Laser device using a laser beam amplifier

Abstract

Laser device, comprising: - a master oscillator (51) for generating a pulsed laser beam; - a preamplifier (20) for amplifying the pulsed laser beam provided by the master oscillator, comprising a laser beam amplifier, comprising: - a container for holding the laser medium; - a pair of electrodes (22, 22b, 23, 23b) for performing a discharge in the laser medium to form a gain region (30, 30a, 30b) for a laser beam in the laser medium; and - an optical system (37, 37a, 38, 38a, 42, 43) for forming a beam path between a first point (34) at which the laser beam is incident and a second point (35) from which the amplified laser beam is output, so that the amplification region lies in the beam path between the first point and the second point, wherein the first point and the second point are conjugate to each other and the laser beam incident at the first point is amplified during at least two passes through the amplification region and is transferred to the second point; - a first relay optics (55) for adjusting a size and an expansion angle of the pulsed laser beam output from the preamplifier; - a main amplifier (60) for amplifying the pulsed laser beam provided by the preamplifier via the first relay optics; and - a second relay optics (57) for coordinating the pulsed laser beam output from the main amplifier to output parallel light.

Description

BACKGROUND OF THE INVENTIONField of the InventionThe present invention relates to a laser apparatus using a laser beam amplifier for amplifying a seed laser beam. More particularly, the present invention relates to a driving laser apparatus for irradiating a target material with a laser beam to convert the target material into a plasma in an extreme ultraviolet (EUV) light source apparatus.Prior ArtIn recent years, as semiconductor processes have become finer, photolithography has made a fast progress toward finer production. In the next generation, copolymerization at 60 nm to 45 nm, and copolymerization at 32 nm and smaller are required. Accordingly, for example, exposure equipment is expected to be generated by combining an EUV light source for generating EUV light at a wavelength of about 13 nm and reflection optics for reduced projection.As the EUV light source, there is an LPP (Laser Produced Plasma) light source which is generated by irradiating a target with a laser beam (hereinafter referred to as an LLP type EUV light source device). The LLP-type light source device irradiates a target material, for example, tin (Sn) provided in the vacuum chamber, with a driving laser beam to excite the target material and generate plasma. From the generated plasma, various wavelength components including EUV light are radiated, and a desired EUV component among them is selectively collected using a collector mirror (EUV collector mirror) and output to an apparatus using EUV light such as an exposure unit.An output power of more than 100 watts is required for an EUV light source. Even in the case of an EUV light source for relatively efficiently generating EUV light by using a carbon dioxide (CO 2) - laser device and a tin (Sn) target, a higher efficiency of the CO 2- laser device for outputting a driving laser beam for plasma generation in industrial application is required.As a driving laser for generating the plasma, a pulsed laser beam is used, and therefore, a master oscillator power amplifier type laser used as a driving laser device includes, for example, a laser oscillator for generating a short pulse CO 2- laser beam and a laser beam amplifier for amplifying the short pulse CO 2- laser beam.The laser beam amplifier has a discharge unit for exciting a CO 2- laser gas by discharge, which contains carbon dioxide (CO 2), nitrogen (N 2), helium (He), and additionally, as needed, hydrogen (H 2), carbon monoxide (CO), xenon (Xe), and so on. In the laser beam amplifier, a seed laser beam generated by the laser oscillator is amplified to a laser beam having desired energy. The amplified laser beam is focused by laser beam focusing optics and is deposited on a target material such as tin (Sn), xenon (Xe), or the like.DE 10 2009 024 360 A1 discloses a slab-type laser device having a laser beam amplifier part comprising a plurality of rear-view mirrors. DE 100 25 874 A1 discloses an optical amplifier arrangement with a plurality of highly reflective mirrors, between which an amplification medium is arranged. US 2002 / 0167974 A1 discloses a Q-switched CO 2 laser with cavity attenuation for material processing. In addition, US 2005 / 0220164 A1 discloses a laser oscillator for generating an axially symmetrical laser beam with suppressed astigmatism.US 5 386 431 A discloses a regenerative laser amplifier arrangement for performing multiple pass amplification. Fig. 26 shows a laser beam amplifier disclosed in US 005386431 A. The laser beam amplifier as shown in FIG. 26 uses a multi-pass system in which an incoming seed laser beam is oscillated a plurality of times in a laser medium, whereby a large amplification factor can be achieved by efficiently using the energy in the laser medium. Thus, downsizing of the driving laser apparatus including the laser beam amplifier can be achieved.In the multi-pass amplification type laser beam amplifier as shown in FIG. 26, it is necessary to make the optical path of the seed laser longer in the discharge region in order to increase the amplification efficiency. Accordingly, the optical system is constructed such that the laser beam is reflected by a rear mirror and a front mirror a plurality of times to make the optical path of the laser beam longer within the discharge medium.However, since the optical path length in the conventional multi-pass amplification type laser beam amplifier may be up to 10 meters, the error between the optical axes (a difference between the reference optical axis and the shifted optical axis) during the reflection and amplification of the seed laser beam increases if the optical axis of the seed laser beam at the input position is shifted with respect to the reference optical axis, as shown in FIG. 27. As a result, due to the large displacement of the optical axis of the laser beam to be amplified, the laser beam amplification is not performed efficiently, and the output power after amplification becomes smaller. In the case where the optical axis is further shifted, the laser beam may be outside the output window and not output from the laser beam amplifier as shown by the dotted line in FIG. 27.Therefore, configuring the laser apparatus using the multiple pass amplification type laser beam amplifier is extremely difficult for the following reasons: (1) it is difficult to properly provide the laser beam amplifier in the optical path of the seed laser beam; (2) it is difficult to correctly guide the seed laser beam to the provided laser beam amplifier; and (3) it is difficult to perform the optical adjustment of the entire laser apparatus. Furthermore, even when the laser device is correctly configured, the beam path within the laser beam amplifier is largely displaced from the optical reference axis due to a slight displacement of the entry position or the entry angle. Accordingly, the power consumption efficiency in the discharge region becomes lower and the boosting state becomes unstable. Due to this, the positional accuracy of the focal point and the focusing ability of the amplified laser beam are deteriorated. Further, the following problems arise if the multi-pass amplification type laser beam amplifier is used in the driving laser apparatus of the EUV light source apparatus. That is, when the position of the focal point of the driving laser beam is shifted by a large amount, the target material is not irradiated with the laser beam and EUV light is not generated. Even when the target material is irradiated with the laser beam, the energy of the EUV light decreases and the energy stability decreases.SUMMARY OF THE INVENTIONThe present invention has been made in view of the above problems. An object of the present invention is to provide a laser apparatus including a laser beam amplifier having high optical axis stability for easy adjustment and output of a stable amplified pulse laser beam, particularly a driver laser apparatus for irradiating a target material with a laser beam to convert the target material into a plasma in an EUV light source apparatus.In order to achieve the above object, a laser device is provided having the features of claim 1.The laser apparatus includes a laser beam amplifier including: a container for accommodating the laser medium; a pair of electrodes for generating discharge in the laser medium to form an amplification region for the laser beam in the laser medium; and an optical system for forming an optical path between a first point at which the laser beam is incident and a second point at which the laser beam is output such that the amplification region in the optical path is between the first point and the second point, wherein the first point and the second point are conjugate with each other, and the laser beam incident at the first point is amplified at least twice passing through the amplification medium and then transferred to the second point.Further, the laser apparatus includes: a master oscillator for generating the pulsed laser beam; a laser beam preamplifier including the laser beam amplifier as described above for amplifying the pulsed laser beam provided from the master laser at the first point and outputting the amplified pulsed laser