Compensation of thermally induced depolarization in laser amplifiers

Abstract

A laser amplifier compensated for thermally induced depolarization comprises a laser multi-pass amplifier cavity having therein a pair of gas cooled multi-slab amplifier assemblies. The two amplifier assemblies each have a plurality of gain slabs and polarization rotators therein. In some designs, the two amplifiers have opposite handed polarization rotators (e.g. quartz polarization rotators) therein. Such a design allows depolarization compensation over a broad amplification spectrum, which may be useful for realization of wide bandwidth, high energy, and high repetition rate amplification of light.

Description

LLNS.041WOZ IL- 13924-02 PATENT COMPENSATION OF THERMALLY INDUCED DEPOLARIZATION IN LASER AMPLIFIERSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority to U.S. Provisional Application No. 63 / 656,002, titled “COMPENSATION OF THERMALLY INDUCED DEPOLARIZATION IN LASER AMPLIFIERS,” filed June 4, 2024. The entirety of each application referenced above is incorporated herein by reference. Additionally, any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application are hereby incorporated by reference under 37 CFR 1.57.FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government support under Contract No. DE-AC52-07NA27344 awarded by the United States Department of Energy. The Government has certain rights in the invention.BACKGROUNDField

[0003] The present disclosure relates generally to compensation of thermally induced depolarization in laser amplifiers, and more particularly, for example, to thermally induced depolarization compensation in wide bandwidth, high energy, high repetition rate gas cooled multi- slab laser amplifiers.Description of the Related Art

[0004] Recent advances in inertial confinement fusion (ICF) with demonstration of "Ignition", along with the emerging field of high energy density (HED) science will benefit from development of laser amplifiers with increasing energies as well as high repetition rates to transform the proof-of-principle results (e.g., Ignition) to real-world applications (e.g., Inertial fusion energy - (IFE) power plants). In addition, to develop lasers for fast ignitionICF or to achieve ultrashort pulse duration for high-intensity laser-plasma interactions, optical amplifiers that also support wide bandwidth amplification will be required. High energy coupled with high repetition rate, however, increases the heat load as well as the temperature seen by the gain medium within the power amplifier section of the laser. The substantial thermal load experienced by laser materials in such high average power laser systems can lead to detrimental effects, including aberrations and stress-induced birefringence. For the latter, the material behaves as a non-uniform retardation element, where the effect on the polarization state of the incoming light varies across the clear aperture of the optic. This non-uniform retardation causes beam depolarization, that is, a degradation of the polarization purity of a beam propagating through the amplifier head. Low polarization purity of the beam can reduce the efficiency for polarization-sensitive processes such as frequency conversion in nonlinear crystals and pulse compression using diffraction gratings. Thus, the next generation of laser amplifiers will benefit from compensating for the thermally induced depolarization not just for the central wavelength, but also for a large portion or possibly the full bandwidth of the amplification spectrum.SUMMARY

[0005] The present disclosure relates generally to improvements and alternative designs for laser amplifiers such as for high energy, high repetition rate laser amplifiers, that produce a substantial thermal load and consequent non-uniform depolarization in gain materials that cause depolarization of the laser light.

[0006] For example, designs and methods for compensation of thermally induced depolarization in a wide bandwidth, high energy, high repletion rate laser system based on gas cooled multi-slab two head architecture is provided. Opposite-handed quartz rotators are included inside first and second amplifier assemblies or amplifier heads within the laser amplifier allowing depolarization compensation of a large portion (e.g., majority) or even entire amplification spectrum used for realization of wide bandwidth, high energy, and repetition rate amplification of light. In various designs described herein, the amplifier comprises a multi-pass amplifier cavity, and the first and second amplifier assemblies or heads are within the amplifier cavity. Some implementations described herein provide thermallyinduced depolarization compensation in wide bandwidth, high energy, high repetition rate gas cooled multi-slab laser amplifiers although other implementations arc possible.

[0007] One implementation, for example, comprises a laser amplifier configured to reduce thermally induced depolarization. The laser amplifier comprises first and second optical amplifier assemblies. The first optical amplifier assembly comprises a first plurality of gain elements and a first polarization rotator inserted between gain elements of the first plurality of gain elements in said first optical amplifier assembly. The second optical amplifier assembly comprises a second plurality of gain elements and a second polarization rotator inserted between gain elements of said second plurality of gain elements in said second optical amplifier assembly. In some designs, the first and second polarization rotators have the same handedness. In other designs, the first and second polarization rotators have opposite handedness. In some implementations, the laser amplifier further comprises a least one reflector wherein the first optical amplifier assembly and the reflector form an optical path with the second optical amplifier assembly included therebetween such that light propagates along the optical path through the first optical amplifier assembly through the second optical amplifier assembly and reflects off the reflector back through the second optical amplifier assembly and through the first optical amplifier assembly.

[0008] This disclosure also describes results of numerical simulation of an image-relayed multi-pass Nd:Glass based diode pumped amplifier operating at 10Hz repetition rate.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

[0010] Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

[0011] Figure 1 A is a schematic diagram of a laser amplifier comprising an optical amplifier assembly comprising a plurality of gain elements (e.g., gain slabs) configured to be cooled by flowing gas or liquid across (e.g., between) the slabs.

[0012] Figure IB is a schematic diagram of a laser amplifier comprising first and second optical amplifier assemblies concatenated together in series in a multi-pass amplifierconfiguration. In this example, the laser amplifier is configured such that the laser beam passes twice through the first and second optical amplifier assemblies.

[0013] Figures 2A and 2B are simulation results of depolarization and depolarization loss, respectively, for an amplified beam output from a laser amplifier, like the system in Figure IB, which includes first and second amplifier assemblies without any depolarization compensation components such as quartz rotators. The simulation results are for a multi-pass system configured such that the laser light passed twice through each of the first and second optical amplifier assemblies.

[0014] Figures 3A and 3B are schematic diagrams of a laser amplifier comprising first and second optical amplifier assemblies concatenated together in series in a multi-pass amplifier configuration, further comprising a polarization rotator between the first and second optical amplifiers. Figure 3A shows the light propagating in a first direction along a first pass while Figure 3B shows the light propagating in a second direction along a second pass after reflecting from a reflector.

[0015] Figures 4A and 4B are simulation results of depolarization and depolarization loss, respectively, for an amplified beam output from a laser amplifier, like the system in Figures 3A and 3B, that includes first and second amplifier assemblies with a quartz rotator placed between the amplifier assemblies and cut at a thickness to rotate the central or design wavelength’s polarization by 90 degrees. The simulation results are for a multi-pass system such as shown in Figures 3A and 3B configured such that the laser light passes twice through each of the first and second optical amplifier assemblies.

[0016] Figure 5 is a schematic diagram of a laser amplifier comprising an optical amplifier assembly having a polarization rotator (e.g., quartz rotator) therein between gain elements (e.g., gain slabs).

[0017] Figures 6A and 6B are schematic diagrams of a laser amplifier comprising first and second optical amplifier assemblies concatenated together in series in a multi-pass amplifier configuration, further comprising a polarization rotator (e.g., quartz rotator) within each of the first and second optical amplifiers. The polarization rotators have the same handedness; for example, each are right-handed (R) in the example shown in Figures 5 A and 5B. Figure 5 A shows the light propagating in a first direction along a first pass while Figure5B shows the light propagating in a second direction along a second pass after reflecting from a reflector.

[0018] Figures 7 A and 7B are simulation results of depolarization and depolarization loss, respectively, for an amplified beam output from a laser amplifier, like the system in Figures 6A and 6B, which includes first and second amplifier assemblies with respective first and second polarization rotators, e.g., quartz rotators (QR1, QR2), having the same handedness (e.g., right-handed, R), inside the first and second optical amplifier assemblies. The simulation results are for a multi-pass system such as shown in Figures 6A and 6B configured such that the laser light passes twice through each of the first and second optical amplifier assemblies.

[0019] Figures 8A and 8B are schematic diagrams of a laser amplifier comprising first and second optical amplifier assemblies concatenated together in series in a multi-pass amplifier configuration, further comprising a polarization rotator within each of the first and second optical amplifier assemblies. The polarization rotators have the opposite handedness; for example, the first optical amplifier assembly includes a quartz rotator (QR1) that is right-handed (R) while the second optical amplifier assembly includes a quartz rotator (QR2) that is left-handed (L) in the example shown in Figures 8 A and 8B. Figure 8 A shows the light propagating in a first direction along a first pass while Figure 8B shows the light propagating in a second direction along a second pass after reflecting from a reflector.

[0020] Figures 9A and 9B are simulation results of depolarization loss versus beam width for an amplified beam output from a laser amplifier for multi-pass amplification systems such as shown in Figures 6A and 6B and Figures 8 A and 8B. The simulation results shown in Figure 9 A are for a laser amplifier wherein both optical amplifier assemblies include polarization rotators of same handedness, e.g., both the first and second optical amplifier assemblies have right-handed polarization rotators (quartz rotators). The simulation results shown in Figure 9B are for a laser amplifier wherein the optical amplifier assemblies include polarization rotators of different handedness, e.g., the first optical amplifier assembly has a right-handed polarization rotator (e.g., quartz rotator, QR1) and second optical amplifier assembly has a left-handed polarization rotator (e.g., quartz rotator, Q2).

[0021] Figures 10A and 10B are simulation results of depolarization loss versus beam width similar to those of Figures 9A and 9B with a larger multiplexing angle of 30 mrad used in the model.

[0022] Figures 11 A and 1 IB are schematic diagrams of a laser amplifier similar to that shown in Figures 8A and 8B, further comprising a waveplate having uniform retardance across the clear aperture thereof for compensation of residual retardance introduced by the laser amplifier at larger multiplexing angles (e.g., a multiplexing angle of 30 mrad). The second opposite handed (left-handed) polarization rotator or quartz rotator (QR) in the second optical amplifier assembly dramatically reduces the dispersion such that most of the wavelengths (e.g. possibly all wavelengths) experience a similar polarization error.

