Optical amplifier

Abstract

1. An optical amplifier configured to amplify a laser beam, the optical amplifier comprising: an optical path for the laser beam including a first portion and a second portion; and a reflector configured to reflect the laser beam to direct the laser beam between the first and second portions of the optical path, the reflector configured to reflect the laser beam such that the laser beam reflected by the reflector is parallel to the laser beam incident on the reflector.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Application No. 22205638.4, filed November 4, 2022, which is incorporated herein by reference in its entirety.

[0002] The present invention relates to optical amplifiers and related systems. [Background technology]

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern in a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on the substrate.

[0004]

[0004] Electromagnetic radiation may be used in lithographic apparatus to project a pattern onto a substrate. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Lithographic apparatuses that use extreme ultraviolet (EUV) radiation, having a wavelength in the range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, allow smaller features to be formed on the substrate than lithographic apparatuses that use radiation with a wavelength of, for example, 193 nm.

[0005]

[0005] The lithography system may include a radiation system and a lithography apparatus. The radiation system may include a laser system configured to generate a laser beam, and an EUV radiation source. The EUV radiation may be generated, for example, by directing a laser beam towards the radiation source so that the laser beam is incident on a fuel in the radiation source. The laser beam may deposit energy in the fuel to create a plasma. Radiation including EUV radiation may be emitted from the plasma.

[0006]

[0006] A laser system may include a laser configured to generate a laser beam and an optical amplifier configured to amplify the laser beam. During use, the laser beam passing through the optical amplifier may be subject to one or more pointing errors or pointing variations. The pointing errors or variations may cause one or more errors or variations in the laser beam position. These errors or variations may be proportional to the optical path of the laser beam within the optical amplifier. Thus, when directing a laser beam through two or more optical amplifiers, the errors or variations in the laser beam position may increase. The errors or variations in the laser beam position may cause a reduction in the power output of the amplified laser beam. Summary of the Invention

[0007]

[0007] According to a first aspect of the present invention, there is provided an optical amplifier configured to amplify a laser beam, the optical amplifier comprising an optical path of the laser beam including a first portion and a second portion, and a reflector configured to reflect the laser beam so as to guide the laser beam between the first portion and the second portion of the optical path, the reflector configured to reflect the laser beam such that the laser beam reflected by the reflector is parallel to the laser beam incident on the reflector.

[0008]

[0008] During use, an optical amplifier may be subjected to a thermal load. For example, the thermal load acting on the optical amplifier may cause movement and / or deformation of one or more components of the optical amplifier. The movement and / or deformation of one or more components of the optical amplifier may be referred to as internal movement and / or deformation. The one or more components of the optical amplifier may include one or more optical elements and / or a frame configured to support the one or more optical elements. The movement or deformation of one or more components of the optical amplifier may cause a beam pointing error or beam pointing variation of a laser beam passing through the optical amplifier. The beam pointing error or beam pointing variation of the laser beam may cause a reduction in the power of the amplified laser beam. By configuring a reflector so that the laser beam reflected by the reflector is parallel to the laser beam incident on the reflector, it is possible to reduce or prevent the beam pointing error or beam pointing variation of the laser beam passing through the optical amplifier. Therefore, it is possible to reduce or prevent a reduction in the power of the amplified laser beam.

[0009]

[0009] The reflector may be configured to reflect the laser beam at least twice, for example to direct the laser beam between the first and second portions of the optical path.

[0010]

[0010] The reflector may include a retroreflector. The retroreflector may include at least one of a corner reflector and / or a conical reflector. The corner reflector may include a cube corner.

[0011]

[0011] The reflector may include one or more reflective surfaces. The one or more reflective surfaces may include a first position, e.g., on or at which the laser beam is incident. The one or more reflective surfaces may include a second position, e.g., at which the laser beam is reflected away from the reflector. At least one or each of the one or more reflective surfaces may include or define a flat or curved surface. The first and second positions of the one or more reflective surfaces may be spaced apart from each other. The distance between the first and second positions of the one or more reflective surfaces may correspond to the space between the first and second portions of the optical path. The one or more reflective surfaces may be arranged such that the laser beam is reflected at least three times, e.g., to guide the laser beam between the first and second portions of the optical path. The one or more reflective surfaces may be arranged such that at least one reflective surface is disposed between at least two other reflective surfaces. The one or more reflective surfaces may be arranged such that at least one reflective surface faces a first direction. The one or more reflective surfaces may be arranged such that at least one other reflective surface faces a second direction. The first direction and the second direction may be different and / or may be opposite (e.g., substantially opposite) directions. The one or more reflective surfaces may be arranged such that an M-shape is defined by at least one of the laser beam reflected by the reflector, the laser beam incident on the reflector, and / or a path of the laser beam between the first and second positions of the one or more reflective surfaces.

[0012]

[0012] According to a second aspect of the present invention, there is provided an amplifier system for use in a laser system, the system comprising a plurality of optical amplifiers, at least one of the plurality of optical amplifiers or each optical amplifier comprising an optical amplifier according to the first aspect, an optical system configured to optically couple at least one of the plurality of optical amplifiers to at least one other of the plurality of optical amplifiers, and a frame configured to support the optical system and configured as a separate entity from the plurality of optical amplifiers.

[0013] During use, a thermal load may act on at least one or each of the optical amplifiers of the plurality of optical amplifiers. For example, a thermal load acting on at least one or each of the optical amplifiers of the plurality of optical amplifiers may cause movement and / or deformation of one or more components of at least one or each of the optical amplifiers of the plurality of optical amplifiers. The one or more components of at least one of the optical amplifiers of the plurality of optical amplifiers may include one or more optical elements and / or a frame configured to support the one or more optical elements. When an optical system (e.g., one or more components thereof) is connected to each of the optical amplifiers of the plurality of optical amplifiers, movement and / or deformation of one or more components of each of the optical amplifiers of the plurality of optical amplifiers may cause movement of one or more components of the optical system connected thereto. Such movement of one or more components of the optical system connected to each of the optical amplifiers of the plurality of optical amplifiers may be referred to as external movement. This external movement may result in beam pointing errors or beam pointing variations of the amplified laser beam, which may be directed to another optical amplifier or exit the amplifier system. By configuring the frame separately from the optical amplifiers and configuring the reflector such that the laser beam reflected by the reflector is parallel to the laser beam incident on the reflector, it is possible to reduce or prevent beam pointing errors or beam pointing variations of the amplified laser beam that may be caused by internal movements and / or variations and external movements, and therefore reduce or prevent power degradation of the amplified laser beam.

[0014]

[0014] The frame may be configured to support the optical system such that the or each optical amplifier of the plurality of optical amplifiers is movable relative to the optical system. The frame may be configured to support the optical system such that the or each optical amplifier of the plurality of optical amplifiers is decoupled from the optical system.

[0015]

[0015] The frame may include or be formed from at least one of a metal material, a metal alloy material, a fiber material, a ceramic material, and / or a glass material. The metal material may include aluminum. The metal alloy material may include at least one of steel and / or stainless steel. The fiber material may include carbon fiber.

[0016]

[0016] The system may include an additional frame. The additional frame may be configured to mount a plurality of optical amplifiers. The frame may be connected to the additional frame. The frame may be integral with the additional frame.