beam from the second point; a first relay optical unit for adjusting a size and an expansion angle of the pulsed laser beam output from the preamplifier; a main amplifier for amplifying the pulsed laser beam provided from the preamplifier via the first relay optical unit; and a second relay optical unit for collimating the laser beam output from the main amplifier to output pallellel light.In the laser beam amplifier, the optical path between the first point and the second point is formed so that the amplification region in the optical path is located between the first point and the second point which are conjugate with each other, and the laser beam incident at the first point is amplified and transferred to the second point during the at least two times of passage through the amplification region. Consequently, the image of the laser beam incident at the first point is transferred to the second point, and the transfer image is focused. As a result, the error of the optical axis is not largely increased, and the errors in the position and the incident angle of the laser beam at the incident position with respect to the reference optical axis are substantially equal to the errors at the output position.Further, since the laser beam amplifier has high optical path stability, the error of the optical axis with respect to the reference optical axis is suppressed, and the power consumption efficiency in the entire laser apparatus is not reduced. In the case where optical adjustment is performed between the laser beam amplifier and further optics, the optical adjustment can be easily and correctly performed by using the first point and the second point as a reference for the optical axis. Thus, by focusing the amplified high-light intensity laser beam output from the laser device according to the present invention on the plasma emission point of the LLP type EUV light source device, EUV light can be generated with high efficiency and the energy stability of the EUV light can be improved.Brief Description of the DrawingsFIG. 1 shows a configuration of a laser beam amplifier for use in a laser apparatus according to the first embodiment of the present invention; FIG. 2 is a perspective view of a structure within a chamber of the laser beam amplifier as shown in FIG. 1; FIG. 3 is a perspective view showing a structure inside a chamber of the laser beam amplifier according to a modified example of the first embodiment; FIG. 4 is a front view for explaining an operation of the laser beam apparatus as shown in FIG. 3 ; FIG. 5 shows a configuration of the laser beam amplifier for use in a laser apparatus according to a second embodiment of the present invention; FIG. 6 is a perspective view showing a structure of a fast axial flow type laser beam amplifier to which the second embodiment of the present invention is applied; FIG. 7 shows electrodes of the fast axial flow type laser beam amplifier as shown in FIG. 6; FIG. 8 shows a modified example of the electrodes of the fast axial flow type laser beam amplifier as shown in FIG. 6; FIG. 9 is a side view of a structure of a triaxial orthogonal type laser beam amplifier to which the second embodiment of the present invention is applied; FIG. 10 is a sectional view along the A-A line of the triaxial orthogonal type laser beam amplifier as shown in FIG. 9; FIG. 11 shows a configuration of a laser beam amplifier according to a modified example of the second embodiment; FIG. 12 is a plan sectional view of a laser beam amplifier according to a first example as viewed from above; FIG. 13 is a diagram for explaining an operation of the laser beam amplifier according to a first example having an optical system in which reflective optical elements are replaced with transmissive optical elements; FIG. 14 is a plan sectional view of a laser beam amplifier according to a second example as viewed from above; FIG. 15 is a diagram for explaining an operation of the laser beam amplifier according to the second example having an optical system in which reflecting optical elements are replaced with light transmitting optical elements; FIG. 16 is a plan sectional view of a laser beam amplifier according to a third example as viewed from above; FIG. 17 is a diagram for explaining an operation of the laser beam amplifier according to the third example having an optical system in which reflecting optical elements are replaced with light transmissive optical elements; FIG. 18 is a plan sectional view of a laser beam amplifier according to a fourth example as viewed from above; FIG. 19 is a diagram for explaining an operation of the laser beam amplifier according to the fourth example having an optical system in which reflective optical elements are replaced with transmissive optical elements, and the transmissive optical elements are arranged in series; FIG. 20 is a plan sectional view of a laser beam amplifier according to a fifth example as viewed from above; FIG. 21 is a diagram for explaining an operation of the laser beam amplifier according to the fifth example with an optical system in which reflective optical elements are replaced with transmissive optical elements and the transmissive optical elements are arranged in series; FIG. 22 is a plan sectional view of a laser beam amplifier according to a sixth example as viewed from above; FIG. 23 shows a configuration of an EUV light source apparatus using a driving laser apparatus according to the third embodiment of the present invention; FIG. 24 shows a configuration of an EUV light source apparatus using a driving laser apparatus according to the fourth embodiment of the present invention; FIG. 25 shows a configuration of an EUV light source apparatus using a driving laser apparatus according to the fifth embodiment of the present invention; FIG. 26 shows a conventional multi-pass amplification type laser beam amplifier; and Fig. 27 is a diagram for explaining a problem in the conventional multi-pass amplification type laser beam amplifier.DESCRIPTION OF THE PREFERRED EMBODIMENTSHereinafter, the preferred embodiments of the present invention will be explained in detail with reference to the drawings. The same reference numerals are assigned to the same subcomponents, and their explanation is omitted.(Embodiment 1)FIG. 1 shows a configuration of a laser beam amplifier for use in a laser apparatus according to a first embodiment of the present invention, and shows the case where the present invention is applied to a plate-like laser beam amplifier to explain the principle of a laser beam amplifier for use in a laser apparatus of the present invention in a manner that is easy to understand.As shown in FIG. 1, the laser beam amplifier according to the first embodiment includes a chamber 21 as a container for accommodating the laser medium, a pair of electrodes for generating the discharge in the laser medium to generate a gain region 30 for a laser beam in the laser medium, and an optical system having a pair of reflecting mirrors 37 and 38 disposed opposite to each other with the gain medium therebetween, and the laser beam amplifier performs multi-pass amplification. The chamber 21 is provided with an input window 33 through which a laser beam is incident and an output window 36 through which the laser beam is output. As the laser medium, for example, a CO 2- laser gas containing carbon dioxide (CO 2), nitrogen (N 2), helium (He), and additionally, as needed, hydrogen (H 2), carbon monoxide (CO), xenon (Xe), etc. is used.FIG. 2 is a perspective view illustrating the structure inside the chamber of the laser beam amplifier as shown in FIG. 1. As shown in Fig. 2, a pair of plate electrodes 22 and 23 are arranged so that the laser medium enclosed within the chamber is sandwiched therebetween. When a radio frequency (RF) power supply 24 provides a radio frequency voltage between the plate electrode 22 and the plate electrode 23, the plate electrodes 22 and 23 generate a radio frequency electric field to generate a radio frequency discharge in the laser medium. Thereby, the laser medium is excited, and a seed laser beam (incident beam) incident into the chamber through the input window 33 is amplified. The seed laser beam is reflected by the reflecting mirrors 37 and 38 during the amplifying, and then outputted as an output beam from the output window 36 to the outside of the chamber.Here, the laser beam amplification region is a discharge region interposed between the electrode plate 22 and the electrode plate 23. By applying a radio frequency voltage between the electrode plate 22 and the electrode plate 23, a part of the chamber filling laser medium sandwiched between the electrode plate 22 and the electrode plate 23 is excited, and a plate-shaped discharge region is formed. The gap between the electrode plate 22 and the electrode plate 23 is about 0.5 mm to 2 mm, and the region of the discharge region has a thin rectangular shape. For cooling the electrode plates 22 and 23 and the laser medium, cooling water is provided to the electrode plates 22 and 23.Referring back to FIG. 1, the reflecting mirrors 37 and 38 are HR (high-reflection) concave mirrors with focusing ability that largely focus the laser beam, and face each other with the amplification region 30 interposed therebetween. for example, the reflecting mirrors 37 and 38 are placed with a small displacement in the vertical direction in FIG. 1 so that the line connecting the center of the reflecting surface and the center of curvature of the reflecting mirror 37 and the line connecting the center of the reflecting