Wavefront correction for ophthalmic surgical lasers

Abstract

A surgical laser system includes a laser engine, configured to generate a laser beam of laser pulses; a proximal optics and a distal optics, together configured to direct the laser beam to a target region, and to scan the laser beam in the target region through a scanning-point sequence; and an aberration sensor, configured to sense aberration by an aberration layer; a compensation controller, coupled to the aberration sensor, configured to generate compensation-point-dependent phase compensation control signals based on the sensed aberration; and a spatial phase compensator, positioned between the proximal optics and the distal optics, at a conjugate aberration surface, conjugate to the aberration layer, and coupled to the compensation controller, configured to receive the compensation-point-dependent phase compensation control signals, and to alter a phase of the laser beam in a compensation-point-dependent manner to compensate the sensed aberration.

Claims

1. A surgical laser system, comprising: a laser engine, configured to generate a laser beam of laser pulses; a proximal optics and a distal optics, together configured to direct the laser beam to a target region within an eye, and to scan the laser beam in the target region through a scanning-point sequence; an aberration sensor, configured to sense aberration by an aberration layer, wherein the aberration layer comprises a boundary of a cornea of the eye, and wherein the aberration sensor comprises an optical coherence tomographic (OCT) imaging system configured to sense aberration by generating an in-depth image of the cornea that includes the aberration layer; a compensation controller, coupled to the aberration sensor and comprising an image processor, configured to: determine, from the image generated by the OCT imaging system, an aberration optical path length or aberration phase shift ΔS(r.sub.a) for a plurality of aberration points r.sub.a; identify, for each of the plurality of aberration points r.sub.a, a corresponding conjugate compensation point r.sub.e based on a mapping which accounts for magnification, demagnification, or spatial distortion of the aberration layer, determine, for each conjugate compensation point r.sub.c, an aberration-compensating phase shift ΔS(r.sub.c(r.sub.a))=ΔS(r.sub.a); and for each compensation point r.sub.c, generate a phase compensation control signal based on the determined aberration-compensating phase shift ΔS(r.sub.c(r.sub.a)) to cause a spatial phase compensator to alter a phase of the laser beam independently at each compensation point r.sub.c; and the spatial phase compensator, positioned between the proximal optics and the distal optics, at a conjugate aberration surface, conjugate to the aberration layer, and coupled to the compensation controller, configured to receive the phase compensation control signal for each compensation point r.sub.c, and to alter a phase of the laser beam independently at each compensation point r.sub.c to compensate the sensed aberration.

2. The surgical laser system of claim 1, wherein: the distal optics includes a patient interface with a contact lens, and the aberration layer has a fixed relationship with the contact lens.

3. The surgical laser system of claim 2, wherein: the aberration layer tracks a distal surface of the contact lens at a distance between 0.1 mm-1 mm.

4. The surgical laser system of claim 1, wherein: at least one of the distal optics and the proximal optics is designed so that the conjugate aberration surface is essentially flat.

5. The surgical laser system of claim 1, wherein: the aberration sensor and the compensation controller are integrated into an aberration controller.

6. The surgical laser system of claim 1, wherein: the compensation controller is configured to generate the phase compensation control signals before the proximal optics and the distal optics scan the laser beam in the target region.

7. The surgical laser system of claim 1, wherein: the phase compensation control signals are the same for at least two different scanning points.

8. The surgical laser system of claim 1, wherein: the phase compensation control signals are independent from a scanning point for an interval of the scanning points.

9. The surgical laser system of claim 1, wherein: the spatial phase compensator is configured to alter a phase of the laser beam at a particular compensation point r.sub.c for at least two different scanning points.

10. The surgical laser system of claim 1, wherein: the spatial phase compensator is configured to alter a phase of the laser beam at a particular compensation point r.sub.c independent from a scanning point for an interval of the scanning points.

11. The surgical laser system of claim 1, wherein: the proximal optics comprises an XY scanner, and the distal optics comprises a Z scanner, and an objective.

12. The surgical laser system of claim 1, wherein: the proximal optics comprises a beam expander, and the distal optics comprises an XY scanner, a Z scanner, and an objective.