[0017]

[0017] According to a third aspect of the present invention, there is provided an amplifier system for use in a laser system, the system comprising: a plurality of optical amplifiers configured to amplify a laser beam, wherein at least one or each of the plurality of optical amplifiers includes an optical path of the laser beam, the optical path including a first portion and a second portion; an optical system configured to optically couple at least one of the plurality of optical amplifiers to at least one other of the plurality of optical amplifiers; and a frame configured to support the optical system and configured separately from the plurality of optical amplifiers, wherein at least one or each of the plurality of optical amplifiers includes a retroreflector configured to guide the laser beam between the first and second portions of the optical path.

[0018]

[0018] According to a fourth aspect of the present invention, there is provided a laser system comprising a laser configured to generate a laser beam and an amplifier system according to the second and / or third aspects, which amplifier system is configured to amplify the laser beam.

[0019]

[0019] The frame may be connected to the laser, for example to the housing of the laser.

[0020] According to a fifth aspect of the present invention, there is provided a radiation system comprising an EUV radiation source and a laser system according to the fourth aspect.

[0021] According to a sixth aspect of the present invention there is provided a radiation system comprising an EUV radiation source and a laser system including an optical amplifier according to the first aspect.

[0022] According to a seventh aspect of the present invention there is provided a lithography system comprising a radiation system according to the fifth aspect and a lithography system.

[0023]

[0023] The various aspects and features of the invention described above or below may be combined with various other aspects and features of the invention, as will be apparent to those skilled in the art. [Brief explanation of the drawings]

[0024]

[0024] Some embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: [Figure 1] 1 depicts a lithography system comprising a lithographic apparatus and a radiation system; [Figure 2] 2 illustrates an exemplary amplifier system for use in the laser system of the radiation system of FIG. 1; [Figure 3] 3 illustrates an exemplary optical path of a laser beam through the amplifier system of FIG. 2. [Figure 4] 3 illustrates an exemplary optical amplifier for use in the amplifier system of FIG. 2, the optical amplifier including a reflector. [Figure 5] 5 illustrates an exemplary optical path of a laser beam through the optical amplifier of FIG. 4. [Figure 6] 5 illustrates an exemplary amplifier frame used in the optical amplifier of FIG. 4; [Figure 7] 5A and 5B illustrate schematic diagrams of exemplary optical paths of laser beams through the optical amplifier of FIG. 4, where the reflector comprises a reflective prism. [Figure 8A]5 shows a schematic side view of the optical amplifier of FIG. 4, where the reflector comprises a reflective prism. [Figure 8B] 5 shows a schematic plan view of the optical amplifier of FIG. 4, where the reflector comprises a reflective prism. [Figure 9A] 5 shows a schematic side view of the optical amplifier of FIG. 4 and one or more components of the optical system of the amplifier system of FIG. 1 , where the one or more components are supported on a frame separate from the optical amplifier, and the reflector comprises a reflective prism. [Figure 9B] 5 shows a schematic plan view of the optical amplifier of FIG. 4 and one or more components of the optical system of the amplifier system of FIG. 1 , where the one or more components are supported on a frame separate from the optical amplifier, and the reflector comprises a reflective prism. [Figure 10A] The optical path of the laser beam through the optical amplifier of FIG. 4 is shown schematically in the x, y plane in FIG. 7, where the reflector comprises a reflective prism. [Figure 10B] The optical path of the laser beam through the optical amplifier of FIG. 4 is shown schematically in the y,z plane in FIG. 7, where the reflector comprises a reflective prism. [Figure 11] 5 schematically illustrates another exemplary optical path of a laser beam through the optical amplifier of FIG. 4, where the reflector comprises a retroreflector. [Figure 12A] 5 shows a schematic plan and side view of the optical amplifier of FIG. 4, where the reflector comprises a retroreflector. [Figure 12B] 5 shows schematic top and side views of the optical amplifier of FIG. 4 and one or more components of the optical system of the amplifier system of FIG. 1, where the one or more components are supported on a frame separate from the optical amplifier, and the reflector comprises a retroreflector. [Figure 13] The optical path of the laser beam through the optical amplifier of FIG. 4 is shown schematically in the x, y and y, z planes in FIG. 11, where the reflector comprises a retroreflector. [Figure 14A] 2 shows an exterior side view of a portion of the amplifier system of FIG. 1. [Figure 14B] 2 shows an interior side view of the portion of the amplifier system of FIG. 1. [Figure 14C] A portion of the amplifier system shown in FIG. 14B is shown, including the optical amplifier. [Figure 15A] 5 schematically illustrates another exemplary reflector for use in the optical amplifier of FIG. 4; [Figure 15B] 5 schematically illustrates another exemplary reflector for use in the optical amplifier of FIG. 4; [Figure 15C] 5 schematically illustrates another exemplary reflector for use in the optical amplifier of FIG. 4; [Figure 16] 5 schematically illustrates another exemplary reflector for use in the optical amplifier of FIG. 4; [Figure 17] 5 schematically illustrates another exemplary reflector for use in the optical amplifier of FIG. 4; DETAILED DESCRIPTION OF THE INVENTION

[0025] 1 shows a lithography system including a radiation system RS and a lithography apparatus LA. The radiation system RS includes a radiation source RO. The radiation source RO is configured to generate a beam of EUV radiation B and to provide this beam of EUV radiation B to the lithography apparatus LA. The lithography apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

[0026] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. Furthermore, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. Together, the facetted field mirror device 10 and the facetted pupil mirror device 11 impart a desired cross-sectional shape and a desired intensity distribution to the EUV radiation beam B. The illumination system IL may include other mirrors or devices in addition to or instead of the facetted field mirror device 10 and the facetted pupil mirror device 11.

[0027]

[0027] Having been so conditioned, the EUV radiation beam B interacts with the patterning device MA. This interaction results in a patterned EUV radiation beam B'. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. To this end, the projection system PS may include a number of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held on the substrate table WT. The projection system PS may apply a demagnification factor to the patterned EUV radiation beam B', thereby forming an image having smaller features than corresponding features on the patterning device MA. For example, a demagnification factor of 4 or 8 may be applied. Although in Figure 1 the projection system PS is depicted as having only two mirrors 13, 14, the projection system PS may include a different number of mirrors (e.g. 6 or 8 mirrors).

[0028] The substrate W may include a previously formed pattern, in which case the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the previously formed pattern on the substrate W.

[0029] A relative vacuum, ie a small amount of gas (eg hydrogen) at a pressure significantly below atmospheric pressure, may be provided within the source SO, illumination system IL and / or projection system PS.

[0030] The radiation source SO shown in FIG. 1 is of a type that may be referred to as a laser-produced plasma (LPP) source, for example. The laser system 1 may include, for example, a CO laser, and is arranged to deposit energy via a laser beam 2 in a fuel, such as tin (Sn), provided by a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may be, for example, in liquid form and may be, for example, a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin droplets, for example, along a trajectory toward a plasma formation region 4. The laser beam 2 is incident on the tin in the plasma formation region 4. Deposition of the laser energy in the tin creates a tin plasma 7 in the plasma formation region 4. During de-excitation and recombination of ions and electrons of this plasma, radiation including EUV radiation is emitted from the plasma 7.

[0031]

[0031] The EUV radiation from the plasma is collected and focused by a collector 5. The collector 5 may, for example, comprise a near-normal incidence radiation collector 5 (sometimes more commonly referred to as a normal incidence radiation collector). The collector 5 may be a multi-layer mirror structure arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength, such as 13.5 nm). The collector 5 may be an elliptical configuration having two foci. As described below, a first of the foci may be at the plasma formation region 4 and a second of the foci may be at an intermediate focus 6.