surface and the center of curvature of the reflecting mirror 38 are parallel to each other. Thereby, the incident beam passes through the side where the reflecting mirror 38 is located and is incident on the reflecting mirror 37, and the output beam passes through the side where the reflecting mirror 37 is located and is output.The seed laser beam incident on the laser beam is transmitted through the input window 33 to enter the chamber 21, and passes through an incident beam position 34 along a reference beam path 40, as shown by solid lines in FIG. 1. The incident beam position 34 is a predetermined first point in the beam path. The seed laser beam that has passed through the incident beam position (first point) is amplified during passage through the amplification region 30, is incident on the reflecting mirror 37 and is reflected with high reflectivity, is amplified during repass through the amplification region 30, and is reflected with high reflectivity by the opposing mirror 38.The laser beam reflected by the reflecting mirror 38 is amplified during repass through the amplifying region 30, reflected again by the reflecting mirror 37, while passage through the amplifying region 30, reflected by the reflecting mirror 38, transmitted through the output window 36, and output from the laser beam amplifier. Here, the image of the laser beam incident at the first point 34 is transferred to a predetermined second point 35 in the vicinity of the position where the reference optical axis 40 passes through the side where the reflecting mirror 37 is located, and an incident beam transfer image is formed.As described above, the optical system including the reflecting mirrors 37 and 38 forms a light path between the first point 34 at which the laser beam is incident and the second point 35 from which the laser beam is output, so that the amplification region 30 is disposed in the light path between the first point 34 and the second point 35 which are conjugate with each other, and the laser beam incident at the first point 34 is amplified during at least twice passage through the amplification region 30 and then transferred to the second point 35. Here, it is desirable that the first point 34 at which the laser beam is incident is disposed adjacent to the input window 33, and that the second point 35 from which the laser beam is output is disposed adjacent to the output window 36.Assuming that an object is placed at the position of the first point 34, the first point 34 and the second point 35 correspond to an object point and an imaging point, respectively, of the optical system including the reflecting mirrors 37 and 38, and are determined based on the focusing capability and the spatial arrangement ratio of the reflecting mirrors 37 and 38. The positions of the first point 34 and the second point 35 are determined in advance, and thereby the radii of curvature "R" and the positions of the reflecting mirrors 37 and 38 corresponding to these positions can be determined.In FIG. 1, the optical path 41 shown by the dotted line is an optical path in which the optical axis (position and / or propagation direction) of the seed laser beam incident on the laser beam amplifier is shifted from that of the reference optical axis 40. In the case of using the optical system in which the first point 34 and the second point 35 are determined as described above, even if the optical axis of the seed laser beam is shifted from the reference optical axis 40, the spatial positional relationship between the incident beam and the first point 34 in the spatial positional relationship between the outgoing beam and the second point 35 is reconstructed from the relationship that the image of the incident beam is transferred to the second point 35 at the first point 34, and thereby the optical axis of the amplified laser beam does not substantially deviate from the reference beam path 40.Thus, in the laser beam amplifier according to this embodiment, even if the optical axis of the incident beam has an error with respect to the reference optical axis 40, the optical axis of the outgoing beam only has the error equal to the error relative to the reference axis 40 at the entrance position. Accordingly, the stability of the optical axis is improved, the amplification efficiency does not vary remarkably, and the seed laser beam can be amplified stably. As a result, the position and shape of the focal point become stable.In particular, it is desirable that the transfer magnification of the incident beam transfer image of the incident beam be substantially equal to "1.". In the case where the transfer magnification is "1", even if the incident beam position at the first point 34 has an error with respect to the reference optical axis 40, the position of the outgoing beam at the second point 35 has substantially only the same error with respect to the reference optical axis 40.Further, even if the angle of the incident beam at the first point 34 has an error with respect to the reference optical axis 40, the angle of the outgoing beam at the second point 35 has only substantially the same error with respect to the reference optical axis 40. In a conventional laser beam amplifier having an optical path length of 30 meters in 10 passes, assuming that the incident position is the reference position and only the incident angle has an error of 1 mrad with respect to the reference optical axis 40, for example, the output position has a position error of about 30 mm with respect to the reference optical axis 40, which has an output angle error of 1 mrad with respect to the reference beam path 40.As described above, the laser beam amplifier according to the embodiment has a great advantage in stabilizing the optical axis. Furthermore, the beam path of the laser beam in the amplification region 30 does not deviate substantially from the provided reference beam path 40, and as a result the utilization efficiency of the energy stored in the laser medium is not reduced.Further, in the case where another optical element is connected upstream or downstream of the optical axis, the stabilization of the optical axis of the entire laser device can be realized when the optical connection is made with respect to the first point 34 and the second point 35. For example, in the case where the seed laser beam is generated by a master oscillator and is incident as an incident beam of the laser beam amplifier, adjusting the laser beam outgoing from the laser beam amplifier can be easily adjusted by adjusting the positional relationship and the angular relationship between the master oscillator and the laser beam amplifier so that the optical axis of the seed laser beam passes through the first point 34. Further, in the case where relay optics for redirecting the laser beam output from the laser beam amplifier are provided, the stabilization of the optical axis of the entire laser apparatus can be further achieved by adjusting the positional relationship and the angular relationship between the laser beam amplifier and the relay optics so that the laser beam whose optical axis passes through the second point 35 reaches the relay optics.In the laser beam amplifier, the first point 34 and the second point 35 behave like the object point and the image point and are conjugate with each other, and therefore, even if the input and the output of the laser beam amplifier are interchanged for use, they are optically equivalent if the transfer magnification is "1".In the embodiment, the configuration in which the amplification region 30 is disposed between the pair of reflecting mirrors 37 and 38 has been explained for the sake of simplicity of explanation. However, the present invention is not limited to this configuration as long as an optical system is provided in which the first point 34 and the second point 35 are conjugate with each other and the laser beam incident on the first point 34 is amplified during the at least two times of passage through the amplification region 30 and then transferred to the second point 35.FIG. 3 is a perspective view illustrating a structure inside a chamber of a laser beam amplifier according to a modified example of the first embodiment, and shows the case where the present invention is applied to an equiaxed plate type laser beam amplifier.As shown in FIG. 3, the laser beam amplifier according to the modified example of the first embodiment includes a pair of cylindrical electrodes 22a and 23a for forming an amplification region for a laser in the laser medium by generating a discharge in the laser medium, and an optical system having a first group of reflecting mirrors 37a and a second group of reflecting mirrors 38a opposing each other with an amplification region therebetween in a chamber in which the laser medium is enclosed and performs multi-amplification. In the chamber, an input window 33a through which a laser beam is incident and an output window 36a from which a laser beam is emitted are provided.FIG. 4 is a front view for explaining an operation of the laser beam amplifier shown in FIG. 3. As shown in FIG. 4, the pair of cylindrical electrodes 22 aand 23 aare arranged coaxially. When a radio frequency (RF) power supply 24a applies an RF voltage between the cylindrical electrode 22a and the cylindrical electrode 23a, the cylindrical electrodes 22 and 23 generate an RF electric field to generate an RF discharge in the laser medium. Thereby, the laser medium is excited, and the seed laser beam (incident beam) incident through the input window 33 is amplified. The seed laser beam is reflected by the reflecting mirrors 37a and 38a during amplification and then outputted as a outgoing beam through the output window 36a to the outside of the chamber.In the modified example of the first embodiment, the optical system including the reflecting mirrors 37a and 38a also forms an optical path between a first point 34a at which the laser beam is incident and a second point 35a at which the laser beam is outputted, so that the amplification region is located in the optical path between the first point 34a and the second point 35a which are conjugate with each other, and the laser beam incident at the first point 34a is amplified during the at least twice passage through the amplification region and then transferred to the second point. Here, it is desirable that the first point 34 aat which the laser beam is incident be located adjacent to the input window 33 aand the second point at which the laser beam is output be located adjacent to the output window 36 a.