Optical amplifier

Abstract

An optical amplifier configured to amplify a laser beam, the optical amplifier comprising: an optical path of the laser beam, the optical path comprising a first portion and a second portion; and a reflector configured to reflect the laser beam so as to direct 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.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to European Application No. 22205638.4, filed on November 4, 2022, the entire content of which is incorporated herein by reference. Technical field

[0003] The present invention relates to an optical amplifier and an associated system. Background art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can project a pattern present on a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate, for example.

[0005] To project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features that can be formed on the substrate. Compared to a lithographic apparatus that uses radiation having a wavelength of, for example, 193 nm, a lithographic apparatus that uses extreme ultraviolet (EUV) radiation having a wavelength in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on the substrate.

[0006] A lithographic system can include a radiation system and a lithographic apparatus. The radiation system can include a laser system configured to generate a laser beam and an EUV radiation source. EUV radiation can be generated, for example, by directing the laser beam into the radiation source such that the laser beam impinges on fuel in the radiation source. The laser beam can deposit energy into the fuel to generate a plasma. Radiation including EUV radiation can be emitted from the plasma.

[0007] The laser system can include a laser configured to generate a laser beam and an optical amplifier configured to amplify the laser beam. In use, the laser beam passing through the optical amplifier can be affected by one or more pointing errors or fluctuations. The pointing errors or fluctuations can cause one or more errors or fluctuations in the position of the laser beam. These errors or fluctuations can be proportional to the optical path of the laser beam in the optical amplifier. Thus, when the laser beam is directed through more than one optical amplifier, the errors or fluctuations in the position of the laser beam can increase. The errors or fluctuations in the position of the laser beam can result in power loss of the amplified laser beam. Summary of the invention

[0008] 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, the optical path comprising a first portion and a second portion; and a reflector configured to reflect the laser beam so as to direct the laser beam between the first portion and the second portion of the optical path, the reflector being 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.

[0009] In use, a thermal load may act on the optical amplifier. For example, the thermal load acting on the optical amplifier may cause movement and / or deformation of one or more parts of the optical amplifier. The movement and / or deformation of one or more parts of the optical amplifier may be referred to as internal movement and / or deformation. One or more parts of the optical amplifier may include one or more optical elements and / or a frame configured to support one or more optical elements. The movement or deformation of one or more parts of the optical amplifier may cause beam pointing error or beam pointing fluctuations of the laser beam passing through the optical amplifier. The beam pointing error or beam pointing fluctuations of the laser beam may cause power loss of the amplified laser beam. By configuring the reflector such that the laser beam reflected by the reflector is parallel to the laser beam incident on the reflector, the beam pointing error or beam pointing fluctuations of the laser beam passing through the optical amplifier can be reduced or prevented. Thus, the power loss of the amplified laser beam can be reduced or prevented.

[0010] The reflector may be configured to reflect the laser beam at least twice, for example to direct the laser beam between the first portion and the second portion of the optical path.

[0011] The reflector may include a retroreflector. The retroreflector may include at least one of the following: a corner reflector and / or a conical reflector. The corner reflector may include a corner cube prism.

[0012] The reflector may include one or more reflective surfaces. The one or more reflective surfaces may include a first position, such as where or at which the laser beam is incident. The one or more reflective surfaces may include a second position, such as where 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 planar surface or a curved surface. The first position and the second position of the one or more reflective surfaces may be spaced apart from each other. The distance between the first position and the second position of the one or more reflective surfaces may correspond to the spacing between the first portion and the second portion of the optical path. The one or more reflective surfaces may be arranged such that the laser beam is reflected at least three times, such as to direct the laser beam between the first portion and the second portion of the optical path. The one or more reflective surfaces may be arranged such that at least one reflective surface is arranged between at least two other reflective surfaces. The one or more reflective surfaces may be arranged such that at least one reflective surface is arranged to face a first direction. The one or more reflective surfaces may be arranged such that at least one other reflective surface is arranged to face a second direction. The first direction and the second direction may be different from and / or opposite to each other (e.g., substantially opposite). The one or more reflective surfaces may be arranged such that an M-shaped shape is defined by at least one of: 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.

[0013] According to a second aspect of the present invention, there is provided an amplifier system for a laser system, the system comprising: a plurality of optical amplifiers, at least one or each of the plurality of optical amplifiers including 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 another of the plurality of optical amplifiers; and a frame configured to support the optical system, the frame being configured to be separate from the plurality of optical amplifiers.

[0014] In use, a thermal load can act on at least one or each of a plurality of optical amplifiers. For example, the thermal load acting on at least one or each of the plurality of optical amplifiers may cause movement and / or deformation of one or more portions of at least one or each of the plurality of optical amplifiers. One or more portions of at least one of the plurality of optical amplifiers include one or more optical elements and / or a frame configured to support one or more optical elements. When an optical system (e.g., one or more portions of the optical system) is connected to each of the plurality of optical amplifiers, movement and / or deformation of one or more portions of each of the plurality of optical amplifiers may cause movement of one or more portions of the optical system connected to one or more portions of each of the plurality of optical amplifiers. Such movement of one or more portions of the optical system connected to each of the plurality of optical amplifiers may be referred to as external movement. This external movement may cause beam pointing errors or beam pointing fluctuations of the amplified laser beam that can be directed to another optical amplifier in the optical amplifier or out of the amplifier system. By configuring the frame to be separate from the plurality of optical amplifiers and by configuring the reflector such that the laser beam reflected by the reflector is parallel to the laser beam incident on the reflector, beam pointing errors or beam pointing fluctuations attributable to internal movement and / or deformation and external movement of the amplified laser beam can be reduced or prevented. Thus, power loss of the amplified laser beam can be reduced or prevented.

[0015] The frame can be configured to support the optical system such that at least one or each of the plurality of optical amplifiers can move relative to the optical system. The frame can be configured to support the optical system such that at least one or each of the plurality of optical amplifiers is separate from the optical system.

[0016] The frame can include at least one of or be formed of at least one of the following: a metallic material, a metal alloy material, a fiber material, a ceramic material, and / or a glass material. The metallic material can include aluminum. The metal alloy material can include at least one of the following: steel and / or stainless steel. The fiber material can include carbon fiber.

[0017] The system can include an additional frame. The additional frame can be configured to mount the plurality of optical amplifiers. The frame can be connected to the additional frame. The frame can be integral with the additional frame.