[0023] Figures 12A and 12B are simulation results of depolarization loss versus beam width similar to those of Figures 10A and 10B. The plot in Figure 12A is a reproduction of the plot in Figure 10A. The plot in Figure 12B is for a laser amplifier such as shown in Figures 11A and 11B having a waveplate having uniform retardance across a range of wavelengths (e.g., most of the wavelength in the amplification band or bandwidth of the laser beam) for compensation of residual retardance introduced by the laser amplifier at larger multiplexing angles (e.g., a multiplexing angle of 30 mrad).

[0024] Figure 13 is a plot of depolarization loss (%) versus wavelength (nm) of the laser beam output by a laser amplifier such as shown in Figures 6A and 6B and Figures 8A and 8B having same and opposite handed polarization rotators (quartz rotators) in respective first and second optical amplifier assemblies. The plot includes curves for different multiplex angle. The plot depicted in Figure 12 is based on a spectrally resolved simulation for depolarization loss for a 60 mm beam width.

[0025] Figure 14 is a schematic diagram of laser amplifier similar to that shown in Figures 8A and 8B configured for thermally induced depolarization compensation further comprising a polarization multiplexing system in addition to an angle multiplexing system. Figure 14 additionally shows the front-end seed light source, a spatial filter for cleaning up the beam and a pulse compressor together with the target of the amplified laser beam.DETAILED DESCRIPTION

[0026] This disclosure provides a variety of designs and methods for compensating for thermally induced depolarization of an amplified beam produced by a laser optical amplifier. These solutions are particularly useful for laser optical amplifiers for amplifying laser beams having one or more of the following: a relatively wide spectral bandwidth, high energy, and / or high repetition rate. Various implementations described herein comprise gas or liquid cooled multi-slab laser amplifiers such as shown in Figure 1.

[0027] In particular, Figure 1 depicts a laser amplifier 10 comprising an optical amplifier assembly or head 12 comprising a plurality of gain elements (e.g., slabs) 14 comprising gain medium that produces optical gain for wavelengths of light at which the laser amplifier operates. The gain medium may comprise a solid state gain medium for example doped with a dopant, e.g., crystals or doped crystals, such as, for example Yb:S-FAP, Yb:YAG, Ti:Sapphire etc., glasses or ceramics such as doped glass, e.g., Nd:Glass, although other hosts materials, crystals, ceramics, glasses and / or other dopants as well as other types of gain mediums may be employed.

[0028] As illustrated, an effective way to increase the cooling of the gain medium is to increase the number of surfaces by splitting a bulk gain medium into multiple slabs 14 and flowing coolant 16 through the slabs. The coolant may comprise, for example, gas or liquid and may be cryogenically cooled gas or liquid in some implementations. Accordingly, various designs include one or more conduits or manifolds to flow the coolant across the gain elements or slabs 14. The amplifier assembly or head 12 may be operated at room temperature or at cooled temperatures such as cryogenically cooled temperatures.

[0029] Figure 1A shows an incident laser beam 24a (represented by an arrow) as well as a laser beam output 24b (also represented by an arrow). The amplitude or intensity of the light 24b exiting the optical amplifier 10 or optical amplifier assembly or head 12 may be increased with respect to the input light 24a as a result of optical amplification provided by the plurality of gain elements 14. The input laser beam 24a may be referred to as the seed laser beam in some cases. The laser beam may, in various implementations, comprise a pulsed laser beam comprising a plurality of optical or laser pulses, however, the laser amplifier can be used to amplify continuous wave (CW) laser light as well.

[0030] To provide optical gain, the gain medium may be pumped, for example, optically pump. An optical pump source 18, which may comprise, for example, a plurality or array of laser diodes is shown in Figure 1A. Pump radiation 20 output by the optical pump source 18 is directed to the plurality of gain elements 14 with a reflector 22, which may comprise, for example, a beam combiner (or beamsplitter) such as a dichroic beam combiner (or dichroic beam splitter). In the example shown, the beam combiner 22 reflects the pump light 20 toward the optical amplifier assembly 12 and transmits the seed light 24a through the beam combiner to the optical amplifier assembly. Likewise, the beam combiner 22 may comprise a dichroic beam combiner in some implementations configured to reflect light having a wavelength of the pump light 20 or optical pump source 18 and to transmit light having a wavelength of the seed light 24a. However, other designs are possible. For example, the beam combiner 22 may be configured to reflect the seed light 24a and transmit the pump light 20.

[0031] To increase amplification, multiple optical amplifier assemblies or heads 12 may be employed. Figure IB, for example, shows first and second optical amplifier assemblies or heads 12a, 12b arranged in series such that the input light 24a passes through both optical amplifier assemblies / heads to be amplified. Figure IB, for example, depicts an optical path including the first and second optical amplifier assemblies 12a, 12b such that the optical seed 24a can pass through the first optical amplifier assembly 12a to be output as amplified light 24b. This amplified light 24b may be input into the second optical amplifier assembly 12b to be output as amplified light 24c. Such a configuration having multiple, for example, two, optical amplifier assemblies or heads 12a, 12b may increase the amplification, for example, by two times (2X) on one pass through the laser amplifier or through the optical amplifier assemblies / heads. Additional amplifier heads 12 may be included in other designs.

[0032] As illustrated in Figure IB, multiple (e.g., first and second) pump light sources 18a, 18b may be included in the laser amplifier 10 and configured to pump (e.g., optically pump) the multiple, e.g., respective first and second optical amplifier assemblies 12a, 12b. Likewise, multiple (e.g., first and second) beam combiners 22a, 22b may be employed to couple light from the respective (first and second) optical pump sources 18a, 18b, into the respective (e.g., first and second) optical amplifier assemblies 12a, 12b.

[0033] FIG. IB additionally illustrates a multi-pass configuration where the laser beam propagates multiple times through the optical amplifier assemblies 12a, 12b. Inparticular, the example laser amplifier 10 shown in Figure IB additionally includes a reflector or mirror 28 positioned to receive light 24c output from the optical amplifier assemblies 12a, 12b on a first pass therethrough and to redirect the light 24d back through the optical amplifier assemblies again for a second pass. In the configuration shown in Figure IB, the reflector 28 is disposed to receive the light 24c from the second optical amplifier assembly 12b and to reflect light 24d back to the second optical amplifier assembly. In the example shown in Figure IB, this light 24d is transmitted through the second beam combiner 22b used to couple pump light 20 from the second optical pump source 18b into the second optical amplifier assembly 12b. This light 24d propagates through the second optical amplifier assembly 12b and is amplified by the plurality of gain elements 14 therein. Light 24e after having passed through the second optical amplifier assembly 12b is directed to the first optical amplifier assembly 12a. This light 24e propagates through the first optical amplifier assembly 12a and is amplified by the plurality of gain elements 14 therein. Light 24f after passing through the first optical amplifier assembly 12a a second time, is output by the laser amplifier 10. In the example shown in Figure IB, this light 24f is transmitted through the beam combiner 22a used to couple pump light 20 from the first optical pump source 18a into the first optical amplifier assembly 12a.

[0034] The configuration shown in Figure IB may be referred to as including an amplifier cavity, an optical enclosure where the field repeats itself. In Figure IB, the optical cavity includes the plurality of optical amplifier assemblies 12a, 12b and the reflector 28 such that the seed beam can propagate multiple times (e.g., twice) through the gain medium. In some designs, the amplifier cavity may include additional optical elements, for example, to provide for more than two passes (e.g., three passes, four passes, or more) through the first and second optical amplifier assemblies 12a, 12b.

[0035] Some multi-pass laser amplifiers include angle multiplexing wherein the light beam is directed at different angles on different passes. Similarly, some multi-pass laser amplifiers include polarization multiplexing wherein the polarization of the light beam such as the orientation of the polarization is specifically altered on different passes. A quarter waveplate may, for example, be used to rotate the orientation of the beam on different passes. In some implementations, both angle multiplexing and polarization multiplexing are employed. Both angle multiplexing and polarization multiplexing may be employed tofacilitate multiple passes through the laser amplifier 10. The angle, polarization, or both may change with different passes through the laser amplifier 10 and may be used to facilitate injection of light into the laser amplifier and / or extraction of high power light from laser amplifier.

[0036] Accordingly Figure IB shows a laser amplifier 10 having a multi-pass configuration that included multiples optical amplifier assemblies 12a, 12b each comprising gain medium (e.g., multiple gain elements 14). As illustrated, the laser amplifier 10 further comprises appropriate cooling (gas, liquid or conductive) and may include an amplifier cavity to propagate the seed beam multiple times through the gain medium.

[0037] As discussed above, the laser amplifier 10 has a thermal load that can be sufficiently high to induce depolarization of the light across the beam. High energy, exacerbated on lasers with high repetition rate, increases the heat load as well as the temperature of the gain medium or gain elements 14 within the optical amplifier assemblies 12. The substantial thermal load experienced by laser materials in such high average power laser systems can lead to detrimental effects including stress-induced birefringence. Increased temperature that produces stress-induced birefringence causes the gain material to have non-uniform retardation across the gain element 14. The polarization state of the incoming light varies across the clear aperture of gain elements 14 as a result of this stress-induced birefringence. This non-uniform retardation causes beam depolarization, a non-uniform degradation of the polarization purity across the beam propagating through the optical amplifier head(s) 12a, 12b.

[0038] Figures 2A-2B show this depolarization caused by heating of optical components within the laser amplifier 10. The laser amplifier 10 that was simulated is similar to that shown in Figure IB; the laser amplifier included two amplifier heads 12a, 12b and was configured to provide two passes through each of the amplifier assemblies.

