration is generally desirable to expedite the overall procedure time. The repetition rate of the laser may be increased to generally decrease the scan time (e.g., for forming an incision in the material). In some laser systems, the repetition rate is typically limited by operating constraints of the amplifier or other components of the system. A simple increase in repetition rate may also affect other characteristics of the pulsed laser beam. For example, the pulse wavelength, the pulse duration, and the pulse intensity are inter-related. More specifically, the pulse intensity is proportional to the pulse energy and inversely proportional to the pulse duration, and the pulse energy is inversely proportional to the pulse wavelength.
• Accordingly, it is desirable to provide a system and method for photoaltering a material that increases the effective repetition rate of the pulsed laser beam. It is also desirable to provide a system and method for photoaltering a material with a pulsed laser beam that improves dissection quality and while reducing scanning speed associated with the photoalteration. Additionally, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.
SUMMARY OF THE INVENTION
• The present invention is directed towards photoaltering a material using a pulsed laser beam. In one embodiment, system is provided for photoaltering a material. The system includes a laser source operable to produce a primary pulsed beam, a holographic optical element configured to receive the primary pulsed beam and transmit a plurality of secondary beams, and a scanner operable to direct the plurality of secondary beams to the material. The plurality of secondary beams is based on the primary pulsed beam.
• In another embodiment, the system includes a laser source operable to produce a primary pulsed beam, a first optical element configured to receive the primary pulsed beam and transmit a first secondary beam, a first polarizing beam splitter configured to reflect a first polarized beam and transmit a second polarized beam, a subsystem configured to produce a first transmission beam and a second transmission beam based on the first and second polarized beams, and a scanner operable to direct the first and second transmission beams to the material. The first secondary beam is based on the primary pulsed beam and phase shifted from the primary pulsed beam by about a first quarter wavelength. The first and second polarized beams are together based on the first secondary beam. The first transmission beam is angularly separated from the second transmission beam.
• In another embodiment, the system includes a laser source operable to produce a primary pulsed beam, an optical element having a reflectivity less than about 100%, a mirror oriented non-parallel to the optical element, and a scanner operable to direct the first and second transmission beams to the material. The optical element is configured to receive the primary pulsed beam, reflect a first transmission beam, and transmit a secondary beam. The first transmission beam and the secondary beam are together based on the primary pulsed beam. The mirror is configured to reflect the secondary beam to the optical element, and the optical element is further configured to transmit a second transmission based on the secondary beam.
• In another embodiment, the system includes a laser source operable to produce a primary pulsed beam, an optical element configured to receive the primary pulsed beam and transmit a phase shifted beam, a first Wollaston prism configured to receive the phase shifted beam and transmit a first secondary beam and a second secondary beam, and a scanner operable to direct a first transmission beam and a second transmission beam to the material. The phase shifted beam is orthogonally polarized with the primary pulsed beam. The first and second secondary beams are together based on the phase shifted beam, and the first secondary beam angularly separated from the second secondary beam. The first and second transmission beams are based on the first and second secondary beams.
• In another embodiment, a method of photoaltering a material is provided. The method includes phase shifting a pulsed laser beam to produce an input beam, holographically altering the input beam to produce a plurality of transmission beams, and scanning a portion of the material with the plurality of transmission beams.
BRIEF DESCRIPTION OF THE DRAWINGS
• In the drawings, wherein like reference numerals refer to similar components:
• FIG. 1 is a block diagram of a laser scanner system in accordance with one embodiment of the present invention;
• FIG. 2 is an elevational view of a holographic optical element in accordance with one embodiment;
• FIG. 3 is a block diagram of an optical subsystem in accordance with one embodiment;
• FIG. 4 is a block diagram of an optical subsystem in accordance with another embodiment;
• FIG. 5 is a block diagram of an optical subsystem in accordance with another embodiment;
• FIG. 6 is a block diagram of an optical subsystem in accordance with one embodiment;
• FIG. 7 is a block diagram of an optical subsystem in accordance with another embodiment;
• FIG. 8 is a top view of raster patterns formed in a material in accordance with one embodiment;
• FIG. 9 is a side view of an optical subsystem in accordance with one embodiment illustrating an “ON” configuration; and
• FIG. 10 is a side view of the optical subsystem shown inFIG. 9 illustrating an “OFF” configuration.
DETAILED DESCRIPTION
• The present invention provides systems and methods for photoaltering a material with multiple beam scanning. Photoalteration of a material may be accomplished using a pulsed laser beam that is directed (e.g., via a scanner) at a desired region of the material. For example, a pulsed laser beam may be controlled to scan the desired region and to create a separation of the material (e.g., which may be used to produce a flap of the material, to separate a portion of the material for transplants, or for a variety of other uses). With the systems and methods of the present invention, multiple output laser beams are optically derived from a single input laser beam, and these output laser beams are simultaneously scanned in the desired region. In one embodiment, the input laser beam is converted to multiple laser beams using a holographic optical element, and the multiple beams are then scanned in the desired region of the material. The term “holographic optical element” (HOE) is defined herein to be an optical element that controls the production of a plurality of beams from a source beam via diffraction. In another embodiment, the multiple beams are produced from the single input laser beam using phase-based optical subsystems.