[0018] According to a third aspect of the present invention, there is provided an amplifier system for 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 another of the plurality of optical amplifiers; a frame configured to support the optical system, the frame being configured to be separated from the plurality of optical amplifiers, wherein at least one or each of the plurality of optical amplifiers includes a retroreflector configured to direct the laser beam between the first portion and the second portion of the optical path.

[0019] According to a fourth aspect of the present invention, there is provided a laser system, the laser system comprising: a laser configured to generate a laser beam; and an amplifier system according to the second aspect and / or the third aspect, wherein the amplifier system is configured to amplify the laser beam.

[0020] The frame may be configured to be connected to the laser, such as the housing of the laser.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] Those skilled in the art will appreciate that the various aspects and features of the present invention described above or below can be combined with various other aspects and features of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0026] - Figure 1 a lithography system including a lithography apparatus and a radiation system is depicted;

[0027] - Figure 2 an exemplary amplifier system in a laser system for a radiation system for Figure 1 is depicted;

[0028] - Figure 3 Depict an exemplary optical path of a laser beam passing through Figure 2 the amplifier system;

[0029] - Figure 4 Depict an exemplary optical amplifier in the amplifier system for Figure 2 wherein the optical amplifier includes a reflector;

[0030] - Figure 5 Depict an exemplary optical path of a laser beam passing through Figure 4 the optical amplifier;

[0031] - Figure 6 Depict an exemplary amplifier frame in the optical amplifier for Figure 4 ;

[0032] - Figure 7 Schematically depict an exemplary optical path of a laser beam passing through Figure 4 the optical amplifier, wherein the reflector includes a reflecting prism;

[0033] - Figure 8A Depict Figure 4 a schematic side view of the optical amplifier, wherein the reflector includes a reflecting prism;

[0034] - Figure 8B Depict Figure 4 a schematic plan view of the optical amplifier, wherein the reflector includes a reflecting prism;

[0035] - Figure 9A Depict Figure 4 a schematic side view of one or more parts of the optical system of the optical amplifier and Figure 1 the amplifier system, the one or more parts being supported on a frame separated from the optical amplifier, wherein the reflector includes a reflecting prism;

[0036] - Figure 9B Depict Figure 4 a schematic plan view of one or more parts of the optical system of the optical amplifier and Figure 1 the amplifier system, the one or more parts being supported on a frame separated from the optical amplifier, wherein the reflector includes a reflecting prism;

[0037] - Figure 10A Schematically depict the optical path of a laser beam passing through Figure 7 the optical amplifier in the x, y plane of Figure 4 , wherein the reflector includes a reflecting prism;

[0038] -Figure 10B Schematically depict the Figure 7 optical path of the laser beam passing through the Figure 4 optical amplifier in the y, z plane, wherein the reflector includes a reflecting prism;

[0039] - Figure 11 Schematically depict another exemplary optical path of the laser beam passing through the Figure 4 optical amplifier, wherein the reflector includes a retroreflector;

[0040] - Figure 12A Depict Figure 4 a schematic plan view and a side view of the

[0041] - Figure 12B Depict Figure 4 optical amplifier and Figure 1 one or more parts of the optical system of the

[0042] - Figure 13 Schematically depict in the Figure 11 x, y plane and y, z plane of the Figure 4 optical path of the laser beam passing through the

[0043] - Figure 14A Depict Figure 1 outer side of a part of the

[0044] - Figure 14B Depict Figure 1 inner side of the said part of the

[0045] - Figure 14C Depict Figure 14B part of the

[0046] - Figures 15A to 15C Schematically depict another exemplary reflector for the Figure 4 optical amplifier;

[0047] - Figure 16 Schematically depict another exemplary reflector for the Figure 4 optical amplifier; and

[0048] - Figure 17 Schematically depict another exemplary reflector for the Figure 4 optical amplifier. Detailed Description

[0049] Figure 1 A lithographic system is shown including a radiation system RS and a lithographic apparatus LA. The radiation system RS includes a radiation source SO. The radiation source SO is configured to generate an EUV radiation beam B and supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA includes 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.

[0050] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident on the patterning device MA. Additionally, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and the faceted pupil mirror device 11 together provide an EUV radiation beam B having a desired cross-sectional shape and a desired intensity distribution. In addition to or instead of the faceted field mirror device 10 and the faceted pupil mirror device 11, the illumination system IL may also include other mirrors or devices.

[0051] After being so conditioned, the EUV radiation beam B interacts with the patterning device MA. Due to this interaction, a patterned EUV radiation beam B’ is produced. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For this purpose, the projection system PS may include a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image having features smaller than the corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although in Figure 1 the projection system PS is illustrated as having only two mirrors 13, 14, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0052] The substrate W may include a previously formed pattern. In such a 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.

[0053] A relative vacuum may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS, i.e., a small amount of gas (e.g., hydrogen) at a pressure far below atmospheric pressure.

[0054] Figure 1The radiation source SO shown belongs to a type that can be referred to as, for example, a laser-produced plasma (LPP) source. A laser system 1, which can include, for example, a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel (such as tin (Sn)) provided by, for example, a fuel emitter 3. Although tin is mentioned in the following description, any suitable fuel can be used. The fuel can be, for example, in liquid form and can be, for example, a metal or an alloy. The fuel emitter 3 can include a nozzle configured to guide the tin, for example, in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident on the tin at the plasma formation region 4. Depositing the laser energy into the tin generates a tin plasma 7 at the plasma formation region 4. During the de-excitation and recombination of the electrons and ions of the plasma, radiation including EUV radiation is emitted from the plasma 7.

[0055] The EUV radiation from the plasma is collected and focused by a collector 5. The collector 5 includes, for example, a near-normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). The collector 5 can have a multilayer mirror structure arranged to reflect EUV radiation (for example, EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 can have an elliptical configuration having two foci. As discussed below, the first focus of the foci can be at the plasma formation region 4, and the second focus of the foci can be at an intermediate focus 6.

[0056] The laser system 1 can be spatially separated from the radiation source SO. In such a case, the laser beam 2 can be transmitted from the laser system 1 to the radiation source SO by means of a beam delivery system (not shown) including, for example, suitable steering mirrors and / or beam expanders and / or other optical devices. The laser system 1, the radiation source SO, and the beam delivery system can be considered together as a radiation system.

[0057] The radiation reflected by the collector 5 forms an EUV radiation beam B. The EUV radiation beam B is focused at the intermediate focus 6 to form an image at the intermediate focus 6 of the plasma existing at the plasma formation region 4. The image at the intermediate focus 6 serves as a virtual radiation source for an illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near an opening 8 in an enclosure structure 9 of the radiation source SO.