(Embodiment 2)FIG. 5 shows a configuration of a laser beam amplifier according to the second embodiment, and shows the case where the present invention is applied to a dual pass amplification type laser beam amplifier for use in a laser apparatus.As shown in FIG. 5, the laser beam amplifier for use in a laser apparatus according to the second embodiment includes a discharge tube 25 for accommodating a laser medium, a pair of electrodes for performing discharge in the laser medium to generate a gain region 30 for a laser beam in the laser medium, an optical system having a concave HR mirror 37 and an HR mirror prism 43 disposed opposite to each other with the gain region 30 therebetween, and a saturable absorber 31 such as a sulfur hexafluoride (SF 6) - gas cell that suppresses self and parasitic oscillations.Reflecting surfaces 44 and 45 of the HR mirror prism 43 are coated with a coating for high-grade reflection of the laser beam. The discharge tube 25 is provided with a first window 46 through which the laser beam is incident and a second window 47 through which the laser beam is output. As the laser medium, for example, a CO 2- laser gas using carbon dioxide (CO 2), nitrogen (N 2), helium (He), and additionally, as needed, hydrogen (H 2), carbon monoxide (CO), xenon (Xe), etc. is used.The laser beam amplifier allows the laser beam to propagate back and forth through the gain region 30 at once to amplify the laser beam. In the dual pass amplification type laser beam amplifier, the concave HR mirror 37 having focusing property and the HR mirror prism 43 for reflecting the incident beam and the outgoing beam are opposed to each other with the amplification region 30 interposed therebetween. Thus, by providing the concave HR mirror 37 and the HR mirror prism 43 facing each other, the incident beam position (first point 34) and the incident beam transfer image position (second point 35) conjugate with each other can be obtained.That is, in the dual-pass amplification type laser beam amplifier, as shown in FIG. 5, the distance "D" between the incident beam position 34 where the incident beam is incident on a reflecting surface 44 of the HR mirror prism 43 and the HR concave mirror 37 is substantially equal to the radius of curvature "R" (focal length f=R / 2) of the HR concave mirror 37 (D≈R=2f). Thereby, the image of the incident beam at the incident beam position (first point) 34 can be transferred to the incident beam transfer image position (second point) 35 on the other reflecting surface 45 of the HR mirror prism 43 in the optical path of the output beam, and the transfer image can be focused. Here, the first point 34 and the second point 35 behave like the object point and the image point and are conjugate to each other.In Fig. 5, the seed laser beam is incident on the reflective surface 44 of the HR mirror prism 43 at an incident angle slightly greater than 45 degrees. Then, the seed laser beam is reflected at an angle slightly larger than 45 degrees, transmitted through the first window 46, amplified by the amplification region 30, further transmitted through the second window 47, and passes through the saturable absorber 31 that suppresses self and parasitic oscillations.The laser beam that has passed through the saturable absorber 31 is incident on the concave HR mirror 37 at an angle slightly larger than 0 degrees, is reflected at an angle slightly larger than 0 degrees, again passes through the saturable absorber 31, is transmitted through the second window 47, and passes through the gain region 30 for further amplification. Then, the amplified laser beam is transmitted again through the first window 46, reaches the other reflecting surface 45 of the HR mirror prism 43, is reflected at a reflection angle slightly larger than 45 degrees, and is output as an amplified laser beam.Here, since the incident beam position (first point) 34 and the incident beam transfer image position (second point) 35 are conjugate with each other, the image of the incident beam at the incident beam position (first point) 34 on the reflecting surface 44 can be transferred to the incident beam transfer image position (second point) 35 on the reflecting surface 45 and the transfer image can be focused.The dual-pass amplification can be applied to a fast axial flow type laser beam amplifier and a triaxial orthogonal type laser beam amplifier. The laser beam amplifier according to the second embodiment has an advantage that the laser beam amplifier can be used in an amplifier having a small amplification range (circular shape, square shape, rectangular shape with a small aspect ratio), for example, a hose-type amplifier.FIG. 6 is a perspective view showing a structure of a fast axial flow type laser beam amplifier to which the second embodiment of the invention is applied, and FIG. 7 shows electrodes of the fast axial flow type laser beam amplifier shown in FIG. 6. As shown in FIGS. 6 and 7, a pair of electrodes 22 band 23 bare provided with the discharge tube 25 filled with a laser medium interposed therebetween. When an RF power supply 24 bapplies an RF voltage between the electrode 22 band the electrode 23 b, the electrodes 22 band 23 bgenerate an RF electric field to generate RF discharge in the laser medium, and a discharge region is formed between the electrodes 22 band 23 b.Thereby, the laser medium is excited, and a seed laser beam (incident beam) reflected by the HR mirror prism 43 and entering the discharge tube 25 through the first window 46 is amplified. Further, the seed laser is transmitted through the second window 47, reflected by the concave HR mirror 37, again incident on the discharge tube 25 through the second window 47, and is amplified. The amplified laser is transmitted through the first window 46, incident on the HR mirror prism 43, reflected by the HR mirror prism 43, and output as an outgoing beam. The laser medium inside the discharge tube 25 is kept circulating by a laser gas circulating pump 26 and cooled by a heat exchanger 27.FIG. 8 shows a modified example of the electrodes of the fast axial flow type laser beam amplifier as shown in FIG. 6. In FIG. 8, each of the electrodes 22 band 23 bhas a spiral shape. Thereby, the discharge region between the electrodes 22b and 23b is made uniform, and uniform amplification of the laser beam can be performed.FIG. 9 is a side view showing a structure of a triaxial orthogonal type laser beam amplifier to which the second embodiment of the present invention is applied, and FIG. 10 is a sectional view along the A-A line of the triaxial orthogonal type laser beam amplifier as shown in FIG. 9. As shown in FIGS. 9 and 10, a pair of electrodes 22 cand 23 care provided inside a chamber 21 cfilled with a laser medium. When an RF power supply applies an RF voltage between the electrode 22c and the electrode 23c, electrodes 22c and 23c generate an RF electric field to generate an RF discharge in the laser medium, and a discharge region is formed between the electrodes 22c and 23c.Thereby, the laser medium is excited, and the seed laser beam (incident beam) reflected by the HR mirror prism 43 and incident into the chamber 21 cthrough the first window 46 is amplified. Further, the seed laser beam is transmitted through the second window 47, reflected by the concave HR mirror 37, re-incident into the chamber 21 cthrough the second window 47, and is amplified. The amplified laser beam is transmitted through the first window 46, incident on the HR mirror prism 43, reflected by the HR mirror prism 43, and output as an output beam. The laser medium within the chamber 21c is kept in circulation by a cross-flow fan driven by a motor and cooled by a heat exchanger 29.FIG. 11 shows a configuration of a laser beam amplifier according to a modified example of the second embodiment. The laser beam amplifier as shown in Fig. 11 is different from the laser beam amplifier in Fig. 5 in that the discharge tube 25 as shown in Fig. 5 is divided into a plurality of discharge tubes (two discharge tubes 25a and 25b are shown in Fig. 11), and thereby the laser beam is amplified while propagating longitudinally through a plurality of amplification regions (two amplification regions 30a and 30b are shown in Fig. 11) in series. The laser beam can be efficiently amplified by setting the diameters of the discharge tubes 25a and 25b according to the intervals at which the laser beam passes through the discharge tubes 25a and 25b. Further, a saturable absorber 31 suppressing self and parasitic oscillations may be disposed between the