• Referring to the drawings, asystem10 for photoaltering amaterial12 is shown inFIG. 1. Thesystem10 includes, but is not necessarily limited to, alaser source14 capable of generating apulsed laser beam18, anenergy control module16 for varying the pulse energy of thepulsed laser beam18, anoptical subsystem32 for selectively converting thepulsed laser beam18 to a plurality of beams36 (e.g., multiple pulsed laser beams), ascanner20, acontroller22, and focusingoptics28 that direct one or morefocal points30 of thepulsed laser beam18 or thebeams36 onto the surface of or within the material12 (e.g., sub-surface). The controller22 (e.g., a processor operating suitable control software) communicates with one or more of thescanner20,optical subsystem32, and focusingoptics28 to control the direction of thefocal point30 as it is scanned along the material. In one scanning configuration, theoptical subsystem32 converts thepulsed laser beam18 to the plurality ofbeams36 which is then provided to thescanner20. In another scanning configuration, thepulsed laser beam18 passes through theoptical subsystem32 to thescanner20. Thesystem10 may switch between these scanning configurations (e.g., by manual selection, automatic selection in response to a programmed scanning procedure, any combination of manual and automatic selection, or other selection).
• In one embodiment, theoptical subsystem32 is manually inserted into or removed from the propagation path of thepulsed laser beam18 to thescanner20 to receive thepulsed laser beam18. In another embodiment, theoptical subsystem32 is mechanically guided into the propagation path to capture thepulsed laser beam18. Control signals may be automatically transmitted to the controller22 (e.g., during execution of one or more programmed scanning procedures) or user input provided to thecontroller22, which then actuates a servo component (not shown) coupled to theoptical subsystem32. Various combinations of automated and manual control of theoptical subsystem32 or other mechanisms may also be used to selectively produce thebeams36 from thepulsed laser beam18 via theoptical subsystem32. In operation, thesystem10 may have multiple scanning modes with at least one of the scanning modes engaging theoptical subsystem32 to produce thebeams36 when selected. The selective conversion of thepulsed laser beam18 to thebeams36 using theoptical subsystem32 may thus be implemented in a variety of embodiments.
• To impart at least a portion of the system control, software, firmware, or the like, can be used to command the actions and placement of the scanner via a motion control system, such as a closed-loop proportional integral derivative (PID) control system. In this embodiment, thesystem10 further includes abeam splitter26 and adetector24 coupled to thecontroller22 to provide a feedback control mechanism for thepulsed laser beam18 or thebeams36. Thebeam splitter26 anddetector24 may also be omitted in other embodiments, for example, with different control mechanisms.
• Thecontroller22 includes computer hardware and/or software, often including one or more programmable processor unit running machine readable program instructions or code for implementing some or all of one or more of the methods described herein. The code is often embodied in a tangible media such as a memory (optionally a read only memory, a random access memory, a non-volatile memory, or the like) and/or a recording media (such as a floppy disk, a hard drive, a CD, a DVD, a memory stick, or the like). The code and/or associated data and signals may also be transmitted to or from the processor via a network connection (such as a wireless network, an Ethernet, an internet, an intranet, or the like), and some or all of the code may also be transmitted between components of thesystem10 and within the controller via one or more bus, and appropriate standard or proprietary communications cards, connectors, cables, and the like will often be included in the controller. Thecontroller22 is often configured to perform the calculations and signal transmission steps described herein at least in part by programming the controller with the software code, which may be written as a single program, a series of separate subroutines or related programs, or the like. Thecontroller22 may include standard or proprietary digital and/or analog signal processing hardware, software, and/or firmware, and will typically have sufficient processing power to perform the calculations described herein during treatment of the patient. Thecontroller22 optionally includes a personal computer, a notebook computer, a tablet computer, a proprietary processing unit, or a combination thereof. Standard or proprietary input devices (such as a mouse, keyboard, touchscreen, joystick, etc.) and output devices (such as a printer, speakers, display, etc.) associated with modern computer systems may also be included, and processors having a plurality of processing units (or even separate computers) may be employed in a wide range of centralized or distributed data processing architectures.
• Movement of the focal point(s)30 of thepulsed laser beam18 orbeams36 is accomplished via thescanner20 in response to thecontroller22. In one embodiment, thescanner20 scans thepulsed laser beam18 or thebeams36 to produce an incision in the material. For a pre-determined scan rate, scanning thebeams36 to produce an incision reduces the overall scan time in comparison with scanning the singlepulsed laser beam18 to produce a substantially similar incision. For example, for a pre-determined scan spot distribution (e.g., as a result of scanning thebeam18 in a pre-determined area), a similar scan spot distribution can be more rapidly produced by scanning thebeams36 in the same area. When theoptical subsystem32 outputs thebeams36, thebeams36 are simultaneously scanned to more rapidly produce a similar distribution of scan spots.
• To provide thepulsed laser beam18, a chirped pulse laser amplification system, such as described in U.S. Pat. No. RE37,585, may be used for photoalteration. U.S. Pat. Publication No. 2004/0243111 also describes other methods of photoalteration. Other devices or systems may be used to generate pulsed laser beams. For example, non-ultraviolet (UV), ultrashort pulsed laser technology can produce pulsed laser beams having pulse durations measured in femtoseconds. Some of the non-UV, ultrashort pulsed laser technology may be used in ophthalmic applications. For example, U.S. Pat. No. 5,993,438 discloses a device for performing ophthalmic surgical procedures to effect high-accuracy corrections of optical aberrations. U.S. Pat. No. 5,993,438 discloses an intrastromal photodisruption technique for reshaping the cornea using a non-UV, ultrashort (e.g., femtosecond pulse duration), pulsed laser beam that propagates through corneal tissue and is focused at a point below the surface of the cornea to photodisrupt stromal tissue at the focal point. Thepulsed laser beam18 is preferably linearly polarized, but may be configured in a different polarization state (e.g., circularly polarized).
• Although thesystem10 may be used to photoalter a variety of materials (e.g., organic, inorganic, or a combination thereof), thesystem10 is suitable for ophthalmic applications in one embodiment. In this case, the focusingoptics28 direct thepulsed laser beam18 toward an eye (e.g., onto a cornea) for plasma mediated (e.g., non-UV) photoablation of superficial tissue, or into the stroma for intrastromal photodisruption of tissue. In this embodiment, thesystem10 may also include an applanation lens (not shown) to flatten the cornea prior to scanning thepulsed laser beam18 toward the eye. A curved, or non-planar, lens may be substituted for the applanation lens to contact the cornea in other embodiments.
