Laser beam wavefront correction using adaptive optics and intermediate field monitoring.

Abstract

The system for correcting the wavefront (182) of a laser beam (180) includes a beam splitter (120) for splitting off a small portion of the laser beam to be used as a diagnostic beam (180D), a focusing element (140) for focusing the diagnostic beam, a measurement subsystem (150) for measuring the size of the diagnostic beam at upstream and / or downstream locations relative to a nominal location of the focal point, and at least one adaptive optics system (110) located upstream from the beam splitter for correcting the wavefront of the laser beam based, at least in part, on the measured size of the diagnostic beam at the upstream and / or downstream locations. The upstream 152(1) and downstream 152(2) locations correspond to intermediate field locations within the laser beam as imaged by the focusing element. The system utilizes the sensitivity of the laser beam size to waist location deviations, which are greatest at one Rayleigh length from the nominal waist location.

Description

[Technical Field]

[0001] (Priority) This application claims priority to U.S. Patent Application No. 17 / 102,201, filed November 23, 2020, the disclosure of which is incorporated herein by reference in its entirety.

[0002] (Technical field of the invention) The present invention relates generally to laser beam wavefront correction, and more particularly to techniques for stabilizing the size and divergence properties of a laser beam using adaptive optics. [Background technology]

[0003] (Discussion of Background Art) The laser quality of a laser beam characterizes its focusability. The wavefront of a high-quality laser beam has a smooth and simple shape. A high-quality laser beam can be more tightly focused than a low-quality laser beam. Different measures of beam quality are used in the art. One measure is the beam parameter product (BPP), which is defined as the product of (i) the beam radius at the beam waist and (ii) the far-field beam divergence angle. The highest possible beam quality is achieved over a diffraction-limited Gaussian beam. Therefore, another measure of beam quality is the M, which is defined as the ratio of (i) the BPP for the laser beam under consideration to (ii) the BPP of a diffraction-limited Gaussian beam of the same wavelength. 2 It is a factor. M 2 The actual measurement of the factor, or equivalently, BPP, is typically quite tedious. Such measurements generally involve focusing a laser beam to a waist and measuring the beam size at and near the waist, i.e., at several locations in the near field, and at several locations at least two Rayleigh lengths from the waist, i.e., in the far field. A fitting function is fitted to the beam size measurements, M 2 Factors are derived from the fitted parameters.

[0004] Some optical elements perform their intended function by altering the wavefront of an incident laser beam. A prime example of this is a lens, which operates by changing the wavefront curvature of the incident light. However, optical elements can also cause undesired wavefront changes. For example, light-induced heating of a gain crystal in a high-power solid-state laser can lead to thermal lensing. Thermal lensing is caused by laser radiation, which heats the gain crystal unevenly and creates a temperature-induced gradient in the gain crystal's refractive index. In another example, the generation of ultraviolet (UV) laser light in a nonlinear crystal can degrade the nonlinear crystal if the UV laser light is sufficiently intense. Both thermal lensing and UV-induced degradation, when mild, cause the laser beam wavefront curvature to change as a function of time, which leads to temporal instability in the size and divergence properties of the output laser beam. When more severe, these effects degrade beam quality and / or power output.

[0005] Adaptive optics are optical elements that can be adjusted to change the wavefront of a bright field (e.g., a laser beam). For example, a deformable mirror can be deformed to impart a desired wavefront change on reflected light. Similarly, a deformable lens can be deformed to impart a desired wavefront change on transmitted light. While open-loop operation is sufficient in some scenarios, adaptive optics are typically incorporated into an active feedback loop that iteratively adjusts the adaptive optics according to measurements of the resulting wavefront.

[0006] Both MEMS-based and piezoelectric-based deformable mirrors designed for laser beam wavefront correction are now commercially available. These deformable mirrors have a flexible mirror surface, a supporting substrate, and an actuator array positioned between the backside of the flexible mirror surface and the supporting substrate. Each actuator can adjust the local distance between the supporting substrate and the mirror surface. The actuator array has a large number of actuators, typically dozens or more, arranged in a two-dimensional array to provide general-purpose, high-resolution wavefront control. Summary of the Invention [Means for solving the problem]

[0007] (Summary of the Invention) Disclosed herein are systems and methods for correcting the wavefront of a laser beam in the presence of one or more sources of undesirable wavefront distortion. It is recognized that in one common type of laser system, the dominant wavefront distortion problem is a shift in the location and size of the laser beam waist. In addition, it is also recognized that the highest sensitivity of beam size measurements to shifts in waist location is achieved in the intermediate field, i.e., about one Rayleigh length from the waist, as opposed to in the near or far field. Thus, the present systems and methods take an unconventional approach of monitoring the beam size in the intermediate field relative to the waist and use the intermediate field measurements to correct for wavefront distortion. 2 In contrast to the many locations required for conventional measurements of factors, the present approach to wavefront correction requires measurements at only one location, although two locations are preferred for accuracy, and some embodiments further implement far-field measurements to achieve additional functionality.

[0008] The present intermediate-field monitoring technique is generally applicable to wavefront monitoring and correction in situations where a primary issue is laser beam waist location shifts. The intermediate-field monitoring technique has been found to be particularly useful for actively stabilizing the waist location of ultraviolet (UV) laser beams generated in nonlinear crystals, which are subject to UV degradation over time. As is true for many laser system applications, such UV laser systems are often required to maintain stable beam parameters over operating times as long as thousands of hours, and the present wavefront correction technique can be implemented to help meet at least this requirement.

[0009] We have further found that the wavefront correction required to stabilize waist location based on intermediate field of view monitoring can be implemented using relatively simple adaptive optics, e.g., one or more deformable cylindrical mirrors, each with as few as three actuators. This represents a significant reduction in cost and complexity compared to systems relying on deformable mirrors commercially available today.

[0010] In one aspect, a system for correcting the wavefront of a laser beam includes a primary beam splitter for splitting off a small portion of the laser beam to be used as a diagnostic beam. The laser beam has a waist at a nominal waist location. The system further includes a focusing element for focusing the diagnostic beam and a measurement subsystem configured to measure a size of the diagnostic beam at at least one of upstream and downstream locations relative to the nominal location of the focal point of the diagnostic beam. Each of the upstream and downstream locations of the diagnostic beam corresponds to an image of a distinct intermediate field location of the laser beam. Additionally, the system includes at least one adaptive optics system located within the laser beam upstream from the primary beam splitter for correcting the wavefront of the laser beam based, at least in part, on the measured size of the diagnostic beam relative to at least one of the upstream and downstream locations.