[0032]

[0032] The laser system 1 may be spatially separated from the radiation source SO. In this case, the laser beam 2 may be routed from the laser system 1 to the radiation source SO using, for example, a beam delivery system (not shown) including appropriate directing mirrors and / or beam expanders, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system may be considered together as a radiation system.

[0033]

[0033] The radiation reflected by the collector 5 forms an EUV radiation beam B. The EUV radiation beam B is focused to an intermediate focus 6 and forms an image of the plasma present in the plasma formation region 4 at the intermediate focus 6. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is positioned such that the intermediate focus 6 is located at or near an opening 8 in an enclosing structure 9 of the radiation source SO.

[0034] 2 shows an exemplary amplifier system 16 used in laser system 1 of radiation system RS. This amplifier system 16 may be referred to as an optical amplifier system. Laser system 1 may include a laser 18, such as a CO2 laser. Laser 18 may be configured to generate a laser beam 20. Laser beam 20 may include a plurality of pulses. Laser system 1 may include one or more optical elements 22, such as one or more mirrors, beam expanders, lenses, and / or other elements, for directing the laser beam into amplifier system 16.

[0035]

[0035] Amplifier system 16 includes a plurality of optical amplifiers 24a-24d, four of which are shown in Figure 2. However, it will be understood that in other embodiments, the system may include more or less than four optical amplifiers. Each optical amplifier 24a-24d is configured to amplify laser beam 20.

[0036] The amplifier system 16 includes an optical system 26. The optical system 26 may also be referred to as a relay optical system. The optical system 26 is configured to optically couple or connect at least one of the optical amplifiers 24a-24d to at least one other of the optical amplifiers 24a-24d. In this embodiment, the optical system 26 is configured to optically couple or connect the amplifiers 24a-24d to each other in series. For example, the optical system 26 may be configured to optically couple or connect the optical amplifiers 24a-24d to each other such that the laser beam 20 is guided from the first amplifier 24a to the second amplifier 24b, from the second amplifier 24b to the third amplifier 24c, and from the third amplifier 24c to the fourth amplifier 24d. The optical system 26 may include multiple optical elements, such as multiple mirrors, lenses, telescopes, etc. The optical amplifiers 24a-24d may be arranged to sequentially amplify the laser beam 20.

[0037]

[0037] The amplifier system 16 may include, for example, an input 25a at which the laser beam 20 enters the amplifier system 16. The amplifier system 16 may include, for example, an output 25b at which the amplified laser beam 21 exits the amplifier system 16. At the input 25a of the amplifier system 16, the laser beam may have a power output of approximately 100 W to 200 W, for example, approximately 140 W. At the output 25b of the amplifier system 16, the amplified laser beam 21 may have a power output of approximately 25 kW to 50 kW. The amplifier system 16 includes a frame 28 configured to support the optical system 26. For example, the optical system 26 may be attached to the frame 28. The frame 28 may be configured separately from the optical amplifiers 24a to 24d. The frame 28 may be configured to support the optical system 26 such that each of the optical amplifiers 24a to 24d can be separated from and / or moved relative to the optical system 26. For example, there may be no physical connection between each of the optical amplifiers 24a to 24d and the optical system 26. By configuring the frame 28 as a separate body from the optical amplifiers 24a to 24d, it is possible to reduce or prevent pointing errors or pointing fluctuations of the laser beam 20 that may be caused by, for example, thermal deformation of a part of each of the optical amplifiers 24a to 24d.

[0038]

[0038] The frame 28 may be configured to couple to the laser 18, for example, to the housing of the laser. For example, the frame 28 may be configured to extend in a direction parallel (e.g., substantially parallel) to the optical amplifiers 24a-24d. The frame 28 may be configured to extend and couple to the laser 18. The frame 28 may be configured to support at least a portion or all of the input portion 25a. By configuring the frame 28 to couple to the laser, movement of the laser 18 relative to the optics 26 may be reduced or prevented. That is, the optics 26 and the laser 18 may move in unison, thereby reducing or preventing pointing errors or pointing fluctuations of the laser beam 20. It will be appreciated that in other embodiments, the frame may be configured differently.

[0039] FIG. 3 illustrates an exemplary optical path of the laser beam 20 passing through the amplification system 16 shown in FIG. 2. In FIG. 3, each optical amplifier 24a-24d is indicated by a dashed-line box. Each optical amplifier 24a-24d includes or defines a respective optical path 30a-30d. Each optical path 30a-30d includes a first portion 32a and a second portion 32b. The first portion 32a and the second portion 32b of each optical path 30a-30d may be spaced apart from each other. For example, the first portion 32a and the second portion 32b of each optical path 30a-30d may be spaced apart from each other in a direction perpendicular (e.g., substantially perpendicular) to the planes of the first portion 32a and the second portion 32b. In FIG. 3, a space E between the first portion 32a and the second portion 32b is illustrated. In the embodiment shown in Figure 3, the first portion 32a is above the second portion 32b, although it will be appreciated that in other embodiments the first and second portions of each optical path may be positioned differently.

[0040] Each amplifier 24a-24d includes a respective reflector 34a-34d, each indicated by a box in FIG. 3 . As described in more detail below, each reflector 34 is configured to reflect the laser beam 20, e.g., at least twice, to direct the laser beam 20 between the first portion 32a and the second portion 32b of the respective optical path 30a-30d. For example, each reflector 34a-34d may be configured to direct the laser beam 20 from the first portion 32a to the second portion 32b of the respective optical path 30a-30d, or from the second portion 32b to the first portion 32a. The first portion 32a and / or the second portion 32b of each optical path 30a-30d of each optical amplifier 24a-24d may also be referred to as a level.

[0041] FIG. 4 illustrates an exemplary optical amplifier 24 used in the amplifier system 16 shown in FIG. 2. The optical amplifier 24 may include multiple amplifier frames, e.g., a first amplifier frame 36a, a second amplifier frame 36b, and a third amplifier frame 36c. It will be understood that in other embodiments, the optical amplifier may include more or less than three amplifier frames. The first amplifier frame 36a may be configured to support a first tube 38a. The second amplifier frame 36b may be configured to support a second tube 38b. The first tube 38a and the second tube 38b may contain a gas mixture. In some embodiments, the gas mixture may be sealed within the first tube 38a and / or the second tube 38b. In other embodiments, the gas mixture may be pumped, for example, continuously pumped, through the first tube and / or the second tube. The gas mixture may include carbon dioxide, oxygen, carbon monoxide, nitrogen, hydrogen, xenon, and / or helium. The optical amplifier 24 may be configured to guide the laser beam 20 through the gas mixture. One or more radio frequency (RF) signals may be used to excite the molecules of the gas mixture. The one or more radio frequency signals may include one or more radio frequency pulses. Alternatively, the one or more radio frequency signals may include continuous wave radio frequency signals. Applying the one or more radio frequency signals to the gas mixture can create a population inversion of the molecules of the gas mixture. This allows the laser beam 20 passing through the gas mixture to be amplified.