• Thesystem10 is capable of generating thepulsed laser beam18 with physical characteristics similar to those of the laser beams generated by a laser system disclosed in U.S. Pat. No. 4,764,930, U.S. Pat. No. 5,993,438, or the like. For example, thesystem10 can produce a non-UV, ultrashort pulsed laser beam for use as an incising laser beam. This pulsed laser beam preferably has laser pulses with durations as long as a few nanoseconds or as short as a few femtoseconds. For intrastromal photodisruption of the tissue, thepulsed laser beam18 has a wavelength that permits thepulsed laser beam18 to pass through the cornea without absorption by the corneal tissue. The wavelength of thepulsed laser beam18 is generally in the range of about 3 μm to about 1.9 nm, and preferably between about 400 nm to about 3000 nm. For accomplishing photodisruption of stromal tissues at the focal point, the irradiance of thepulsed laser beam18 is preferably greater than the threshold for optical breakdown of the tissue. Although a non-UV, ultrashort pulsed laser beam is described in this embodiment, thepulsed laser beam18 may have other pulse durations and different wavelengths in other embodiments.
• Scanning is accomplished with thescanner20 via thecontroller22 by selectively moving the focal point(s)30 to produce a structured scan pattern (e.g., a raster pattern, a spiral pattern, or the like) of scan spots. Operating thescanner20 to scan this structured pattern is particularly useful for controlling the spacing between scan spots of the pattern. The step rate at which thefocal point30 is moved is referred to herein as the scan rate. For example, thescanner20 can operate at scan rates between about 10 kHz and about 400 kHz, or at any other desired scan rate. In one embodiment, thescanner20 generally moves the focal point of thepulsed laser beam18 through the desired scan pattern at a substantially constant scan rate while maintaining a substantially constant separation between adjacent focal points. Further details of laser scanners are known in the art, such as described, for example, in U.S. Pat. No. 5,549,632, the entire disclosure of which is incorporated herein by reference.
• In one embodiment, thescanner20 includes, but is not necessarily limited to, a pair of scanning mirrors or other optics (not shown) to angularly deflect and scan one or more input beams (e.g, thepulsed laser beam18 or beams36). For example, scanning mirrors driven by galvanometers may be employed where each of the mirrors scans along different orthogonal axes (e.g., an x-axis and a y-axis). A focusing objective (not shown), having one or more lenses, images the input beam onto a focal plane of thesystem10. Thefocal point30 may thus be scanned in two dimensions (e.g., along the x-axis and the y-axis) within the focal plane of thesystem10. Scanning along the third dimension, i.e., moving the focal plane along an optical axis (e.g., a z-axis), may be achieved by moving the focusing objective, or one or more lenses within the focusing objective, along the optical axis.
• For ophthalmic applications (e.g., preparing a cornea for flap separation, corneal transplant, or the like), an area (e.g., substantially circular, oval, or other shape) may be scanned with a scan pattern based on the movement of the scanning mirrors. As thefocal point30 is scanned along a corneal bed, thepulsed laser beam18 orbeams36 photoalter the stromal tissue. Using structured patterns, the distribution of scan spots is generally determined by the pulse frequency, the scan rate, the amount of scan line separation, and the number ofbeams18,36 selected for scanning. Generally, higher scan rates, enable shorter procedure times by increasing the rate at which corneal tissue can be photoaltered. For example, the scan rates may be selected from a range between about 30 MHz and about 1 GHz with a pulse width in a range between about 300 picoseconds and about 10 femtoseconds, although other scan rates and pulse widths may be used.
• Thesystem10 may additionally acquire detailed information about optical aberrations to be corrected, at least in part, using thesystem10. Examples of such detailed information include, but are not necessarily limited to, the extent of the desired correction, and the location in the cornea of the eye associated with the correction (e.g., where the correction can be made most effectively). The refractive power of the cornea may be used to indicate corrections. Wavefront analysis techniques, made possible by devices such as a Hartmann-Shack type sensor (not shown), can be used to generate maps of corneal refractive power. Other wavefront analysis techniques and sensors may also be used. The maps of corneal refractive power, or similar refractive power information provided by other means, such as corneal topographs or the like, can then be used to identify and locate the optical aberrations of the cornea that require correction.
• In general, when thelaser source14 is activated, thefocal spot30 is selectively directed (e.g., via the scanner20) along a beam path to photoalter stromal tissue. For example, thefocal spot30 is moved along a predetermined length of the beam path in one reference area. Thepulsed laser beam18 orbeams36 are then redirected through another reference area, and the process of photoalteration is repeated. The sequence for directing thepulsed laser beam18 orbeams36 through individually selected reference areas can be varied, and the extent of stromal tissue photoalteration while the incising laser beam is so directed, can be varied. Specifically, as indicated above, the amount of photoalteration can be based on the refractive power map. On the other hand, the sequence of reference areas that is followed during a customized procedure will depend on the particular objectives of the procedure.
• Thescanner20 may also scan a predetermined pattern using one or more scan patterns to one or more combinations of these reference areas or scan a single line (e.g., to produce a sidecut). One example of an ophthalmic scanning application is a laser assisted in-situ keratomilieusis (LASIK) type procedure where a flap is cut from the cornea to establish extracorporeal access to the tissue that is to be photoaltered. The flap may be created using random scanning or one or more scan patterns of pulsed laser beams. To create the corneal flap, a sidecut is created around a desired perimeter of the flap such that the ends of the sidecut terminate, without intersection, to leave an uncut segment. This uncut segment serves as a hinge for the flap. The flap is separated from the underlying stromal tissue by scanning the laser focal point across a resection bed, the perimeter of which is approximately defined by and slightly greater than the sidecut. Once this access has been achieved, photoalteration is completed, and the residual fragments of the photoaltered tissue are removed from the cornea. In another embodiment, the sidecut may be created completely around a desired perimeter (e.g., with the ends terminating with one another) to separate a portion of corneal tissue (e.g., for corneal transplant or the like) from the cornea. In another embodiment, intrastromal tissue may be photoaltered by thesystem10 so as to create an isolated lenticle of intrastromal tissue. The lenticle of tissue can then be removed from the cornea.