[0011] In another aspect, a method for correcting a wavefront of a laser beam includes splitting off a small portion of the laser beam as a diagnostic beam. The laser beam has a waist at a nominal waist location. The method further includes focusing the diagnostic beam and measuring a size of the diagnostic beam at at least one of upstream and downstream locations relative to the nominal location of the focal point of the diagnostic beam. Each of the upstream and downstream locations of the diagnostic beam corresponds to an image of a distinct intermediate field location of the laser beam. The method also includes correcting the wavefront of the laser beam using at least one adaptive optics system located within the laser beam upstream of the split between the laser beam and the diagnostic beam based, at least in part, on the measured size for the at least one of the upstream and downstream locations. The present invention provides, for example, the following. (Item 1) 1. A system for correcting a wavefront of a laser beam, comprising: a primary beam splitter for splitting off a small portion of the laser beam to be used as a diagnostic beam, the laser beam having a waist at a nominal waist location; a focusing element for focusing the diagnostic beam; a measurement subsystem configured to measure a size of the diagnostic beam at at least one of upstream and downstream locations relative to a nominal location of a focal point of the diagnostic beam, each of the upstream and downstream locations of the diagnostic beam corresponding to an image of a respective intermediate field location of the laser beam; at least one adaptive optics system located within the laser beam upstream from the primary beam splitter for correcting the wavefront of the laser beam based, at least in part, on the measured size of the diagnostic beam relative to the at least one of the upstream and downstream locations; A system comprising: (Item 2) Item 10. The system of item 1, wherein each of the intermediate field locations is displaced from the nominal waist location by 90% to 110% of the nominal Rayleigh length of the laser beam relative to the nominal waist location. (Item 3) the at least one adaptive optics system includes a first adaptive mirror configured to correct the wavefront relative to the laser beam in a first transverse axis; 3. The system of claim 1, wherein the corresponding size measured by the measurement subsystem at each of the at least one of the upstream and downstream locations includes a first lateral dimension of the diagnostic beam in the first lateral axis. (Item 4) The first adaptive mirror comprises: a mirror surface; only three actuators distributed along the first transverse axis and adjusting the mirror surface in a dimension parallel to a plane defined by the first transverse axis and the direction of propagation of the laser beam; Item 4. The system according to item 3, comprising: (Item 5) The measurement subsystem includes: an image sensor; first and second secondary beam splitters for directing respective first and second subportions of the diagnostic beam to different respective first and second locations on the image sensor, the first and second secondary beam splitters being arranged in series such that the image sensor captures the first and second subportions of the diagnostic beam at the upstream and downstream locations, respectively; Item 5. The system according to item 3 or 4, comprising: (Item 6) the at least one adaptive optics system further includes a second adaptive mirror downstream from the first adaptive mirror and configured to correct the wavefront relative to the laser beam in a second transverse axis, the second transverse axis being orthogonal to the first transverse axis; 6. The system of any one of items 3 to 5, wherein the corresponding size measured by the measurement subsystem at each of the at least one of the upstream location and the downstream location further includes a second lateral dimension of the diagnostic beam in the second lateral axis. (Item 7) 1. A laser device with wavefront correction, comprising: Item 6. The system according to item 6, a laser source for generating the laser beam with a waist at a waist location within the laser source; a telescope upstream of the at least one adaptive optics system for forming an image of the laser beam at the waist location at an image plane; and Equipped with The at least one adaptive optics system is located at the image plane. (Item 8) 8. The laser device of claim 7, wherein the laser source includes a nonlinear crystal, and the waist location is within the nonlinear crystal. (Item 9) 9. The laser apparatus of claim 7 or 8, further comprising a feedback controller configured to actively adjust the at least one adaptive optics system to stabilize a location of the second waist of the laser beam at the image plane and maintain a nominal value of the size measured for the at least one of the upstream and downstream locations. (Item 10) 10. The system of claim 1, wherein the measurement subsystem is configured to further measure a size of the diagnostic beam at a focal plane of the focusing element. (Item 11) The measurement subsystem includes: an image sensor; first, second, and third secondary beam splitters for directing respective first, second, and third subportions of the diagnostic beam to different respective first, second, and third locations on the image sensor, the first, second, and third secondary beam splitters being arranged in series such that the image sensor captures the first, second, and third subportions of the diagnostic beam at the upstream location, the focal plane, and the downstream location, respectively; 10. The system of any of the preceding items, comprising: (Item 12) Item 12. The system of item 10 or 11, further comprising a feedback controller configured to actively adjust the at least one adaptive optics system to maintain a nominal value of the size measured for the at least one of the upstream and downstream locations and at the focal plane. (Item 13) 13. The system of any of items 10 to 12, wherein the at least one adaptive optics system includes a first pair of adaptive mirrors arranged in series and each configured to correct the wavefront relative to the laser beam in a first horizontal axis. (Item 14) Each adaptive mirror of the first pair comprises: a mirror surface; only three actuators distributed along the first transverse axis and adjusting the mirror surface in a dimension parallel to a plane defined by the first transverse axis and the direction of propagation of the laser beam; Item 14. The system according to item 13, comprising: (Item 15) Item 15. The system of item 13 or 14, wherein the at least one adaptive optics system further includes a second pair of adaptive mirrors arranged in series and each configured to correct the wavefront relative to the laser beam in a second horizontal axis, the second horizontal axis being orthogonal to the first horizontal axis. (Item 16) 1. A laser device with wavefront correction, comprising: Item 15. The system according to item 15, a laser source for generating said laser beam; a beam shaping module upstream of the at least one adaptive optics system, the beam shaping module being configured to achieve a predefined waist size and far-field divergence of the laser beam when the wavefront has nominal properties at the beam shaping module; a feedback controller configured to adjust the first and second pair of adaptive optics systems based, at least in part, on the measured size of the diagnostic beam at the focal plane and relative to the at least one of the upstream and downstream locations to achieve the predefined waist size and far-field divergence of the laser beam when the wavefront does not have the nominal properties at the beam shaping module; and A laser device comprising: (Item 17) Item 17. The laser device of item 16, wherein the laser source includes a nonlinear crystal. (Item 18) 1. A method for correcting a wavefront of a laser beam, comprising: splitting off a small portion of the laser beam as a diagnostic beam, the laser beam having a waist at a nominal waist location; focusing the diagnostic beam; measuring a size of the diagnostic beam at at least one of upstream and downstream locations relative to a nominal location of a focal point of the diagnostic beam, each of the upstream and downstream locations of the diagnostic beam corresponding to an image of a respective intermediate field location of the laser beam; correcting a wavefront of the laser beam using at least one adaptive optics system located within the laser beam upstream of the split between the laser beam and the diagnostic beam based, at least in part, on the size measured relative to at least one of the upstream and downstream locations; A method comprising: (Item 19) Item 19. The method of item 18, wherein each of the intermediate field locations is displaced from the nominal waist location by 90% to 110% of the nominal Rayleigh length of the laser beam relative to the nominal waist location. (Item 20) The laser beam has a first waist, and the method further comprises: forming an image of the first waist at an image plane located at the at least one adaptive optics system; measuring a first lateral dimension of the diagnostic beam at the at least one of the upstream and downstream locations in a first lateral axis relative to the laser beam; adjusting a first adaptive mirror to maintain a nominal value of the first lateral dimension relative to the at least one of the upstream and downstream locations so as to stabilize a location of a second waist of the laser beam in the first lateral axis relative to the image plane, in the correcting step; 20. The method according to item 18 or 19, comprising: (Item 21) 21. The method of claim 20, wherein the first waist is in a nonlinear crystal, and the correcting step stabilizes the location of the second waist in the presence of wavefront deformation of the laser beam in the nonlinear crystal. (Item 22) The measuring step includes: using two beam splitters arranged in series along the diagnostic beam to direct two different and distinct subportions of the diagnostic beam to two different and distinct locations on a single image sensor such that the image sensor captures two different and distinct subportions at the upstream location and the downstream location, respectively; extracting a size of the diagnostic beam at each of the upstream and downstream locations from an image captured by the image sensor; 22. The method according to item 20 or 21, comprising: (Item 23) measuring a second transverse dimension of the diagnostic beam at each of the upstream and downstream locations in a second transverse axis relative to the laser beam, the second transverse axis being orthogonal to the first transverse axis; adjusting a second adaptive mirror to maintain a nominal value of the second lateral dimension at the upstream and downstream locations so as to stabilize a location of a second waist of the laser beam in the second lateral axis relative to the image plane, in the correcting step; 22. The method of claim 20 or 21, further comprising: (Item 24) a focusing element performing the step of focusing the diagnostic beam; the measuring step further comprises measuring a size of the diagnostic beam at a focal plane of the focusing element; 24. The method according to any one of items 18 to 23, wherein the correcting step further bases the wavefront correction on a size of the diagnostic beam measured at the focal plane. (Item 25) Item 25. The method of item 24, wherein the correcting step adjusts a plurality of adaptive optics systems to maintain a predefined waist size and far-field divergence of the laser beam. (Item 26) generating said laser beam in a nonlinear crystal by frequency conversion; shaping the laser beam upstream of the plurality of adaptive optics systems, the shaping configured to achieve the predefined waist size and far-field divergence of the laser beam in the absence of wavefront deformation by the nonlinear crystal; further comprising 26. The method of claim 25, wherein the correcting step provides additional shaping to correct for the presence of wavefront deformation due to the nonlinear crystal. (Item 27) the measuring step includes measuring a first lateral dimension of the diagnostic beam at each of the at least one of the upstream and downstream locations and the focal plane in a first lateral axis relative to the laser beam; 27. The method of any one of items 24 to 26, wherein the correcting step adjusts a first pair of serially arranged adaptive mirrors based on the first lateral dimension to maintain (a) a predefined location of a waist of the laser beam on the first lateral axis and (b) a predefined size of the waist on the first lateral axis. (Item 28) the measuring step further includes measuring a second transverse dimension of the diagnostic beam at each of the at least one of the upstream and downstream locations and the focal plane in a second transverse axis relative to the laser beam, the second transverse axis being orthogonal to the first transverse axis; Item 28. The method of item 27, wherein the correcting step further comprises adjusting a first pair of serially arranged adaptive mirrors based on the second lateral dimension to maintain (a) a predefined location of a waist of the laser beam in the second lateral axis and (b) a predefined size of the waist in the second lateral axis. (Item 29) The measuring step includes: directing three different and distinct portions of the diagnostic beam to three different and distinct locations on a single image sensor using three beam splitters arranged in series, such that the image sensor captures the three different and distinct portions at the upstream location, the focal plane, and the downstream location, respectively; extracting from the image captured by the image sensor the size of the diagnostic beam at each of the upstream location, the focal plane, and the downstream location; 29. The method according to any one of Items 24 to 28, comprising: (Item 30) monitoring higher order transverse mode properties of the diagnostic beam at the focal plane in the measuring step; In the correcting step, further adjusting the at least one adaptive optics system according to the higher-order transverse mode properties. 30. The method according to any one of items 24 to 29, further comprising: [Brief explanation of the drawings]

[0012] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate diagrammatically preferred embodiments of the present invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

[0013] [Figure 1] 1 illustrates a system for correcting the wavefront of a laser beam, according to one embodiment, which utilizes beam size measurement at an intermediate field and adaptive optics.

[0014] [Figure 2] Figures 2A and 2B illustrate the sensitivity of beam size to waist location shifts as a function of distance from the nominal waist location, demonstrating that maximum sensitivity is achieved when the beam size is measured at one Rayleigh length from the nominal focus.

[0015] [Figure 3]FIG. 3 illustrates a measurement subsystem that uses a single stationary image sensor to simultaneously measure beam size at both upstream and downstream locations, at any given time, and at the focal point, according to one embodiment.

[0016] [Figure 4] 4 is a flowchart of a method for correcting the wavefront of a laser beam, according to one embodiment, that utilizes beam size measurement at an intermediate field and adaptive optics.

[0017] [Figure 5] FIG. 5 illustrates a laser device incorporating a telescope.

[0018] [Figure 6] FIG. 6 illustrates a laser apparatus that uses intermediate field beam size measurements to stabilize the laser beam waist relative to the telescope's image plane, according to one embodiment.

[0019] [Figure 7] FIG. 7 illustrates an adaptive optics module configured to perform 2D wavefront correction in the system of FIG. 6, according to one embodiment.