[0039] Figures 2A and 2B shows the estimated depolarization loss for the central or design wavelength if no compensation, for example, without any depolarization compensation components such as quartz rotators or a Faraday rotator, e.g., in the amplifier cavity, as discussed below. Figure 2A shows the spatial variation of the local polarization state after two passes. This spatial variation is visualized by a grid of polarization ellipses, with green (light) ellipses 19 and red (dark) ellipses 21 representing right- and left-handedpolarization states, respectively. The ellipses 19, 21 are overlaid on a grayscale image of the local depolarization, which corresponds to the energy that is rejected by the polarizer after the second pass, with a value of 1 representing 100% energy loss at a given location. The right plot shows the cumulative integrated energy loss as a function of beam width. The beam after propagating through the two head amplifier section (both first and second optical amplifier assemblies 12a, 12b) for 2 passes without any compensation, experiences about 50% depolarization-induced energy loss for a 60 mm beam width. (The plots are for a central wavelength of 1060 nm with lineal' polarization initially propagating. The laser amplifier 10 simulated also included angle multiplexing with a multiplexing angle of 15 mrad used in the model.)

[0040] As illustrated by Figures 3 A and 3B, thermally induced depolarization compensation within a laser cavity can be achieved by the introduction of a polarization rotator 26 such as a quartz rotator (QR) in the optical path between two (e.g., first and second) amplifier heads 12a, 12b. The polarization rotator 26 may be configured to rotate the polarization state of the beam emerging out of first amplifier head 12a (e.g., by 90 degrees) before injecting the partially amplified light into the second head 12b. This optical compensation approach theoretically works better when the heat loads and temperature profiles of the first and second amplifier assemblies 12a, 12b are identical and thus the same stresses are induced in both amplifier heads along with co-propagating beams inside the laser amplifier 10 and / or laser cavity. The above conditions, however, are difficult to achieve in experiments due to the multiplexing angle of the amplifier cavity (multi-pass amplifier) as well as the cooling conditions of the heads 12a, 12b (which may differ due to manufacturing tolerances). Furthermore, the polarization rotation of the quartz rotator (QR) is wavelength-dependent. The QR is designed and cut to a thickness that will rotate only the central wavelength of the seed spectrum by the selected amount, e.g., by 90 degrees or thereabouts. Other wavelengths, especially wavelengths at the extremes of the seed spectrum, experience non-optimal polarization rotation, thus rendering the thermally induced depolarization compensation less effective for a high energy, wide-bandwidth, large aperture amplifier at high rep rates.

[0041] Figure 3A shows the light propagating in a first direction along a first pass while Figure 3B shows the light propagating in a second direction along a second pass after reflecting from a reflector 28. As illustrated and discussed above, a reflector 28 is positionedwithin the laser amplifier 10 to cause light propagating through the first and second optical amplifier assemblies 12a, 12b on a first pass (see Figure 3A) to return back through said second and first optical amplifier assembly on a second return pass (see Figure 3B). As illustrated, light that passes through the second optical amplifier assembly 12b on the first pass through the laser amplifier 10 will be incident on the reflector 28. Consequently, the reflector 28 reflects light 24d back to the second optical amplifier assembly. Light 24d propagates through the second optical amplifier assembly 12b being further amplified as a result. Light 24e output from the second optical amplifier assembly 12b is transmitted through the polarization rotator 26 and to the first optical amplifier assembly 12a. Light 24f propagates through and is output by the first optical amplifier assembly 12a being further amplified as a result.

[0042] On the first pass through the laser amplifier 10 and / or amplifier cavity shown in Fig. 3A, the polarization rotation optic 26, e.g., the 90-degree QR, in the optical path between two identical amplifier heads 12a, 12b, causes the second optical amplifier assembly to compensate for thermally induced depolarization from the first optical amplifier assembly. The spatially varying polarization orientation exiting the first optical amplifier assembly 12a is rotated by 90 degrees and propagated through the second optical amplifier assembly 12b, which should at least partially compensate for the retardation introduced by the first amplifier, if the second amplifier assembly is similar, if not virtually identical, in construction and thermal and / or optical properties (e.g., amount of optical absorption and / or heat load and / or amount of thermally / stress induced birefringence and / or retardance).

[0043] Likewise on the second (return) pass through the laser amplifier 10 and / or amplifier cavity shown in Fig. 3B, the polarization rotation optic or polarization rotator 26, e.g., the 90-degree QR, in the optical path between two identical amplifier heads 12a, 12b causes the first optical amplifier assembly to compensate for thermally induced depolarization from the second optical amplifier assembly. The spatially varying polarization orientation exiting the second optical amplifier assembly 12b is rotated again by 90 degrees and propagated through the first optical amplifier assembly 12a, which should at least partially compensate for the retardation introduced by the second optical amplifier assembly if the first amplifier assembly is similar, if not virtually identical, in construction and thermal and / or optical properties (e.g., amount of optical absorption and / or heat load and / or amount of thermally / stress induced birefringence and / or retardance).

[0044] Figures 4A and 4B are simulation results of depolarization and depolarization loss, respectively, for an amplified beam output from a laser amplifier 10 such as shown in Figures 3A and 3B, which includes first and second amplifier assemblies 12a, 12b with a polarization rotation optic or polarization rotator (e.g., quartz rotator) placed between the amplifier heads and configured (e.g., cut at a thickness) to rotate the central wavelength’s polarization by 90 degrees. The plots are for a central wavelength of 1060 nm with linear polarization initially propagating. As referenced above, these simulation results are for a multipass laser amplifier such as shown in Figures 3A and 3B configured such that the laser light passes twice through each of the first and second optical amplifier assemblies 12a, 12b. The laser amplifier 10 simulated also included angle multiplexing with a multiplexing angle of 15 mrad used in the model.

[0045] The results of the numerical model for the 2-pass amplifier cavity with two amplifier heads 12a, 12b for the central wavelength (1060nm) in Figure 4A and 4B show a reduction in depolarization as compared to the simulation results presented in Figures 2A and 2B for the laser amplifier 10 with no depolarization compensation. Nevertheless, the depolarization loss shown in Figure 4B increases with the aperture of the beam. Also, although a substantial reduction is observed for the depolarization loss with a 90 degrees rotating quartz rotator between the amplifier assemblies 12a, 12b, the compensation is not perfect due to the multiplexing angle within the amplifier heads. Furthermore, the model assumes perfect alignment, identical heat loads and temperature profiles in both the first and second optical amplifiers assemblies 12a, 12b, which is generally not true for an experimental case. Depolarization-induced energy loss exceeding 1% are calculated for a 60 mm beam width and 15 mrad multiplexing angle.

[0046] In another design shown in Figure 5, the laser amplifier 10 comprises a single optical amplifier assembly 12 with polarization rotating optics or a polarization rotator (such as a QR) 27 inserted within the optical amplifier assembly 12. The polarization rotation optics or polarization rotator 27 are inserted between gain elements 14 such that gain material is on both sides of the polarization rotation optics or polarization rotator. In certain such multislab designs, for example, the polarization rotation optics or polarization rotator 27 is positioned with equal numbers of gain slabs or elements 14 on both sides of the polarization rotation optics or polarization rotator (e.g., QR). With such a design, thermally induceddepolarization in the first group of the gain elements / slabs 14 on the first side of the polarization rotation optics or polarization rotator 27 can be at least partially compensated for by the depolarization induced in the second group of gain medium / slabs within the optical amplifier assembly 12 on the second opposite side of the polarization rotation optics or polarization rotator. However, this configuration has limitations when used in laser amplifiers 10 that include angle multiplexing as well as with amplification of wide-bandwidth seed spectrums in a real-life or practical amplifier cavity.

[0047] Accordingly, various laser amplifiers 10 described herein comprise a plurality of optical amplifier assemblies 12a, 12b that form an optical path within the optical amplifier 10 such that laser light amplified by the first optical amplifier assembly 12a is directed to the second optical amplifier assembly 12b to be further amplified. Some such designs are multi-pass optical amplifiers configured such that light amplified by the second optical amplifier assembly 12b is returned back to the second optical amplifier assembly and then directed onto the first optical amplifier for further amplification by both the first and second optical amplifier assemblies. As discussed above, at least one reflector 28 may be included to form a multi-pass amplifier cavity. In some designs, the light passes twice through the first and second optical amplifier assemblies. In other designs, the light passes more than twice (e.g., three, four, or more times) through the first and second optical amplifier assemblies. The laser amplifier may have different number of passes depending on the gain of the system. For example, the laser amplifier can have as low as one pass and as high as practically possible. In some cases, opto-mechanical considerations affect the design and number of passes. In various implementations, the multi-pass amplifier has 4 or 8 passes.

[0048] In various implementations described herein, however, the first and second optical amplifier assemblies 12a, 12b additionally include polarization rotation optics or a polarization rotator (e.g., a quartz rotator) 27 therein to rotate the polarization of light propagating through the respective optical amplifier assembly. As discussed above, the polarization rotation optic s / polarization rotator 27 may be included within the optical amplifier assemblies 27a, 27b between gain elements 14 in the respective optical amplifier assembly 12. In some designs, for example, an equal number of gain elements or slabs 14 may be on opposite sides of the polarization rotation optic s / polarization rotator 27 in the optical amplifier assembly

[0049] Figures 6 A and 6B show such a laser amplifier 10 having first and second optical amplifier assemblies 12a, 12b, each of which have polarization rotation optics or a polarization rotator (e.g., a quartz rotator) 27 therein between gain elements or gain slabs 14. The first and second optical amplifier assemblies 12a, 12b are included in an optical path such that light amplified by the first optical amplifier assembly is directed to the second optical amplifier assembly for further amplification. In the example shown in Figures 6A and 6B, an equal number of gain elements or slabs 14 are on opposite sides of the polarization rotation optics or polarization rotator 27 in the optical amplifier assembly 12. In this particular' example, six (6) gain elements or gain slabs 14 are on each side of the polarization rotation optics or polarization rotator 27 within the optical amplifier assembly 12a, 12b. However, the number of gain elements 14 on each side may be larger or smaller. The number of gain slabs may depends upon the design of the amplifier, such as the pumped energy, pumped fluence, extraction energy, extraction fluence, doping concentration, transverse amplifier spontaneous emission (TASE) (e.g., the pumping and controlling the TASE), number of passes etc.