[0058] Figure 2An exemplary amplifier system 16 in a laser system 1 for a radiation system RS is shown. The amplifier system 16 may also be referred to as an optical amplifier system. The laser system 1 may include a laser 18, such as a CO2 laser. The laser 18 may be configured to generate a laser beam 20. The laser beam 20 may include a plurality of pulses. The 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 guiding the laser beam to the amplifier system 16.

[0059] The amplifier system 16 includes a plurality of optical amplifiers 24a to 24d, four of which are shown in Figure 2 However, it should be understood that in other embodiments, the system may include more or fewer than four optical amplifiers. Each of the optical amplifiers 24a to 24d is configured to amplify the laser beam 20.

[0060] 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 to 24d to at least another of the optical amplifiers 24a to 24d. In the present embodiment, the optical system 26 is configured to optically serially couple or connect the amplifiers 24a to 24d to each other. For example, the optical system 26 may be configured to optically couple or connect the optical amplifiers 24a to 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 a plurality of optical elements, such as a plurality of mirrors, lenses, telescopes, and / or the like. The optical amplifiers 24a to 24d may be arranged to continuously amplify the laser beam 20.

[0061] The amplifier system 16 may include an input end 25a, for example, where a laser beam 20 enters the amplifier system 16. The amplifier system 16 may include an output end 25b, for example, where the amplified laser beam 21 exits the amplifier system 16. At the input end 25a of the amplifier system 16, the laser beam may have a power between about 100 W and 200 W (such as about 140 W). At the output end 25b of the amplifier system 16, the amplified laser beam 21 may have a power between about 25 kW and 50 kW. The amplifier system 16 includes a frame 28 configured to support an optical system 26. For example, the optical system 26 may be mounted to the frame 28. The frame 28 is configured to be separated 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 is separate from and / or may be movable 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 to be separated from the optical amplifiers 24a to 24d, pointing errors or fluctuations due to thermal deformation of a portion of the laser beam 20 that may be attributable to each of the optical amplifiers 24a to 24d can be reduced or prevented.

[0062] The frame 28 may be configured to be connected to the laser 18, such as the housing of the laser. For example, the frame 28 may be configured to extend in a direction parallel (such as substantially parallel) to the optical amplifiers 24a to 24d. The frame 28 may be configured to extend and be connected to the laser 18. The frame 28 may also be configured to support at least a part or all of the input end 25a. By configuring the frame 28 to be connected to the laser, movement of the laser 18 relative to the optical system 26 can be reduced or prevented. In other words, the optical system 26 and the laser 18 may move consistently. Therefore, pointing errors or fluctuations of the laser beam 20 can be reduced or prevented. It should be understood that in other embodiments, the frame may be configured differently.

[0063] Figure 3 shows the laser beam 20 passing through Figure 2 the exemplary optical path of the amplifier system 16 shown in Figure 3is indicated by a dashed box. Each optical amplifier 24a to 24d includes or defines a corresponding optical path 30a to 30d. Each optical path 30a to 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 to 30d may be spaced apart from each other. For example, the first portion 32a and the second portion 32b of each optical path 30a to 30d may be spaced apart in a direction perpendicular (e.g., substantially perpendicular) to the plane of each of the first portion 32a and the second portion 32b. The spacing E between the first portion 32a and the second portion 32b is in Figure 3 is indicated. In Figure 3 the embodiment shown, the first portion 32a is above the second portion 32b. However, it should be understood that in other embodiments, the first portion and the second portion of each optical path may be arranged differently.

[0064] Each amplifier 24a to 24d includes a corresponding reflector 34a to 34d, each of the corresponding reflectors 34a to 34d being indicated by a box in Figure 3 As will be described in more detail below, each reflector 34 is configured to reflect the laser beam 20 at least twice, for example, so as to guide the laser beam 20 between the first portion 32a and the second portion 32b of the corresponding optical paths 30a to 30d. For example, each reflector 34a to 34d may be configured to guide the laser beam 20 from the first portion 32a to the second portion 32b or from the second portion 32b to the first portion 32a of the corresponding optical paths 30a to 30d. The first portion 32a and / or the second portion 32b of each optical path 30a to 30d of each optical amplifier 24a to 24d may also be referred to as a horizontal portion.

[0065] Figure 4 is shown for Figure 2The exemplary optical amplifier 24 in the amplifier system 16 shown in. The optical amplifier 24 may include a plurality of amplifier frames, such as a first amplifier frame 36a, a second amplifier frame 36b, and a third amplifier frame 36c. It should be understood that in other embodiments, the optical amplifier may include more or fewer than three amplifier frames. The first amplifier frame 36 may be configured to support a first conduit 38a. The second amplifier frame 36b may be configured to support a second conduit 38b. The first conduit 38a and the second conduit 38b may enclose a gas mixture. In some embodiments, the gas mixture may be sealed in the first conduit 38a and / or the second conduit 38b. In other embodiments, the gas mixture may be pumped (e.g., continuously pumped) through the first conduit and / or the second conduit. The gas mixture may include carbon dioxide, oxygen, carbon monoxide, nitrogen, hydrogen, xenon, and / or helium. The optical amplifier 24 may be configured such that the laser beam 20 is directed through the gas mixture. One or more radio frequency (RF) signals may be used to excite the molecules of the gas mixture. One or more radio frequency signals may include one or more radio frequency pulses. Alternatively, one or more radio frequency signals may include continuous wave radio frequency signals. One or more radio frequency signals may be applied to the gas mixture to produce population inversion of the molecules of the gas mixture. This may result in amplification of the laser beam 20 passing through the gas mixture.

[0066] The amplifier 24 may include a plurality of optical elements 40, one of the plurality of optical elements 40 being indicated in Figure 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 direct the laser beam 20 through the gas mixture in the first conduit 38a and the second conduit 38b. The optical elements 40 may be arranged 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 arranged at three corners of each of the first frame 36a and the second frame 36b. Reflectors 34 may be arranged at the remaining corners of each of the first frame 36a and the second frame 36b. The first conduit 38a and some of the optical elements 40 may define a first portion 32a of the optical path 30 of the amplifier 24. The second conduit 38b and some other of the optical elements 40 may define a second portion 32b of the optical path 30 of the amplifier 24. The first conduit 38a may be arranged on the periphery of the first frame 36a. The second conduit 38b may be arranged on the periphery of the second frame 36b.

[0067] The third amplifier frame 36c can be configured to support the optical element 40 and the reflector 34. The third amplifier frame 36c can be arranged between the first amplifier frame 36a and the second amplifier frame 36b.