13. The surgical laser system of claim 1, the spatial phase compensator comprising: at least one of a transmissive system, an absorptive system and a reflective system.

14. The surgical laser system of claim 13, the spatial phase compensator comprising: an array of electronically controllable electro-optical beam modulators.

15. The surgical laser system of claim 13, the spatial phase compensator comprising: an array of electronically controllable liquid crystal display elements.

16. The surgical laser system of claim 13, the spatial phase compensator comprising: an array of electronically controllable micro-reflectors.

17. The surgical laser system of claim 13, the spatial phase compensator comprising: an array of electronically controllable pixels.

18. The surgical laser system of claim 13, wherein: the compensation controller and a scanning controller are separate.

19. A method of reducing aberrations in a surgical laser system, the method comprising: generating, with an optical coherence tomographic (OCT) imaging system, an in-depth image of an aberration layer, the aberration layer comprising cornea of an eye; determining, by a compensation controller coupled to the aberration sensor, an aberration optical path length or aberration phase shift ΔS(r.sub.a) for a plurality of aberration points r.sub.a from the generated image; identifying, by the compensation controller, for each of the plurality of aberration points r.sub.a, a corresponding conjugate compensation point r.sub.c based on a mapping which accounts for magnification, demagnification, or spatial distortion of the aberration layer, determining, by the compensation controller, for each conjugate compensation point r.sub.c, an aberration-compensating phase shift ΔS(r.sub.c(r.sub.a))=ΔS(r.sub.a); generating, by the compensation controller, a phase compensation control signal for each compensation point r.sub.c based on the aberration characteristic aberration-compensating phase shift ΔS(r.sub.c(r.sub.a)); and altering a phase of a scanned laser beam at each compensation point r.sub.c according to the phase compensation control signals to compensate the sensed aberration by a spatial phase compensator, positioned between the proximal optics and the distal optics, at a conjugate aberration surface, conjugate to the aberration layer, and coupled to the compensation controller to receive the phase compensation control signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A-E illustrate an effect of a wrinkled cornea on an ophthalmic surgical laser beam.

(2) FIGS. 2A-C illustrate an enlargement of a focus spot because of corneal wrinkling.

(3) FIGS. 3A-B illustrate an effect of corneal wrinkling on a two dimensional cut.

(4) FIG. 4 illustrates an ophthalmic surgical laser system and its aberration layer.

(5) FIG. 5 illustrates an ophthalmic surgical laser system with a spatial phase compensator.

(6) FIG. 6 illustrates an embodiment of the ophthalmic surgical laser system with a spatial phase compensator.

(7) FIGS. 7A-B illustrate embodiments of spatial phase compensators.

DETAILED DESCRIPTION

(8) Methods are known to manipulate or correct a distorted waveform or wavefront of light to counteract the effect of distortions due to the optics, the medium or the target, as this problem arises from medicine to astronomy, where the atmospheric distortions need to be compensated. However, correcting the distorted wavefront with a phase modulator remains a difficult challenge for multiple reasons.

(9) First, such systems need to have a suitable wavefront analyzer and associated optics. Such analyzers are expensive, increase the complexity and the size of the system and are often cumbersome to use.

(10) Second, even if a wavefront analyzer is implemented, the distorted wavefront reaching the ocular target, such as the lens of the eye, is not available for analysis, as a substantial portion of the distortion can occur after the beam left the optical system. For example, the earlier described wrinkling of the cornea can distort the laser beam to a substantial degree after the beam exited the optical system and entered the eye. Some systems attempt to capture this additional distortion by analyzing the beam reflected from the retina. However, the beam reflected from the retina potentially picks up additional distortions from ocular regions past the target such as the lens, and it gets distorted twice, on its way in and out of the eye. Also, more advanced cataracts, such as class 4 cataracts, may largely absorb the reflected beam.

(11) Third, the detection and analysis of the distorted wavefront needs to be processed to determine the phase modulation response needed to correct the distorted wavefront. This determined response then needs to be coupled back to an aberration controller by a feedback control loop. These requirements are particularly demanding to implement in a scanning system like an ophthalmic surgical laser. The detection, analysis, processing and feeding back all need to be done in real time during scanning Doing so requires substantial signal processing bandwidth and advanced electronics.