discharge tube 25a and the discharge tube 25b.Also in this case, the distance "D" between the incident beam position 34 where the seed laser beam (incident beam) is incident on the HR mirror prism 43 and the HR concave mirror 37 is made substantially equal to the radius of curvature "R" of the HR concave mirror 37 (D≈R=2f). Thereby, the image of the incident beam at the incident beam position (first point) 34 can be transferred to the incident beam transfer image position (second point) 35 on the other reflecting surface 45 of the HR mirror prism 43, and the transfer image can be focused. Here, the first point 34 and the second point 35 behave like the object point and the image point and are conjugate to each other.In Fig. 11, the seed laser beam is incident on the reflecting surface 44 of the HR mirror prism 43 coated with a coating highly reflecting the seed laser beam at an incident angle slightly larger than 45 degrees. Then, the seed laser beam is reflected at a reflection angle slightly larger than 45 degrees, transmitted through a first window 46 aof the discharge tube 25 a, amplified by the amplification region 30 a, further transmitted through a second window 47 a, and passes through the saturable absorber 31 that suppresses self and parasitic oscillations. Further, the laser beam having passed through the saturable absorber 31 is transmitted through a first window 46b of the discharge tube 25b, amplified by the amplifying region 30b, and further transmitted through a second window 47b.The laser beam transmitted through the second window 47 bis incident on the concave HR mirror 37 at an incident angle slightly larger than 0 degrees, is reflected at a reflection angle slightly larger than 0 degrees, and is amplified again by the amplification region 30 b. The laser beam that has passed through the amplification region 30 brepass through the saturable absorber 31 again, is then amplified by the amplification region 30 a, and reaches the other reflecting surface 45 of the HR mirror prism 43, is reflected at a reflection angle slightly larger than 45 degrees, and is output as an amplified laser beam.Here, since the incident beam position (first point) 34 and the incident beam transfer image position (second point) 35 are conjugate with each other, the image of the incident laser beam at the position of the incident laser beam (first point) 34 on the reflecting surface 44 can be transferred to the incident beam transfer image position (second point) 35 on the reflecting surface 45 and the transfer image can be focused.An advantage of the modified example of the second embodiment is that a discharge tube is divided into a plurality of discharge tubes and the saturable absorber 31 may be disposed between the plurality of discharge tubes, and therefore the seed laser beam can be amplified with high efficiency while suppressing parasitic oscillations and self-oscillations.Several examples obtained by further using the embodiments of the present invention will be explained below.<Example 1>FIG. 12 is a cross-sectional plane view of a laser beam amplifier according to a first example as viewed from above. In the laser beam amplifier, an RF voltage is applied between two wide plate electrodes to generate an RF discharge in a CO 2- laser gas, thereby forming an amplification region 30. Further, both of the two reflecting mirrors 37 and 38 sandwiching the gain region 30 are concave HR mirrors.The seed laser beam (incident beam) is obliquely transmitted through the input window 33 and amplified by the amplification region 30. The seed laser beam is incident on the reflecting mirror 37 at an incident angle greater than 0 degrees, is reflected with high reflectivity, and is re-amplified in the amplification region 30. Then, the laser beam is incident on the reflecting mirror 38 disposed opposite to the reflecting mirror 37 at an incident angle greater than 0 degrees, reflected with high reflectivity, further amplified in the amplification region 30, transmitted through the output window 36, and output.In the optical system including the reflecting mirrors 37 and 38 as shown in Fig. 12, an image of the laser beam at the incident beam position (first point) 34 located adjacent to the input window 33 is also transferred to the incident beam transfer image position (second point) 35 located in the outgoing beam path adjacent to the output window 36, and an incident beam transfer image is focused.FIG. 13 is a diagram for explaining the operation of the laser beam amplifier according to the first example having an optical system in which reflecting optical elements are replaced with light transmissive optical elements.In general, a combined focal length "F" of a complex lens system including two thin lenses M 1 and M 2 is expressed by the following equation (1), and the distance ZH between a principal point of the first lens M 1 and a front principal point of the complex lens system is described by the following equation (2): where f 1 is a focal length of the first lens M 1, f 2 is a focal length of the rear lens M 2, and "t" is a distance between the front lens M 1 and the rear lens M 2.Assuming that the distance between the object position and the front lens M1 is equal to the distance between the lenses "t", the magnification "M" of the transfer image of the object in the complex lens system is expressed by the following equation (3).Accordingly, as shown in Fig. 13, an optical system is adopted in which the reflecting mirrors 37 and 38 are replaced with two thin lenses M1 and M2 having focusing characteristics as shown in Fig. 12. In FIG. 13, provided that the distance "t" between the lens M1 and the lens M2 is "L", the distance Li between the incident beam position (first point) 34 corresponding to the object position and the front lens M1 is "L", the distance Lo between the incident beam transfer image position (second point) 35 where the transfer image of the beam incident at the incident beam position (first point) 34 is formed and the rear lens M2 is "L", and the image is transferred at a magnification of M=1. Further, the lenses M1 and M2 have the same focal length f=R / 2.By substituting these relationships into the above equations, the following equations (4) to (6) are obtained:From the equations (4) to (6), f=R / 2=L is obtained. That is, by providing two spherical concave HR mirrors having radii of curvature R=2L which are opposed to each other at a distance of "L", the image of the incident beam at the incident beam position (first point) 34 is transferred to the incident beam transfer image position (second point) 35 in the outgoing beam path at a ratio of 1:1. Here, the first point 34 and the second point 35 are conjugate with each other.The first example can be applied to a plate type CO 2- laser beam amplifier. Since the transfer image of the incident beam in the outgoing beam path varies only to the same extent as the incident beam, the laser beam amplifier according to the first example can amplify an incident beam even if the optical axis of the incident beam is shifted to some extent, and the stability of the optical axis of the outgoing beam is improved.<Example 2>FIG. 14 is a plan sectional view of the laser beam amplifier according to a second example as viewed from above. The second example is applied to a plate type CO 2- laser beam amplifier similar to that shown in FIG. 12. The laser beam amplifier according to the second example is configured such that the laser beam is amplified in five times by using an optical system including reflecting mirrors between which an amplification region 30 is located. The second example is characterized in that an image of the beam incident at the first point 34 is transferred to a third point 39 in the center of the zigzag optical path to focus a first transfer image, and the first transfer image is further transferred to the second point 35 in the optical path of the outgoing beam to generate a second transfer image. However, the second example is within the technical scope of the first embodiment in that the optical axis is changed by changing the image of the beam incident at the first point 34 to the second point 35 in the beam path of the outgoing laser beam.The laser beam amplifier according to the second example transfers the image of the incident beam at the incident beam position (first point 34) in the path of the incident beam to the first incident beam transfer image position (third point 39) in the center of the zigzag path to focus the first transfer image, and further transfers the first transfer image at the third point 39 to the second incident beam transfer image position (second point 35) in the path of the outgoing beam to focus the second transfer image. Specifically, the optical system of the laser beam amplifier is configured such that the incident beam position (first point 34) substantially coincides with the second incident beam transfer image position (second point) 35. By using the optical system, it becomes easier to form the optical system in the entire laser device.In the laser beam amplifier, a pair of reflecting mirrors 37 and 38 are disposed at a distance "L" opposite to each other with the amplification region 30 therebetween. Each of the reflecting mirrors 37 and 38 is an HR concave mirror having a radius of curvature "R" as shown in FIG. 14, the reflecting mirror 37 is tilted so that its lower end is closer to the reflecting mirror 38 in the drawing.A seed laser beam (incident beam) provided by a master oscillator is obliquely transmitted through the