[0068] Figure 5 showing the laser beam 20 passing through Figure 4 the exemplary optical path 30 of the optical amplifier 24 shown in Figure 6 showing for Figure 4 the exemplary third amplifier frame 36c in the optical amplifier 24 shown in. The optical path 30 of the laser beam 20 passing through the optical amplifier 24 is also shown in Figure 6 by the dashed line with respect to the third amplifier frame 36c.

[0069] In this embodiment, each of the first portion 32a and the second portion 32b of the optical path 30 of the laser beam 20 includes three optical elements 40. It will be appreciated that in other embodiments, at least one or each of the first and second paths of the laser beam may include more or fewer 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 corner cube prism. It will be appreciated that in other embodiments, the reflector may be implemented differently. For example, in other embodiments, the retroreflector may be provided in the form of a conical reflector, a cat's eye configuration, and / or the like.

[0070] As visible in Figure 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 arranged to direct the laser beam 20 onto the first portion 32a of the optical path 30. The first optical element 40a may be arranged to direct the laser beam 20 onto 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 of the optical path 30 to the second portion 32b. In this embodiment, the reflector 34 is configured to reflect the laser beam 20 three times, as will be described in more detail below. 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 the reflector 34 may be the reverse direction of the laser beam incident on the reflector 34. The laser beam incident on the reflector 34 may also be referred to as the incident laser beam 20a. The laser beam reflected by the reflector 34 may also be referred to as the reflected laser beam 20b. The spacing E between the first portion 32a and the second portion 32b of the optical path 30 is shown in Figure 5is indicated in

[0071] The second optical element 40b may be arranged to direct the laser beam 20 onto a second part 32b of the optical path 30. The second optical element 40b may be arranged to direct the reflected laser beam 20b away from the reflector 34, such that, for example, the reflected laser beam 20b exits the optical amplifier 24 in the vicinity of the reflector 34. The laser beam exiting the optical amplifier 24 may be regarded as, for example, 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 arranged such that the laser beam 20d enters the optical amplifier 24 via the first optical window 41a and the amplified laser beam 20c exits the optical amplifier via the second optical window 41b.

[0072] 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 the plurality of first support elements being shown in Figure 6 Each of the first support elements 42 of the first support elements 42 may be provided in the form of a block or mounting block. Each first support element 42 may be configured to support the first optical element 40a and the second optical element 40b.

[0073] 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 mounting block. The second support element 44 may be configured to support the reflector 34.

[0074] The first support element 42 and the second support element 44 may be arranged 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 arranged at one or more corners of the rectangular or square shape.

[0075] In use, a thermal load may act on one or more of the first amplifier frame 36a, the second amplifier frame 36b, and the third amplifier frame 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 cause movement of the first support element 42 and / or the second support element 44, and the movement of the first support element 42 and / or the second support element 44 may cause the first optical element 40a and / or the second optical element 40b and / or the reflector 34 to become optically misaligned. This may cause one or more drifts and / or pointing errors or fluctuations of the laser beam 20, as will be described in more detail below. The movement and / or deformation of the third amplifier frame 36c is indicated in Figure 6 is indicated.

[0076] Figure 7 Schematically shows the laser beam 20 passing through Figure 4 the exemplary optical path 30 of the optical amplifier 24 shown in. 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 reflecting prism (such as a porro prism). Figure 7 Shows the first portion 32a and the second portion 32b of the optical path 30 with respect to a coordinate system.

[0077] Figure 8A Shows Figure 4 the optical amplifier 24 shown in, for example, in Figure 7 a schematic side view in the y, z plane of the coordinate system shown in. Figure 8B Shows Figure 4 the optical amplifier 24 shown in, for example, in Figure 7 a schematic plan view in the x, y plane of the coordinate system shown in. For clarity, Figure 8A and Figure 8B only show some of the optical elements of the first optical element 40a and the second optical element 40b. The reflector 34 has been omitted from Figure 8A and Figure 8B . It will be understood that Figure 8A and Figure 8B the arrangement of the first optical element 40a and the second optical element 40b of the optical amplifier 24 shown in is the same as the arrangement described above. As described above, the optical amplifier 24 may be configured such that the laser beam 20 enters and exits the optical amplifier 24 at substantially the same location (e.g., near the reflector 34). However, for clarity, the laser beam 20 is shown entering and exiting the optical amplifier 24 at different locations in Figure 8A and Figure 8B .

[0078] InFigure 8A and Figure 8B In the example shown in, two parts 26a of the optical system 26 are connected to the optical amplifier 24, for example, connected to one or all of the first amplifier frame 36a, the second amplifier frame 36b, and the third amplifier frame 36c. When the part 26a of the optical system 26 is connected to the optical amplifier 24, the movement and / or deformation of the optical amplifier 24 (for example, the third frame 36c indicated by the arrow M in Figure 8A and Figure 8B ) may cause the movement of the part 26a of the optical system 26. This can be referred to as external movement and may cause beam pointing error or beam pointing fluctuation. For example, when the optical amplifier 24 remains stationary, in Figure 8A and Figure 8B , the amplified laser beam 20c indicated by the solid line may leave the optical amplifier 24 at an angle β with respect to the amplified laser beam 20c indicated by the dashed line in Figure 8A and Figure 8B . The angle of the amplified laser beam 20c may cause beam pointing error or beam pointing fluctuation. The beam pointing error or beam pointing fluctuation may cause an error or fluctuation in the beam position, and the error or fluctuation may be proportional to the optical length of the amplifier system 16. Thus, as the optical length of the amplifier system 16 increases, the beam positioning error or beam positioning fluctuation may increase. The beam pointing error or beam pointing fluctuation may cause power loss of the amplified laser beam leaving the optical amplifier, which in turn may cause power loss of the amplified laser beam leaving the amplifier system.

[0079] Figure 9A shows Figure 4 a schematic side view of the optical amplifier 24 shown in, for example, in the y, z plane of the coordinate system shown in Figure 7 . Figure 9B shows Figure 4 a schematic plan view of the optical amplifier 24 shown in, for example, in the x, y plane of the coordinate system shown in Figure 7 . Figure 9A and Figure 9B are similar to Figure 8A and Figure 8B . Thus, only the differences will be described below. In Figure 9A and Figure 9BIn the example shown, the optical system 26 (e.g., its part 26a) is supported by the frame 28, as described above. Thus, the optical system 26 (e.g., its part 26a) is optically coupled or connected to the optical amplifier 24. However, the optical amplifier 24 is separate from the optical system 26 (e.g., its part 26a) and / or may be movable relative to the optical system 26 (e.g., its part 26a). In other words, there is no physical connection between the optical system 26 (e.g., its part 26a) and the optical amplifier 24.