(12) Fourth, the requirement of real-time phase modulation is particularly pressing in a modern, high speed XYZ scanning ophthalmic surgical system, where the repetition rate of the pulsed laser beam can be 50 kHz or even 150 kHz. As the beam is scanned through target points with different XYZ coordinates in a target volume, its distortion continues to change, caused by centers throughout the target volume. Accordingly, the phase modulation response needs to be continuously detected, re-analyzed and re-processed, and the re-processed phase modulation response has to be continuously fed back to the phase modulator. The high scanning speed of the ophthalmic surgical lasers places strong demands on the processing speed and signal bandwidth of all components of the system, the wavefront detector, analyzer, processor, and the phase modulator.

(13) To address the above described problem of corneal wrinkling distorting the surgical laser beam, causing no-cut regions in a way different from the existing wavefront modulators, this patent document describes embodiments of a surgical laser system with a spatial phase modulator.

(14) Embodiments of the surgical laser system are developed from the following considerations.

(15) (1) The distortion of the laser beam in these ophthalmic systems is caused not by a volumetric distortion, but by a layer, the wrinkled distal boundary of the cornea. This will be referred to as the aberration layer.

(16) (2) According to general principles of optical design, for imaging optics that create real images, an object layer or surface is imaged to a conjugate or image surface. This means that all the rays that go through a particular aberration point r.sub.a of the aberration layer also go through a corresponding conjugate, or compensation point r.sub.c on its conjugate surface. Since the location of the aberration layer relative to the optics is known to a substantial precision, the conjugate surface of this aberration layer inside the optics is known as well. Even in an optical system where the target's own conjugate surface is at infinity, the optical system can be designed such that the conjugate surface of the aberration layer is at a finite distance from the aberration surface, inside the optical system itself.

(17) (3) At any given moment, the optics of a surgical laser focuses the rays of a typically expanded laser beam to a specific scanning point r.sub.s in the target region. Since typically the aberration layer is proximal to the target region, the rays that intersect each other in the target region in the specific scanning point r.sub.s intersect the aberration layer at aberration points r.sub.a that cover an extended area. Correspondingly, rays that go through the same aberration point r.sub.a are directed to different scanning points r.sub.s. Since the aberration at the aberration point r.sub.a only depends on the source of aberration (e.g. a corneal wrinkle) specifically at r.sub.a, the aberrations can be compensated by an r.sub.a-dependent compensation which can be largely independent of the scanning point r.sub.s.

(18) (4) Because of the strong correspondence and mapping between the aberration points r.sub.a and the conjugate compensation points r.sub.c, the aberrations at r.sub.a can be compensated by placing a spatial phase compensator at the conjugate surface. The spatial phase compensator can alter the phase of the rays at the compensation point r.sub.c that corresponds to the aberration point r.sub.a by an amount that compensates the aberration caused by the aberration layer at r.sub.a.

(19) Combining (3) and (4) illustrates that it is possible to compensate the aberrations caused by an aberration layer to rays directed to scanning points r.sub.s by positioning a spatial phase compensator at the conjugate surface of the aberration layer, and configuring the spatial phase compensator to introduce phase shifts that depend on the compensation points r.sub.c, but not on the scanning points r.sub.s.

(20) (5) Typically the conjugate surface of the curved distal corneal boundary is curved. It is challenging to fabricate a phase modulator that is positioned at such a curved conjugate surface. However, embodiments have been developed from the recognition that it is possible to design an optical system where the conjugate surface of an aberration surface with a cornea-like curvature is essentially flat.

(21) (6) Since the aberrations are dominantly caused by corneal wrinkling at the known location of the distal boundary of the cornea, it is possible to calculate the aberration optical path length from the knowledge of the shape and location of the wrinkles alone. This information can be gleaned from an optical coherence tomographic (OCT) image, without using a wavefront aberrometer. And since many ophthalmic laser systems already include an OCT imaging system, the aberration optical path can be determined without the need to insert additional and costly wavefront aberrometers.

(22) Embodiments implement one or more of the above design considerations as follows.