input window 33 of the laser beam amplifier and amplified during passage through the amplification region 30 (first passage). A point at which the distance "L" between the reflecting mirrors 37 and 38 in the path of the incident beam is divided into two equal parts is set to the incident beam position (first point) in the optical system. The distance Li between the first point 34 in the path of the incident beam and the reflecting mirror 37 is L / 2.The laser beam amplified by passing the first point 34 is incident on the reflecting mirror 37 at an incident angle greater than 0 degrees, is obliquely reflected to the lower left side in the drawing, passes the amplification region 30 again, and is further amplified (second pass). Further, the laser beam falls on the reflecting mirror 38 on the left side of the drawing at an incident angle larger than 0 degrees, passes through the amplification region 30 substantially horizontally in the drawing, and is amplified (third pass). By adjusting the configuration of the optical system, the image of the incident beam at the first point 34 can be transferred to the third point 39 located in the center of the reflecting mirror 37 to focus the first transfer image. Here, the first point 34 and the second point 35 behave like the object point and the image point and are conjugate to each other.Further, the laser beam is reflected by the right reflecting mirror 37 in the upper left direction in the drawing and amplified by the amplifying region 30 (fourth pass), reflected by the left reflecting mirror 38 in the upper right direction in the drawing, amplified by the amplifying region 30 (fifth pass), transmitted through the output window, and output as an outgoing beam. Here, the optical system including the reflecting mirrors 37 and 38 transfers the first transfer image at the third point 39 to the second point 35 located at the center of the reflecting mirrors 37 and 38 in the optical path of the laser beam from the reflecting mirror 38 to the output window 36 to focus the second transfer image of the incident beam. The second incident beam transfer image position (second point) 35 may be superimposed on the incident beam position (first point) 34. The distance Lo between the reflecting mirror 38 and the second point 35 in the outgoing beam path is L / 2.FIG. 15 is a diagram for explaining an operation of the laser beam amplifier according to the second example having an optical system in which reflective optical elements have been replaced with transmissive optical elements. This optical system is different from the optical systems shown in Figs. 5 and 12, and it is not possible to apply the equations (1) to (3) because the combined focal length becomes infinitely large when the relationship f1=f2=L / 2 and t=L is substituted into the equation (1). Such an optical system is generally referred to as an afocal system.In the case where the focal lengths of the lens M 1 and the lens M 2 are f 1 and f 2, respectively, and the lenses M 1 and M 2 are arranged at a distance (f 1+f 2) from each other, assuming that the object position is a location upstream of the lens M 1 at a distance f 1 from the lens M 1, the position of the transfer image of the object is a location downstream of the lens M 2 at a distance f 2 of the lens M 2.The gain "M" in the case of an afocal system is expressed by the following equation:When f1=f2as in the second example, the magnification is "M" = "1". Here, assuming that the focal lengths of the lenses M1 to M4 correspond to the same focal length "f" and the distance between two adjacent lenses is "L", the optical system shown in Fig. 15 is obtained if the relationship L = 2f is satisfied. Here, the optical system can be realized by satisfying the relationship L=R, provided that the radii of curvature of the concave mirrors 37 and 38 correspond to "R".Referring to FIG. 15, the seed laser beam (incident beam) is transmitted through the incident beam position (first point) 34 in the optical path of the incident beam and is incident on the lens M 1. The incident beam position (first point) 34 is located upstream of the lens M1 at a distance Li = R / 2 from the lens M2. The lens M1 has the same focusing characteristic as the right reflecting mirror 37 in Fig. 14, and the lens M2 has the same focusing characteristic as the left reflecting mirror 38 as in Fig. 14. The laser beam that has been transmitted through the lenses M1 and M2 focuses the first transfer image at the third point 39 located downstream of the lens M2 at a distance L / 2 from the lens M2.Further, a lens M3 is provided at a position downstream of the third point 39 at a distance L / 2 from the third point 39, and further, a lens M4 is provided at a position downstream of the lens M3 at a distance "L" from the lens M3. The lens M3 has the same focusing characteristic as the right reflecting mirror 37 in Fig. 14, and the lens M4 has the same focusing characteristic as the left reflecting mirror 38 in Fig. 14, thereby transmitting the laser beam outgoing from the first transfer image at the third point 39 through the lens M3 and the lens M4, and then forming the second transfer image at the second point 35 which is located at a distance of Lo = L / 2 from the lens M4.In the second example, the image of the incident beam is transferred twice. However, the present invention is not limited to this example, but the image of the incident beam may be transferred a plurality of times, so that the image of the incident beam at the first point of the optical path of the incident beam may be transferred to the second point in the optical path of the outgoing beam to focus the transfer image of the incident beam.According to the second example, the transfer image of the incident beam at the second point in the beam path of the outgoing beam varies only to the same extent as the incident beam itself, and therefore the stability of the outgoing beam is improved. Further, the gain can be maintained at a high level and the output of the laser beam amplifier is stabilized.Further, the laser beam amplifier according to the second example employs the afocal optical system because the first point in the optical path of the incident beam coincides with the second point in the optical path of the outgoing beam and is therefore output while maintaining the optical quality characteristics of the incident beam (beam size, beam expansion, etc.). As a result, there is an advantage that the adjustment between optical elements to be connected is extremely simplified in the case where the laser device for amplifying the laser beam at a large amplification factor is formed by connecting many laser beam amplifiers in series.<Example 3>FIG. 16 is a plan sectional view of the laser beam amplifier according to a third example as viewed from above. In the third example, a plate type CO 2- laser beam amplifier similar to those shown in FIGS. 12 and 14 is adopted. The laser beam amplifier according to the third example allows the seed laser beam (incident beam) to pass through the amplification region in multiple passes in a zigzag manner by using the optical system constituted by two reflecting mirrors arranged opposite to each other with the strengthening region 30 interposed therebetween, each mirror having a radius of curvature "R". By amplifying on the long optical path length in this way, the amplification efficiency can be improved, and further, the energy of the incident beam can be highly amplified and output. An image of the incident beam at the incident beam position (first point) 34 is transferred to the second point 35 in the beam path of the outgoing beam, and thereby a transfer image of the incident beam is focused.As shown in FIG. 16, the seed laser beam (incident beam) obliquely entered into the optical system through the input window 33 passes the first point 34 and is amplified by the amplification region 30 (first pass). Then, the laser beam is incident on the right reflecting mirror 37 in the drawing at an incident angle larger than 0 degrees, is reflected downward to the left, and is amplified by the amplification region 30 (second pass). Further, the laser beam is incident on the left reflecting mirror 38 in the drawing at an incident angle greater than 0 degrees, is reflected downward to the right, and is amplified by the amplification region 30 (third pass). The process in which the laser beam is incident on the reflecting mirror at an incident angle greater than 0 degrees, obliquely reflected downward, and amplified by the amplification region 30 is repeated from the fourth pass to the sixth pass. Then, the laser beam is reflected in the substantially horizontal direction by the left reflecting mirror 38 whose reflecting surface is directed upward at the incident position because the reflecting surface is concave, and is amplified by the amplifying region 30 (seventh pass).As shown by dotted lines in FIG. 16, the process in which the laser beam is incident on the reflecting mirror at an incident angle greater than 0 degrees, is obliquely reflected upward, and is amplified by the amplification region 30 is repeated from the eighth pass to the eleventh pass. Then, the laser beam is transmitted through the output window 36 and output as an outgoing beam in the eleventh pass. An image of the incident beam at the first point 34 located adjacent to the input window 33 is transferred to the incident beam transfer image position (second point) 35 in the eleventh pass, and thereby the transfer image of the incident beam is focused. Here, the