[0050] In the example shown in Figure 6A and 6B, the polarization rotation optics polarization rotator 27a, 27b in both the first and second optical amplifier assemblies or heads 12a, 12b have the same handedness. In particular, both the first and second optical amplifier assemblies or heads 12a, 12b are right-handed (R) rotators. Right-handed polarization rotation optics or polarization rotators 27 rotate the orientation of the polarization of the light propagating through the polarization rotation optics / polarization rotator clockwise (as seen from facing at the approaching light). Conversely, left-handed (L) polarization rotation optics / polarization rotators 27 rotate the orientation of the polarization of the light propagating through the polarization rotation optics / polarization rotator counter-clockwise (as seen from facing the approaching light). Although in the example shown, the polarization rotation optics 27a, 27b in both the first and second optical amplifier assemblies or heads 12a, 12b are right-handed polarization rotators, in other designs the polarization rotation optics 27a, 27b in both the first and second optical amplifier assemblies or heads 12a, 12b are left-handed polarization rotators. In either case, the handedness of the polarization rotation optics / polarization rotator 27a, 27b in both the first and the second optical amplifier assemblies 12a, 12b is the same in these designs.

[0051] Additionally, in the example shown in Figures 6A and 6B, the laser amplifier 10 is a multi-pass laser amplifier configured such that the light passes multiple times through each of the first and second optical amplifier assemblies 12a, 12b. In this example design, the laser amplifier 10 includes a reflector or mirror 28 in an optical path of the first and second optical amplifier assemblies 12a, 12b thereby forming an amplifier cavity. The reflector 28 is positioned such that amplified light 24c output from the second optical amplifier assembly 12b is received by the reflector as shown in Figure 6A. Light 24d is reflected from the reflector 28 and directed back to the second optical amplifier assembly 12b as illustrated in Figure 6B. This light 24d propagates through the second optical amplifier assembly 12b thereby being amplified once again. This amplified light 24e is directed back to the first optical amplifier assembly 12a and propagates through the first optical amplifier assembly 12a thereby being amplified more. This amplified light 24f is output from the first optical amplifier assembly 12a and exits the laser amplifier 10.

[0052] The example laser amplifier 10 shown in Figures 6A and 6B is a two-pass laser amplifier. Light passes twice through both the first and second optical amplifier assemblies 12a, 12b. Other configurations, however, are possible. For example, the laser amplifier 10 may be configured for a single pass, for example, by removing the reflector 28. Alternatively, the laser amplifier may be configured for more than two passes (e.g., three, four or more) and may employ angle and / or polarization multiplexing to assist in providing multiple passes).

[0053] Additionally, the example laser amplifier 10 shown in Figures 6A and 6B further comprises first and second optical pump sources 18a, 18b configured to provide pump radiation to the first and second optical amplifier assemblies 12a, 12b, respectively. The laser amplifier 10 includes first and second beam combiners (e.g. beam splitters) 22a, 22b such as dichroic beam combiners (e.g., dichroic beam splitters) to direct the pump radiation 20 to the respective gain media. Other configurations, however, are possible.

[0054] In various implementations, the polarization rotation optics or polarization rotator 27 comprises material with optical activity having a suitable thickness to rotate the polarization by a selected amount. In various implementations, that selected amount is 90°. The thickness of the material with optical activity may be such that the polarization is rotation by 90°. This thickness can include thickness where the polarization is rotated by 360°n+90°,where n is an integer. Faraday optical rotators (like TGG or GGG) can also be used to rotate the polarization. Accordingly, in various implementations the polarization rotating optics / polarization rotator 27a, 27b comprises 90° polarization rotation optics or a polarization rotator configured to rotate the orientation of the polarization of the light by 90°. The polarization rotator 27a, 27b will rotate linearly polarized light 90° about the axis of propagation, e.g., convert vertically polarized light into horizontally polarized light or vice versa or convert light from s -polarization to p-polarization or vice versa and will also rotate the major axis of elliptical polarization by 90°. (Additionally, the 90° polarization rotation optical will convert right-handed circular polarization light into left-handed circularly polarized light or vice versa.) In some cases, the material with optical activity is quartz and said first and second polarization optics or polarization rotators 27a, 27b comprise quartz rotators.

[0055] The introduction of polarization rotating optics (e.g., quartz rotators) 27a, 27b inside each amplifier head 27a, 27b (instead of a QR placed between the amplifier heads), further reduces the depolarization losses by an order of magnitude. This reduction is produced by compensating each amplifier head 12a, 12b individually by itself. Thermally induced depolarization caused by the gain elements 14 on one side of the polarization rotation optics / polarization rotator 27 is compensated for by thermally induced depolarization caused by the gain elements on the other side of the polarization rotation optics / polarization rotator. This design is also less affected by having non-identical heat loads and temperature profiles for the two amplifier heads 12a, 12b as compared, for example, to the laser amplifier design shown in Figures 3 A and 3B.

[0056] Figures 7 A and 7B are simulation results of depolarization and depolarization loss, respectively, for an amplified beam output from a laser amplifier 10 such as shown in Figures 6A and 6B, which includes first and second amplifier assemblies 12a, 12b each with polarization rotation optics or a polarization rotator (e.g., quartz rotator) 27a, 27b placed therein with gain elements 14 on either side. The polarization rotation optics or polarization rotator (e.g., quartz rotators) are configured (e.g., cut at a thickness) to rotate the central wavelength’s polarization by 90 degrees. Both have the same handedness and are right-handed (R) in this case.

[0057] Figure 7 A shows the spatial variation of the local polarization state after two passes. This spatial variation is visualized by a grid of polarization ellipses, with green (light) ellipses 19 and red (dark) ellipses 21 representing right- and left-handed polarization states, respectively. The ellipses 19, 21 are overlaid on a grayscale image of the local depolarization, which corresponds to the energy that is rejected by a polarizer after the second pass, with a value of 1 representing 100% energy loss at a given location. The right plot shows the cumulative integrated energy loss as a function of beam width.

[0058] The results of the numerical model shows a reduction in depolarization as compared to the simulation results presented in Figures 4 A and 4B for the laser amplifier 10 wherein the polarization rotation optics or polarization rotator 26 is between first and second optical amplifier assemblies 12a, 12b, instead of within the first and second optical amplifier assemblies. This reduction may be an order of magnitude.

[0059] The plots are for a central wavelength of 1060 nm with linear polarization initially propagating. The simulation results are for a multi-pass system such as shown in Figures 6 A and 6B configured such that the laser light passes twice through each of the first and second optical amplifier assemblies 12a, 12b. The system simulated also included angle multiplexing with a multiplexing angle of 15 mrad used in the model.

[0060] Figure 7A and 7B shows the calculated depolarization loss at the central wavelength only. However, the quartz rotators 27 used inside the amplifier heads 12a, 12b have wavelength dependent rotation of polarization for a fixed thickness. Accordingly, the effect of non-optimal polarization rotation at the extremes of the amplified spectrum has been modeled to predict the depolarization loss at those wavelengths.

[0061] Additionally, the laser amplifier design shown in Figures 6A and 6B can be modified to instead include polarization rotation optics or polarization rotators (e.g., quartz rotators) of opposite handedness as shown in Figures 8A and 8B. The laser amplifier 10 of Figures 8A and 8B is the same as the laser amplifier of Figures 6A and 6B with the exception that the polarization rotation optics or polarization rotator in the second optical amplifier assembly 27b is left-handed (instead of being right-handed and matching the handedness of the polarization rotation optics / polarization rotator in the first optical amplifier assembly 12a). Likewise, the various components, features, characteristics, variations, and options described above with respect to the laser amplifier 10 such as shown in Figure 6 A and 6B havingpolarization rotation optics or polarization rotators of the same handedness are applicable to laser amplifiers having polarization rotation optics or polarization rotators of different handedness in different optical amplifier assemblies 12a, 12b such as for example shown in Figure 8 A and 8B.

[0062] Likewise, as illustrated, for example, the laser amplifier 10 of Figures 8 A and 8B is a multi-pass laser amplifier with a multi-pass cavity including a reflector or mirror 28 configured to return light back to the first and second optical amplifier assemblies 12a, 12b for another pass of additional amplification. The laser amplifier 10 also includes first and second pump sources 18a, 18b to provide pump radiation to the first and second optical amplifier assemblies 12a, 12b, respectively.

[0063] As discussed above, in various implementations, the polarization rotating optics or polarization rotator comprises material with optical activity having a suitable thickness to rotate the polarization by a selected amount. In various implementations, that selected amount is 90°. The thickness of the material with optical activity may be such that the polarization is rotation by 90°. The thickness of the material with optical activity may be such that the polarization is rotation by 90°. As discussed above, this thickness can include thickness where the polarization is rotated by 360°n+90°, where n is an integer. Faraday optical rotators (like TGG or GGG) can also be used to rotate the polarization. Accordingly, in various implementations the polarization rotating optics or polarization rotator 27a, 27b comprises 90° polarization rotation optics or a 90° polarization rotator configured to rotate the orientation of the polarization of the light by 90°. In some cases, the material with optical activity is quartz and said first and second polarization rotators 12a, 12b comprise quartz rotators.

[0064] Although in the example shown in Figure 8 A and 8B, the polarization rotation optics 27a in the first optical amplifier assembly or head 12a is a right-handed polarization rotator and the polarization rotation optics 27b in the second optical amplifier assembly or head is a left-handed polarization rotator, in other designs the polarization rotation optics 27a in the first optical amplifier assembly or head is a left-handed polarization rotator and the polarization rotation optics 27b in the second optical amplifier assembly or head is a right-handed polarization rotator. In either case, the handedness of the polarization rotation optics or polarization rotators 27a, 27b in the first and the second optical amplifier assemblies 12a, 12b are different from each other in these designs.