• FIG. 2 is an elevational view of a holographicoptical element42 in accordance with one embodiment. The holographicoptical element42 may be used as theoptical subsystem36, shown inFIG. 1. Afirst transmission beam44 and asecond transmission beam46 are produced from an incident beam (i.e., the pulsed laser beam18) on the holographicoptical element42. In this embodiment, the phase shift between grating lines of the holographicoptical element42 is about one-hundred and eighty degrees (180°) and results in secondary order diffracted beams from the incident beam while suppressing the zero order. The secondary order diffracted beams (e.g., the −1 and +1 order) have about eighty percent (80%) of the energy of the incident beam. Multi-level holographic optical elements can be produced which have lower losses (e.g., approaching about ninety-seven percent (97%) efficiency). The holographicoptical element42 may be constructed to vary the angle of departure (i.e., from the angle of the incident beam) of the transmission beams44,46 as well as the number of output beams.
• For example, each of the transmission beams44,46 may have a departure angle of about 0.25 rad from the incidence angle. In this example, the intensity (I₁) associated with thefirst transmission beam44 is substantially similar to the intensity (I₂) associated with the second transmission beam46 (e.g., an average deviation from intensity symmetry of about 5%), and the average intensity associated with the transmission beams44,46 is greater than about 80% (e.g., (I₁+I₂)/I₀) where I₀is the intensity associated with the pulsed laser beam18). Total losses of the output beams44,46 in comparison with thepulsed laser beam18 are less than about 200%. Thus, multiple beams are provided by the holographicoptical element42 from thepulsed laser beam18, which may then be scanned to form an incision.
• FIG. 3 is a block diagram of anoptical subsystem50 in accordance with one embodiment. In this embodiment, theoptical subsystem50 includes, but is not necessarily limited to, a first quarter-wave plate52, a firstpolarizing beam splitter54, and an optics arrangement configured to produce afirst transmission beam67 and asecond transmission beam72 based on a beam incident on the quarter-wave plate52, such as thepulsed laser beam18. Thus, theoptical subsystem50 provides two beams (e.g., the transmission beams67 and72) from a single beam (e.g., the pulsed laser beam18), which may then be scanned, such as using thescanner20 shown inFIG. 1.
• A wave plate or retarder is an optical device that alters the polarization state of an input light wave traveling therethrough. In one embodiment, the wave plate operates by shifting the phase of the input light wave between two perpendicular polarization components. One example of a wave plate is a birefringent crystal having a a pre-determined thickness. This crystal is typically cut such that the extraordinary axis (i.e., polarized parallel to the axis of anisotropy) is parallel to the surfaces of the wave plate. When the extraordinary index is less than the ordinary (i.e., polarized perpendicularly to the axis of anisotropy) index, such as in calcite, the extraordinary axis is referred to as the “first axis,” and the ordinary axis is referred to as the “slow axis.” In general, light polarized along the first axis propagates faster than light polarized along the slow axis. Depending on the thickness of the crystal, input light with polarization components along both axes emerge from the crystal in a different polarization state.
• The wave plate is characterized by the amount of relative phase imparted on the two polarization components, which is related to the birefringence and the thickness of the crystal. For example, a quarter-wave plate creates a quarter wavelength phase shift and can change linearly polarized light to circular and vice versa (e.g., by adjusting the plane of the incident light to about a 45° angle with the fast axis). The resulting light has equal amplitude ordinary and extraordinary waves. A half-wave plate retards one polarization component by a half wavelength, or 180°, and thus rotates the polarization direction of linear polarized light.
• In operation, the quarter-wave plate52 converts thepulsed laser beam18 from linear polarization to circular polarization, or more particularly, a circularly polarized pulsed laser beam. Thepolarizing beam splitter54 receives the circularly polarized light and reflects light having one polarization (e.g., a first polarized beam64) while transmitting light having a different polarization (e.g., a second polarized beam68) than the reflected light. The first polarized beam64 (e.g., linearly polarized along the plane ofFIG. 3) is reflected to a first optics group of the optics assembly, and the second polarized beam68 (e.g., linearly polarized in/out of the plane ofFIG. 3) is transmitted to a second optics group of the optics assembly.
• The first optics group includes a second quarter-wave plate56 and afirst mirror reflector58. The firstpolarized beam64 is incident on the quarter-wave plate56, which converts the firstpolarized beam64 to a circularly polarizedbeam66, and themirror reflector58 reflects the circularly polarizedbeam66 back to the quarter-wave plate56, which converts the circularly polarizedbeam66 to thefirst transmission beam67. Thefirst transmission beam67 is linearly polarized orthogonal to the first polarized beam64 (e.g., linearly polarized in/out of the plane ofFIG. 3) and is transmitted through thepolarizing beam splitter54 to thescanner20.
• The second optics group includes a third quarter-wave plate60 and asecond mirror reflector62 that is tiltable or may be pivoted. The secondpolarized beam68 is incident on the quarter-wave plate60, which converts the secondpolarized beam68 to a circularly polarizedbeam70, and themirror reflector62 reflects the circularly polarizedbeam70 back to the quarter-wave plate60, which converts the circularly polarizedbeam70 to thesecond transmission beam72. Thesecond transmission beam72 is linearly polarized orthogonal to the second polarized beam68 (e.g., linearly polarized along the plane ofFIG. 3) and is reflected by thepolarizing beam splitter54 to thescanner20. Thefirst transmission beam67 is angularly separated, and preferably divergent, from thesecond transmission beam72 by the degree of tilt or pivot of thesecond mirror reflector62, thereby controlling the separation between the focal points of the transmission beams67,72.
• FIG. 4 is a block diagram of anoptical subsystem80 in accordance with another embodiment. In this embodiment, theoptical subsystem80 includes, but is not necessarily limited to, a firstpolarizing beam splitter84, afirst mirror reflector90 that is tiltable or may be pivoted, asecond mirror reflector94, and a secondpolarizing beam splitter92. In one embodiment, abeam82 incident on thepolarizing beam splitter84, such as thepulsed laser beam18, may have a polarization of about 45°±θ, where θ is selected for intensity ratio control. Thepolarizing beam splitters84 and94 are configured to reflect light having one polarization while transmitting light having a different polarization than the reflected light. For example, thepolarizing beam splitter84 transmits a first polarized beam86 (e.g., linearly polarized in/out of the plane ofFIG. 4) to themirror reflector90 and reflects a second polarized beam88 (e.g., linearly polarized along the plane ofFIG. 4) to themirror reflector92. Both of thepolarized beams86 and88 are based on theincident beam82. Themirror reflector90 reflects the firstpolarized beam86 to thepolarizing beam splitter94, and themirror reflector92 reflects the secondpolarized beam88 to thepolarizing beam splitter94. Thepolarizing beam splitter94 transmits the firstpolarized beam86 and reflects the secondpolarized beam88. Thus, theoptical subsystem80 provides two beams (e.g., thepolarized beams86 and88) from a single beam (e.g., the pulsed laser beam18), which may be provided to thescanner20 shown inFIG. 1.