[0020] [Figure 8] FIG. 8 illustrates a three-actuator adaptive cylindrical mirror according to an embodiment.

[0021] [Figure 9] FIG. 9 illustrates a method for stabilizing a laser beam waist relative to a telescope image plane using one or more adaptive optics systems and based on intermediate field beam size measurements, according to an embodiment.

[0022] [Figure 10]FIG. 10 illustrates a laser apparatus incorporating a wavefront correction system using intermediate and far-field beam size measurements and adaptive optics to stabilize the size and divergence properties of the laser beam, according to one embodiment.

[0023] [Figure 11] FIG. 11 illustrates an adaptive optics module configured to perform 2D wavefront correction within the wavefront correction system of the laser apparatus of FIG. 10, according to an embodiment.

[0024] [Figure 12] 12 illustrates a method for stabilizing the size and divergence properties of a laser beam using one or more adaptive optics systems, according to one embodiment. The method utilizes beam size measurements in the intermediate and far fields.

[0025] [Figure 13] FIG. 13 illustrates a four-actuator adaptive cylindrical mirror according to an embodiment.

[0026] [Figure 14] 14 illustrates another system for correcting the wavefront of a laser beam, according to an embodiment, which utilizes beam size measurements within the intermediate field of view of the diagnostic beam. DETAILED DESCRIPTION OF THE INVENTION

[0027] (Detailed Description of the Invention) Referring now to the drawings, in which like components are designated by like numbers, Figure 1 illustrates one system 100 for correcting a wavefront 182 of a laser beam 180. System 100 utilizes beam size measurements within an intermediate field of view of laser beam 180. Figure 1 shows system 100 in an example scenario in which system 100 is incorporated into a laser device 102.

[0028] The laser device 102 includes a laser source 170 that generates a laser beam 180. The laser source 170 may include one or more elements that cause a distortion of a wavefront 182. As used herein, "distortion" of a wavefront refers to an unwanted wavefront change, as opposed to an intended wavefront change imparted by, for example, a lens. For example, the laser source 170 may include a nonlinear crystal (NLC) 172 that generates the laser beam 180 through frequency conversion but also undesirably distorts the wavefront 182. In one implementation, the laser beam 180 is a high-power ultraviolet (UV) laser beam that causes the nonlinear crystal 172 to gradually undergo UV degradation. The UV degradation distorts the wavefront 182, and the system 100 corrects for this wavefront distortion to maintain stable beam parameters for the laser beam 180.

[0029] System 100 includes one or more adaptive optics (AO) systems 110, a beam splitter 120, and a diagnostic module 130. Beam splitter 120 splits off a small portion of laser beam 180 to be used as diagnostic beam 180D. One or more adaptive optics systems 110 are located upstream of beam splitter 120 and correct wavefront 182 according to measurements obtained from diagnostic module 130. In one embodiment, each adaptive optics system 110 is an adaptive mirror, e.g., a deformable mirror including a flexible mirror surface, a support substrate, and multiple actuators therebetween. Alternatively, system 100 may implement at least one adaptive optics system 110 as an adaptive lens. In some embodiments of system 100, an adaptive mirror may provide a simpler and less expensive solution than an adaptive lens.

[0030] The caustic of laser beam 180 passing through beam splitter 120 is characterized by a waist 188 and a Rayleigh extent. Waist 188 can be upstream or downstream from beam splitter 120, for example, at one of the locations indicated by arrows 188A, 188B, and 188C. Waist 188 can be real or virtual. Deformation of wavefront 182 can be associated with a deviation of the far-field divergence of laser beam 180 from its nominal far-field divergence. This deviation causes waist 188 of laser beam 180 to deviate away from its nominal waist location.

[0031] 2A and 2B illustrate the sensitivity of beam size to waist location shifts as a function of distance from the nominal waist location. 2A and 2B show that maximum sensitivity occurs when the beam size is one Rayleigh length z from the nominal waist. R This demonstrates that the performance is achieved when measured at

[0032] FIG. 2A plots the caustics of Gaussian beam 200 as a function of longitudinal coordinate z. FIG. 2A considers one horizontal axis of Gaussian beam 200, here the y-axis. Gaussian beam 200 has a waist at location z. FIG. 2A also plots the caustics of Gaussian beam 210. Gaussian beam 210 is identical to Gaussian beam 200 except that it is shifted in the positive z-axis direction by an amount Δz. At any given location z, Gaussian beam 200 has a radius w n (z), and the Gaussian beam 210 has a radius w s (z) w n (z) and w s The difference Δw(z) between (z) and (z) increases with distance away from the waist region of Gaussian beams 200 and 210. However, the beam radius w n (z) and w s (z) also increases with distance away from the waist region. In the far field, both beam radii increase linearly with distance. As a result, the relative difference Δw(z) / w(z) is a significant function of z.

[0033] FIG. 2B mathematically explores the relationship Δw(z) / w(z) and plots the normalized beam size change δw(z) as a function of z.

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[0034] 1 , it may be impractical to measure the size of laser beam 182 at a Rayleigh length away from waist 188. Even if waist 188 is in an accessible location, the Rayleigh length may be impractically long. For example, a well-collimated laser beam 180 may have a Rayleigh length of 10 meters or more. Diagnostic module 130 serves to image the Rayleigh range of laser beam 180 to a nearby accessible location.

[0035] Diagnostic module 130 includes focusing element 140, e.g., a lens, and measurement subsystem 150. Focusing element 140 focuses diagnostic beam 180D. Measurement subsystem 150 measures the size of diagnostic beam 180D at at least one of upstream location 152(1) and downstream location 152(2) relative to a nominal location 142 of the focal point of diagnostic beam 180D. Nominal focal location 142 is the location of the waist of diagnostic beam 180D as generated by focusing element 140 in the absence of deformation of wavefront 182. Upstream location 152(1) is selected to be the location at which focusing element 140 causes diagnostic beam 180D to form an image of a nominal intermediate field location of laser beam 180 downstream of waist 188. Downstream location 152(2) is selected to be the location where focusing element 140 causes diagnostic beam 180D to form an image of a nominal intermediate field location of laser beam 180 upstream of waist 188. These selections of upstream and downstream locations 152(1) and 152(2) optimize the sensitivity of measurement subsystem 150 to deviations in waist 188 of laser beam 180.

[0036] Beam size determination by measurement subsystem 150 may utilize various beam size measurements as known in the art. For example, the beam size may be determined by the 1 / e 2The second moment has the advantage of being well-defined, regardless of the profile of the power density distribution.

[0037] During operation of system 100, one or more adaptive optics systems 110 correct wavefront 182 based, at least in part, on the measured size of diagnostic beam 180D at locations 152(1,2) to achieve a corrected wavefront 182' of laser beam 180 downstream of adaptive optics systems 110. To this end, system 100 may include feedback controller 160 that actively adjusts adaptive optics systems 110 according to measurements made by measurement subsystem 150. Feedback controller 160 may be operated essentially continuously according to sustained or frequently repeated measurements by measurement subsystem 150.

[0038] 2A and 2B, because achieving maximum sensitivity at one Rayleigh length from the nominal focus relies on a small deviation in the location of waist 188 compared to the Rayleigh length, it may be advantageous to actively adjust adaptive optics 110 on a near-continuous basis to keep any deviation between the actual and nominal location of waist 188 small relative to the Rayleigh length. System 100 is highly suitable for maintaining the nominal location of waist 188 with a high degree of accuracy.

[0039] In embodiments in which measurement subsystem 150 measures beam size at only one of upstream location 152(1) and downstream location 152(2), the corrections performed by adaptive optics 110 serve to maintain an absolute target value for the measured beam size at the selected one of upstream location 152(1) and downstream location 152(2). Improved accuracy and robustness may be achieved in embodiments in which measurement subsystem 150 measures beam size at both upstream location 152(1) and downstream location 152(2). Here, the corrections performed by adaptive optics 110 may be configured to maintain an absolute target value for the measured beam size at each of upstream location 152(1) and downstream location 152(2). Alternatively, measurement subsystem 150 may monitor the ratio between the corresponding beam sizes at upstream location 152(1) and downstream location 152(2), and the corrections performed by adaptive optics 110 are configured to maintain a target value for that ratio.

[0040] In one class of embodiments, measurement subsystem 150 is configured to measure beam size only at one or both of locations 152(1, 2). In another class of embodiments, measurement subsystem 150 also measures beam size at focal plane 148 of focusing element 140, and one or more adaptive optics systems 110 may further base their correction of wavefront 182 on the beam size measured at focal plane 148 (such embodiments are discussed in more detail below with reference to FIGS. 10 and 12 ). Each of these classes of embodiments may be implemented in a one-dimensional (1D) or two-dimensional (2D) array. In a 1D array, measurement subsystem 150 obtains measurements in a single axis transverse to the propagation direction of diagnostic beam 180D, and one or more adaptive optics systems 110 correct wavefront 182 in the corresponding transverse axis relative to laser beam 180. In a 2D array, the measurement subsystem 150 acquires measurements in two axes transverse to the propagation direction of the diagnostic beam 180D, and one or more adaptive optics systems 110 correct the wavefront 182 in two transverse axes relative to the laser beam 180.