(23) FIGS. 4-5 illustrate a surgical laser system 100 that can include a laser engine 110, configured to generate a laser beam of laser pulses; a proximal optics 120 and a distal optics 130, together configured to direct the laser beam to a target region, and to focus and scan the laser beam in the target region through a sequence of scanning points r.sub.s. The distal optics 130 can also include a patient interface 140 with a contact lens 150 that makes contact with the cornea 2. As discussed above, in ophthalmic embodiments where the corneal wrinkling is a major source of the beam aberration, the aberration layer can be the distal boundary of the cornea 2, interfacing with the anterior chamber of the eye 1. In what follows, the general notion of the aberration layer is denoted by 2A, a specific example being the distal boundary of a wrinkled cornea 2. In these and other embodiments, the aberration layer 2A can have a fixed and known relationship with the contact lens 150 and therefore with the distal optics 130. For example, given that the average thickness of cornea 2 is in the range of 0.4 mm-0.8 mm, the aberration layer 2A can track a distal surface of the contact lens 150 at a distance between 0.1 mm-1 mm. The points of the aberration layer 2A are denoted by r.sub.a.

(24) As discussed before, FIG. 4 also illustrates that the aberration layer 2A has a conjugate aberration surface 2B whose points are denoted by r.sub.c. By construction, the rays that go through a particular aberration point r.sub.a of the aberration layer 2A all intersect each other at the compensation point r.sub.c at the conjugate aberration surface 2B that corresponds to, or can be mapped to, the aberration point r.sub.a. This is indicated for a specific aberration point r.sub.a—compensation point r.sub.c pair by the thick solid lines in FIG. 4. The above recognitions (1)-(2), that for layer-like aberration sources like the aberration layer 2A such conjugate surfaces 2B exist, or the optical system can be designed such that they exist, are part of the platform on which the aberration-compensating embodiments of the surgical laser system 100 are developed.

(25) It is recapped here for clarity that the optics 120-130 expands and then focuses the rays, or beam components, of the laser beam to a specific scanning point r.sub.s in the target region, such as the lens 5. Since typically the aberration layer 2A, such as the cornea 2, is proximal to the target region or lens 5, the rays that intersect each other in the target region at the specific scanning point r.sub.s go through the aberration layer at aberration points r.sub.a that cover an extended area. Correspondingly, rays that go through the same aberration point r.sub.a are directed to different scanning points r.sub.s. Since the aberration at the aberration point r.sub.a only depends on the source of aberration specifically at r.sub.a (e.g. a corneal wrinkle), the aberrations can be compensated by an r.sub.a-dependent compensation which can be largely independent of the scanning point r.sub.s. Finally, there is a strong correspondence, or mapping, between the aberration points r.sub.a and the conjugate compensation points r.sub.c=r.sub.c(r.sub.a). Therefore, it is possible to compensate the aberrations caused by the aberration layer 2A at points r.sub.a during scanning the laser beam through a sequence of scanning points r.sub.s by altering the phase of the rays at the conjugate compensation points r.sub.c=r.sub.c(r.sub.a) of the conjugate aberration surface 2B that correspond to the aberrations points r.sub.a, but are largely independent from the scanning points r.sub.s. Here, in general the scanning points r.sub.s, the aberration points r.sub.a and the compensation points r.sub.c all represent three dimensional vectors: r.sub.s=(x.sub.s,y.sub.s,z.sub.s), r.sub.a=(x.sub.ay.sub.az.sub.a), and r.sub.c=(x.sub.cy.sub.cz.sub.c).

(26) It is noted here that during a typical cataract procedure the laser beam is focused into a lens target region that is distal to the corneal aberration layer 2A. There are some procedures where the target of the surgery is the cornea and thus the target region and the aberration layer can be quite close. In such cases the distinction that the aberration compensation is r.sub.a/r.sub.c dependent but r.sub.s independent may appear hollow. However, it is recalled that the aberrations are caused by the corneal wrinkles at the distal boundary of the cornea, distal to the surgical target region. Accordingly, the corneal wrinkles do not introduce substantial aberration for corneal surgical procedures.

(27) It is further noted that even if the optics of the surgical laser system 100 is such that the image or conjugate surface of the surgical target is at infinity, corresponding to a parallel laser beam, the optics can be designed such that the conjugate aberration surface 2B of the aberration layer 2A can be inside the optics.