• The firstpolarized beam86 is angularly separated, and preferably divergent, from the secondpolarized beam88 by the degree of tilt or pivot of themirror reflector90, thereby controlling the separation between the focal points of thepolarized beams86,88. Although this embodiment of theoptical subsystem80 generates two focal points (or two beams from a single beam), a different numbers of focal points (or multiple beams from a single beam) can be generated in other embodiments. For example, a tandem embodiment based on the optical subsystem80 (e.g., series or parallel combinations of two of the optical subsystem80) can be used to generate 4 focal points.
• FIG. 5 is a block diagram of anoptical subsystem100 in accordance with another embodiment. Theoptical subsystem100 is based on a Fizeau wedge to produce two transmission beams from a single incident beam. A Fizeau wedge is generally formed by two optical elements at an angle to one another. Typically, an incident beam impinges on the front surface of the Fizeau wedge at an angle with the normal to the front surface. As this beam passes into the wedge, a portion of the incident beam is refracted as a second beam through the thickness of the wedge. The other portion of the incident beam not entering the wedge is reflected from the front surface. The second beam impinges on the internal side of the rear surface of the wedge and is reflected off the internal side of the rear surface towards the internal side of the front surface. Part of the reflected second beam exits the front surface of the wedge as a third beam. The other part of the reflected second beam is reflected back off the internal side of the front surface to the rear surface of the wedge as a fourth beam.
• In this embodiment, theoptical subsystem100 includes, but is not necessarily limited to, anoptical element102 and amirror reflector108 oriented at a predetermined angle with respect to theoptical element102. This angle may be adjusted depending on a desired focal point separation between at least two transmission beams. Theoptical element102 has a predetermined reflectivity (R) that allows a majority of the incident light to pass through theoptical element102. In one embodiment, the reflectivity is about 0.382 [R=(3/2²)/2=0.382?], although theoptical element102 may be configured with other values of reflectivity.
• Based on this reflectivity, theoptical element102 reflects a portion of a beam incident on the optical element102 (e.g., the pulsed laser beam18) and transmits another portion of thebeam18. For example, theoptical element102 reflects afirst beam106 while reflecting asecond beam104, where both the first andsecond beams106 and104 are based on thebeam18. Themirror reflector108 reflects thesecond beam104 back to theoptical element102, and theoptical element102 transmits a portion of thesecond beam104 while reflecting another portion of thesecond beam104 based on the reflectivity, R. For example, theoptical element102 transmits athird beam109 while reflecting afourth beam110, where both of thebeams109 and110 are based on thesecond beam104. This reflection bymirror reflector108 and partial transmission and reflection byoptical element102 can continue indefinitely. For example, themirror reflector108 reflects thefourth beam110 back to theoptical element102. Based on the reflectivity, R, and thefourth beam110, theoptical element102 transmits afifth beam112 while reflecting asixth beam114. Themirror reflector108 reflects thesixth beam114 back to theoptical element102. Based on the reflectivity, R, and thesixth beam114, theoptical element102 transmits aseventh beam116 while reflecting aneighth beam118. Themirror reflector108 reflects theeighth beam118 back to theoptical element102. Based on the reflectivity, R, and theeighth beam118, theoptical element102 transmits aninth beam120 while reflecting atenth beam122.
• In this embodiment, the first andthird beams106 and109 have substantially similar intensities, and the spacing between the focal points of thebeams106 and109 may be controlled by the angle between theoptical element102 and themirror reflector108. The beams other than the first andthird beams106 and109 (e.g., thebeams112,116,120) produced by theoptical subsystem100 have intensities (e.g., per pulse) that are substantially diminished from the intensities associated with each of the first andthird beams106 and109. For example, when the first andthird beams106 and109 each have an intensity of about 1 μJ, the intensity of thefifth beam112 may be represented by about 0.38 μJ, and the intensity of the seventh beam may be represented by about 0.147 μJ. The total percentage energy loss associated with thebeams112,116,120 can be represented by (0.145+0.056+0.021+ . . . )*100=23.6%. In a non-overlapping scanning procedure (e.g., non-overlapping focal points of adjacent scan spots), the beams other than the first andthird beams106 and109 (e.g., thebeams112,116,120) are preferably not used for scanning. Thus,multiple beams106,109 are produced from the singlepulsed laser beam18 and can be directed to thescanner20, shown inFIG. 1.
• FIG. 6 is a block diagram of anoptical subsystem130 in accordance with one embodiment. In this embodiment, theoptical subsystem130 includes, but is not necessarily limited to, a half-wave plate132 and aWollaston prism136 configured to produce afirst transmission beam140 and asecond transmission beam142 based on a beam incident on the half-wave plate132, such as thepulsed laser beam18. Thus, theoptical subsystem130 provides two beams (e.g., the transmission beams140 and142) from a single beam (e.g., the pulsed laser beam18), which may then be scanned, such as using thescanner20 shown inFIG. 1.
• In general, a Wollaston prism manipulates polarized light by separating randomly polarized or unpolarized light into two orthogonal, linearly polarized outgoing beams. In one configuration, the Wollaston prism includes two calcite prisms that are coupled together to form two right trianglular prisms with orthogonal optical axes. Outgoing light beams diverge from the Wollaston prism resulting in the two differently polarized beams, with the angle of divergence determined by the prisms' wedge angle and the wavelength of the light. For example, light striking the surface of incidence at right angles is refracted in a first prism (e.g., the first of the two coupled prisms encountered along the propagation direction) into an ordinary beam (O) and an extraordinary beam (A). However, these two beams continue to propagate in the same direction. With the optical axis of the second prism being perpendicular to the optical axis of the first prism, the ordinary beam (O) becomes an extraordinary beam at the boundary surface (e.g., between the two prisms). The opposite applies to the original extraordinary beam (A), which becomes an ordinary beam.