[0041] In some applications of system 100, wavefront deformation properties differ along two transverse axes of laser beam 180. For example, when nonlinear crystal 172 is birefringent, the beam parameters of laser beam 180 are likely to exhibit one behavior along an axis parallel to the walk-off direction and another behavior along an axis perpendicular to the walk-off direction. In such applications, system 100 may advantageously implement each adaptive optics 110 as a cylindrical mirror. Accordingly, the 1D array of system 100 discussed above may implement one, two, or more adaptive cylindrical mirrors. Similarly, the 2D array of system 100 discussed above may implement two mirror sets, each arranged to correct wavefront 182 along two separate, mutually orthogonal transverse axes. Each mirror set may consist of a single adaptive cylindrical mirror or several adaptive cylindrical mirrors.

[0042] Measurement subsystem 150 may use beam size measurement techniques known in the art, optionally in conjunction with higher order wavefront measurement techniques known in the art. In one embodiment, measurement subsystem 150 includes a conventional beam profiler, such as a scanning slit beam profiler, a charge-coupled device (CCD) camera beam profiler, or, for laser beam 180 in the infrared spectrum, a pyrometer-based beam profiler. This embodiment of measurement subsystem 150 may also include a conventional wavefront sensor, such as a Shack-Hartmann wavefront sensor.

[0043] Without departing from the scope of this specification, diagnostic module 130 may be provided as a stand-alone diagnostic module, implemented within a third-party laser system, or used for uncorrected monitoring of beam parameters, or to assist in wavefront correction using third-party adaptive optics or other methods of wavefront correction.

[0044] 3 illustrates one measurement subsystem 300 that uses a single stationary image sensor 320 to simultaneously measure the beam size at both the upstream location 152(1) and the downstream location 152(2) at any given time and at the focal point 148. Measurement subsystem 300 is one embodiment of measurement subsystem 150.

[0045] Measurement subsystem 300 includes two beam splitters 310(1) and 310(2) arranged in series along the propagation path of diagnostic beam 180D, spaced a distance 318 apart from each other. Beam splitters 310(1) and 310(2) split diagnostic beam 180D into separate subportions 382D(1) and 382D(2). Image sensor 320 captures both diagnostic beam subportions 382D(1) and 382D(2). However, the different path lengths of diagnostic beam subportions 382D(1) and 382D(2) between focusing element 140 and image sensor 320 cause image sensor 320 to capture diagnostic beam subportions 382D(1) and 382D(2) at different longitudinal distances from nominal focal location 142. Distance 318 and the propagation paths of diagnostic beam subportions 382D(1) and 382D(2) are configured such that image sensor 320 captures diagnostic beam subportions 382D(1) and 382D(2) at upstream location 152(1) and downstream location 152(2), respectively. As a result, each image 328 captured by image sensor 320 includes (a) spot 382S(1), which shows the lateral profile of diagnostic beam 180D at upstream location 152(1), and (b) spot 382S(2), which shows the lateral profile of diagnostic beam 180D at downstream location 152(2).

[0046] 3, measurement subsystem 300 includes two folding mirrors 330 and 340 to achieve a desired form factor. One or both of folding mirrors 330 and 340 may be omitted or arranged differently, and / or measurement subsystem 300 may include additional folding mirrors, without departing from the scope of this specification. The depicted embodiment of measurement subsystem 300 also includes beam block 350. Beam block 350 may advantageously be in the form of a diagnostic power meter.

[0047] 3 shows a Cartesian coordinate system 390 indicating the longitudinal axis (z-axis) and transverse axes (x-axis and y-axis) of laser beam 180. As diagnostic beam 180D splits from laser beam 180, and diagnostic beam subportions 382D(1) and 382D(2) split from diagnostic beam 180D, coordinate system 390 is passed over diagnostic beam subportions 382D(1) and 382D(2). The orientation of coordinate system 390 changes as the associated beam changes direction. Ultimately, at image sensor 320, coordinate system 390 may have a different orientation than laser beam 180 upstream from beam splitter 120. Nevertheless, image 328 shows spot 382S(1,2) in the transverse x-y plane of coordinate system 390 relative to laser beam 180. For each spot 328(1,2), lateral dimension 388X corresponds to the x-axis relative to laser beam 180, and lateral dimension 388Y corresponds to the y-axis relative to laser beam 180.

[0048] An embodiment of system 100 implementing measurement subsystem 300 may therefore operate adaptive optics 110 to (a) correct wavefront 182 in the x-axis according to lateral dimension 388X of spots 382(1) and 382(2), and (b) correct wavefront 182 in the y-axis according to lateral dimension 388Y of spots 382(1) and 382(2).

[0049] In one embodiment, measurement subsystem 300 includes an additional beam splitter 312 located between beam splitters 310(1) and 310(2). Beam splitter 312 splits off a subportion 384D of diagnostic beam 180D. Diagnostic beam subportion 384D propagates to image sensor 320 with a path length that causes image sensor 320 to capture diagnostic beam subportion 384D at focal plane 148 of focusing element 140 (see FIG. 1 ). A corresponding spot 384S in image 328 thus enables beam size measurement at focal plane 148. Spot 384S may also provide other information, such as higher-order transverse mode properties of diagnostic beam 180D, and therefore laser beam 180. As used herein, “higher-order transverse mode properties” refers to properties higher than beam size or divergence. In an alternative embodiment not depicted in FIG. 3, a small portion of diagnostic beam 180D is directed to a conventional wavefront sensor such as a Shack-Hartmann sensor for evaluation of higher order transverse mode properties.

[0050] Although FIG. 3 depicts the diagnostic beam subportions as propagating alongside one another, different diagnostic beam subportions may instead take different, separate paths, e.g., non-parallel paths, to image sensor 320, as long as the path lengths are such that image sensor 320 intercepts diagnostic beam subportions 382D(1) and 382D(2) at locations 152(1) and 152(2), respectively (and, in embodiments including beam splitter 312, image sensor 320 intercepts diagnostic beam subportion 384D at focal plane 148).

[0051] Without departing from the scope of this specification, the measurement subsystem 300 may be modified or operated to measure the beam size at the focal plane 148 and only one of the upstream location 152(1) and the downstream location 152(2).

[0052] 4 is a flowchart of one method 400 for correcting the wavefront of a laser beam. Method 400 may be performed by system 100. Method 400 includes steps 410, 420, 430, and 440.

[0053] Step 410 splits off a small portion of the laser beam as a diagnostic beam. In one embodiment of step 410, beam splitter 120 splits off diagnostic beam 180D from laser beam 180, as discussed above with reference to FIG.

[0054] Step 420 focuses the diagnostic beam. The focal point is at a nominal focal location when the laser beam wavefront is in a nominal state. If the laser beam wavefront is deformed, the focal location may be displaced from the nominal focal location. In one implementation of step 420, focusing element 140 focuses diagnostic beam 180D as discussed above with reference to FIG. 1.

[0055] Step 430 measures the size of the diagnostic beam at at least one of an upstream location and a downstream location relative to the nominal focus location. The upstream location corresponds to a downstream intermediate field location of the laser beam as imaged through the diagnostic beam by step 420, and the downstream location corresponds to an upstream intermediate field location of the laser beam as imaged through the diagnostic beam by step 420. In one implementation of step 430, measurement subsystem 150 measures the size of diagnostic beam 180D at one or both of locations 152(1) and 152(2), as discussed above with reference to FIG. 1 . In one embodiment, step 430 includes step 432, which measures the size using a single image sensor. In one implementation of this embodiment, step 432 utilizes measurement subsystem 300 to measure the size using image sensor 320.

[0056] Step 440 corrects the laser beam wavefront using at least one adaptive optics system located upstream of where the diagnostic beam was split in step 410. The corrections performed by the adaptive optics system are based, at least in part, on the size measured in step 430. In one implementation of step 440, one or more adaptive optics systems 110 correct wavefront 182 of laser beam 180 based, at least in part, on the beam size measured by measurement subsystem 150, as discussed above with reference to FIG. 1 . Step 440 may utilize a feedback controller, such as feedback controller 160, to control the adaptive optics system according to the measurements made in step 430.

[0057] In one embodiment, method 400 is used to correct wavefront deformations caused by a nonlinear crystal. In this embodiment, step 440 includes step 442 of correcting the wavefront for such deformations. In one implementation of step 442, one or more adaptive optics systems 110 correct wavefront 182 of laser beam 180 for deformations caused by nonlinear crystal 172. This embodiment of method 400 can stabilize the wavefront of a laser beam in the presence of changing wavefront deformation behavior in the nonlinear crystal over time, for example, as shown diagrammatically by wavefront 182′ in FIG. 1 . In one scenario, wavefront correction by adaptive optics is sufficient to meet a set of performance requirements. In another scenario, degradation of the nonlinear crystal is severe enough that wavefront correction by adaptive optics is sufficient for only a limited period of time, after which the position of the nonlinear crystal is shifted so that the laser beam passes through an undegraded portion of the nonlinear crystal. When the nonlinear crystal is shifted, the wavefront is likely to undergo discrete changes. In one embodiment, step 440 is further configured to maintain or re-establish the wavefront properties of the laser beam when the nonlinear crystal is shifted.

[0058] In one embodiment, steps 430 and 440 include separate steps 434 and 444. For each of the upstream and downstream locations, step 434 measures a first lateral dimension of the diagnostic beam in a first lateral axis relative to the laser beam. Step 444 then uses a first adaptive optics system or a first set of adaptive optics systems to correct the wavefront in the first lateral axis. In one example of such an embodiment, measurement subsystem 300 measures lateral dimension 388X of each of spots 382S(1) and 382S(2), and one or more adaptive optics systems 110 (e.g., one or more adaptive cylindrical mirrors) then correct the wavefront of laser beam 180 in the x-axis. Without departing from the scope of this specification, this example may utilize another type of measurement subsystem to measure lateral dimension 388X, for example, based on a Shack-Hartmann wavefront sensor.