[0042] The amplifier 24 may include multiple optical elements 40, one of which is shown in FIG. 4 . The optical elements 40 may be provided in the form of one or more reflective elements, such as one or more mirrors. The optical elements 40 may be configured to guide the laser beam 20 through the gas mixture in the first tube 38a and the second tube 38b. The optical elements 40 may be disposed at one or more corners of each of the first frame 36a and the second frame 36b. For example, the optical elements 40 may be disposed at three corners of each of the first frame 36a and the second frame 36b. Reflectors 34 may be disposed at the remaining corners of each of the first frame 36a and the second frame 36b. The first tube 38a and several optical elements 40 may define a first portion 32a of the optical path 30 of the amplifier 24. The second tube 38b and several other optical elements 40 may define a second portion 32b of the optical path 30 of the amplifier 24. The first tube 38a may be disposed around the first frame 36a, and the second tube 38b may be disposed around the second frame 36b.

[0043] The third amplifier frame 36c may be configured to support the optical element 40 and the reflector 34. The third amplifier frame 36c may be disposed between the first amplifier frame 36a and the second amplifier frame 36b.

[0044]

[0044] Figure 5 shows an exemplary optical path 30 of the laser beam 20 passing through the optical amplifier 24 shown in Figure 4. Figure 6 shows an exemplary third amplifier frame 36c used in the optical amplifier 24 shown in Figure 4. In Figure 6, the optical path 30 of the laser beam 20 passing through the optical amplifier 24 relative to the third amplifier frame 36c is also shown by a dashed line.

[0045] In this embodiment, the first and second portions 32a and 32b of the optical path 30 of the laser beam 20 each include three optical elements 40. It will be appreciated that in other embodiments, at least one of the first and second paths of the laser beam, or each path, may include more or less than three optical elements. The optical amplifier 24 includes a reflector 34, which in this embodiment is provided in the form of a retroreflector, such as a corner reflector. For example, the corner reflector may be provided in the form of a cube corner. It will be appreciated that in other embodiments, the reflector may be implemented in a different manner. For example, in other embodiments, the retroreflector may be provided in the form of a conical reflector, a cat-eye configuration, or the like.

[0046] 5, the laser beam 20 enters the optical amplifier 24 near the reflector 34. The optical element 40 may include a first optical element 40a and a second optical element 40b. The first optical element 40a may be positioned to direct the laser beam 20 on the first portion 32a of the optical path 30. The first optical element 40a may be positioned to direct the laser beam 20 to the reflector 34. The reflector 34 may be configured to reflect the laser beam 20 at least twice, for example, to direct the laser beam 20 from the first portion 32a to the second portion 32b of the optical path 30. As described in more detail below, in this embodiment, the reflector 34 is configured to reflect the laser beam 20 three times. The reflector 34 is configured to reflect the laser beam 20 such that the laser beam reflected by the reflector 34 is parallel (e.g., substantially parallel) to the laser beam incident on the reflector 34. In other words, the direction of the laser beam reflected by reflector 34 may be opposite to the direction of the laser beam incident on reflector 34. The laser beam incident on reflector 34 may also be referred to as incident laser beam 20a. The laser beam reflected by reflector 34 may also be referred to as reflected laser beam 20b. In Figure 5, a space E between first portion 32a and second portion 32b of optical path 30 is shown.

[0047] The second optical element 40b may be positioned to guide the laser beam 20 on the second portion 32b of the optical path 30. The second optical element 40b may be positioned to guide the reflected laser beam 20b away from the reflector 34, for example, so that the reflected laser beam 20b exits the optical amplifier 24 near the reflector 34. The laser beam exiting the optical amplifier 24 may be considered, for example, as an amplified laser beam 20c relative to the laser beam 20d entering the optical amplifier 24. The optical amplifier 24 may include a first optical window 41a and a second optical window 41b. The first optical window 41a and the second optical window 41b may be positioned so that the laser beam 20d enters the optical amplifier 24 through the first optical window 41a and the amplified laser beam 20c exits the optical amplifier through the second optical window 41b.

[0048] The third amplifier frame 36c may be configured to support the first optical element 40a and the second optical element 40b. For example, the third amplifier frame 36c may include a plurality of first support elements 42, three of which are shown in FIG. 6. Each of the first support elements 42 may be provided in the form of a block or a mounting block. Each of the first support elements 42 may be configured to support the first optical element 40a and the second optical element 40b.

[0049] The third amplifier frame 36c may be configured to support the reflector 34. For example, the third amplifier frame 36c may include a second support element 44. The second support element 44 may be provided in the form of a block or a mounting block. The second support element 44 may be configured to support the reflector 34.

[0050] The first support element 42 and the second support element 44 may be disposed at one or more corners of the third amplifier frame 36c. For example, the third amplifier frame 36c may define or include a rectangular or square shape. The first support element 42 and the second support element 44 may be disposed at one or more corners of the rectangle or square.

[0051] During use, thermal loads may act on one or more of the first, second, and third amplifier frames 36a-36c. For example, a thermal load acting on the third amplifier frame 36c may cause movement and / or deformation (e.g., bending) of at least the third amplifier frame 36c. This may result in movement of the first support element 42 and / or the second support element 44, which may cause the first optical element 40a and / or the second optical element 40b and / or the reflector 34 to become optically misaligned. As a result, one or more drifts and / or pointing errors or pointing variations of the laser beam 20 may occur, as described in more detail below. Movement and / or deformation of the third amplifier frame 36c is illustrated in FIG. 6.

[0052]

[0052] Figure 7 shows a schematic diagram of an exemplary optical path 30 of the laser beam 20 passing through the optical amplifier 24 shown in Figure 4. The optical path 30 includes a first portion 32a and a second portion 32b. In this embodiment, the reflector 34 is provided in the form of a reflective prism, such as a Porro prism. Figure 7 shows the first portion 32a and the second portion 32b of the optical path 30 relative to a coordinate system.

[0053]

[0053] Figure 8A shows a schematic side view of the optical amplifier 24 shown in Figure 4, for example, in the y-z plane of the coordinate system shown in Figure 7. Figure 8B shows a schematic plan view of the optical amplifier 24 shown in Figure 4, for example, in the x-y plane of the coordinate system shown in Figure 7. For clarity, only a portion of the first optical element 40a and the second optical element 40b are shown in Figures 8A and 8B. The reflector 34 is omitted in Figures 8A and 8B. It will be understood that the arrangement of the first optical element 40a and the second optical element 40b of the optical amplifier 24 shown in Figures 8A and 8B is the same as that described above. As described above, the optical amplifier 24 can be configured so that the laser beam 20 enters and exits the optical amplifier 24 at substantially the same position, for example, near the reflector 34. However, for clarity, in Figures 8A and 8B, laser beam 20 is shown as entering and exiting optical amplifier 24 at different locations.

[0054] In the example shown in FIGS. 8A and 8B, two components 26a of the optical system 26 are connected to the optical amplifier 24, e.g., to one or all of the first, second, and third amplifier frames 36a-36c. Because the component 26a of the optical system 26 is connected to the optical amplifier 24, movement and / or deformation of the optical amplifier 24 (e.g., the third frame 36c), as indicated by arrow M in FIGS. 8A and 8B, can cause the component 26a of the optical system 26 to move. This may be referred to as external movement and can cause beam pointing errors or fluctuations. For example, the amplified laser beam 20c, shown by the solid line in FIGS. 8A and 8B, can exit the optical amplifier 24 at an angle β relative to the amplified laser beam 20c when the optical amplifier 24 is held stationary, as indicated by the dashed line in FIGS. 8A and 8B. This angle of the amplified laser beam 20c can cause beam pointing errors or fluctuations. Beam pointing errors or beam pointing variations may cause errors or variations in the beam position, which may be proportional to the optical length of the amplifier system 16. Thus, the longer the optical length of the amplifier system 16, the greater the potential for beam positioning errors or variations. Beam pointing errors or beam pointing variations may lead to reduced power output of the amplified laser beam exiting the optical amplifier, which may result in reduced power output of the amplified laser beam exiting the amplifier system.