[0065] The modeling was further extended to understand the wavelength dependence of the quartz rotators and its effect on the depolarization compensation in the real-life or practical amplifier cavity. Figures 9 A and 9B are plots based on simulation results of depolarization loss versus beam width for an amplified beam output from laser amplifiers multi-pass configurations such as shown in Figures 6A and 6B, and 8A and 8B. Curves are provided for a range of wavelengths (e.g., from 1045 nm to 1075 nm) to observe the performance of the two laser amplifier designs over a broader range of wavelengths.

[0066] The simulation results are for a multi-pass system such as shown in Figures 6A and 6B, and 8A and 8B configured such that the laser light passes twice through each of the first and second optical amplifier assemblies 12a, 12b. The first and second amplifier assemblies 12a, 12b include respective first and second polarization rotation optics or polarization rotators, e.g., quartz rotators (QR1, QR2) 27a, 27b, configured (e.g. cut at a thickness) to rotate the central wavelength’s polarization by 90 degrees within each of the first and second optical amplifier assemblies. The quartz rotator has a thickness (e.g., is cut) for 90-degree polarization rotation at 1060 nm. However, wavelength dependence of depolarization loss vs beam width was determined and plotted for wavelengths ranging from 1045 nm to 1075 nm. The system simulated also included angle multiplexing with a multiplexing angle of 15 mrad used in the model.

[0067] Figures 9A and 9B show the wavelength and beam width dependent depolarization loss for the laser amplifier 10 when the QRs 27a, 27b in the two amplifier heads 12a, 12b have the same handedness and opposite handedness, respectively. A comparison of Figures 9A and 9B demonstrates that including polarization rotation optics or polarization rotators of opposite handedness in the first and second optical amplifier assemblies 12a, 12b, respectively, provide for reduced de-polarization over a wider range of wavelengths.

[0068] In particular, as illustrated by Figure 9A, the lasers amplifier 10 that includes right-handed QRs in both amplifier heads show significantly higher depolarization losses towards the wings of the spectrum (3% at 1045 nm for 60 mm beam width) compared to the central wavelength (0.1% at 1060 nm for 60 mm beam width). Similarly, the curves 32, 34 for wavelengths near 1045 nm and for wavelengths near 1075 nm are at the top, exhibiting high de-polarization loss, while the curves 36 for wavelengths near 1060, the more central design wavelength, are at the bottom, exhibiting low de-polarization loss. This variation withwavelength will alter the spectral content going to the compressor and will deteriorate (e.g., increase) the temporal pulse width at the output of the compressor.

[0069] By contrast, without subscribing to any particular scientific theory, when a right-handed QR 27a is included in first optical amplifier assembly 12a and a left-handed QR 27b is included in second optical amplifier assembly 12b, the sub-optimal counterclockwise rotation of polarization by one is compensated by the clockwise rotation by the other at the same wavelength throughout the spectrum. Thus, compensation is provided within the laser amplifier for a portion of the full spectrum, possibly most of the spectrum or even the full spectrum or almost the entire spectrum. In some cases, the range of the spectrum can extend to the absorption edge of the QR. See, e.g., Figure 9B, which shows a more uniform depolarization loss amongst the various wavelengths and the amount of de-polarization loss is lower on average for the range of wavelengths between 1045 nm to 1075 nm, a 30 nm band. The techniques described herein, for example, for using a LH-QR and a RH-QR in the respective optical amplifier assemblies 12a, 12b can work for other broadband gain medium and other bandwidths. The temporal pulse width at the output of the compressor and the laser system can thus be better maintained.

[0070] As discussed above, in various implementations, the multi-pass laser amplifiers include angle multiplexing wherein the light beam is directed at different angles on different passes. In various examples discussed above, angle multiplexing has been employed and the multiplexing angle has been 15 mrad. To understand the impact on the depolarization loss with increasing multiplexing angles, the model was re-run with a higher multiplexing angle of 30 mrad.

[0071] Figures 10A and 10B are simulation results of depolarization loss versus beam width similar to those of Figures 9A and 9B with a larger multiplexing angle of 30 mrad used in the model. Figures 10A and 10B are simulation results of depolarization loss versus beam width for an amplified beam output from a laser amplifier 10 for a multi-pass system such as shown in Figures 6A and 6B, and 8A and 8B. The simulation results shown in Figure 10A are for a laser amplifier wherein both optical amplifier assemblies 12a, 12b include polarization rotators 27a, 27b of same handedness, e.g., both the first and second optical amplifier assemblies have right-handed polarization rotators (quartz rotators). The simulation results shown in Figure 10B are for a laser amplifier 10 wherein the optical amplifierassemblies 12a, 12b include polarization rotators 27a, 27b of different handedness, e.g., the first optical amplifier assembly has a right-handed polarization rotator (c.g., quartz rotator, QR1) and second optical amplifier assembly has a left-handed polarization rotator (e.g., quartz rotator, Q2).

[0072] As shown by a comparison of Figure 10A with Figure 9A, an increase in multiplexing angle from 15 mrad to 30 mrad increases the depolarization loss marginally for the case with same handed QRs at both the amplifier heads. (The de-polarization loss, however, varies with wavelength in both Figures 9A and 10A. For example, the curves 32, 34 for wavelengths near 1045 nm and for wavelengths near 1075 nm, respectively, are at the top, exhibiting high de-polarization loss, while the curves 36 for wavelengths near 1060, the more central design wavelength, are at the bottom, exhibiting low de-polarization loss.)

[0073] The second opposite handed (left-handed) polarization rotator or quartz rotator (QR) 27b dramatically reduces the dispersion such that most of the wavelengths (e.g. possibly all wavelengths) experience a similar polarization error. A comparison of Figure 10B with Figure 9B, however, shows that the depolarization losses are increased substantially (from 0.5% to 3.3% for 60 mm beam width) for the case with opposite handed QRs 27a, 27b in the two heads 12a, 12b. Even though the overall depolarization loss is significantly increased for 30 mrad multiplex angle, the output polarization is nearly the same for most if not all the wavelengths within the spectrum. Therefore, this static, spatially uniform polarization error can be compensated by a fixed retardation for a range (e.g., most or all) the spectral components by introducing a waveplate with an amount of retardance to at least partially or possibly more significantly offset the de-polarization. Alternatively, a combination of halfwaveplate ( / 2) and quarter-waveplate (X / 4), which when one is rotated with respect to the other can provide a selected amount of retardance, may be employed. In some implementations, an angled rotator such as an angled quartz rotator is used to provide an amount of retardance to offset or at least partially this de-polarization. For example, depending upon the angle used, the angled quartz rotator effects the polarization both with its birefringence and its optical activity.

[0074] Figures HA and 11B show a laser amplifier 10 similar to that shown in Figures 8A and 8B, further comprising such retardance correction optics 38, e.g., a waveplate or angled quartz rotator, having uniform retardance across a range of wavelengths forcompensation of residual retardance introduced by the laser amplifier at larger multiplexing angles (c.g., a multiplexing angle of 30 mrad). Such a wavcplatc, angled rotator, or quartz rotator may have an amount of retardance to offset or at least partially offset the residual retardance introduced by the first and second optical amplifier assemblies 12a, 12b and / or a retardance useful for offsetting or at least partially offsetting residual retardance in the beam after passing through the first and second optical amplifier assemblies. Such a waveplate, angled rotator, or quartz rotator need not therefore be a half-wave waveplate (X / 2) or quarterwave waveplate ( / 4) as the amount of residual retardance may be different for different optical amplifiers 10. As discussed above, in some alternative designs, this retardance correction optics 38 may also comprise a combination of half-wave waveplate (X / 2) and quarter-wave waveplate (X / 4), which when one is rotated with respect to the other, can provide a selected amount of retardance. The retardation correction optics 38 may comprise other components that introduce retardance to offset at least a portion of the residual retardance.

[0075] The laser amplifier 10 shown in Figures 11A and 11B further comprises a beamsplitter 39 configured to redirect the light output from the laser amplifier to the retardance correction optics 38. In some implementations, for example, this beamsplitter 39 may comprise a polarization beamsplitter. The laser amplifier 10 may include a quarter-waveplate (X / 4) in the optical path of the laser beam to rotate the polarization when the laser beam undergoes two passes through the quarter-wave waveplate. As illustrated, the laser amplifier shown in Figures 11A and 1 IB is a double pass laser amplifier that includes a reflector 28 disposed in the optical path to cause the laser beam to undergo a second optical path through the first and second optical amplifier assemblies 12a, 12b. In various implementations, the quarter-waveplate (X / 4) (not shown) may be included in the optical path, for example, between the second optical amplifier assembly 12b and the reflector 28. In such cases, the beamsplitter 39 may comprise a polarization beam splitter, e.g., that transmits light having a first polarization and reflects light having a second polarization. In such cases, two passes through the quarter- waveplate (X / 4) may transform the light of the first polarization into light of the second polarization. Such a configuration may cause light such as the seed laser beam that is at first transmitted through the beamsplitter 39 to be ultimately reflected to the retardance correction optics 38 after being amplified by multiple passes through the first and second optical amplifier assemblies 12a, 12b. This is an example of polarization multiplexing.

[0076] In the example shown in Figures 11 A and 11B, the retardation correction optics 38 is disposed to receive the laser beam after passing through the first and second optical amplifier assemblies 12a, 12b. In the example shown, the retardation correction optics 38 is disposed to receive the laser beam after passing through the first and second optical amplifier assemblies 12a, 12b a second time on a second pass therethrough. In the example shown, the retardation correction optics 38 is shown positioned to receiving the laser beam output from the first optical amplifier assembly 12a, however, the retardation correction optics 38 can be located elsewhere in the optical path, for example at the output of the second optical amplifier assembly, for example, in single pass laser amplifier or in multiple pass laser amplifiers having an odd number of passes. Likewise, the retardation correction optics 38 is shown positioned to receive the laser beam output from the first optical amplifier assembly 12a in multiple pass laser amplifiers 10 having an even number of passes.