[0059] In one embodiment, method 400 is configured for 1D wavefront correction. In this embodiment, steps 430 and 440 operate on only the first axis. In another embodiment, method 400 is configured for 2D wavefront correction using separate corrections in each of two mutually orthogonal horizontal axes. In this embodiment, step 430 includes steps 434 and 436, and step 440 includes steps 444 and 446. Steps 436 and 446 are similar to steps 434 and 444, except that they apply to orthogonal axes. In one example, steps 434 and 444 operate on the x-axis of coordinate system 390, while steps 436 and 446 operate on the y-axis.

[0060] FIG. 5 illustrates one laser apparatus 502 incorporating a telescope. Laser apparatus 502 includes a laser source 570 and a telescope 540. Laser source 570 is an embodiment of laser source 170 that generates laser beam 580 such that laser beam 580 has a waist w within laser source 570. In one embodiment, laser source 570 includes nonlinear crystal 172, where waist w is inside nonlinear crystal 172. Telescope 540 images waist w onto image plane 550. Telescope 540 may be comprised of two lenses 542 and 544. Laser source 570 includes a wavefront modification source. For example, a gradual change in the refractive index of nonlinear crystal 172 can gradually modify the wavefront of laser beam 580. Laser source 570 is configured such that wavefront deformation does not significantly change the size of laser beam 580 at the nominal location of waist w, but may shift the location and size of second waist w, as shown by waist w of wavefront-deformed laser beam 580. Even in the presence of such wavefront deformation, telescope 550 ensures that the size of laser beam 580 remains the same at image plane 550.

[0061] Figure 6 illustrates one laser apparatus 602 that uses intermediate-field beam size measurements to stabilize the laser beam waist relative to the telescope's image plane. Laser apparatus 602 is an extension of laser apparatus 502 that incorporates a system 600 for wavefront correction that stabilizes the waist location of laser beam 580 relative to image plane 550. System 600 is an embodiment of system 100. Figure 6 shows laser apparatus 602 with laser source 570 generating wavefront-deformed laser beam 580'.

[0062] In laser device 602, beam splitter 120 is downstream of image plane 550, and one or more adaptive optics 110 are located within image plane 550 or at least within a range 618 thereof. Range 618 is within 0.1z of image plane 550. R or 0.2z R 0.1z from upstream of image plane 550 R or 0.2z RIt may be straddled downstream, z R is the Rayleigh length of laser beam 580 relative to waist w1. These spans of range 618 allow adaptive optics 110 to collimate laser beam 580 while also maintaining the size of waist w1 close to its nominal value. One or more adaptive optics 110 correct the wavefront of laser beam 580 to ensure that the nominal divergence of laser beam 580 is maintained downstream of image plane 550, even when laser beam 580 is deformed (e.g., as shown by wavefront-deformed laser beam 580' in FIG. 6 propagating to image plane 550), thereby stabilizing waist w1 relative to image plane 550.

[0063] Stabilization of waist w1 relative to image plane 550 may be based solely on intermediate-field measurements of beam size. Accordingly, system 600 is an embodiment of system 100 that adjusts adaptive optics 110 based solely on intermediate-field measurements of beam size. System 600 includes diagnostic module 630, an embodiment of diagnostic module 130. Diagnostic module 630 may be configured to measure beam size only at upstream location 152(1) and / or downstream location 152(2). System 600 may stabilize waist w1 relative to image plane 550 by stabilizing one or more beam sizes measured at upstream location 152(1) and / or one or more corresponding beam sizes measured at downstream location 152(2) to their nominal values ​​that correspond to waist w1 at image plane 550. In one implementation, system 600 includes feedback controller 160, which actively adjusts adaptive optics 110 to stabilize the lateral beam dimensions measured at locations 152(1) and 152(2) relative to their nominal values.

[0064] In one implementation, the beam size is monitored in terms of absolute values, as discussed above with reference to FIG. 1. In another implementation, the beam size value is monitored in terms of an upstream-to-downstream beam size ratio. In this implementation, the measurement subsystem 150 of the diagnostic module 630 monitors the ratio between (a) one or more beam sizes at the upstream location 152(1) and (b) the corresponding beam size at the downstream location 152(2) and maintains each such ratio at a nominal value. If the location of the waist w1 deviates from the image plane 550, the location of the focal point of the diagnostic beam 180D deviates away from the nominal focal point location 142, and the ratio between the corresponding beam sizes at the upstream location 152(1) and the downstream location 152(2) changes away from its nominal value. If the beam size of the laser beam 580 is within ±z from the waist w1, R , but the beam size of diagnostic beam 180D at corresponding upstream and downstream locations 152(1) and 152(2) is the same as ±z R The beam sizes at the upstream and downstream locations 152(1) and 152(2) are typically not the same because the locations are at different path lengths from the focusing element 140. Therefore, the nominal ratio between the corresponding beam sizes at the upstream and downstream locations 152(1) and 152(2) is typically not one.

[0065] System 600 can be configured for 1D or 2D wavefront correction. When configured for 1D wavefront correction, adaptive optics 110, and optionally measurement subsystem 150 of diagnostic module 630, can operate on only a single horizontal axis (e.g., the x-axis). When system 600 is configured for 2D wavefront correction, measurement module 150 and adaptive optics 110 operate on two horizontal axes. For simplicity of operation, these two horizontal axes are preferably mutually orthogonal (e.g., the x- and y-axes). System 600, including laser source 570 and telescope 540, may be provided as a stand-alone wavefront correction system configured for integration into a third-party laser device.

[0066] FIG. 7 illustrates one adaptive optics module 700 configured to perform 2D wavefront correction within system 600. Adaptive optics module 700 is an embodiment of adaptive optics system 110. Adaptive optics module 700 includes serially arranged adaptive cylindrical mirrors 710X and 710Y. Mirror 710X has a mirror surface 712X that is adjustable to change the wavefront of laser beam 580 in the x-axis. For example, mirror surface 712X can be adjusted from an initial planar shape to a cylindrical shape indicated by dashed line 712X′ to impart focusing onto laser beam 580 in the x-axis. Similarly, mirror 710Y has a mirror surface 712Y that can be adjusted to change the wavefront of laser beam 580 in the y-axis (this adjustability is not in the plane of FIG. 7 and is therefore not depicted).

[0067] In one embodiment, the actions of mirrors 710X and 710Y are decoupled from one another such that mirror 710X has no, or at least a negligible, effect on the y-axis of the wavefront, and mirror 710Y has no, or at least a negligible, effect on the x-axis of the wavefront. In this embodiment, for optimal decoupling between the x-axis and y-axis effects, mirrors 710X and 710Y may be advantageously arranged to fold the path of laser beam 580 in either the xy-plane or the yz-plane (as depicted in FIG. 7). Mirrors 710X and 710Y may be the same mirror, with the orientation of mirror 710X about the z-axis preferably differing from that of mirror 710Y by 90 degrees to decouple that of mirror 710X from that of mirror 710Y.

[0068] Mirrors 710X and 710Y may each be deformable mirrors with a flexible mirror surface, a supporting substrate, and multiple actuators between them. Because mirrors 710X and 710Y each require adjustment in only one axis, a 1D array of actuators is sufficient. More complex commercially available adaptive mirrors with 2D actuator arrays are not required within adaptive optics module 700.

[0069] Adaptive optics module 700 may be modified to perform 1D wavefront correction within laser device 602. In this modification, adaptive optics module 700 omits one of mirrors 710X and 710Y. The omitted mirror may be replaced by a rigid mirror, for example, a planar mirror.

[0070] FIG. 8 illustrates a three-actuator adaptive cylindrical mirror 800. The mirror 800 includes (a) a mirror substrate 810 with a mirror surface 812, (b) a support substrate 830, and (c) three actuators 820 connecting the mirror substrate 810 to the support substrate 830. The lengths of at least some of the actuators 820 are adjustable in direction 890. Direction 890 is generally orthogonal to the mirror surface 812. The actuators 820 are distributed along direction 892, which is orthogonal to the orthogonal direction 890. In one embodiment, all of the actuators 820 have adjustable lengths. In another embodiment, only actuators 820(1) and 820(3) have adjustable lengths. In yet another embodiment, only actuator 820(2) has an adjustable length. Each of these embodiments is capable of changing the radius of curvature of the mirror surface 812. FIG. 8 depicts an example in which actuators 820(1) and 820(3) are expanded relative to actuator 820(2), changing mirror surface 812 from an initial planar shape to a lesser concave shape 812′ or a more strongly concave shape 812″. The radius of curvature of concave shape 812″ is less than the radius of curvature of concave shape 812′, and the radius of curvature of the initial planar shape is infinite. Mirror surface 812 can also achieve a convex shape.

[0071] Mirrors 710X and 710Y in adaptive optics module 700 may each be implemented as individual mirrors 800, with one of mirrors 710X and 710Y oriented in a direction 892 parallel to the x-axis and the other of mirrors 710X and 710Y oriented in a direction 892 parallel to the y-axis. Because mirror 800 has only three actuators, mirror 800 can be produced at a much lower cost than currently commercially available deformable mirrors, which are equipped with a much larger number of actuators. Furthermore, we have found that the three-actuator design of mirror 800 is sufficient to maintain the waist of laser beam 580 at image plane 550 in laser device 602, at least under most circumstances.