[0055] 9A shows a schematic side view of the optical amplifier 24 shown in FIG. 4, for example, in the y-z plane of the coordinate system shown in FIG. 7. FIG. 9B shows a schematic plan view of the optical amplifier 24 shown in FIG. 4, for example, in the x-y plane of the coordinate system shown in FIG. 7. FIGS. 9A and 9B are similar to FIGS. 8A and 8B. Therefore, only the differences will be described below. In the example shown in FIGS. 9A and 9B, the optical system 26, for example, the part 26a thereof, is supported by the frame 28 as described above. Therefore, the optical system 26 (for example, the part 26a thereof) is optically coupled or connected to the optical amplifier 24. However, the optical amplifier 24 is decoupled from the optical system 26 (for example, the part 26a thereof) and / or is movable relative to the optical system 26 (for example, the part 26a thereof). That is, there is no physical connection between the optical system 26 (for example, the part 26a thereof) and the optical amplifier 24.

[0056] 9A, movement and / or deformation of the optical amplifier 24 relative to the optical system 26 (e.g., part 26a thereof) may cause the amplified laser beam 20c to shift (e.g., translationally shift) relative to the amplified laser beam 20c when the optical amplifier 24 is held stationary. That is, movement and / or deformation of the optical amplifier 24 may cause the amplified laser beam 20c to become parallel (e.g., substantially parallel) relative to the amplified laser beam 20c when the optical amplifier 24 is held stationary. By configuring the frame 28 as a separate entity from the optical amplifier 24, pointing errors or pointing variations of the laser beam 20 in at least one plane of the optical amplifier 24, which may be caused by external movement, may be reduced or prevented. In the above and following descriptions, the expression "movement and / or deformation of the optical amplifier 24" may be considered to encompass movement and / or deformation of the third frame 36c of the optical amplifier 24. The expression "movement and / or deformation of the optical amplifier 24" may be considered to encompass movement and / or deformation of the first frame 36a and / or the second frame 36b of the optical amplifier 24. It will be understood that movement and / or deformation of the optical amplifier 24 may cause optical misalignment of the optical amplifier 24 relative to the optical system 26 (e.g., part 26a thereof).

[0057]

[0057] Referring to Figure 9B, movement and / or deformation of the optical amplifier 24 may cause the amplified laser beam 20c to exit the optical amplifier 24 at an angle β in the x, y plane shown in Figure 7 relative to the amplified laser beam 20c when the optical amplifier 24 is held stationary.

[0058]

[0058] Figure 10A schematically illustrates the optical path 30 of the laser beam 20 passing through the optical amplifier 24 in the x- and y-planes shown in Figure 7. Figure 10B schematically illustrates the optical path 30 of the laser beam 20 passing through the optical amplifier 24 in the y- and z-planes shown in Figure 7. For clarity, only a portion of the first optical element 40a is shown in Figures 10A and 10B. However, it will be understood that the optical amplifier 24 shown in Figures 10A and 10B may include a first optical element 40a and a second optical element 40b, as described above and below. In the example shown in Figures 10A and 10B, for example, one of the first optical elements 40a may move due to movement and / or deformation of the optical amplifier 24. This movement and / or deformation may be referred to as internal movement and / or deformation. The movement of the first optical element 40a is indicated by arrow M in Figures 10A and 10B. It will be appreciated that in other embodiments, two or more first optical elements and / or one or more second optical elements may move, for example due to movement and / or deformation of an optical amplifier. The expression "movement of the first optical element and / or the second optical element" may be considered to encompass optical misalignment of the first optical element and / or the second optical element.

[0059] 10A and 10B show the optical path 30 of the laser beam 20 when the first optical element 40a is kept stationary. The solid line in FIG. 10A shows the optical path 30 of the laser beam 20 when the first optical element 40a is moved. Referring to FIG. 10A, the movement of the first optical element 40a can cause the amplified laser beam 20c to exit the optical amplifier 24 at an angle β. In this example, the reflector 34 can be thought of as functioning as a plane mirror in the x, y plane shown in FIG. 7.

[0060]

[0060] Referring to Figure 10B, movement of the first optical element 40 can cause the amplified laser beam 20c to be shifted (e.g., translationally shifted) and / or become parallel (e.g., substantially parallel) relative to the amplified laser beam 20c when the optical element is kept stationary.

[0061]

[0061] Figure 11 schematically illustrates another exemplary optical path 30 of the laser beam 20 passing through the optical amplifier 24 shown in Figure 4. The optical path 30 includes a first portion 32a and a second portion 32b. In this embodiment, the reflector 34 is provided in the form of a retroreflector, e.g., a cube corner, as described above in connection with the embodiment shown in Figure 5. It will be understood that the reflector may be oriented and / or positioned in numerous ways different from those disclosed herein. However, in each of these ways, the reflector may be considered to be configured to reflect the laser beam such that the laser beam reflected by the reflector is parallel to the laser beam incident on the reflector.

[0062]

[0062] Figure 11 shows the optical path 30 of the laser beam 20 relative to a coordinate system. Figure 12A shows schematic plan and side views of the optical amplifier 24 shown in Figure 4, for example, in the y-z and x-y planes of the coordinate system shown in Figure 11. For clarity, only a portion of the first optical element 40a is shown in Figure 12A. However, it will be understood that the optical amplifier 24 shown in Figures 10A and 10B may include the first optical element 40a and the second optical element 40b, as described above and below. In the example shown in Figure 12A, the component 26a of the optical system 26 is connected to the optical amplifier 24, for example, to one or all of the first, second, and third amplifier frames 36a-36c of the optical amplifier 24. Because the component 26a of the optical system 26 is connected to the optical amplifier 24, movement and / or deformation of the optical amplifier 24, as indicated by arrow M in Figure 12A, may cause movement of the component 26a of the optical system 26. This can result in beam pointing errors or beam pointing variations. For example, movement and / or deformation of optical amplifier 24 can cause amplified laser beam 20c, shown by the solid line in Figure 12A, to exit optical amplifier 24 at an angle β relative to the amplified laser beam 20c if optical amplifier 24 were held stationary, shown by the dashed line in Figure 12A.

[0063]

[0063] Figure 12B shows another schematic plan and side view of the optical amplifier 24 shown in Figure 4, for example, in the y, z and x, y planes of the coordinate system shown in Figure 11. Figure 12B is similar to Figure 12A. Therefore, only the differences will be described below. In the example shown in Figure 12B, the optical system 26, for example, the part 26a thereof, is supported by the frame 28 as described above. Therefore, the optical system 26 (for example, the part 26a thereof) is optically coupled or connected to the optical amplifier 24. However, the optical amplifier 24 is decoupled from the optical system 26 (for example, the part 26a thereof) and / or is movable relative to the optical system 26 (for example, the part 26a thereof). That is, there is no physical connection between the optical system 26 (for example, the part 26a thereof) and the optical amplifier 24.