[0077] Figures 12A and 12B are simulation results of depolarization loss versus beam width similar to those of Figures 10A and 10B. Figure 12B is for a laser amplifier such as shown in Figures 11A and 11B having a waveplate 38 having uniform retardance across a range of wavelengths (e.g., most of the wavelength in the band) for compensation of remnant retardance introduced by the laser amplifier 10 at larger multiplexing angles (e.g., a multiplexing angle of 30 mrad). Figure 12 A is the same plot in Figure 10A for the design where the polarization rotation optics 27a, 27b in the first and second optical amplifier assemblies 12a, 12b are the same handedness, reproduced for convenient reference and comparison with Figure 12B. As discussed above, the achromatic waveplate 38 is inserted to provide correction to the polarization state by removing residual retardance for a range (e.g., majority) of the wavelengths in the broadband laser beam.

[0078] Figure 12B shows the result when a waveplate 38 having uniform retardance across a range (e.g., majority) of the wavelengths in the broadband laser beam is utilized for the compensation of the static polarization error for the output shown in Fig. 10B. The same technique will be less effective in compensating for the depolarization loss shown in Fig. 10A as the rotation of polarization changes with wavelength for a same handed polarization rotation optics (QR) 27a, 27b in both heads 12a, 12b. These modeling results indicate that the utilization of opposite handed polarization rotation optics (QRs) 27a, 27b in different heads 12a, 12b for a two-head design can compensate thermally induceddepolarization for large angular multiplexing (e.g., 30 mrad) as well as for large beam sizes compared to same handed polarization rotation optics (QR) in both amplifier heads.

[0079] Figure 13 is a plot of depolarization loss (%) versus wavelength (nm) of the laser beam output by a laser amplifier 10 such as shown in Figures 8A and 8B having opposite handed polarization rotators (quartz rotators) 27a, 27b in respective first and second optical amplifier assemblies 12a, 12b. Figure 13 shows the wavelength dependent depolarization loss for a fixed beam width (60 mm) at various multiplexing angles over the entire amplification spectrum. Curve 52, 54, 56, 60 are for multiplexing angles of 30 mrad, 25 mrad, 20 mrad, and 5 mrad, respectively, for a laser amplifier 10 having polarization rotation optics (QRs) of same handedness in the first and second optical amplifier assemblies. Curve 72, 74, 76, 78, 80 are for multiplexing angles of 30 mrad, 25 mrad, 20 mrad, 15 mrad, and 5 mrad, respectively, for a laser amplifier 10 having polarization rotation optics (QRs) 27a, 27b of opposite handedness in the first and second optical amplifier assemblies 12a, 12b, respectively. The depolarization loss reduces considerably for most if not the entire wavelength spectrum when opposite handed quartz rotators are used inside the amplifier heads for multiplexing angles below 15 mrad. Although the depolarization loss increases significantly above 15 mrad multiplexing angle, significant if not substantially complete compensation is possible by additional uniform waveplate 38 (as discussed above, e.g., with regard to Figures 10B, 11 A, 11B, 12A, and 12B). These results show a strong dependence of depolarization loss on the multiplexing angle as well as the beam size and exhibit the advantage of utilizing opposite-handed QRs 27a, 27b inside the amplifier heads 12a, 12b in a two-head amplifier 10. The plot depicted in Figure 13 is based on a spectrally resolved simulation for depolarization loss for a 60 mm beam width.

[0080] Figure 14 is a schematic diagram of laser amplifier 10 including first and second optical amplifier assemblies 12a, 12b that have polarization rotation optics (e.g., quartz rotators) 27a, 27b of opposite handedness between gain members 14 in the first and second optical amplifier assemblies. As discussed above, this configuration provides compensation for thermally induced depolarization. Figure 14 also shows gas coolant 16 directed into the first and second optical amplifier assemblies 12a, 12b to cool the gain elements 14. In this example, the coolant comprises helium (He) gas.

[0081] Figure 14 additionally shows pump sources 18a, 18b optically coupled to the optical path of the laser beam by beam combiners 22a, 22b, such as dichroic beamcombiners. The beam combiners 22a, 22b are positioned to allow the laser beam to propagate along the optical path within the laser amplifier 10, while pump radiation from the pump sources 18a, 18b is also directed to the gain elements 14 of each of the first and second optical amplifier assemblies 12a, 12b. In the example shown, the beam combiners 18a, 18b are transmissive to the pump radiation and reflect the laser beam propagating along the path within the laser amplifier 10.

[0082] The laser amplifier 10 also includes a spatial filter 84 configured to “clean up” the beam. High spatial frequency components within the beam are at least partially filtered out resulting in a smoother spatial profile. In this design, the spatial filter 84 is between the first and second optical amplifier assemblies 12a, 12b.

[0083] In the example shown in Figure 14, the laser amplifier 10 additionally includes a plurality of optical relays for conveying the laser beam along the optical path within the laser amplifier. The relays are formed by relay lenses 80. In the example shown, the relay lenses 80 form afocal relays. The afocal relays 80 permit the laser beam to propagate along portions of the optical path as a generally collimated laser beam. To form the afocal relay, a first relay lens 80a may be separated from a second relay lens 80b by a distance corresponding to the sum of the focal lengths, fi + 14, of the first and second relay lenses. The afocal relay receives a collimate beam, focuses the beam down to a location a focal length (fi) away from the first relay lens 80a. The second relay lens 80b can be positioned a focal length (1'2) away from the focused beam to collimate the beam again. The relay works in a similar manner in reverse. More than one lenses element, e.g., two lenses, may operate as the first relay lens 80a and likewise, more than one lenses element, e.g., two lenses, may operate as the second relay lens 80b. For example, a pair of lens elements are employed as the first lens 80a and a pair of lens elements are employed as the second lens 80b in the relay surrounding the spatial filter 84. Various such relays, e.g., afocal relays, are employed throughout the laser amplifier 10 shown in Figure 14. The optical amplifier 10 also includes mirrors 82 to redirect the optical path and the laser beam along the optical path.

[0084] The laser amplifier 10 is shown in Figure 14 integrated together with a seed laser source 86 and with a target 88. The seed laser source 86 may comprise a pulsed laser in some implementations. The seed laser source 86 is optically coupled to the laser amplifier 10 to provide laser light to be amplified by the laser amplifier.

[0085] Figure 14 also shows a pulse compressor 92 positioned in the optical path of the light output by the laser amplifier 10. The pulse compressor 92 may be configured to compress optical pulses comprising the amplified laser beam. In the example configuration shown, the output of the pulse compressor 92 is directed to focusing optics (e.g., a focusing mirror) 94 via another beam steering mirror 82 (e.g., a planar mirror or reflector). The focusing mirror 92 is configured to focus the amplified laser beam onto the target 88.

[0086] In various implementations, the output of the seed laser source 86 has an output directed to a deflector 90 that couples the laser beam into the laser amplifier 10. The deflector 90 may comprise a material that reflects the laser beam output of the seed laser source 86. The laser beam from the seed laser source 86 is deflected by the deflector 90 to the laser amplifier 10. Some designs further include a pin hole (not shown) positioned such that the laser beam deflected by the deflector 90 passes through the pin hole prior to being received by the laser amplifier 10. The pinhole works as a spatial filter and removes high frequency components from the spatial profile of the beam. The pin hole also acts as a baffle for stray reflections.

[0087] The laser amplifier 10 shown in Figure 14 also includes angle multiplexing. The seed laser beam from the seed laser 86 is deflected by the deflector 90 such that the seed laser beam enters the laser amplifier 10 at an angle (e.g., a angle of 15 mrad or possibly larger, e.g., 30 mrad). The laser beam will propagate within the laser amplifier 10 and ultimately return back close to the location of the beam deflector 90. The laser beam, however, incident on the optics within the laser amplifier 10 at an angle will return at an angle opposite to where it started. The angle is different and opposite, because the beam will return back from the end mirror 28b (e.g., adaptive optics) and will propagate in an oppose angle when returning back, and will miss the beam deflector 90 and propagate past the beam deflector, and the amplified laser beam will continue to the compressor 92 and / or focusing optics / focusing mirror 94 and onto the target 88. As discussed above, the angle multiplexing can provide for multiple passes of the beam through the optical amplifier assemblies 12a, 12b.

[0088] The laser amplifier 10 additionally includes polarization multiplexing. The input of the laser amplifier 10 includes a polarizer, in particular, a polarizing beamsplitter 96 that receives the light from the seed laser source 86. In particular, the laser beam from the seed laser source 86 is deflected by the deflector 90 to the polarizing beam splitter 96. The light ispolarized and, in this example, reflected toward the first and second optical amplifier assemblies 12a, 12b. Light output by the second optical amplifier 12b is transmitted through a quarter waveplate (X / 4 waveplate) and then to a reflector 28b which reflects the amplified beam back through the quarter waveplate (X / 4 waveplate) again and back to the first and second optical amplifier assemblies 12a, 12b in reverse. The two passes through the quarter waveplate (X / 4 waveplate) introduces a half wave of retardance which causes the rotation of vertical polarized light to horizontally polarized light or vice versa. As a result, light having a first polarization that was reflected by the polarizing beam splitter 96 will be instead transmitted by polarizing beam splitter. As illustrated in Figure 14, this light transmitted through the polarization beamsplitter 96 will continue onto a reflector 28a and be reflected again back through the first and second optical amplifier assemblies 12a, 12b for further amplification.