[0080] Reference Figure 9A , when the optical amplifier 24 remains stationary, movement and / or deformation of the optical amplifier 24 relative to the optical system 26 (e.g., its part 26a) may cause a shift of the amplified laser beam 20c relative to the amplified laser beam 20c, such as a translational shift. In other words, when the optical amplifier 24 remains stationary, movement and / or deformation of the optical amplifier 24 may cause the amplified laser beam 20c to be parallel, e.g., substantially parallel, relative to the amplified laser beam 20c. By configuring the frame 28 to be separated from the optical amplifier 24, pointing errors or fluctuations attributable to external movement in at least one plane of the optical amplifier 24 of the laser beam 20 can be reduced or prevented. In the descriptions above and below, the term "movement and / or deformation of the optical amplifier 24" may be regarded as covering movement and / or deformation of the third frame 36c of the optical amplifier 24. The term "movement and / or deformation of the optical amplifier 24" may also be regarded as covering movement and / or deformation of the first frame 36a and / or the second frame 36b of the optical amplifier 24. It will be appreciated 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., its part 26a).

[0081] Reference Figure 9B , when the optical amplifier 24 is Figure 7 stationary in the x, y plane shown, 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 relative to the amplified laser beam 20c.

[0082] Figure 10A Schematically shows the optical path 30 of the laser beam 20 passing through the optical amplifier 24 in the x, y plane shown in Figure 7 . Figure 10B Schematically shows the optical path 30 of the laser beam 20 passing through the optical amplifier 24 in the y, z plane shown in Figure 7 . For clarity, Figure 10A and Figure 10B only show some of the first optical elements in the first optical element 40a. However, it will be appreciated that Figure 10A and Figure 10BThe optical amplifier 24 shown in can include a first optical element 40a and a second optical element 40b, as described above and below. In Figure 10A and Figure 10B In the example shown in, one of the first optical elements 40a may move, for example, due to movement and / or deformation of the optical amplifier 24. Such 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 an arrow M in Figure 10A and Figure 10B It will be appreciated that in other embodiments, more than one first optical element and / or one or more second optical elements may move, for example, due to movement and / or deformation of the optical amplifier. The term "movement of the first optical element and / or the second optical element" may be considered to cover the first optical element and / or the second optical element becoming optically misaligned.

[0083] When the first optical element 40 remains stationary, Figure 10A and Figure 10B The dashed lines in indicate the optical path 30 of the laser beam 20. When the first optical element 40a has moved, Figure 10A The solid line in indicates the optical path 30 of the laser beam 20. Referring to Figure 10A , the movement of the first optical element 40a may cause the amplified laser beam 20c to exit the optical amplifier 24 at an angle β. In this example, the reflector 34 may be considered to act as a plane mirror in the x, y plane shown in Figure 7 .

[0084] Referring to Figure 10B , when the optical element remains stationary, the movement of the first optical element 40 may cause the amplified laser beam 20c to be displaced (e.g., translationally displaced) and / or parallel (e.g., substantially parallel) with respect to the amplified laser beam 20c.

[0085] Figure 11 Schematically shows 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 such an embodiment, the reflector 34 is provided in the form of a retroreflector, such as a corner cube prism, as described above with respect to the embodiment shown in Figure 5 . It will be appreciated that the reflector may be oriented and / or arranged in a variety of ways different from those disclosed herein. However, in each of those 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.

[0086] Figure 11Shows the optical path 30 of the laser beam 20 relative to the coordinate system. Figure 12A Shows Figure 4 The optical amplifier 24 shown in, for example, Figure 11 Schematic plan views and side views in the y,z plane and the x,y plane of the coordinate system shown in. For clarity, Figure 12A Only some of the first optical elements in the first optical element 40a are shown in. However, it will be understood that Figure 10A And Figure 10B The optical amplifier 24 shown in may include a first optical element 40a and a second optical element 40b, as described above and below. In the Figure 12A Example shown in, a part 26a of the optical system 26 is connected to the optical amplifier 24, for example to one or all of the first amplifier frame 36a, the second amplifier frame 36b and the third amplifier frame 36c of the optical amplifier 24. When the part 26a of the optical system 26 is connected to the optical amplifier 24, the movement and / or deformation of the optical amplifier 24 (which is indicated by the arrow M in Figure 12A ) may cause the movement of the part 26a of the optical system 26. This may cause beam pointing errors or beam pointing fluctuations. For example, when the optical amplifier 24 remains stationary, the movement and / or deformation of the optical amplifier 24 may cause the amplified laser beam 20c indicated by the solid line in Figure 12A to leave the optical amplifier 24 at an angle β with respect to the amplified laser beam 20c indicated by the dashed line in Figure 12A .

[0087] Figure 12B Shows Figure 4 The optical amplifier 24 shown in, for example, Figure 11 Another schematic plan view and side view in the y,z plane and the x,y plane of the coordinate system shown in. Figure 12B Similar to Figure 12A . Thus, only the differences will be described below. In the Figure 12B Example shown in, the optical system 26 (for example, its part 26a) is supported by the frame 28, as described above. Thus, the optical system 26 (for example, its part 26a) is optically coupled or connected to the optical amplifier 24. However, the optical amplifier 24 is separate from the optical system 26 (for example, its part 26a) and / or may be movable relative to the optical system 26 (for example, its part 26a). In other words, there is no physical connection between the optical system 26 (for example, its part 26a) and the optical amplifier 24.

[0088] Refer to [[ID= , when the optical amplifier 24 is in ​When stationary in the x, y plane and the y, z plane shown, movement and / or deformation of the optical amplifier 24 relative to the optical system 26 (such as its part 26a) may cause a shift of the amplified laser beam 20c relative to the amplified laser beam 20c, such as a translational shift. Thus, beam pointing errors or fluctuations attributable to external movement can be reduced or prevented.

[0089] ​ Schematically shown in ​ the optical path 30 of the laser beam 20 passing through the optical amplifier 24 in the x, y plane and the y, z plane shown in ​ For clarity, ​ only some of the first optical elements in the first optical element 40a are shown in ​ And ​ the optical amplifier 24 shown in ​ may include a first optical element 40a and a second optical element 40b, as described above and below. In the example shown in ​ one of the first optical elements in the first optical element 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 the arrow M in ​ Although ​ and ​ the description refers to the movement of one first optical element 40a, it will be understood that in other embodiments, more than one first optical element and / or one or more second optical elements may move, for example, due to movement and / or deformation of the optical amplifier.