[0064] 12B, movement and / or deformation of the optical amplifier 24 relative to the optical system 26 (e.g., part 26a thereof) can cause the amplified laser beam 20c to shift (e.g., translationally shift) in the x, y and y, z planes shown in FIG. 11 relative to the amplified laser beam 20c when the optical amplifier 24 is held stationary. Thus, beam point errors or variations that may result from external movement can be reduced or prevented.

[0065] FIG. 13 schematically illustrates the optical path 30 of the laser beam 20 passing through the optical amplifier 24 shown in FIG. 4 in the x, y, and y, z planes shown in FIG. 11. For clarity, only some of the first optical elements 40a are shown in FIG. 13. However, it will be understood that the optical amplifier 24 shown in FIGS. 10A and 10B may include first optical elements 40a and second optical elements 40b, as described above and below. In the example shown in FIG. 13, one of the first optical elements 40a may move, for example, due to movement and / or deformation of the optical amplifier 24. The movement of the first optical element 40a is indicated by arrow M in FIG. 13. While the descriptions in FIGS. 10A, 10B, and 13 refer to movement of one first optical element 40a, it will be understood that in other embodiments, two or more first optical elements and / or one or more second optical elements may move, for example, due to movement and / or deformation of the optical amplifier.

[0066]

[0066] The dashed lines in Figure 13 show the optical path 30 of the laser beam 20 when the first optical element 40a is held stationary. The solid lines in Figure 13 show the optical path 30 of the laser beam 20 when the first optical element 40a is moved. As can be seen from Figure 13, movement of the first optical element 40a can cause the amplified laser beam 20c to be shifted (e.g., translationally shifted) and / or become parallel (e.g., substantially parallel) relative to the amplified laser beam 20c when the first optical element 40a is held stationary. Configuring the reflector 34 so that the reflected laser beam 20b is parallel to the incident laser beam 20a can reduce or prevent beam pointing errors or beam pointing variations of the laser beam 20 passing through the optical amplifier 24, which may be due to internal deformations and / or movement, for example, in both the x, y and y, z planes. Thus, by configuring frame 28 separately from optical amplifier 24 and by configuring reflector 34 so that reflected laser beam 20b is parallel to incident laser beam 20a, it is possible to reduce or prevent beam pointing errors or beam pointing variations of laser beam 20, which may be due to internal movement and / or deformation and external movement, for example, in both the x,y and y,z planes of optical amplifier 24. The above-described x,y and y,z planes shown in Figures 8A-13 are for illustrative purposes only, and the features of the optical amplifier should not be understood to be limited to these planes. For example, it will be understood that any features described with respect to the y,z plane of optical amplifier 24 are also applicable to the x,z plane of the optical amplifier. Therefore, by configuring the frame 28 separately from the optical amplifier 24 and by configuring the reflector 34 so that the reflected laser beam 20b is parallel to the incident laser beam 20a, it is possible to reduce or prevent beam pointing errors or beam pointing variations of the laser beam 20 that may be caused by internal movement and / or deformation and external movement, even within the x, z plane of the optical amplifier 24.

[0067]

[0067] Figures 14A and 14B show a portion of the amplifier system 16 shown in Figure 1 excluding the optical amplifiers 24a to 24d. Figure 14A shows the outer side of the portion of the amplifier system 16 shown in Figure 1. Figure 14B shows the inner side of the portion of the amplifier system 16.

[0068] 14A and 14B show a frame 28 configured to support the optical system 26. In the embodiment shown in Figures 14A and 14B, the frame 28 is not connected to the laser 18. Instead, the frame 28 is configured to support only the optical system 26. However, as discussed above in connection with Figure 2, in some embodiments, the frame 28 may be configured to connect to the laser 18.

[0069]

[0069] The frame 28 may be formed from materials such as metal materials, metal alloy materials, and / or fiber materials. Such materials may be selected to be thermally stable. For example, metal materials may include aluminum, etc. Metal alloy materials may include steel, stainless steel, etc. Fiber materials may include carbon fiber, etc. It will be understood that in other embodiments, materials may include ceramic materials, glass materials, etc.

[0070]

[0070] The amplifier system 16 may include an additional frame 45. The frame 28 may be connected to or attached to the additional frame 45. For example, the frame 28 may be welded to the additional frame 45 or at least a portion thereof. In other embodiments, the frame and the additional frame may be integral. The frame 28 may be attached to or connected to the additional frame 45 so as to extend along at least a portion of the additional frame 45.

[0071] The additional frame 45 may be configured to mount the optical amplifiers 24a-24d, for example, in series. For example, the additional frame 45 may include a plurality of additional support elements 46a, 46b, eight of which are shown in Figures 14A and 14B. It will be appreciated that in other embodiments, the additional frame may include more or less than eight additional support elements and / or may be configured to mount the amplifiers in a different arrangement.

[0072] Each of the additional support elements 46a, 46b may be configured to protrude or extend from the additional frame 45, for example, toward the inside of the amplifier system 16. Each of the additional support elements 46a, 46b may be arranged to protrude or extend from the additional frame 45 in a direction perpendicular (e.g., substantially perpendicular) to the longitudinal axis B of the additional frame 45. For example, each of the additional support elements 46a, 46b may be arranged to protrude or extend upward, for example, when the amplifier system 16 is in use.

[0073] At least two additional support elements 46a, 46b may be associated with each optical amplifier 24. The two additional support elements 46a, 46b may be configured to mount the respective amplifiers 46a, 46b. The two additional support elements 46a, 46b may have different sizes and / or shapes from one another. For example, the first support element 46a of the two additional support elements 46a, 46b may include or define an L-shape (e.g., substantially L-shape) and / or may be larger than the second support element 46b of the two additional support elements. The second support element 46b of the two additional support elements 46a, 46b may include or define a T-shape (e.g., substantially T-shape) and / or may be smaller than the first support element 46a.

[0074]

[0074] Figure 14C shows the portion of the amplifier system 16 shown in Figure 14B, including the optical amplifiers 24a-24d. Two additional support elements 46a, 46b may be positioned to mount opposite sides, e.g., diagonally opposite sides, of each optical amplifier 24.

[0075]

[0075] The amplifier system 16 may include a cooling system. The cooling system may be configured to cool at least one or all of the frame and the additional frame. For example, the frame 28 and / or the additional frame 45 may include a plurality of coolant channels (not shown). The coolant channels may be arranged to circulate a coolant through the frame 28 and / or the additional frame 45. The coolant may include a cooling fluid or liquid (e.g., water). The coolant channels may be arranged to maintain the frame 28 and the additional frame 45 at the same temperature. The coolant channels may be arranged to achieve a uniform temperature throughout the frame and / or the additional frame.

[0076]

[0076] Figures 15A-15C schematically illustrate another exemplary reflector 34 for use in an optical amplifier (e.g., the optical amplifier 24 shown in Figure 4). This reflector 34 is provided in the form of a corner cube. The reflector shown in Figures 15A-15C is similar to the reflector 34 described above in relation to Figures 5 and 11-13. Thus, any of the features described above in relation to Figures 5 and 11-13 may also apply to the reflector shown in Figures 15A-15C.

[0077] The reflector 34 may include one or more reflective surfaces 48a, 48b, 48c, three of which are shown in Figures 15A-15C. It will be appreciated that in other embodiments, the reflector may include more or less than three reflective surfaces. In this embodiment, each of the reflective surfaces 48a, 48b, 48c is provided in the form of a flat surface. It will be appreciated that in other embodiments, at least one or each of the reflective surfaces may include a curved surface.