• In this embodiment, the half-wave plate132 changes the polarization direction of light incident on the half-wave plate132 (e.g., the pulsed laser beam18). By retarding one polarization component of thepulsed laser beam18 by a half wavelength, the linear polarized light is rotated to produce afirst beam134. At aboundary interface138 of theWollaston prism136, the first transmission beam140 (e.g., linearly polarized along the plane ofFIG. 6) and the second transmission beam142 (e.g., linearly polarized in/out of the plane ofFIG. 6) are produced from thefirst beam134 and angularly separated by φ. In one embodiment, theWollaston prism136 is constructed to produce an angle of separation φ=6, although the angle of separation may vary in other embodiments. Each of the transmission beams140 and142 has an intensity associated therewith, I_iand I₂, respectively, and the intensity ratio of I₁/I₂can be continuously adjusted by the half-wave plate132 (e.g., to provide a substantially unity ratio).
• FIG. 7 is a block diagram of anoptical subsystem150 in accordance with another embodiment. Theoptical subsystem150 includestandem Wollaston prisms152 and154 to provide four beams (e.g., afirst transmission beam160, asecond transmission beam162, athird transmission beam164, and a fourth transmission beam166) from a single beam (e.g., the pulsed laser beam18), which may then be scanned, such as using thescanner20 shown inFIG. 1. In this embodiment, afirst Wollaston prism152 has orthogonal optical axes, and asecond Wollaston prism154 has optical axes that are orthogonal to one another and together rotated about 45° from the optical axes of thefirst Wollaston prism152. In operation, thefirst Wollaston prism152 receives thepulsed laser beam18 and produces a firstintermediate beam154 having a first polarization and a secondintermediate beam156 having a second polarization orthogonal to the first polarization. Thesecond Wollaston prism158 produces the first andsecond transmission beam160 and162, respectively, from the firstintermediate beam154 and the third andfourth transmission beam164 and166, respectively, from the secondintermediate beam156. The first and second transmission beams160 and162 have orthogonal polarizations, and the third and forth transmission beams164 and166 have orthogonal polarizations.
• FIG. 8 is a top view ofraster scan patterns170,180 formed on a material (e.g., glass) in accordance with one embodiment. Referring toFIGS. 1 and 8, thepulsed laser beam18 may be directed (e.g., without producing multiple beams by the optical subsystem32) to thescanner20 to produce theraster scan pattern170. Theraster scan pattern170 hasmultiple raster lines172,174,176, each corresponding to a different scan line, and eachraster line172,174,176 is formed from multiple spots. Theraster scan pattern170 was formed in glass with a 60 kHz laser and has a spot separation of about 7 μm (i.e., between adjacent spots along a raster line) and a scan line separation of about 14 μm (i.e., between adjacent raster lines).
• When theoptical subsystem32 produces themultiple beams36 from thepulsed laser beam18, thebeams36 are directed to thescanner20 to produce the raster scan pattern180. The raster scan pattern180 has multiple pairs of raster lines, such as a first pair ofraster lines182 and183, a second pair ofraster lines184 and185, and a third pair ofraster lines186 and187, and eachraster line132,183,184,185,186, and187 is formed from multiple spots. Each of the pairs ofraster lines182 and183,184 and185,186 and187 corresponds to a different scan line. For example, thescanner20 scans along a first scan line (S₁) to produce the first pair ofraster lines182 and183, along a second scan line (S₂) to produce the second pair ofraster lines184 and185, and along a third scan line (S₃) to produce the third pair ofraster lines186 and187. The raster scan pattern180 was also formed in glass with the 60 kHz laser and has a spot separation of about 7 μm (i.e., between adjacent spots along a raster line) and a scan line separation of about 14 μm (i.e., between the adjacent scan lines). Using theoptical subsystem32 to produce themultiple beams36 effectively doubles the scan spots and/or the raster lines while scanning at the same rate in comparison to scanning with thepulsed laser beam18 to produce theraster scan pattern170. Thus, for a pre-determined scan rate, the scan spots within a desired scan region can be increased without incurring additional scan time using theoptical subsystem32. Additionally, for a pre-determined scan rate and scan spot distribution, the total scan time can be decreased using theoptical subsystem32.
• Theoptical subsystem32 may also be configured to dynamically modify thepulsed laser beam18 to produce the multiple beams36.FIG. 9 is a side view of anoptical subsystem190 in accordance with one embodiment illustrating an “ON” configuration.FIG. 10 is a side view of theoptical subsystem190 shown inFIG. 9 illustrating an “OFF” configuration. Referring toFIGS. 1,9, and10, theoptical subsystem190 may be implemented with the system10 (e.g., substituted for the optical subsystem32) and includes a first phase grating191 substantially adjacent to a second phase grating192. Each of thephase gratings191,192 hasgrooves193 and194, respectively, with a periodicity based on the wavelength of thepulsed laser beam18. Thegrooves193 of the first phase grating191 face thegrooves194 of the second phase grating192. In the ON configuration shown inFIG. 9, thegrooves193 are aligned with thegrooves194 to phase shift thepulsed laser beam18 and thereby produce multiple beams, such as thebeams44 and46 shown inFIG. 2. In the OFF configuration, thegrooves193 are staggered with respect to thegrooves194 to shift light associated with each of thegrooves193,194 by about a half period and thereby prevent the production of multiple beams. Thus, thephase gratings191 and192 may be positioned with respect to one another to produce multiple beams from the pulsed laser beam18 (e.g., using the ON configuration) or to pass the pulsed laser beam18 (e.g., using the OFF configuration). Thephase gratings191 and192 may also be positioned between the ON and OFF configurations to create a compound phase pattern.
• Thus, systems and methods of photoaltering a material with multiple pulsed laser beams originating from a single input pulsed laser beam are disclosed. The systems and methods are suited to remove material, photoalter corneal tissue, micromachine materials, surface profile various biological tissues, or the like. Examples of some refractive eye surgery applications for thesystem10 include, but are not necessarily limited to, photorefractive keratectomy (PRK), LASIK, laser assisted sub-epithelium keratomileusis (LASEK), or the like. Theoptical subsystem32 is simple to implement and integrate within thesystem10 and improves the effective repetition rate of the pulsed laser beam by at least a factor of 2, 3, or greater. When theoptical subsystem32 is used in thesystem10, at least one of the following advantages may result: the overall procedure time associated with scanning is decreased, the energy associated with each scan spot is decreased, the relative smoothness of the scan bed is improved, and the scanning speed of the galvo is reduced.
• While embodiments of this invention have been shown and described, it will be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the following claims.