[0090] When the first optical element 40a remains stationary, ​ the dashed line in ​ indicates the optical path 30 of the laser beam 20. When the first optical element 40a has moved, ​As can be seen, when the first optical element 40a remains stationary, movement of the first optical element 40a can cause the amplified laser beam 20c to shift (e.g., translate) and / or be parallel (e.g., substantially parallel) relative to the amplified laser beam 20c. By configuring the reflector 34 such that the reflected laser beam 20b is parallel to the incident laser beam 20a, beam pointing errors or beam pointing fluctuations of the laser beam 20 passing through the optical amplifier 24 that can be attributed to internal deformation and / or movement can be reduced or prevented, for example, in both the x, y plane and the y, z plane. Thus, by configuring the frame 28 to be separated from the optical amplifier 24 and by configuring the reflector 34 such that the reflected laser beam 20b is parallel to the incident laser beam 20a, beam pointing errors or beam pointing fluctuations of the laser beam 20 that can be attributed to internal movement and / or deformation and external movement can be reduced or prevented, for example, in both the x, y plane and the y, z plane of the optical amplifier 24. As described above and ​ The x, y plane and the y, z plane shown in are for illustrative purposes only and should not be understood as restricting the features of the optical amplifier to these planes. For example, it will be appreciated that any feature described with respect to the y, z plane of the optical amplifier 24 can also apply to the x, z plane of the optical amplifier. Thus, by configuring the frame 28 to be separated from the optical amplifier 24 and by configuring the reflector 34 such that the reflected laser beam 20b is parallel to the incident laser beam 20a, beam pointing errors or beam pointing fluctuations of the laser beam 20 that can be attributed to internal movement and / or deformation and external movement can also be reduced or prevented in the x, z plane of the optical amplifier 24.

[0091] ​ and ​ shown ​ A portion of the amplifier system 16 shown in, where the optical amplifiers 24a to 24d are not present. ​ shown ​ The outer side of a portion of the amplifier system 16 shown in. ​ shows the inner side of the said portion of the amplifier system 16.

[0092] ​ and ​ show the frame 28 configured to support the optical system 26. In ​ and ​ In the embodiment shown in, the frame 28 is not connected to the laser 18. Instead, the frame 28 is configured to only support the optical system 26. However, as described above with respect to ​ In some embodiments, the frame 28 can be configured to be connected to the laser 18.

[0093] The frame 28 can be formed of materials such as metallic materials, metal alloy materials, and / or fiber materials. The materials can be selected to be thermally stable. For example, the metallic materials can include aluminum and / or the like. The metal alloy materials can include steel, stainless steel, and / or the like. The fiber materials can include carbon fiber and / or the like. It will be appreciated that in other embodiments, the materials can include ceramic materials, glass materials, and / or the like.

[0094] The amplifier system 16 can include an additional frame 45. The frame 28 can be connected to or mounted on the additional frame 45. For example, the frame 28 can be welded to the additional frame 45 or at least a portion of the additional frame 45. In other embodiments, the frame and the additional frame can be integral. The frame 28 can be mounted or connected to the additional frame such that the frame 28 extends along at least a portion of the additional frame 45.

[0095] The additional frame 45 can be configured to serially mount, for example, the optical amplifiers 24a to 24d. For example, the additional frame 45 can include a plurality of additional support elements 46a, 46b, eight of the support elements being shown in ​ and ​ . It will be appreciated that in other embodiments, the additional frame can include more or fewer than eight additional support elements and / or can be configured to mount the amplifiers in a different arrangement.

[0096] Each of the additional support elements 46a, 46b can be configured to project or extend from the additional frame 45, for example, towards the interior of the amplifier system 16. Each of the additional support elements 46a, 46b can be arranged to project 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 can be arranged to project or extend in an upward direction, for example, when the amplifier system 16 is in use.

[0097] At least two of the additional support elements 46a, 46b can be associated with a corresponding optical amplifier 24. The two additional support elements 46a, 46b can be configured to mount the corresponding amplifiers 46a, 46b. The two additional support elements 46a, 46b can include different dimensions and / or shapes relative to each other. For example, the first support element 46a of the two additional support elements 46a, 46b can include or define an L shape (e.g., substantially L shape), and / or can 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 can include or define a T shape (e.g., substantially T shape) and / or can be smaller than the first support element 46a.

[0098] ​ shown ​ The portion of the amplifier system 16 shown in ​ that includes optical amplifiers 24a to 24d. Two additional support elements 46a, 46b may be arranged to mount opposite sides (e.g., diagonally opposite sides) of the corresponding optical amplifier 24.

[0099] 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 coolant through the frame 28 and / or the additional frame 45. The coolant may include a cooling fluid or liquid, such as 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 such that the temperature on the frame and / or the additional frame is uniform.

[0100] ​ Schematically shown for an optical amplifier (such as ​ the optical amplifier 24 shown in ​ ). The reflector 34 is provided in the form of a corner prism. ​ The reflector shown in ​ is the same as the reflector 34 described above with respect to ​ , ​ . Thus, any features described above with respect to ​ , ​ may also apply to the reflector shown in ​ ​ .

[0101] The reflector 34 may include one or more reflective surfaces 48a, 48b, 48c, and three of the one or more reflective surfaces 48a, 48b, 48c are shown in ​ ​ . It will be appreciated that in other embodiments, the reflector may include more or fewer than three reflective surfaces. In such embodiments, each of the reflective surfaces 48a, 48b, 48c is provided in the form of a planar surface. It will be appreciated that in other embodiments, at least one or each of the reflective surfaces may include a curved surface.

[0102] The reflective surfaces 48a, 48b, 48c may be arranged perpendicular to each other, e.g., substantially perpendicular to each other. In such an 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 reflective surface 48a to the second reflective surface 48b. The incident beam 20a may then be reflected from the second reflective surface 48b to the third reflective surface 48c. The third reflective surface 48c may be arranged to reflect the laser beam away from the reflector 34. The reflective surfaces 48a, 48b, 48c may be arranged such that the reflected laser beam 20b is parallel to the incident laser beam 20a. In other words, the reflective surfaces 48a, 48b, 48c may be arranged such that the direction of the reflected laser beam 20b is the reverse direction of the incident laser beam 20a. Although ​ the first reflective surface, the second reflective surface, and the third reflective surface are shown as being arranged perpendicular to each other, it will be appreciated that in other embodiments, the first reflective surface, the second reflective surface, and the third reflective surface may be arranged differently.