[0078] The reflecting surfaces 48a, 48b, and 48c may be positioned to be perpendicular (e.g., substantially perpendicular) to one another. In this embodiment, the reflector 34 is configured to reflect the laser beam 20 three times. The incident laser beam 20a may be reflected from the first reflecting surface 48a to the second reflecting surface 48b. The incident beam 20a may then be reflected from the second reflecting surface 48b to the third reflecting surface 48c. The third reflecting surface 48c may be positioned to reflect the laser beam in a direction away from the reflector 34. The reflecting surfaces 48a, 48b, and 48c may be positioned such that the reflected laser beam 20b is parallel to the incident laser beam 20a. In other words, the reflecting surfaces 48a, 48b, and 48c may be positioned such that the direction of the reflected laser beam 20b is opposite to that of the incident laser beam 20a. Although Figures 15A-15C show the first, second and third reflective surfaces as being arranged perpendicular to one another, it will be appreciated that in other embodiments the first, second and third reflective surfaces may be arranged differently.

[0079] The first, second, and third reflective surfaces 48a-48c may be arranged such that the angle of incidence of the laser beam 20 on each of the first, second, and third reflective surfaces is the same and equal to approximately 55.7 degrees. Such an arrangement of the first, second, and third reflective surfaces 48a-48c may be considered a symmetrical arrangement. However, it will be understood that in other embodiments, the arrangement of the first, second, and third reflective surfaces may be asymmetrical and / or the first, second, and third reflective surfaces may be arranged such that the angle of incidence on the first, second, and / or third reflective surfaces is different from 55.7 degrees.

[0080] The first reflecting surface 48a may include or define a first position at which the laser beam 20 is incident. The third reflecting surface 48c may include or define a second position at which the laser beam 20 is reflected from the reflector 34. The first and second positions on the first and third reflecting surfaces 48a, 48c may be spaced apart from one another. A distance C between the first and second positions on the first and third reflecting surfaces 48a, 48c shown in FIG. 15C may correspond to the space E between the first and second portions 32a, 32b of the optical path 30 shown in FIG. 3 for the first optical amplifier 24a.

[0081] As can be seen from FIGS. 15A to 15C, second reflecting surface 48b is disposed between first reflecting surface 48a and third reflecting surface 48c.

[0082]

[0082] Figures 15D and 15E schematically illustrate another exemplary optical path 30 of the laser beam 20 passing through the optical amplifier 24 shown in Figure 4, including the reflector 34 shown in Figures 15A-15C. Figures 15D and 15E illustrate the optical path 30, including the first and second optical elements 40a, 40b, the first and second optical windows 41a, 41b, and the space E between the first and second portions 32a, 32b of the optical path 30. However, in the embodiment shown in Figures 15D and 15E, the order of the first, second, and third reflecting surfaces 48a-48c is reversed relative to the order of the first, second, and third reflecting surfaces 48a-48c shown in Figures 15A-15C.

[0083]

[0083] Figure 16 schematically illustrates another exemplary reflector 34 for use in an optical amplifier (e.g., the optical amplifier 24 shown in Figure 4). The reflector 34 shown in Figure 16 is similar to the reflector 34 shown in Figures 5, 11-13, and 15A-15E. Accordingly, any of the features described above in relation to Figures 5, 11-13, and 15A-15E may also apply to the reflector 34 shown in Figure 16. Only the differences will be described below. In the embodiment shown in Figure 16, the reflector 34 is provided in the form of a retroreflector, for example a conical reflector.

[0084] The reflector 34 can be considered to include a single reflective surface 48. The reflective surface 48 may be configured so that the reflected laser beam 20b is parallel to the incident laser beam 20a. In other words, the reflective surface 48 may be configured so that the direction of the reflected laser beam 20b is opposite to the direction of the incident laser beam 20a. For example, the diameter D1 and / or depth D2 of the reflective surface 48 may be selected so that the reflected laser beam 20b is parallel to the incident laser beam 20a. The depth D2 of the reflective surface 48 can be considered to be the distance measured along the axis of symmetry from the vertex V to the plane of the periphery R of the reflective surface 48. While FIG. 16 illustrates the reflector as including a single reflective surface, it will be understood that in other embodiments, the reflector may include multiple reflective surfaces.

[0085] 16, the reflective surface 48 may include or define a first location at which the laser beam 20 is incident and a second location at which the laser beam 20 is reflected from the reflector 34. The first and second locations of the reflective surface 48 may be spaced apart from one another. Although omitted from FIG. 16, the distance between the first and second locations of the reflective surface 48 may correspond to the space E between the first and second portions 32a, 32b of the optical path 30.

[0086]

[0086] Figure 17 schematically illustrates another exemplary reflector 34 for use in an optical amplifier (e.g., the amplifier 24 shown in Figure 4). The reflector 34 shown in Figure 17 is similar to the reflector 34 shown in Figures 5, 11-13, 15A-15E, and 16. Thus, any of the features described above in relation to Figures 5, 11-13, 15A-15E, and 16 may also apply to the reflector 34 shown in Figure 17. Only the differences will be described below.

[0087] In the embodiment shown in FIG. 17, the reflector 34 is configured to reflect the laser beam 20 three times. The reflector 34 may include one or more reflective surfaces 48a-48c, three of which are shown in FIG. 17. The first reflective surface 48a may include or define a first position at which the laser beam 20 is incident. The third reflective surface 48c may include or define a second position at which the laser beam 20 is reflected from the reflector 34. The first and second positions of the first and third reflective surfaces 48a, 48c may be spaced apart from each other. A distance C between the first and second positions on the first and third reflective surfaces 48a, 48c shown in FIG. 17 may correspond to the space E between the first and second portions 32a, 32b of the optical path 30 shown in FIG. 3 for the first optical amplifier 24a.

[0088] Each of the first, second, and third reflective surfaces 48-48c may include or define a curved surface. The curvature of each of the first and third reflective surfaces 48a and 48c and the arrangement of the first and third reflective surfaces 48a and 48c may be selected such that the first and third reflective surfaces 48a and 48c define a common parabola or lie on a common parabola. For example, the first and third reflective surfaces 48a and 48c may be arranged to define a common cone or lie on a common cone.

[0089] The reflecting surfaces 48a-48c may be arranged such that an M-shape (e.g., a substantially M-shape) as shown in FIG. 17 is defined by the incident laser beam 20a, the reflected laser beam 20b, and a path 50 of the laser beam 20 between the first position and the second position, e.g., between the first reflecting surface 48a and the third reflecting surface 48c. For example, the second reflecting surface 48b may be arranged between the first reflecting surface 48a and the third reflecting surface 48c. The second reflecting surface 48b may be arranged to face a first direction. The first reflecting surface 48a and the third reflecting surface 48c may be arranged to face a second direction. The first direction and the second direction may be different from each other and / or may be opposite (e.g., substantially opposite) to each other. For example, the second reflecting surface 48b may be arranged to face a common parabola defined by the first reflecting surface 48a and the third reflecting surface 48c. The reflector 34 shown in Figure 17 can be thought of as defining a cat's eye configuration.

[0090]

[0090] It will be appreciated that the placement of the retroreflector is not limited to the embodiments or examples disclosed in this specification and that in other embodiments the retroreflector may be placed differently.

[0091]

[0091] It will be understood that the terms "optical amplifier" and "amplifier" can be used interchangeably.