Claims (27)

1. A system for photoaltering a material, the system comprising:

a laser source operable to produce a primary pulsed beam;

a holographic optical element configured to receive the primary pulsed beam and transmit a plurality of secondary beams, the plurality of secondary beams based on the primary pulsed beam; and

a scanner operable to direct the plurality of secondary beams to the material.

2. The system ofclaim 1, wherein the primary pulsed beam has a first intensity, wherein the holographic optical element is further configured to transmit a first secondary beam and a second secondary beam, the first secondary beam having a second intensity, the second secondary beam having a third intensity, and wherein a sum of the second and third intensities is greater than about 80% of the first intensity.

3. The system ofclaim 1, wherein the primary pulsed beam has a first intensity, wherein the holographic optical element is further configured to transmit a first secondary beam and a second secondary beam, the first secondary beam having a second intensity, the second secondary beam having a third intensity, and wherein a difference between the second intensity and the third intensity is less than about 5% of an average of the second and third intensities.

4. The system ofclaim 1, wherein the holographic optical element comprises:

a first phase grating having a first surface and a second surface, the second surface of the first phase grating opposing the first surface of the first phase grating, the first surface of the first phase grating configured to receive the primary pulsed beam; and

a second phase grating having a first surface and a second surface, the second surface of the second phase grating opposing the first surface of the second phase grating, the first surface of the second phase grating adjacent to the second surface of the first phase grating, the second surface of the second phase grating configured to:

transmit about 100% of the plurality of secondary beams when the first phase grating and the second phase grating are in a first alignment;

transmit about 0% of the plurality of secondary beams when the first phase grating and the second phase grating are in a second alignment; and

transmit a variable portion of each of the plurality of secondary beams when the first phase grating and the second phase grating are between the first alignment and the second alignment.

5. The system ofclaim 1, wherein the holographic optical element is further configured to vary a spatial separation between each of the plurality of secondary beams.