[0089] Once again, after the light passes through the second optical amplifier 12b, this amplified beam will continue onto the quarter waveplate (X / 4 waveplate) and to the reflector 28b which reflects the laser beam back through the quarter waveplate (X / 4 waveplate) again. The two passes through the quarter waveplate (X / 4 waveplate) introduces a half wave of retardance and cause the rotation of vertical polarized light to horizontally polarized light or vice versa. As a result, light having the second polarization that was transmitted by the polarizing beam splitter 96 will instead be reflected by polarizing beam splitter out of the laser amplifier 10 and toward the beam compressor 92 and / or focusing optics / focusing mirror 94 and ultimately to the target 88. As discussed above, the light beam exiting the laser amplifier 10 might be propagating at an angle such that the laser beam misses the deflector and proceeds to the beam compressor 92 and / or focusing optics / focusing mirror 94 instead of being directed back to the pulse laser source 86. The polarization multiplexing thus permits that light to pass back and forth through the first and second optical amplifier assemblies 12a, 12b, twice such that the light propagates through each of the first and second optical amplifier assemblies four times.

[0090] As discussed above, the laser amplifier 10 may comprise a cavity to provide for multiple passes. As discussed above, this cavity may include a reflector 28 to reflect light back to the first and second optical amplifier assemblies 12a, 12b for a second pass. In the example configuration shown in Figure 14, the laser amplifier 10 and the cavity includes two reflectors 28a and 28b.

[0091] In various laser amplifier designs such as in the laser amplifier 10 shown in Figure 14, an adaptive optical clement is included, for example, to reduce optical aberration. The adaptive optical element may comprise reflective adaptive optical element such as a deformable mirror, which can be configured, e.g., deformed, to counter and / or reduce aberration. In the example shown in Figure 14, the reflector 28b is closer to the output of the second optical amplifier assembly 12b than the first optical amplifier assembly 12a, comprises adaptive optics such as a deformable mirror.

[0092] As discussed above, the first and second optical amplifier assemblies 12a, 12b each have therein polarization rotation optics (e.g., quartz rotators) that results in compensation of thermally induced depolarization. In various implementations described herein such polarization rotation optics (e.g., quartz rotators) in the first and second optical amplifier assemblies 12a, 12b may have opposite handedness (e.g., be right-handed and lefthanded, respectively, or left-handed and right-handed, respectively).

[0093] Accordingly, in various designs described herein, a high temporal contrast, wide spectral bandwidth seed laser beam is propagated through the first amplifier head ("Amplifier 1") 12a, which comprises of multiple gain slabs 14 with a quartz rotator (QR1) 27a as shown in Figure 14. The QR1 27a in the first optical amplifier head (Amplifier 1) 12a is a right-handed rotating QR and is cut at a thickness that rotates the central wavelength of the seed spectrum to 90 degrees. Further, the beam propagates through the spatial filter 84 and high spatial frequency components within the beam are filtered resulting in a smooth spatial profile. The beam then passes through a second amplifier head ("Amplifier 2") 12b, which has the same configuration of gain slabs 14 and a quartz rotator (QR2) 27b. However, the QR2 27b in Amplifier 2 12b is a left-handed rotating QR. This arrangement of QRs 27a, 27b within the amplifier heads 12a, 12b not only compensates the depolarization in the first group of the gain medium 14 by the depolarization induced in the second group of gain medium within one amplifier head; it also compensates for the non-optimal rotation of the wavelengths at the extreme of the seed spectrum in the two amplifier heads due to the opposite handed QR. Note that the QR locations are not fixed and can be interchanged between the amplifier heads 12a, 12b within the cavity or laser amplifier 10, e.g., the results remain the same even if the QR1 27a is a left-handed and QR227b is right-handed.

[0094] Various designs described herein, comprise a practical multi-pass, image relayed amplifier cavity comprising two amplifier heads. A quarter- wave wavcplatc ( / 4 plate) may be positioned between the reflector 28b (e.g., deformable mirror) and the second optical amplifier assembly 12b, e.g., close to the deformable mirror, within the cavity and may be used for polarization flipping at the end of the odd numbered pass (for polarization multiplexing). The individual amplifier heads 12a, 12b comprise multiple gain slabs 14 and a rotating optic (quartz rotator) 27, e.g., at the center of the head. The amplifier cavity shown in Figure 14 is both a polarization multiplexed and angle multiplexed cavity, for example, to keep the multiplexing angle as small as possible. However, for some designs the laser amplifier 10 can work with angle multiplexing and without polarization multiplexing.

[0095] Theoretical simulations were performed for estimating the depolarization losses within this cavity for a Nd:Glass based diode pumped multi-slab amplifier operational at 10Hz as shown in Fig. 14. The central wavelength of amplification was chosen as 1060 nm with a 30 nm bandwidth, and the multiplexing angle was chosen to be 15 mrad (0.86°). The probe beam’s incident electric field was horizontally polarized with a flat-top energy distribution with unit amplitude. The model simulates the depolarization loss for the probe beam that propagates two times through the gain mediums. The losses between the first cavity round trip (two passes) and the subsequent cavity round trip (n-number of two passes) are assumed multiplicative only. The local depolarization after cavity round trip, the fraction of energy that remains horizontally polarized after two passes, was calculated at the output plane. The energy loss as a function of beam width was calculated by integrating the local depolarization over centered square sub-apertures of different sizes.

[0096] The model and design, however, do have limitations. For example, the model accounts for the wavelength dependence of the QR (not the stress birefringence). The model also accounts for the small I incar retardance of the QR resulting from off-axis incidence. Additionally, in a dual angle and polarization multiplexed cavity design such as shown in Fig.14, the angles may be chosen below 15 mrad to reduce or minimize optic sizes and reduce the complexity of the cavity design.

[0097] Nevertheless, various laser amplifier designs and methods are provided for depolarization compensation and generation of high energy, wide bandwidth, and high repetition rate simultaneously in a two-head laser amplifier setup. The theoretical modellingpredicts an order of magnitude reduction in depolarization losses for the central wavelength when the rotating optic configuration (quartz rotator) is moved from between the amplifier heads to within the individual amplifier heads. Further, spectrally resolved simulation results show significant reduction in the depolarization losses for most or all wavelengths within the amplification bandwidth when opposite handed quartz rotators are utilized inside the amplifier heads.

[0098] Various laser amplifier designs described herein comprise image relayed laser amplifiers wherein the distribution of the electric field is mapped from one location to another when the optical field propagates from a first pass to a second pass (e.g., pass 1 to pass 2.) If you propagate the beam for long distances the spatial profile will change due to diffraction and the electric field generally not repeat if the beam keeps propagating freely (without any optics such as a lens). The image or field distribution can be propagated or be relayed using a lens. For example, with two lenses of focal length “F” placed apart by a distance of 2F, the field distribution at the object plane that is at a distance di=F before the first lens will repeat at a distance d2=F after the second lens (e.g., a total of 4F distance). Such an image relay may be referred to as a 4F image relay. In various implementations of the amplifier, the beam image of the input is relayed to the first amplifier then again relayed to the second amplifier and then to the end mirror (e.g., adaptive optics).

[0099] In simulations used to produce various plots presented herein, a two-pass laser amplifier cavity was used. The measured thermal depolarization should be the same for a 4 pass cavity as after the 2-passes the depolarization, is removed from the cavity by polarizers and the depolarization starts again from zero (e.g., linear polarized). The aim is to reduce the losses due to the other polarized light being kicked out (thermal depolarization compensation).

[0100] The laser amplifier 10 can include a wide range of variations. For example, although two amplifier heads 12a, 12b are shown, for example, in Figure 14, the number of amplifier assemblies may be greater than two. The number of optical amplifier assemblies 12 in the laser amplifier may, for example, depend on the amount of gain desired.Examples:

[0101] This disclosure provides various examples of laser amplifiers devices, systems, and methods. Some such examples include but are not limited to the following examples.1. A laser amplifier configured to reduce thermally induced depolarization, said laser amplifier comprising:a first optical amplifier assembly comprising a first plurality of gain elements and a first polarization rotator inserted between gain elements of the first plurality of gain elements in said first optical amplifier assembly; anda second optical amplifier assembly comprising a second plurality of gain elements and a second polarization rotator inserted between gain elements of said second plurality of gain elements in said second optical amplifier assembly.2. The laser amplifier of Example 1 , wherein said first and second polarization rotators have same handedness.3. The laser amplifier of Example 1 , wherein said first and second polarization rotators have opposite handedness.4. The laser amplifier of any of the examples above, wherein said first and second polarization rotators each rotate the polarization of light transmitted through the respective first and second optical amplifier assemblies by 90°.5. The laser amplifier of any of the examples above, wherein said first and second polarization rotators comprise material with optical activity having a suitable thickness to rotate the polarization by a selected amount.6. The laser amplifier of Example 5 wherein said selected amount is 90°.7. The laser amplifier of any of the examples above, wherein said first and second polarization rotators comprise quartz rotators.8. The laser amplifier of any of the examples above, wherein said gain elements comprise slabs of gain medium.9. The laser amplifier of any of the examples above, further comprising a cooling system configured to direct gas or liquid across said gain elements in said first and second optical amplifier assemblies.10. The laser amplifier of any of the examples above, wherein said first plurality of gain elements in said first optical amplifier assembly comprise an even number of gain elements in first and second groups of equal number of gain elements on either side of said first polarization rotator.11. The laser amplifier of any of the examples above, wherein said second plurality of gain elements in said second optical amplifier assembly comprise an even number of gain elements split into first and second groups of equal number of gain elements on either side of said second polarization rotator.12. The laser amplifier of any of the examples above, further comprising a least one reflector, said first optical amplifier assembly and said at least one reflector forming an optical path, said second optical amplifier assembly included in said optical path between said first optical amplifier assembly and said at least one reflector such that light propagates along said optical path through said first optical amplifier assembly through said second optical amplifier assembly and reflects off said at least one reflector back through said second optical amplifier assembly and through said first optical amplifier assembly.13. The laser amplifier of Example 12, wherein said at least one reflector comprises an adaptive optical element.14. The laser amplifier of Example 12 or 13, further comprising a least one optical relay comprising a plurality of lenses between said second optical amplifier assemblies and said reflector.15. The laser amplifier of any of Examples 1-11, further comprising a multi-pass amplifier cavity comprising first and second reflectors, said first and second optical amplifier assemblies in an optical path between said first and second reflectors.16. The laser amplifier of Example 15, wherein at least one of said first and second reflectors comprises an adaptive optical element.17. The laser amplifier of Example 15 or 16, further comprising a least one optical relay comprising a plurality of lenses in said optical path between said first and second reflectors.18. The laser amplifier of any of the examples above, further comprising at least one retarder positioned to receive light having propagated through said first and second optical amplifier assemblies to compensate for the depolarization introduced by said first and second optical amplifier assemblies.19. The laser amplifier of Example 18, wherein said at least one retarder comprises a quartz having a thickness to provide suitable rctardancc to compensate for the depolarization introduced by said first and second optical amplifier assemblies.20. The laser amplifier of any of Examples 18-19, wherein said at least one retarder is not a quarter wave retarder or a half wave retarder.21. The laser amplifier of any of Examples 1-17, further comprising a quarter wave retarder and a half wave retarder that can be rotated with respect to each other to obtain any retardance, said quarter wave retarder and said half wave retarder positioned to receive light having propagated through said first and second optical amplifier assemblies to compensate for the depolarization introduced by said first and second optical amplifier assemblies.22. The laser amplifier of any of the examples above, further comprising a quarter waveplate or faraday rotator in the optical path of the amplified laser beam to provide polarization multiplexing.23. The laser amplifier of any of the examples above, further comprising a spatial filter in said optical path between said first and second optical amplifier assemblies.24. The laser amplifier of any of the examples above, further comprising at least one optical pump configured to pump said gain elements in said first and second optical amplifier assemblies.25. The laser amplifier of any of Example 24, wherein said at least one optical pump comprises at least one laser diode.26. The laser amplifier of any of the examples above, further comprising a least one optical relay comprising a plurality of lenses between said first and second optical amplifier assemblies.27. The laser amplifier of any of Examples 14, 17 or 26, wherein said at least one optical relay comprises an afocal optical relay.28. The laser amplifier of any of the examples above, wherein said laser amplifier is configured to provide angle multiplexing.29. A laser amplifier configured to reduce thermally induced depolarization, said laser amplifier comprising:a first multi-slab amplifier assembly comprising a plurality of gain slabs and a first polarization rotator inserted between gain slabs of said plurality of gain slabs in said first multi-slab amplifier assembly; anda second multi-slab amplifier assembly comprising a plurality of gain slabs and a second polarization rotator inserted between gain slabs of said plurality of gain slabs in said second multi-slab amplifier assembly.30. The laser amplifier of any of Example 29, wherein said first and second polarization rotators have opposite handedness.31. The laser amplifier of any of Example 29, wherein said first and second polarization rotators have same handedness.32. The laser amplifier of any of the examples above, further comprising a least one reflector, said first multi-slab amplifier assembly and said at least one reflector forming an optical path in said laser cavity, said second multi-slab amplifier assembly included in said optical path between said first multi-slab amplifier assembly and said at least one reflector such that light propagates along said optical path through said first multi-slab amplifier assembly and then through said second multi-slab amplifier assembly, reflects off said at least one reflector back through said second multi-slab amplifier assembly, and then continues through said first multi-slab amplifier assembly.33. The laser amplifier of any of Examples 1-28, further comprising at least one retarder configured to compensate for the depolarization introduced by said first and second optical amplifier assemblies.34. The laser amplifier of Example 33, wherein said at least one retarder comprises a quartz having a thickness to provide suitable retardance to compensate for the depolarization introduced by said first and second optical amplifier assemblies.35. The laser amplifier of any of Examples 33-34, wherein said at least one retarder is not a quarter wave retarder or a half wave retarder.36. The laser amplifier of any of Examples 1-28, further comprising a quarter wave retarder and a half wave retarder that can be rotated with respect to each other to obtain any retardance, said quarter wave retarder and said half wave retarder configured to compensate for the depolarization introduced by said first and second optical amplifier assemblies.

[0102] Although the description above contains many details and specifics, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document. The features of the embodiments described herein may be combined in all possible combinations of methods, apparatus, modules, systems, and computer program products. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments.

[0103] Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art. In the claims, reference to an element in the singular is not intended to mean "one and only one" unless explicitly so stated, but rather "one or more." All structural and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element or component in the present disclosure is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase "means for."

Claims

WHAT IS CLAIMED IS:

1. A laser amplifier configured to reduce thermally induced depolarization, said laser amplifier comprising:a first optical amplifier assembly comprising a first plurality of gain elements and a first polarization rotator inserted between gain elements of the first plurality of gain elements in said first optical amplifier assembly; anda second optical amplifier assembly comprising a second plurality of gain elements and a second polarization rotator inserted between gain elements of said second plurality of gain elements in said second optical amplifier assembly.

2. The laser amplifier of Claim 1, wherein said first and second polarization rotators have same handedness.

3. The laser amplifier of Claim 1, wherein said first and second polarization rotators have opposite handedness.

4. The laser amplifier of Claim 1, wherein said first and second polarization rotators each rotate the polarization of light transmitted through the respective first and second optical amplifier assemblies by 90°.

5. The laser amplifier of Claim 1, wherein said first and second polarization rotators comprise material with optical activity having a suitable thickness to rotate the polarization by a selected amount.

6. The laser amplifier of Claim 5, wherein said selected amount is 90°.

7. The laser amplifier of Claim 1, wherein said first and second polarization rotators comprise quartz rotators.

8. The laser amplifier of Claim 1, wherein said gain elements comprise slabs of gain medium.

9. The laser amplifier of Claim 1, further comprising a cooling system configured to direct gas or liquid across said gain elements in said first and second optical amplifier assemblies.

10. The laser amplifier of Claim 1, wherein said first plurality of gain elements in said first optical amplifier assembly comprise an even number of gain elements in first and second groups of equal number of gain elements on either side of said first polarization rotator.

11. The laser amplifier of Claim 1 , wherein said second plurality of gain elements in said second optical amplifier assembly comprise an even number of gain elements split into first and second groups of equal number of gain elements on either side of said second polarization rotator.

12. The laser amplifier of Claim 1, further comprising a least one reflector, said first optical amplifier assembly and said at least one reflector forming an optical path, said second optical amplifier assembly included in said optical path between said first optical amplifier assembly and said at least one reflector such that light propagates along said optical path through said first optical amplifier assembly through said second optical amplifier assembly and reflects off said at least one reflector back through said second optical amplifier assembly and through said first optical amplifier assembly.

13. The laser amplifier of Claim 12, wherein said at least one reflector comprises an adaptive optical element.

14. The laser amplifier of Claim 12, further comprising a least one optical relay comprising a plurality of lenses between said second optical amplifier assembly and said reflector.

15. The laser amplifier of Claim 1, further comprising a multi-pass amplifier cavity comprising first and second reflectors, said first and second optical amplifier assemblies in an optical path between said first and second reflectors.

16. The laser amplifier of Claim 15, wherein at least one of said first and second reflectors comprises an adaptive optical element.

17. The laser amplifier of Claim 15, further comprising a least one optical relay comprising a plurality of lenses in said optical path between said first and second reflectors.

18. The laser amplifier of Claim 1, further comprising at least one retarder positioned to receive light having propagated through said first and second optical amplifier assemblies to compensate for the depolarization introduced by said first and second optical amplifier assemblies.

19. The laser amplifier of Claim 18, wherein said at least one retarder comprises a quartz having a thickness to provide suitable retardance to compensate for the depolarization introduced by said first and second optical amplifier assemblies.

20. The laser amplifier of any of Claim 18, wherein said at least one retarder is not a quarter wave retarder or a half wave retarder.

21. The laser amplifier of Claim 1, further comprising a quarter wave retarder and a half wave retarder that can be rotated with respect to each other to obtain any retardance, said quarter wave retarder and said half wave retarder positioned to receive light having propagated through said first and second optical amplifier assemblies to compensate for the depolarization introduced by said first and second optical amplifier assemblies.

22. The laser amplifier of Claim 1, further comprising a quarter waveplate or faraday rotator in the optical path of the amplified laser beam to provide polarization multiplexing.

23. The laser amplifier of Claim 1, further comprising a spatial filter in said optical path between said first and second optical amplifier assemblies.

24. The laser amplifier of Claim 1, further comprising at least one optical pump configured to pump said gain elements in said first and second optical amplifier assemblies.

25. The laser amplifier of Claim 24, wherein said at least one optical pump comprises at least one laser diode.

26. The laser amplifier of Claim 1, further comprising a least one optical relay comprising a plurality of lenses between said first and second optical amplifier assemblies.

27. The laser amplifier of Claim 26, wherein said at least one optical relay comprises an afocal optical relay.

28. The laser amplifier of Claim 1, wherein said laser amplifier is configured to provide angle multiplexing.

29. A laser amplifier configured to reduce thermally induced depolarization, said laser amplifier comprising:a first multi-slab amplifier assembly comprising a plurality of gain slabs and a first polarization rotator inserted between gain slabs of said plurality of gain slabs in said first multi-slab amplifier assembly; anda second multi-slab amplifier assembly comprising a plurality of gain slabs and a second polarization rotator inserted between gain slabs of said plurality of gain slabs in said second multi- slab amplifier assembly.

30. The laser amplifier of Claim 29, wherein said first and second polarization rotators have opposite handedness.

31. The laser amplifier of Claim 29, wherein said first and second polarization rotators have same handedness.

32. The laser amplifier of Claim 29, further comprising a least one reflector, said first multi-slab amplifier assembly and said at least one reflector forming an optical path in said laser cavity, said second multi-slab amplifier assembly included in said optical path between said first multi-slab amplifier assembly and said at least one reflector such that light propagates along said optical path through said first multi-slab amplifier assembly and then through said second multi-slab amplifier assembly, reflects off said at least one reflector back through said second multi-slab amplifier assembly, and then continues through said first multi-slab amplifier assembly.

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