[0103] The first reflective surface 48a, the second reflective surface 48b, and the third reflective surface 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. This arrangement of the first reflective surface 48a, the second reflective surface 48b, and the third reflective surface 48c may be considered a symmetric arrangement. However, it will be appreciated that in other embodiments, the arrangement of the first reflective surface, the second reflective surface, and the third reflective surface may be asymmetric, and / or the first reflective surface, the second reflective surface, and the third reflective surface may be arranged such that the angle of incidence on the first, second, and / or third reflective surfaces is different from 55.7 degrees.

[0104] The first reflective surface 48a may include or define a first location on which the laser beam 20 is incident. The third reflective surface 48c may include or define a second location at which the laser beam 20 is reflected away from the reflector 34. The first location of the first reflective surface 48a and the second location of the third reflective surface 48c may be spaced apart from each other. In ​ the distance C between the first location on the first reflective surface 48a and the second location on the third reflective surface 48c indicated in ​ may correspond to the spacing E between the first portion 32a and the second portion 32b of the optical path 30 indicated for the first optical amplifier 24a in

[0105] As visible in ​ the second reflective surface 48b is arranged between the first reflective surface 48a and the third reflective surface 48c.

[0106] ​and ​ schematically shows a laser beam 20 passing through another exemplary optical path 30 that includes ​ the reflector 34 shown in ​ and the optical amplifier 24 shown in. ​ and ​ shows the optical path 30, which includes a first optical element 40a and a second optical element 40b, a first optical window 41a and a second optical window 41b, and a spacing E between a first portion 32a and a second portion 32b of the optical path 30. However, in the embodiment shown in ​ and ​ the order of the first reflective surface 48a, the second reflective surface 48b, and the third reflective surface 48c has been reversed with respect to the order of the first reflective element 48a, the second reflective element 48b, and the third reflective element 48c shown in ​ .

[0107] ​ schematically shows another exemplary reflector 34 for use in an optical amplifier such as ​ the optical amplifier 24 shown in. ​ The reflector 34 shown in is similar to the reflector 34 shown in ​ , ​ and ​ . Thus, any features described above with respect to ​ , ​ and ​ can also apply to the reflector 34 shown in ​ . Only the differences will be described hereinafter. In the embodiment shown in ​ the reflector 34 is provided in the form of a retroreflector such as a conical reflector.

[0108] The reflector 34 can be considered to include a single reflective surface 48. The reflective surface 48 can be configured such that the reflected laser beam 20b is parallel to the incident laser beam 20a. In other words, the reflective surface 48 can be configured such that the direction of the reflected laser beam 20b is the reverse direction of the incident laser beam 20a. For example, the diameter D1 and / or the depth D2 of the reflective surface 48 can be selected such 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 as the distance measured along the axis of symmetry from the vertex V to the plane of the edge R of the reflective surface 48. Although ​ the reflector is shown as including a single reflective surface, it will be understood that in other embodiments, the reflector can include more than one reflective surface.

[0109] In ​In the embodiment shown, the reflective surface 48 may include or define a first position and a second position, on which the laser beam 20 is incident and at which the laser beam 20 is reflected away from the reflector 34. The first and second positions of the reflective surface 48 may be spaced apart from each other. The distance that has been omitted between the first and second positions of the reflective surface 48 may correspond to the spacing E between the first portion 32a and the second portion 32b of the optical path 30. ​ The distance between the first and second positions of the reflective surface 48 that has been omitted may correspond to the spacing E between the first portion 32a and the second portion 32b of the optical path 30.

[0110] ​ Schematically shown is another exemplary reflector 34 for use in an optical amplifier (such as ​ the amplifier 24 shown). ​ The reflector 34 shown in ​ , ​ , ​ and ​ is similar to the reflector 34 shown in ​ , ​ , ​ and ​ Accordingly, any features described above with respect to ​ may also apply to the reflector 34 shown in

[0111] In the embodiment shown in ​ , the reflector 34 is configured to reflect the laser beam 20 three times. The reflector 34 may include one or more reflective surfaces 48a to 48c, three of which are shown in ​ . The first reflective surface 48a may include or define a first position on 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 away from the reflector 34. The first position of the first reflective surface 48a and the second position of the third reflective surface 48c may be spaced apart from each other. The distance C between the first position on the first reflective surface 48a and the second position on the third reflective surface 48c indicated in ​ may correspond to the spacing E between the first portion 32a and the second portion 32b of the optical path 30 indicated for the first optical amplifier 24a in ​ .

[0112] Each of the first reflective surface 48, the second reflective surface 48b, and the third reflective surface 48c may comprise or define a curved surface. The curvature of each of the first reflective surface 48a and the third reflective surface 48c and the arrangement of the first reflective surface 48a and the third reflective surface 48c may be selected such that the first reflective surface 48a and the third reflective surface 48c define a common parabola or lie on a common parabola. For example, the first reflective surface 48a and the third reflective surface 48c may be arranged such that the first reflective surface 48a and the third reflective surface 48c define a common cone or lie on a common cone.

[0113] The reflective surfaces 48a to 48c may be arranged such that the path 50 between the first position and the second position (e.g., the first reflective surface 48a and the third reflective surface 48c) of the incident laser beam 20a, the reflected laser beam 20b, and the laser beam 20 defines an M-shaped shape (e.g., a substantially M-shaped shape), as ​ shown. For example, the second reflective surface 48b may be arranged between the first reflective surface 48a and the third reflective surface 48c. The second reflective surface 48b may be arranged to face a first direction. The first reflective surface 48a and the third reflective 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 opposite to each other (e.g., substantially opposite). For example, the second reflective surface 48b may be arranged opposite to the common parabola defined by the first reflective surface 48a and the third reflective surface 48c. ​ The reflector 34 shown may be regarded as defining a cat's eye configuration.

[0114] It will be appreciated that the arrangement of the retroreflector is not limited to the embodiments or examples disclosed herein, and in other embodiments, the retroreflector may be arranged differently.

[0115] It will be appreciated that the terms "optical amplifier" and "amplifier" may be used interchangeably.

[0116] It will be understood that references to multiple features may be used interchangeably with references to singular forms of those features such as (e.g.) "at least one" and / or "each". Singular forms such as (e.g.) "at least one" or "each" of the features may be used interchangeably.

[0117] Although reference may be made specifically herein to the use of a lithographic apparatus in IC manufacture, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include 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, and the like.