6. The system ofclaim 5, wherein the holographic optical element has a transmission plane and an axis within the transmission plane, and wherein the holographic optical element is configured to pivot the transmission plane about the axis.

7. The system ofclaim 1, wherein the system has an optical axis extending between the holographic optical element and the material, and wherein the holographic optical element is further configured to rotate about the optical axis.

8. A system for photoaltering a material, the system comprising:

a laser source operable to produce a primary pulsed beam;

a first optical element configured to receive the primary pulsed beam and transmit a first secondary beam, the first secondary beam based on the primary pulsed beam and phase shifted from the primary pulsed beam by about a first quarter wavelength;

a first polarizing beam splitter configured to reflect a first polarized beam and transmit a second polarized beam, the first and second polarized beams together based on the first secondary beam;

a subsystem configured to produce a first transmission beam and a second transmission beam based on the first and second polarized beams, the first transmission beam angularly separated from the second transmission beam; and

a scanner operable to direct the first and second transmission beams to the material.

9. The system ofclaim 8, wherein the subsystem comprises:

a second optical element configured to receive the first polarized beam and transmit a second secondary beam, the second secondary beam based on the first polarized beam and phase shifted from the first polarized beam by about a second quarter wavelength;

a first mirror configured to reflect the second secondary beam to the second wave plate, the second wave plate further configured to transmit the first transmission beam;

a third optical element configured to receive the second polarized beam and transmit a third secondary beam, the third secondary beam based on the second polarized beam and phase shifted from the second polarized beam by about a third quarter wavelength; and

a second mirror configured to reflect the third secondary beam to the third wave plate, the third wave plate further configured to transmit the second transmission beam.

10. The system ofclaim 9, wherein each of the first, second, and third optical elements comprises a quarter wave plate.

11. The system ofclaim 9, wherein the second mirror has an optical plane and is further configured to pivot the optical plane to angularly separate the first transmission beam from the second transmission beam.

12. The system ofclaim 9, wherein the first polarizing beam splitter is further configured to transmit the first transmission beam from the second wave plate and reflect the second transmission beam from the third wave plate.

13. The system ofclaim 8, wherein the subsystem comprises:

a first mirror configured to reflect the first polarized beam;

a second mirror configured to reflect the second polarized beam; and

a second polarizing beam splitter configured to:

receive the first polarized beam from the first mirror and the second polarized beam from the second mirror;

transmit the first polarized beam to the scanner, the first polarized beam being the first transmission beam; and

reflect the second polarized beam to the scanner, the second polarized beam being the second transmission beam.

14. The system ofclaim 13, wherein the first mirror has an optical plane and is further configured to pivot the optical plane to angularly separate the first transmission beam from the second transmission beam.

15. A system for photoaltering a material, the system comprising:

a laser source operable to produce a primary pulsed beam;

an optical element having a reflectivity less than about 100%, the optical element configured to:

receive the primary pulsed beam;

reflect a first transmission beam; and

transmit a secondary beam, the first transmission beam and the secondary beam together based on the primary pulsed beam;

a mirror oriented non-parallel to the optical element, the mirror configured to reflect the secondary beam to the optical element, the optical element further configured to transmit a second transmission based on the secondary beam; and

a scanner operable to direct the first and second transmission beams to the material.

16. The system ofclaim 15, wherein the reflectivity, R, of the optical element is about 0.382.

17. The system ofclaim 15, wherein the pulsed laser beam has a pulse width of between about 300 picoseconds and about 10 femtoseconds.

18. The system ofclaim 15, wherein the pulsed laser beam has a wavelength between about 400 nm to about 3000 nm.

19. The system ofclaim 15, wherein the pulsed laser beam has a pulse energy less than or equal to about 800 nanojoules/pulse.

20. The system ofclaim 15, wherein the pulsed laser beam has a pulse frequency selected from a range of about 30 MHz to about 1 GHz.

21. The system ofclaim 15, wherein the pulsed laser beam has a pulse width between about 300 picoseconds and about 10 femtoseconds.

22. A system for photoaltering a material, the system comprising:

a laser source operable to produce a primary pulsed beam;

an optical element configured to receive the primary pulsed beam and transmit a phase shifted beam, the phase shifted beam orthogonally polarized with the primary pulsed beam;

a first Wollaston prism configured to:

receive the phase shifted beam; and

transmit a first secondary beam and a second secondary beam, the first and second secondary beams together based on the phase shifted beam, the first secondary beam angularly separated from the second secondary beam; and

a scanner operable to direct a first transmission beam and a second transmission beam to the material, the first and second transmission beams based on the first and second secondary beams.

23. The system ofclaim 22, wherein the first transmission beam is the first secondary beam, and wherein the second transmission beam is the second secondary beam.

24. The system ofclaim 22, wherein the first transmission beam comprises a third secondary beam and a fourth secondary beam, wherein the second transmission beam comprises a fifth secondary beam and a sixth secondary beam, and wherein the system further comprises a second Wollaston prism coupled to the first Wollaston prism, the second Wollaston prism configured to:

receive the first and second secondary beams; and

transmit a third secondary beam, a fourth secondary beam, a fifth secondary beam, and a sixth secondary beam, the third and fourth secondary beams together based on the first secondary beam, the fifth and sixth secondary beams together based on the second secondary beam.

25. A method of photoaltering a material, the method comprising the steps of:

holographically altering an input beam to produce a plurality of transmission beams; and

scanning a portion of the material with the plurality of transmission beams.

26. The method ofclaim 25, wherein the scanning step comprises scanning a portion of a cornea with the plurality of transmission beams.

27. The method ofclaim 26, wherein the scanning step comprises scanning a portion of a cornea at a predetermined depth with the plurality of transmission beams.

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