first point 34 and the second point 35 behave like the object point and the image point and are conjugate to each other.FIG. 17 is a diagram for explaining an operation of the laser beam amplifier according to the third example in the optical system in which reflective optical elements have been replaced with transmissive optical elements.An optical system equipped with two concave mirrors at both ends for performing multipass amplification is equivalent to a compound lens system in which a plurality of lenses having the same focal lengths f=R / 2 are arranged in series at an interval "L", so that an image of the incident beam at the incident beam position (first point 34) is transferred at a distance Li=L from the first lens M1 on the upstream side to the location (second point 35) at a distance Lo=L from the last lens M n to the downstream side, and the transfer image is focused.Here, it is assumed that all (k+1) lenses (M 1, M 2,..., M (k+1)) have the same focal length "F". Then, the combined focal length F k+1, the distance ZH k+1 between the front principal point of the compound lens system including the k lenses (M 1, M 2,..., M k) and the front principal point of the compound lens system including the (k+1) lenses (M 1, M 2,..., M k+1) and the magnification "M" of the (k+1) lenses can be expressed by the following equations (8) to (10) based on the equations (1) to (3):In the case where k = 0 (the case of only one lens M 1) holds for the original value of the combined focal length F l= f. By successively calculating f l to F n using the equations (8) to (10), the focal length F n and the front principal plane position ZH N of the composite lens system including the N lenses (M 1, M 2,..., M N) can be obtained, and further, the focal length "f" of each lens which makes the magnification "M" substantially equal to "1" can be obtained.Assuming that the distance "L" between a pair of reflecting mirrors is 1000 mm and eleven-pass amplification is performed, the focal length "f" of the reflecting mirrors is about 1100 mm and the radius of curvature R (= 2F) is about 22000 mm based on the equations (8) to (10).In the third example, an optical system is employed in which the laser beam obliquely passes through the amplification region 30 from top to bottom in a zigzag manner and then obliquely passes through the amplification region 30 from bottom to top in a zigzag manner, and thereby higher amplification efficiency can be achieved by the multiple pass amplification having many passes to eleven passes. In addition, the image of the incident beam at the incident beam position (first point) 34 is transferred to the second point 35 in the beam path of the outgoing beam, whereby stable beam travel can be realized.<Example 4>FIG. 18 is a plan sectional view of a laser beam amplifier according to a fourth example as viewed from above. In the fourth example, a plate type CO 2- laser beam amplifier similar to those shown in FIGS. 12, 14 and 16 is adopted. The laser beam amplifier according to the fourth example is different from the laser beam amplifier according to the third example in that six-pass amplification is performed using an HR planar mirror as one of the two reflecting mirrors.As shown in FIG. 18, the seed laser beam (incident beam) is obliquely transmitted through the input window 33, passes through the incident beam position (first point) 34, and falls into an optical system including two reflecting mirrors 42 and 38. In the optical system, the reflecting mirror 42 which is a HR planar mirror and the reflecting mirror 38 which is a HR concave mirror are disposed opposite to each other at a distance "D" substantially parallel.The seed laser beam (incident beam) incident on the laser beam amplifier is amplified by the amplification region 30 (first pass), incident on the reflecting mirror 42 at an incident angle greater than 0 degrees, reflected obliquely downward, amplified by the amplification region 30 (second pass), incident on the reflecting mirror at an incident angle greater than 0 degrees, reflected obliquely downward, and amplified by the amplification region 30 (third pass).Further, the laser beam falls on the reflecting mirror 42 at an incident angle greater than 0 degrees, is obliquely reflected downward, is amplified by the amplifying region 30 (fourth pass), falls on the reflecting mirror 38 at an incident angle greater than 0 degrees, is obliquely reflected downward, is amplified by the amplifying region 30 (fifth pass), is obliquely reflected downward by the reflecting mirror 42, is amplified by the amplifying region 30 (sixth pass), and passes through the incident beam transfer imaging position (second point 35), and is output as an outgoing beam through the output window 36.In the fourth example, the reflecting mirror 38 and the reflecting mirror 42 are arranged in parallel opposite to each other at a distance "D". Further, the position where the beam on one side of the reflecting mirror 38 passes in the path of the incident beam is set as the incident beam position (first point) 34, and the position where the beam on the opposite side of the reflecting mirror 38 passes in the path of the outgoing beam is set as the incident beam transfer image position (second point) 35. Further, the optical system is adjusted such that an image of the incident beam at the first point 34 is transferred to the second point 35 to focus the transfer image. Here, the first point 34 and the second point behave like the object point and the image point and are conjugate to each other.FIG. 19 is a diagram for explaining an operation of the laser beam amplifier according to the fourth example having an optical system in which reflective optical elements have been replaced with transmissive optical elements, and the transmissive optical elements are arranged in series. In Fig. 19, the planar transparent plates HR1 HR2 and HR3 representing the reflecting mirror 42 do not have a focusing property, and therefore the optical system has substantially the same configuration as that shown in Fig. 13. Here, the positional relationships that the distance "L" between two lenses M1 and M2 corresponding to the reflecting mirror 38 having focusing characteristics is 2D determine that the first point 34 on the upstream side of the lens M1 is at a distance of Li=2D from the lens M1, and the transfer image of the image of the laser beam at the first point 34 is focused at the second point 35 on the downstream side of the lens M2 at a distance of Lo=2D from the lens M2.The combined focal length "F", the distance between the principal points ZH, and the magnification "M" of the lenses M1 and M2 can be obtained using the following equations (11) to (13) by substituting "L" with 2D in the equations (4) to (6):With respect to the focal length "f" of the reflecting mirror 38, according to the equations (11) to (13), D=F / 2 (L=f) holds to satisfy the magnification condition M=1. Assuming that the radius of curvature of the reflecting mirror 38 is "R", the radius of curvature satisfies the relationshipThe laser beam amplifier according to the fourth example can stably amplify the laser beam because the position and angle of the output beam vary only to the same extent as the position and angle of the incident beam even if the optical axis of the incident beam is shifted relative to the laser beam amplifier to some extent, and the amplification efficiency hardly varies. Further, the stability of the optical axis is improved and the adjustment in combination with another optical apparatus is easy.One of the features of the laser beam amplifier according to the fourth example is further that the incident beam and the outgoing beam pass both sides of the reflecting mirror 38, and the laser beam can be incident on and output from the laser beam on one side.<Example 5>FIG. 20 is a plan sectional view of a laser beam amplifier according to a fifth example as viewed from above. In the example, a plate type CO 2- laser beam amplifier similar to those shown in FIGS. 12, 14, 16 and 18 is adopted. The laser beam amplifier according to the fifth example is different from the laser beam amplifier according to the fourth example in that, instead of the two reflecting mirrors being arranged in parallel opposite to each other, the two reflecting mirrors are arranged such that their principal surfaces are at a small angle to each other to increase the number of passes, and that the incident beam transfer image position (second point) 35 is defined in the outgoing beam path outside the chamber of the laser beam amplifier.The seed laser beam (incident beam) that has obliquely passed upward through the input window 33 provided in the left lower part of the chamber 21 in FIG. 20 passes the first point 34 and then falls into the optical system including the reflecting mirror 42 and the reflecting mirror 32 facing each other at a distance "D". The distance "L" in the optical path from the point of reflection of the laser beam on the reflecting mirror 38 having focusing characteristics to the closest point of incidence of the laser beam on the reflecting mirror 38 is 2D.The reflecting mirror 42 and the reflecting mirror 38 are not parallel to each other but face each other at a small angle. In FIG. 20, the reflecting mirror 38 is tilted so that the upper end in the drawing is closer to the reflecting mirror 42. The reflecting mirror 42 and the reflecting mirror 38 are disposed at a small angle of inclination to each other, and thereby multipass amplification with many passes can be