[0072] Additional details of suitable adaptive mirrors can be found in co-pending U.S. patent application Ser. No. 17 / 101,783, filed November 23, 2020, the disclosure of which is incorporated by reference.

[0073] 9 illustrates one method 900 for stabilizing a laser beam waist relative to an image plane of a telescope using one or more adaptive optics systems. Method 900 may be performed by telescope 540 and system 600 to stabilize waist w1 of laser beam 580 relative to image plane 550 in laser device 602 in the presence of wavefront deformations in laser source 570 caused, for example, by nonlinear crystal 172. Method 900 is an embodiment of method 400 that implements steps 930 and 940 in place of steps 430 and 440, respectively, and further includes step 902.

[0074] Step 902 forms an image of a first waist of the laser beam at an image plane located in the adaptive optics system. In one implementation of step 902, telescope 540 forms an image of waist w at image plane 550, as discussed above with reference to FIG.

[0075] Step 930 is an embodiment of step 430 that includes step 434. Step 930 may also include step 436 and, although not shown in FIG.

[0076] Step 940 is an embodiment of step 440, correcting the laser beam wavefront using at least one adaptive optics system to stabilize the second waist of the laser beam relative to the image plane of step 902. In one implementation of step 940, feedback controller 160 adjusts one or more adaptive optics systems 110 to stabilize waist w1 of laser beam 580 relative to image plane 550 in laser device 602, as discussed above with reference to FIG. 6 . Step 940 includes step 944, which is an embodiment of step 444, adjusting first adaptive optics systems to maintain nominal values ​​of a first lateral dimension at upstream and downstream locations in a first lateral axis relative to the laser beam to stabilize the second waist relative to the image plane. Step 944 relies on measurements made in step 434. In one example of step 944, feedback controller 160 adjusts at least one first adaptive optics system, e.g., mirror 710X, to maintain a nominal value of lateral dimension 388X of spots 382S(1) and 382S(2) (see FIG. 3 ), thereby stabilizing waist w1 of laser beam 580 relative to image plane 550 in the x-axis. The adjustment in step 944 may be based on the absolute value of the lateral dimension or the ratio between the lateral dimensions measured at upstream and downstream locations, as discussed above with reference to FIG. 6 . Step 940 may further include step 946. Step 946 is similar to step 944, except that step 946 preferably operates on a second lateral axis, e.g., orthogonal to the first lateral axis, e.g., the y-axis. Although not shown in FIG. 9 , step 940 may further include step 442.

[0077] In a 1D wavefront correction implementation of method 900, step 940 may omit step 946, and step 930 may omit step 436. In a 2D wavefront correction implementation of method 900, steps 930 and 940 include steps 436 and 946, respectively.

[0078] FIG. 10 illustrates one laser apparatus 1002 that uses intermediate-field and far-field beam size measurements and adaptive optics to stabilize the size and divergence properties of a laser beam. The laser apparatus 1002 includes a laser source 570, a beam shaping module 1040, and a wavefront correction system 1000. The beam shaping module 1040 is located between the laser source 570 and the wavefront correction system 1000. The beam shaping module 1040 shapes a laser beam 1080 generated by the laser source 570. The beam shaping module 1040 may include one or more lenses. When the laser beam 1080 has nominal properties, the beam shaping module 1040 achieves a predefined set of size and divergence properties for the laser beam 1080. The wavefront correction system 1000 ensures that the predefined size and divergence properties are achieved even in the presence of wavefront deformations of the laser beam 1080 within the laser source 570. 10 depicts an example in which laser source 570 generates a wavefront-deformed laser beam 1080′ that does not have nominal wavefront properties at beam shaping module 1040 and therefore would fail to meet predefined size and divergence properties after beam shaping by beam shaping module 1040 unless corrected by system 1000. For example, as depicted in FIG. 10 , wavefront-deformed laser beam 1080′ may be larger and more divergent than nominal laser beam 1080 at downstream location 1084.

[0079] Wavefront correction system 1000 is an embodiment of system 100 that measures beam size at locations 152(1) and 152(2) and focal plane 148. Wavefront correction system 1000 includes beam splitter 120, diagnostic module 1030, and multiple adaptive optics systems 110. Wavefront correction system 1000 may further include feedback controller 160. Diagnostic module 1030 is an embodiment of diagnostic module 130 that is specifically configured to measure beam size not only at locations 152(1) and / or 152(2), but also at focal plane 148. 1 / e at focal plane 148 2 Beam radius w f Is the relationship w f = θ0 / f through 1 / e 2 is related to the far field divergence angle θ0 of 1 / e where f is the focal length of the focusing element 140. 2 The far-field divergence angle θ0 of is 1 / e through the equation θ0=λ / (πw0) 2 where λ is the wavelength of the laser beam 1080. Thus, with the additional availability of beam size measurements at focal plane 148, system 1000 can stabilize not only the waist location but also the waist size. As a result, system 1000 can maintain a general set of predefined size and divergence properties of laser beam 1080. In the presence of wavefront deformations within laser source 570, system 1000 adjusts adaptive optics 110 to provide compensatory beam shaping based on the measured intermediate-field and far-field beam sizes as imaged by focusing element 140. This compensatory beam shaping, in conjunction with beam shaping module 1040, achieves the predefined size and divergence properties of laser beam 1080 downstream of adaptive optics 110, even in the presence of wavefront deformations caused by beam shaping module 1040.

[0080] In contrast to the telescope 540 of the laser apparatus 502, the beam shaping module 1040 does not need to form an image of the laser beam waist, and the adaptive optics module 110 does not need to be located at the corresponding image plane. Compared to the laser apparatus 502, the laser apparatus 1002 benefits from the additional availability of a far-field beam size measurement, as imaged by the focusing element 140, to maintain predefined size and divergence properties while accommodating more general beam shaping functionality. On the other hand, the laser apparatus 1002 requires more adaptive optics modules 110 than the laser apparatus 502, since the system 1000 must be able to adjust both the size and divergence angle of the laser beam.

[0081] In a scenario where the nominal waist radius w of the laser beam 1080 (as probed by the diagnostic module 1000) is known, the beam quality factor M of the laser beam 580 is 2 is the relation M 2 In one embodiment, adaptive optics 110 can be derived from M 2 The control circuit is configured to perform a correction that minimizes or maintains a desired value of .

[0082] The system 1000 may be provided as a stand-alone wavefront correction system configured for incorporation into a third-party laser device, including the laser source 570 and the beam shaping module 1040.

[0083] 11 illustrates one adaptive optics module 1100 configured to perform 2D wavefront correction within system 1000. Adaptive optics module 1100 is an embodiment of adaptive optics 110. Adaptive optics module 1100 includes two adaptive cylindrical mirrors 710X and two adaptive cylindrical mirrors 710Y arranged in series. The order of the series may differ from that shown in FIG.

[0084] In one embodiment, the action of mirror 710X(1,2) is decoupled from that of mirror 710Y(1,2), as discussed with respect to mirrors 710X and 710Y in adaptive optics module 700. Mirrors 710X(1), 710X(2), 710Y(1), and 710Y(2) may be the same, with the alignment of mirror 710X(1,2) about the z-axis preferably differing from the orientation of mirror 710Y(1,2) by 90 degrees to decouple that of mirror 710X(1,2) from that of mirror 710Y(1,2). Corresponding pairs of mirrors 710X(1,2) or 710Y(1,2) in each of the x- and y-axes provide degrees of freedom to independently vary both beam size and divergence to achieve predefined size and divergence properties of laser beam 1080.

[0085] Adaptive optics module 1100 may be modified to perform 1D wavefront correction within laser apparatus 1002. In this modification, adaptive optics module 1100 omits either mirror 710X(1,2) or mirror 710Y(1,2), and the remaining mirror 710X(1,2) or 710Y(1,2) may be rearranged accordingly to simplify the path of laser beam 1080 through adaptive optics module 1100.

[0086] Referring again to system 1000 and laser device 1002, in one scenario, beam shaping module 1040 may be omitted, with its functionality instead being performed by system 1000. In this scenario, adaptive optics 110 of system 1000 is adjustable for a "zero point" configuration that achieves predefined size and divergence properties under nominal conditions. However, there are potential advantages to having both beam shaping module 1040 and adaptive optics 110 of system 1000. For example, if adaptive optics 110 is an adaptive cylindrical mirror, elimination of beam shaping module 1040 may require operating at least some of the cylindrical mirrors with a relatively high curvature. With adaptive mirrors, it may be difficult to reach such a high curvature without introducing higher-order aberrations. Therefore, higher beam quality is likely to be achievable when utilizing the adaptive optics 110 of system 1000 due to the fact that beam shaping module 1040 maintains zero-point beam shaping and only fewer corrections are required to stabilize size and divergence properties in the presence of wavefront deformation.

[0087] 12 illustrates one method 1200 for stabilizing the size and divergence of a laser beam using one or more adaptive optics systems. Method 1200 may be performed by wavefront correction system 1000 to maintain a predefined size and divergence of laser beam 1080 in the presence of wavefront deformations in laser source 570 caused, for example, by nonlinear crystal 172. Method 900 is an embodiment of method 400 that implements steps 1230 and 1240 instead of steps 430 and 440, respectively.

[0088] Step 1230 is an extension of step 930, for example, further obtaining one or more beam size measurements at the focal plane of a focusing element used to perform step 420 at focal plane 148. Step 1230 includes step 1234 and, optionally, also step 1236. Steps 1234 and 1236 are extensions of steps 934 and 936, respectively, which further measure individual lateral dimensions at the focal plane. Step 1230 may also implement step 432.