(28) FIG. 5 illustrates that the surgical laser system 100 can further include an aberration sensor 210, configured to sense an aberration by the aberration layer 2A, and a compensation controller 220 that is coupled to the aberration sensor 210 to generate compensation-point-dependent phase compensation control signals based on the sensed aberration. The surgical laser system 100 can also include a spatial phase compensator 230 that is positioned between the proximal optics 120 and the distal optics 130, at the conjugate aberration surface 2B that is conjugate to the aberration layer 2A. The spatial phase compensator 230 can be coupled to the compensation controller 220 to receive the compensation-point-dependent phase compensation control signals and to alter a phase of the laser beam in a compensation-point-dependent manner to compensate the sensed aberration, sensed by the aberration sensor 210.

(29) Placing the spatial phase compensator 230 at the conjugate aberration surface 2B makes it possible to introduce compensating phases at r.sub.c compensation points that are needed to compensate the aberration caused at the r.sub.a aberration points by, e.g., a corneal wrinkle of the aberration layer 2A, wherein the r.sub.c compensation points are the conjugates of the r.sub.a aberration points: r.sub.c=r.sub.c(r.sub.a).

(30) As discussed above at (3)-(4), with this design the compensating phases are dependent from the r.sub.c compensation points, but can be essentially independent from the r.sub.s scanning points. Therefore, in some embodiments the compensation controller 220 can be a separate or independent subsystem from a scanner control subsystem. In some cases these two systems may use separate computer processors.

(31) In embodiments of the surgical laser system 100 at least one of the proximal optics 120 and the distal optics 130 can be designed so that the conjugate aberration surface 2B is essentially flat. As discussed above at (5), systems with a flat conjugate aberration surface 2B are better suited to have a spatial phase compensator 230 compensate the beam aberrations effectively. In such embodiments, the compensation points can be characterized only by their lateral coordinates: r.sub.c=(x.sub.c, y.sub.c).

(32) As also mentioned above, in some embodiments, the aberration sensor 210 can include an optical coherence tomographic imaging system, or OCT. An OCT embodiment of the aberration sensor 210 can generate an in-depth image of the cornea 2 and its aberration-causing wrinkles. The compensation controller 220 can include an image processor to determine an aberration-optical-path-length ΔS, or a corresponding aberration-phase-shift ΔS from the image generated by the OCT imaging system. In particular, from the image of the wrinkled cornea 2, the aberration-optical-path-length ΔS(r.sub.a) can be determined as: ΔS(r.sub.a)=∫Δn ds, where the integral runs for the Δ(r.sub.a) portion of the corneal thickness that is in excess of the average corneal thickness at the aberration point r.sub.a, and the difference Δn=n.sub.c-n.sub.ah, is the difference between the index of refraction of the cornea (n.sub.c=1.377) and that of the aqueous humor in the anterior chamber (n.sub.ah=1.337). With this definition ΔS(r.sub.a) is a measure of the aberration-optical-path-length, or aberration phase shift, caused by the aberration layer that needs to be compensated by a compensating phase ΔS(r.sub.c(r.sub.a))=−ΔS(r.sub.a), introduced at a corresponding, or mapped conjugate point r.sub.c=r.sub.c(r.sub.a) at the conjugate aberration surface 2B.

(33) The design process of the compensation controller 220 can include a mapping of the r.sub.a aberration points to the r conjugate compensating points to establish the r.sub.c=r.sub.c(r.sub.a) correspondence or mapping. This mapping can be complex, since the image of the aberration surface 2A may be magnified, demagnified, or spatially distorted by the distal optics 130 and other factors. The mapping can be carried out using a transfer matrix or a look-up table and can be determined by several means. By knowing the optical design of the surgical laser system 100, each aberration point r.sub.a at the aberration layer can be numerically ray-traced backwards, opposite to the propagation of the laser beam, to determine the corresponding compensation point r.sub.c=r.sub.c(r.sub.a) at the conjugate aberration surface 2B. Alternatively, the mapping or correspondence can be carried out by a calibration process. A calibration object, such as a grid pattern or an array of pin-holes can be placed at the location of the aberration layer 2A and its real image can be recorded by a camera at the location of the conjugate aberration surface 2B, thus determining the r.sub.c=r.sub.c(r.sub.a) mapping.