[0118] Although specific embodiments of the present invention have been described above, it will be understood that the present invention may be practiced in other ways different from those described. The above description is intended to be illustrative, not restrictive. Thus, those skilled in the art will appreciate that the described present invention may be modified without departing from the scope of the aspects and claims set forth below.

[0119] Aspect:

[0120] 1. An optical amplifier configured to amplify a laser beam, the optical amplifier comprising:

[0121] an optical path of the laser beam, the optical path including a first portion and a second portion; and

[0122] a reflector configured to reflect the laser beam so as to direct the laser beam between the first portion and the second portion of the optical path, the reflector being 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.

[0123] 2. The optical amplifier according to aspect 1, wherein the reflector is configured to reflect the laser beam at least twice to direct the laser beam between the first portion and the second portion of the optical path.

[0124] 3. The optical amplifier according to aspect 1 or 2, wherein the reflector includes a retroreflector.

[0125] 4. The optical amplifier according to aspect 3, wherein the retroreflector includes at least one of the following:

[0126] a corner reflector; and

[0127] a conical reflector.

[0128] 5. The system according to aspect 4, wherein the corner reflector includes a corner cube prism.

[0129] 6. The optical amplifier according to any of the preceding aspects, wherein the reflector includes one or more reflective surfaces, the one or more reflective surfaces including a first position and a second position, the laser beam being incident on the first position and the laser beam being reflected away from the reflector at the second position, at least one or each of the one or more reflective surfaces including a planar surface or a curved surface.

[0130] 7. The optical amplifier according to aspect 6, wherein the first position and the second position of the one or more reflective surfaces are spaced apart from each other, and the distance between the first position and the second position of the one or more reflective surfaces corresponds to the spacing between the first portion and the second portion of the optical path.

[0131] 8. The optical amplifier according to aspect 6 or 7, wherein the one or more reflective surfaces are arranged such that the laser beam is reflected at least three times to direct the laser beam between the first portion and the second portion of the optical path, and the one or more reflective surfaces are arranged such that at least one of the following conditions exists:

[0132] At least one reflective surface is arranged between at least two other reflective surfaces;

[0133] 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 from and / or opposite to each other; and

[0134] 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 define an M-shaped configuration.

[0135] 9. An amplifier system for a laser system, the system comprising:

[0136] A plurality of optical amplifiers, at least one or each optical amplifier of the plurality of optical amplifiers comprising the optical amplifier according to any one of the preceding aspects;

[0137] An optical system configured to optically couple at least one optical amplifier of the plurality of optical amplifiers to at least another optical amplifier of the plurality of optical amplifiers; and

[0138] A frame configured to support the optical system, the frame being configured to be separated from the plurality of optical amplifiers.

[0139] 10. The system according to aspect 9, wherein the frame is configured to perform at least one of the following:

[0140] Support the optical system such that each optical amplifier of the plurality of optical amplifiers is movable relative to the optical system; and

[0141] Support the optical system such that each optical amplifier of the plurality of optical amplifiers is separated from the optical system.

[0142] 11. The system according to any one of aspects 9 to 10, wherein the system includes: an additional frame configured to mount the plurality of optical amplifiers, the frame being connected to the additional frame or the frame and the additional frame being integral.

[0143] 12. A laser system, the laser system including:

[0144] a laser configured to generate a laser beam; and

[0145] an amplifier system according to any one of aspects 9 to 11, wherein the amplifier system is configured to amplify the laser beam.

[0146] 13. The laser system according to aspect 12, wherein the frame is configured to be connected to the laser.

[0147] 14. A radiation system, comprising:

[0148] an EUV radiation source; and

[0149] a laser system according to aspect 12 or 13.

[0150] 15. A radiation system, comprising:

[0151] an EUV radiation source; and

[0152] a laser system including an optical amplifier according to any one of aspects 1 to 8.

[0153] 16. A lithography system, comprising a radiation system and a lithography system according to aspect 14 or 15.

Claims

1. An optical amplifier configured to amplify a laser beam, the optical amplifier comprising: an optical path of the laser beam, the optical path 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 portion and the second portion of the optical path, the reflector being 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 according to claim 1, wherein, The reflector is configured to reflect the laser beam at least twice to direct the laser beam between the first portion and the second portion of the optical path.

3. The optical amplifier according to claim 1 or 2, wherein, The reflector includes a retroreflector.

4. The optical amplifier according to claim 3, wherein The retroreflector includes at least one of the following: a corner reflector, wherein the corner reflector includes a corner cube prism; and a conical reflector.

5. The optical amplifier according to any one of the preceding claims, wherein, The reflector includes one or more reflective surfaces, the one or more reflective surfaces including a first position and a second position, the laser beam being incident on the first position and the laser beam being reflected away from the reflector at the second position, at least one or each of the one or more reflective surfaces including a planar surface or a curved surface.

6. The optical amplifier according to claim 5, wherein, The first position and the second position of the one or more reflective surfaces are spaced apart from each other, and the distance between the first position and the second position of the one or more reflective surfaces corresponds to the spacing between the first portion and the second portion of the optical path.

7. The optical amplifier according to claim 5 or 6, wherein, The one or more reflective surfaces are arranged such that the laser beam is reflected at least three times to direct the laser beam between the first portion and the second portion of the optical path, the one or more reflective surfaces being arranged such that there is at least one of the following: at least one reflective surface is arranged 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 from and / or opposite to each other; and 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 define an M-shaped configuration.

8. An amplifier system for a laser system, the system comprising: a plurality of optical amplifiers, at least one or each of the plurality of optical amplifiers including the optical amplifier according to any one of the preceding claims; an optical system configured to optically couple at least one of the plurality of optical amplifiers to at least another one of the plurality of optical amplifiers; and a frame configured to support the optical system, the frame being configured to be separated from the plurality of optical amplifiers.

9. The system according to claim 8, wherein, The frame is configured to perform at least one of the following: Support the optical system such that each of the plurality of optical amplifiers is capable of moving relative to the optical system; and Support the optical system such that each of the plurality of optical amplifiers is separated from the optical system.

10. The system according to any one of claims 8 to 9, wherein, The system includes an additional frame configured to mount the plurality of optical amplifiers, the frame being connected to the additional frame or the frame and the additional frame being integral.

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

12. The laser system according to claim 11, wherein, The frame is configured to be connected to the laser.

13. A radiation system, comprising: An EUV radiation source; And The laser system according to claim 11 or 12.

14. A radiation system, comprising: An EUV radiation source; And A laser system, the laser system including an optical amplifier according to any one of claims 1 to 7.

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

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