[0089] Step 1240 corrects the laser beam wavefront using multiple adaptive optics systems located upstream of the split between the laser beam and the diagnostic beam to stabilize the size and divergence of the laser beam to a predefined set of sizes and divergences. Step 1240 includes step 1244 and, optionally, step 1246. Step 1244 is an embodiment of step 444, adjusting a first pair of adaptive optics systems to correct the laser beam wavefront in a first horizontal axis. Step 1244 may achieve the required wavefront correction by adjusting the first pair of adaptive optics systems in a manner to maintain nominal values ​​of the first horizontal dimension at the upstream location, the first horizontal dimension at the downstream location, and the first horizontal dimension at the focal plane. Step 1244 relies on measurements made in step 1234. In one implementation of step 1244, feedback controller 160 adjusts a first pair of adaptive optics, e.g., mirrors 710X(1) and 710X(2), to maintain a nominal value for lateral dimension 388X of spots 382S(1) and 382S(2) and also to maintain a nominal value for lateral dimension 388X of spot 384S (see FIG. 3 ). Feedback controller 160 thereby maintains a predefined size and divergence characteristic in the x-axis of laser beam 1080. Step 1246 is similar to step 1244, except that it preferably operates on a second transverse axis, e.g., orthogonal to the first transverse axis, e.g., the y-axis. Although not shown in FIG. 12 , step 1240 may further include step 442.

[0090] Method 1200 may further include step 1202 of shaping the laser beam to achieve a predefined size and divergence in the absence of wavefront deformation. In this embodiment, step 1240 provides additional beam shaping that complements the primary beam shaping in step 1202 to maintain the predefined size and divergence in the presence of wavefront deformation. Step 1202 may be performed by beam shaping module 1040.

[0091] In a 1D wavefront correction implementation of method 1200, step 1240 may omit step 1246, and step 1230 may omit step 1236. In a 2D wavefront correction implementation of method 1200, steps 1230 and 1240 include steps 1236 and 1246, respectively.

[0092] Compared to method 900, method 1200 is more generally applicable because method 1200 does not require step 902 of forming a waist image. For example, method 1200 does not require the laser device to be configured to form a waist image at image plane 550 as in laser device 600. However, method 1200 also requires more complex wavefront correction, including measuring beam size at additional locations and using at least one additional adaptive optics system. Whereas the wavefront correction in step 940 of method 900 can be performed using only a single adaptive optics system per horizontal axis, the wavefront correction in step 1240 of method 1200 requires two adaptive optics systems per horizontal axis.

[0093] Method 900, method 1200, system 600, and system 1000, as discussed above, each primarily serve to provide a laser beam with stable size and divergence properties in the presence of wavefront deformations. Wavefront deformations cause changes in the size and / or divergence properties of the laser beam, and it is not uncommon for these to also introduce higher-order aberrations, such as deviations of the power density distribution from either the TEM00 mode or another Hermite-Gaussian mode. For example, a nonlinear crystal, such as nonlinear crystal 172, can cause wavefront deformations that not only affect the size and divergence properties of the laser beam, but also introduce higher-order aberrations. Method 900, method 1200, system 600, and system 1000, each, serve to provide a laser beam with stable size and divergence properties in the presence of wavefront deformations, such as, for example, a desired beam quality factor M 2 1200。 Systems 100, 600, and 1000 and methods 400, 900, and 1200 may also be adapted to further correct at least some forms of higher order aberrations of the laser beam to achieve M. Such correction is not limited to the schemes of systems 600 and 1000 and methods 900 and 1200, but may be incorporated more generally into system 100 and method 400 by including a beam size measurement at the focal plane of the focusing element (e.g., focal plane 148). Systems 100, 600, and 1000 and methods 400, 900, and 1200 may also correct for higher order aberrations and / or M without implementing adaptive optics-based corrections thereof. 2 may be configured to monitor

[0094] In the context of method 900, correction for higher-order aberrations may be achieved by (a) additionally monitoring the higher-order transverse mode nature of the power density distribution of the diagnostic beam at the focal plane to detect the higher-order aberrations in step 930, and (b) further adjusting each adaptive optics system according to the higher-order transverse mode nature in step 940. This adaptation of method 900 may therefore adjust the adaptive optics system in a manner that reduces or eliminates the higher-order aberrations. Method 1200 may be adapted in a similar manner. In the context of system 600, diagnostic module 630 may further monitor the higher-order transverse mode nature of the power density distribution of the diagnostic beam at the focal plane (e.g., using measurement subsystem 300 and image sensor 320), and adaptive optics 110 may further be adjusted according to the higher-order transverse mode nature. This adaptation of system 600 may adjust adaptive optics 110 in a manner that reduces or eliminates the higher-order aberrations detected by diagnostic module 630 to reduce or eliminate corresponding higher-order aberrations in laser beam 580. System 1000 may be adapted in a similar manner.

[0095] FIG. 13 illustrates a four-actuator adaptive cylindrical mirror 1300. Mirror 1300 is an embodiment of either mirror 710X or 710Y. Mirror 1300 is an extension of mirror 800, including four actuators 820. All four actuators 820 have adjustable lengths. Mirror 1300 has the functionality of mirror 800. In addition, with one additional actuator 820, mirror 1300 can impart higher-order wavefront correction, for example, to reduce or eliminate higher-order aberrations in a laser beam. Mirror 1300 is therefore suitable for use in any one of methods 900, 1200, system 600, and system 1000, which are adapted to further correct higher-order aberrations.

[0096] 14 illustrates another system 1400 for correcting the wavefront 182 of a laser beam 180. System 1400 is similar to system 1000, except that it replaces diagnostic module 130 with a similar diagnostic module 1430 that measures the beam size within the intermediate field of view of diagnostic beam 180D instead of within the intermediate field of view of laser beam 180. Diagnostic module 1430 is similar to diagnostic module 130, except that the upstream and downstream measurement locations are within the intermediate field of view of diagnostic beam 180D relative to the nominal focal location 142 of the waist of diagnostic beam 180D generated by focusing element 140. Depending on the location of waist 188 of laser beam 180 relative to focusing element 140, the upstream and downstream measurement locations of diagnostic module 1430 may be significantly different, similar, or even identical to those of diagnostic module 130.

[0097] Diagnostic module 1430 implements measurement subsystem 1450. Measurement subsystem 1450 is a modification of measurement subsystem 150 that measures the size of diagnostic beam 180D at each of upstream intermediate-field location 1452(1) and downstream intermediate-field location 1452(2) relative to nominal focal location 142. Upstream intermediate-field location 1452(1) is a distance 1444(1) upstream of nominal focal location 142. Downstream intermediate-field location 1452(2) is a distance 1444(2) downstream of nominal focal location 142. Distances 1444(1) and 1444(2) are each the Rayleigh length z of diagnostic beam 180D. RD For example, the distances 1444(1) and 1444(2) are approximately equal to the Rayleigh length z RD Distances 1444(1) and 1444(2) may be the same.

[0098] By measurements performed at upstream and downstream intermediate-field locations 1452(1) and 1452(2) of diagnostic beam 180D, measurement subsystem 1450 optimizes the sensitivity of its beam size measurements to shifts in the waist location of diagnostic beam 180D away from nominal focal location 142. The reasons for this optimal sensitivity are the same as the reasons for the optimal sensitivity of diagnostic module 130 to shifts in waist 188 of laser beam 180 away from its nominal location within laser beam 180. A more detailed discussion is found above with reference to FIGS. 2A and 2B.

[0099] System 1400 is an alternative to system 100 that is optimized to stabilize the position of the waist of diagnostic beam 180D relative to nominal focal location 142, regardless of where diagnostic module 1430 is located relative to waist 188 of laser beam 180. However, the sensitivity of diagnostic module 1430 to shifts in the location of waist 188 depends on where diagnostic module 1430 is located relative to waist 188. Optimal sensitivity of diagnostic module 1430 to shifts in the location of waist 188 is achieved when focusing element 140 is close to waist 188, in the present situation because upstream intermediate-field location 1452(1) and downstream intermediate-field location 1452(2) are relatively close to upstream location 152(1) and downstream location 152(2), respectively.

[0100] Certain embodiments of diagnostic module 1430 are also configured to perform beam size measurements at focal plane 148 of focusing element 140 and / or measure other properties of diagnostic beam 180D at focal plane 148. Such embodiments of diagnostic module 1430 provide additional capabilities similar to those of embodiments of diagnostic module 130 configured to perform measurements at focal plane 148, for example, as discussed above with reference to FIG.

[0101] Without departing from the scope of this specification, method 400 may be modified to utilize beam size measurements at intermediate fields within the diagnostic beam, for example, by using diagnostic module 1450 in step 430.

[0102] The present invention has been described above in terms of preferred and alternative embodiments. However, the present invention is not limited to the embodiments described and depicted herein. Rather, the present invention is limited only by the claims appended hereto.