(34) Since it is possible to compute the distortion- or aberration-compensating phase shift ΔS(r.sub.c(r.sub.a))=ΔS(r.sub.a) based on the image of the OCT embodiment of the aberration sensor 210 which is often already present in surgical laser system 100, the here-described embodiments of the laser system 100 do not require the installation of an additional wavefront analyzer, Shack-Hartmann analyzer, or other additional equipment. This can accelerate the system performance substantially and make it also simpler and cheaper.

(35) As described above, positioning the spatial phase compensator 230 at the conjugate aberration surface 2B makes it possible that the compensation controller 220 is configured to generate the compensation-point-dependent phase compensation control signals before the proximal optics 120 and the distal optics 130 scan the laser beam in the target region, since the phase compensation control signals depend only on the compensation points r.sub.c but not on the scanning points r.sub.s and thus need not be re-calculated for every new scanning point r.sub.s.

(36) Accordingly, in some embodiments the compensation-point (r.sub.c)-dependent phase compensation control signals that correspond to the aberration-compensating phase shift ΔS(r.sub.c) can be the same for at least two different scanning points r.sub.s. Also, in some embodiments the compensation-point (r.sub.c)-dependent phase compensation control signals that correspond to the aberration-compensating phase shift ΔS(r.sub.c) can be independent from a scanning point r.sub.s for an interval of the scanning point r.sub.s.

(37) Similarly, in some embodiments the spatial phase compensator 230 can alter a phase of the laser beam in a compensation-point (r.sub.c)-dependent manner for at least two different scanning points r.sub.s. Further, in some embodiments the spatial phase compensator 230 can alter a phase of the laser beam in a compensation-point (r.sub.c)-dependent manner independent from a scanning point r.sub.s for an interval of the scanning points r.sub.s.

(38) In some embodiments of the surgical laser system 100, the aberration sensor 210 and the compensation controller 220 can be integrated into a single, integrated aberration controller.

(39) FIG. 6 illustrates that in some embodiments of the surgical laser system 100, the proximal optics 120 can include possibly a beam expander 112 and an XY scanner 114, whereas the distal optics 130 can include a Z scanner 132 and an objective 134. (The beams are indicated only partially for clarity.)

(40) Alternatively, the surgical laser system 100 can include an embodiment of the proximal optics 120 that includes a beam expander 112, and the distal optics 130 includes an XY scanner, a Z scanner, and an objective.

(41) FIGS. 7A-B illustrate that the spatial phase compensator 230 can be a transmissive system, an absorptive system, or a reflective system.

(42) FIG. 7A illustrates that in some cases the spatial phase compensator 230 can include an LCD array 231 that includes electronically controllable liquid crystal display elements or pixels 232.

(43) FIG. 7B illustrates that in some cases the spatial phase compensator 230 can include a deformable reflector 236 that includes a substrate 237 and an array of electronically controllable mechanical actuators 238.

(44) Many other embodiments can be included as well. For example, the spatial phase compensator 230 can include an array of electronically controllable micro-reflectors.

(45) Finally, a method 300 of reducing aberrations in a surgical laser system may include:

(46) (310)—generating a laser beam of laser pulses by a laser engine;

(47) (320)—directing the laser beam to a target region by a proximal optics and a distal optics;

(48) (330)—scanning the laser beam in the target region by the proximal optics and the distal optics through a sequence of scanning points;

(49) (340)—sensing aberration, caused by an aberration layer, with an aberration sensor;

(50) (350)—generating compensation-point-dependent phase compensation control signals based on the sensed aberration by a compensation controller, coupled to the aberration sensor; and

(51) (360)—altering a phase of the laser beam in a compensation-point-dependent manner to compensate the sensed aberration by a spatial phase compensator, positioned between the proximal optics and the distal optics, at a conjugate aberration surface, conjugate to the aberration layer, and coupled to the compensation controller to receive the compensation-point-dependent phase compensation control signals.

(52) While this document contains many specifics, these should not be construed as limitations on the scope of an invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or a variation of a subcombination.