[0092] It will be understood that references to plural features may be used interchangeably with references to the singular of those features, e.g., "at least one" and / or "each." Singular forms of certain features, such as "at least one" and "each," can be used interchangeably.

[0093]

[0093] Although specific reference is made in this specification to the use of lithographic apparatus in IC manufacture, it should be understood that the lithographic apparatus described herein may have other applications, including the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0094]

[0094] While specific embodiments of the present invention have been described above, it will be understood that the present invention may be practiced in other ways. The above description is intended to be illustrative and not limiting. Thus, it will be apparent to those skilled in the art that modifications may be made to the invention described herein without departing from the scope of the following clauses and the appended claims.

[0095]

[0095] Clause: 1. An optical amplifier configured to amplify a laser beam, comprising: an optical path of the laser beam, the optical path including a first portion and a second portion; an optical amplifier including: a reflector configured to reflect the laser beam to direct the laser beam between the first and second portions of the optical path, the reflector configured to reflect the laser beam such that the laser beam reflected by the reflector is parallel to the laser beam incident on the reflector. 2. The optical amplifier of clause 1, wherein the reflector is configured to reflect the laser beam at least twice to direct the laser beam between the first and second portions of the optical path. 3. An optical amplifier according to clause 1 or 2, wherein the reflector comprises a retroreflector. 4. Retroreflectors are Corner reflectors, and 4. The optical amplifier of claim 3, comprising at least one of: a. a conical reflector; 5. The system of clause 4, wherein the corner reflector comprises a corner cube. 6. An optical amplifier according to any preceding clause, wherein the reflector comprises one or more reflective surfaces, the one or more reflective surfaces having a first position at which the laser beam is incident and a second position at which the laser beam is reflected away from the reflector, and at least one or each of the one or more reflective surfaces comprises a flat surface or a curved surface. 7. An optical amplifier as described in clause 6, wherein the first and second positions of the one or more reflecting surfaces are spaced apart from each other, and the distance between the first and second positions of the one or more reflecting surfaces corresponds to the space between the first and second portions of the optical path. 8. The one or more reflective surfaces are arranged to direct the laser beam between the first and second portions of the optical path by at least three reflections of the laser beam, and the one or more reflective surfaces are at least one reflective surface is disposed between at least two other reflective surfaces; At least one reflective surface is arranged to face a first direction and at least one other reflective surface is arranged to face a second direction, the first direction and the second direction being different and / or opposite to each other. are arranged so as to satisfy at least one of the following: 8. An optical amplifier according to clause 6 or 7, wherein an M-shape is defined by the laser beam reflected by the reflector, the laser beam incident on the reflector, and / or the path of the laser beam between the first position and the second position of the one or more reflective surfaces. 9. An amplifier system for use in a laser system, comprising: a plurality of optical amplifiers, wherein at least one or each of the plurality of optical amplifiers comprises an optical amplifier according to any of the preceding clauses; and an optical system configured to optically couple at least one optical amplifier of the plurality of optical amplifiers to at least one other optical amplifier of the plurality of optical amplifiers; a frame configured to support the optical system and configured separate from the plurality of optical amplifiers. 10. The frame is the frame supporting the optical system such that each of the plurality of optical amplifiers is movable relative to the optical system; and 10. The system of claim 9, wherein the frame supports the optical system such that each of the plurality of optical amplifiers is decoupled from the optical system. 11. A system described in any one of clauses 9 to 10, wherein the system includes an additional frame configured to mount a plurality of optical amplifiers, and the frame is connected to the additional frame or the frame is integral with the additional frame. 12. A laser system comprising: a laser configured to generate a laser beam; 12. A laser system comprising: an amplifier system according to any one of clauses 9 to 11, the amplifier system being configured to amplify a laser beam. 13. The laser system of clause 12, wherein the frame is configured to connect to the laser. 14. A radiation system comprising: an EUV radiation source; 14. A radiation system comprising: a laser system according to clause 12 or 13. 15. A radiation system comprising: an EUV radiation source; a laser system comprising an optical amplifier according to any one of clauses 1 to 8. 16. A lithography system comprising a radiation system according to clause 14 or 15 and a lithography system.

Claims

1. 1. An optical amplifier configured to amplify a laser beam, comprising: an optical path of the laser beam, the optical path including a first portion and a second portion; a reflector configured to reflect the laser beam to direct the laser beam between the first and second portions of the optical path, the reflector configured to reflect the laser beam such that the laser beam reflected by the reflector is parallel to the laser beam incident on the reflector.

2. 2. The optical amplifier of claim 1, wherein the reflector is configured to reflect the laser beam at least twice to direct the laser beam between the first and second portions of the optical path.

3. 3. The optical amplifier of claim 1, wherein the reflector comprises a retroreflector.

4. The retroreflector is a corner reflector including a corner cube; and Conical reflector 4. The optical amplifier according to claim 3, comprising at least one of:

5. 5. An optical amplifier according to claim 1, wherein the reflector includes one or more reflective surfaces, the one or more reflective surfaces including a first position at which the laser beam is incident and a second position at which the laser beam is reflected in a direction away from the reflector, and at least one of or each of the one or more reflective surfaces includes a flat surface or a curved surface.

6. 6. The optical amplifier of claim 5, wherein the first and second positions of the one or more reflecting surfaces are spaced apart from one another, and the distance between the first and second positions of the one or more reflecting surfaces corresponds to a space between the first and second portions of the optical path.

7. the one or more reflective surfaces are arranged to direct the laser beam between the first and second portions of the optical path by reflecting the laser beam at least three times, and the one or more reflective surfaces are at least one reflective surface is disposed between at least two other reflective surfaces; At least one reflective surface is arranged to face a first direction and at least one other reflective surface is arranged to face a second direction, the first direction and the second direction being different and / or opposite to each other. are arranged so as to satisfy at least one of the following:

7. An optical amplifier according to claim 5 or 6, wherein an M-shape is defined by the laser beam reflected by the reflector, the laser beam incident on the reflector, and / or the path of the laser beam between the first position and the second position of the one or more reflective surfaces.

8. 1. An amplifier system for use in a laser system, comprising: a plurality of optical amplifiers, wherein at least one or each of the plurality of optical amplifiers comprises the optical amplifier according to any one of claims 1 to 7; an optical system configured to optically couple at least one optical amplifier of the plurality of optical amplifiers to at least one other optical amplifier of the plurality of optical amplifiers; a frame configured to support the optical system and configured separate from the plurality of optical amplifiers.

9. The frame is the frame supports the optical system such that each of the plurality of optical amplifiers is movable relative to the optical system; and 9. The system of claim 8, wherein the frame supports the optical system such that each of the plurality of optical amplifiers is decoupled from the optical system.

10. The system of any one of claims 8 to 9, further comprising an additional frame configured to mount the plurality of optical amplifiers, the frame being connected to the additional frame or the frame being integral with the additional frame.

11. 1. A laser system comprising: a laser configured to generate a laser beam; 11. A laser system comprising: an amplifier system according to any one of claims 8 to 10, configured to amplify the laser beam.

12. The laser system of claim 11 , wherein the frame is configured to connect to the laser.

13. 1. A radiation system comprising: an EUV radiation source; and A radiation system comprising: a laser system according to claim 11 or 12.

14. 1. A radiation system comprising: an EUV radiation source; and A laser system comprising an optical amplifier according to any one of claims 1 to 7.

15. A lithography system comprising a radiation system according to claim 13 or 14 and a lithography system.

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