Claims

1. A system for correcting a wavefront of a laser beam from a laser source, the laser beam having a waist at a nominal waist location along a length of the laser beam, the system comprising: a primary beam splitter for splitting off a portion of the laser beam to be used as a diagnostic beam; a focusing element for focusing the diagnostic beam, the focusing element having a focal plane; a measurement subsystem configured to measure a size of the diagnostic beam at at least one of upstream and downstream locations relative to a nominal focal location of the focal point of the diagnostic beam, the nominal focal location being a waist location of the diagnostic beam in the absence of deformation of the wavefront, each of the upstream and downstream locations of the diagnostic beam corresponding to an image of a distinct intermediate field location of the laser beam, each of the intermediate field locations being displaced from the nominal waist location by 80% to 120% of a nominal Rayleigh length of the laser beam with respect to the nominal waist location; at least one adaptive optics system located in the laser beam upstream from the primary beam splitter for correcting the wavefront of the laser beam based at least in part on the measured size of the diagnostic beam relative to the at least one of the upstream and downstream locations; A system comprising:

2. the at least one adaptive optics system includes a first adaptive mirror configured to correct the wavefront relative to the laser beam in a first transverse axis; 2. The system of claim 1, wherein the corresponding size measured by the measurement subsystem at each of the at least one of the upstream and downstream locations includes a first lateral dimension of the diagnostic beam in the first lateral axis.

3. The first adaptive mirror comprises: a mirror surface; only three actuators distributed along the first transverse axis and adjusting the mirror surface in a dimension parallel to a plane defined by the first transverse axis and the direction of propagation of the laser beam; The system of claim 2 , comprising:

4. The measurement subsystem includes: an image sensor; first and second secondary beam splitters for directing respective first and second portions of the diagnostic beam to different respective first and second locations on the image sensor, the first and second secondary beam splitters being arranged in series such that the image sensor captures the first and second portions of the diagnostic beam at the upstream and downstream locations, respectively; 4. The system of claim 2 or claim 3, comprising:

5. the at least one adaptive optics system further includes a second adaptive mirror downstream from the first adaptive mirror and configured to correct the wavefront relative to the laser beam in a second transverse axis, the second transverse axis being orthogonal to the first transverse axis; 5. The system of claim 2, wherein the corresponding size measured by the measurement subsystem at each of the at least one of the upstream and downstream locations further comprises a second lateral dimension of the diagnostic beam in the second lateral axis.

6. 1. A laser device with wavefront correction, comprising: A system according to claim 5; a laser source for generating the laser beam such that the nominal waist location is within the laser source; and a telescope for forming an image of the nominal waist location at an image plane, the telescope being located upstream of the at least one adaptive optics system; and Equipped with The at least one adaptive optics system is located at the image plane.

7. 7. The laser device of claim 6, wherein the laser source includes a nonlinear crystal, and the waist location is within the nonlinear crystal.

8. 8. The laser apparatus of claim 6 or claim 7, further comprising a feedback controller configured to actively adjust the at least one adaptive optics system to stabilize a location of the second waist of the laser beam at the image plane so as to maintain a nominal value of the size measured for the at least one of the upstream and downstream locations.

9. The system of any of claims 1 to 5, wherein the measurement subsystem is further configured to measure a size of the diagnostic beam at the focal plane.

10. The measurement subsystem includes: an image sensor; first, second, and third secondary beam splitters for directing respective first, second, and third portions of the diagnostic beam to different respective first, second, and third locations on the image sensor, the first, second, and third secondary beam splitters being arranged in series such that the image sensor captures the first, second, and third portions of the diagnostic beam at the upstream location, the focal plane, and the downstream location, respectively; The system of any of claims 1 to 5 or 9, comprising:

11. 11. The system of claim 9 or claim 10, further comprising a feedback controller configured to actively adjust the at least one adaptive optics system to stabilize the sizes measured by the measurement subsystem at the at least one of the upstream and downstream locations and at the focal plane relative to their nominal values.

12. 12. The system of claim 9, wherein the at least one adaptive optics system includes a first pair of adaptive mirrors arranged in series and each configured to correct the wavefront relative to the laser beam in a first transverse axis.

13. Each adaptive mirror of the first pair comprises: a mirror surface; only three actuators distributed along the first transverse axis and adjusting the mirror surface in a dimension parallel to a plane defined by the first transverse axis and the direction of propagation of the laser beam; The system of claim 12 , comprising:

14. 14. The system of claim 12 or claim 13, wherein the at least one adaptive optics system further includes a second pair of adaptive mirrors arranged in series and each configured to correct the wavefront relative to the laser beam in a second transverse axis, the second transverse axis being orthogonal to the first transverse axis.

15. 1. A laser device with wavefront correction, comprising: A system according to claim 14; a laser source for generating said laser beam; a beam shaping module upstream of the at least one adaptive optics system, the beam shaping module configured to achieve a predefined waist size and far-field divergence of the laser beam when the wavefront has nominal properties at the beam shaping module; A laser device comprising:

16. 16. The laser device of claim 15, wherein the laser source comprises a nonlinear crystal.

17. A method for correcting a wavefront of a laser beam from a laser source, said laser beam having a waist at a nominal waist location along a length of said laser beam, said method comprising: splitting off a portion of the laser beam as a diagnostic beam; focusing the diagnostic beam; measuring a size of the diagnostic beam at at least one of upstream and downstream locations relative to a nominal focal location of the focal point of the diagnostic beam, the nominal focal location being a waist location of the diagnostic beam in the absence of deformation of the wavefront, each of the upstream and downstream locations of the diagnostic beam corresponding to an image of a distinct intermediate field location of the laser beam, each of the intermediate field locations being displaced from the nominal waist location by 80% to 120% of a nominal Rayleigh length of the laser beam relative to the nominal waist location; correcting the wavefront of the laser beam using at least one adaptive optics system located within the laser beam upstream of a split between the laser beam and the diagnostic beam based at least in part on the size measured for the at least one of the upstream and downstream locations; A method comprising:

18. The method of claim 17, further comprising forming an image of said nominal waist location in an image plane located at said at least one adaptive optics system; measuring a first lateral dimension of the diagnostic beam at the at least one of the upstream and downstream locations in a first lateral axis relative to the laser beam; adjusting a first adaptive mirror to maintain a nominal value of the first lateral dimension relative to the at least one of the upstream and downstream locations so as to stabilize a second waist location of the laser beam in the first lateral axis downstream of the nominal waist location relative to the image plane, in the correcting step; 20. The method of claim 17, further comprising:

19. 20. The method of claim 18, wherein the nominal waist location is within a nonlinear crystal, and the correcting step stabilizes the second waist location in the presence of wavefront deformations of the laser beam within the nonlinear crystal.

20. The measuring step includes: using two beam splitters arranged in series along the diagnostic beam to direct two different and distinct portions of the diagnostic beam to two different and distinct locations on a single image sensor such that the image sensor captures two different and distinct portions at the upstream and downstream locations, respectively; extracting the size of the diagnostic beam at each of the upstream and downstream locations from images captured by the image sensor; 20. The method of claim 18 or claim 19, comprising:

21. measuring a second transverse dimension of the diagnostic beam at each of the upstream and downstream locations in a second transverse axis relative to the laser beam, the second transverse axis being orthogonal to the first transverse axis; adjusting a second adaptive mirror to maintain a nominal value of the second lateral dimension at the upstream and downstream locations so as to stabilize a location of a second waist of the laser beam in the second lateral axis relative to the image plane in the correcting step; 20. The method of claim 18 or claim 19, further comprising:

22. a focusing element performing the step of focusing the diagnostic beam; the measuring step further comprises measuring a size of the diagnostic beam at a focal plane of the focusing element; The method of any of claims 17 to 21, wherein the correcting step further comprises basing the wavefront correction on the size of the diagnostic beam measured at the focal plane.

23. 23. The method of claim 22, wherein the correcting step adjusts a plurality of adaptive optics to maintain a predefined waist size and far-field divergence of the laser beam.

24. generating said laser beam in a nonlinear crystal by frequency conversion; shaping the laser beam upstream of the plurality of adaptive optics systems, the shaping configured to achieve the predefined waist size and far-field divergence of the laser beam in the absence of wavefront deformation by the nonlinear crystal; further comprising 24. The method of claim 23, wherein the correcting step provides additional shaping to correct for the presence of wavefront distortion due to the nonlinear crystal.

25. the measuring step includes measuring a first lateral dimension of the diagnostic beam at each of the at least one of the upstream and downstream locations and the focal plane in a first lateral axis relative to the laser beam; 25. The method of claim 22, wherein the correcting step comprises adjusting a first pair of serially arranged adaptive mirrors based on the first lateral dimension to maintain (a) a predefined location of a waist of the laser beam in the first lateral axis, and (b) a predefined size of the waist in the first lateral axis.

26. the measuring step further measures a second transverse dimension of the diagnostic beam at each of the at least one of the upstream and downstream locations and the focal plane in a second transverse axis relative to the laser beam, the second transverse axis being orthogonal to the first transverse axis; 26. The method of claim 25, wherein the correcting step further comprises adjusting a first pair of serially arranged adaptive mirrors based on the second lateral dimension to maintain (a) a predefined location of the laser beam waist in the second lateral axis, and (b) a predefined size of the waist in the second lateral axis.

27. The measuring step includes: directing three different and distinct portions of the diagnostic beam to three different and distinct locations on a single image sensor using three beam splitters arranged in series, such that the image sensor captures three different and distinct portions at the upstream location, the focal plane, and the downstream location, respectively; extracting the size of the diagnostic beam at each of the upstream location, the focal plane, and the downstream location from an image captured by the image sensor; The method of any one of claims 22 to 26, comprising:

28. monitoring higher order transverse mode properties of the diagnostic beam at the focal plane in the measuring step; In the correcting step, further adjusting the at least one adaptive optics system according to the higher-order transverse mode properties. The method of any of claims 22 to 27, further comprising:

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