realized.Further, the position where the beam passes one side of the reflecting mirror 38 in the path of the incident beam is set as the incident beam position (first point) 34, and the position in the path of the outgoing beam downstream at the distance of 2D from the position where the outgoing beam is last reflected by the reflecting mirror 38 is set as the incident beam transfer image position (second point) 35, and the optical system is adjusted so that a transfer image of the beam incident at the first point 34 is focused on the second point 35. The second point 35 lies within the path of the outgoing beam output through the output window 36.Referring to FIG. 20, the seed laser beam (incident beam) incident into the laser amplifier is obliquely transmitted from below through the input window 33, passes through the first point 34, and enters the optical system. The seed laser beam is amplified by the amplification region 30 (first pass), incident on the reflecting mirror at an incident angle greater than 0 degrees, obliquely reflected upward, amplified by the amplification region 30 (second pass), incident on the reflecting mirror 38 at an incident angle greater than 0 degrees, obliquely reflected upward, and amplified by the amplification region 30 (third pass).Further, the amplified laser beam is obliquely upwardly reflected by the reflecting mirror 42, amplified by the amplifying region 30 (fourth pass), incident on the reflecting mirror 38 at an incident angle greater than 0 degrees, obliquely upwardly reflected, amplified by the amplifying region 30 (fifth pass), incident on the reflecting mirror 42 and at an incident angle greater than 0 degrees, obliquely upwardly reflected, amplified by the amplifying region 30 (sixth pass), and incident on the reflecting mirror 38. the reflecting surface of the reflecting mirror 38 is a concave surface, and the main surface is slightly tilted downward relative to the reflecting mirror 42, and the reflecting surface is tilted downward. Accordingly, the laser beam incident on the reflecting mirror 38 is obliquely reflected downward as shown by the dotted line in Fig. 20.The obliquely downward reflected laser beam is amplified by the amplification region 30 (seventh pass), incident on the reflecting mirror 42 at an incident angle greater than 0 degrees, obliquely downward reflected, amplified by the amplification region 30 (eighth pass), incident on the reflecting mirror 38 at a reflection angle greater than 0 degrees, obliquely downward reflected, and amplified by the amplification region 30 (ninth pass). The amplified laser beam is transmitted through the output window 36 and output as an outgoing beam. In the beam path of the outgoing beam, the incident beam transfer image position (second point) 35 to which the image of the beam incident at the first point 34 is transferred is defined at the downstream location at a distance of 2D from the reflecting mirror 38. Here, the first point 34 and the second point 35 behave like the object point and the image point and are conjugate to each other.FIG. 21 is a diagram for explaining an operation of the laser beam amplifier according to the fifth example having an optical system in which reflective optical elements have been replaced with transmissive optical elements and the transmissive optical elements are arranged in series. By setting a distance "D" between the incident beam position (first point) 34 and the reflecting mirror 42 (plane transparent plate HR 1) the distance Li between the incident beam position (first point) 34 and the first lens M 1 is represented by 2D, and the distance Lo between the last lens M N and the incident beam transfer image position (second point) 35 is represented by 2D.In the case where the distance "D" between the reflecting mirror 38 and the reflecting mirror 42 is used, the combined focal length F K, the distance between the principal points ZH k and the magnification "M" of the k mirrors (lenses) can be obtained by the following equations (14) to (16) by substituting "L" with 2D in the equations (8) to (10):If k = 0, the original value of the combined focal length F is 1= f. By successively calculating F 1 to F N using equations (14) to (16), the combined focal length F N and the front principal position ZH N of the N lenses (M 1, M 2,..., M N) can be obtained, and further, the focal length "f" of each lens having a gain "M" substantially equal to "1" can be obtained. For example, when D = 600 mm and nine-pass amplification is performed, the radius of curvature "R" is about 7200 mm.An advantage of the fifth example is that many passes are made through the amplification region and the energy of the laser medium is utilized more efficiently, thereby obtaining a powerful laser beam and making the optical axis of the laser beam amplifier stable.<Example 6>FIG. 22 is a plan sectional view of a laser beam amplifier according to a sixth example as viewed from above. The optical system of the amplifier according to the sixth example is substantially the same as that shown in Fig. 21, and is omitted here. The laser beam amplifier according to the sixth example has substantially the same configuration as that of a plate type laser beam amplifier according to the fifth example shown in Fig. 20, and a multi-pass amplification type laser beam amplifier in which one reflecting mirror 38 is provided substantially orthogonally to the longitudinal direction of the laser beam amplifier and the other reflecting mirror 42 is opposed to the reflecting mirror 38 at a predetermined angle. In this type of laser beam amplifiers, the number of passes can be changed appropriately by adjusting the focusing capability and length of each reflecting mirror, the angle between the reflecting mirrors, the incident angle of the seed laser beam, and so on. In the sixth example, as shown in FIG. 22, the number of passes is set to eleven.In the laser beam amplifier, the seed laser beam supplied to the laser beam amplifier is incident on the 38 optical system including the reflecting mirror 42 and the reflecting mirror, which are disposed facing each other at a distance "D". The reflecting mirror 38 is arranged orthogonally to the longitudinal direction of the laser beam amplifier, and the reflecting mirror 42 is arranged tilted so that the upper end is closer to the reflecting mirror 38 in the drawing.The seed laser beam (incident beam) that has incident on the laser beam amplifier is transmitted from the bottom left through the input window 33, passes through the first point 30, and enters the optical system. The seed laser beam is amplified in the amplification region 30 (first pass), incident on the reflecting mirror 42 at an incident angle greater than 0 degrees, obliquely reflected upward, amplified by the amplification region 30 (second pass), incident on the reflecting mirror 38 at an incident angle greater than 0 degrees, obliquely reflected upward, and amplified by the amplification region 30 (third pass).Further, the laser beam is incident on the reflecting mirror 42, is obliquely upwardly reflected, is amplified by the amplifying region 30 (fourth pass), is incident on the reflecting mirror 38, is obliquely upwardly reflected, is amplified by the amplifying region 30 (fifth pass), is incident on the reflecting mirror 42, is obliquely upwardly reflected, is amplified by the amplifying region 30 (sixth pass), is incident on the reflecting mirror 38 and is reflected. The laser beam reflected by the reflecting mirror 38 is amplified by the amplifying region 30 (seventh pass) and is incident on the reflecting mirror 42. the reflecting mirror 38 and the reflecting mirror 42 are tilted from each other, and thereby the incident angle of the laser beam incident on the reflecting mirror 42 gradually decreases. Accordingly, the laser beam incident on the reflecting mirror 42 is obliquely reflected downward as shown by the dotted line in FIG. 22.The laser beam that has been obliquely reflected downward by the reflecting mirror 42 is amplified by the amplification region 30 (eighth pass), is incident on the reflecting mirror 38, is obliquely reflected downward, and is amplified by the amplification region 30 (ninth pass). Further, the laser beam is incident on the reflecting mirror 42, is obliquely reflected downward, is amplified by the amplification region 30 (tenth pass), is incident on the reflecting mirror 38, is obliquely reflected downward, is amplified by the amplification region 30 (eleventh pass), and is transmitted through the output window 36 and output as an outgoing beam.The incident beam transfer image position (second point) 35 in the beam path of the outgoing beam to which an image of the beam incident at the incident beam position (first point) 34 is transferred to focus a transfer image is defined at the downstream location at a distance 2D=L from the reflecting mirror 38 from which the outgoing beam is last reflected.By the action of the optical system, the image of the laser beam at the first point 34 is transferred to the second point 35 and the transfer image is focused. Here, the first point 34 and the second point 35 behave like the object point and the image point and are conjugate to each other.The combined focal length F N, the front principal position ZH N and the magnification "M" of the optical system can be obtained from the equations (14) to (16). For example, when D = 1800 mm and eleven-pass amplification is performed, the radius of curvature "R" is about 30000 mm.An advantage of the sixth example is the same as that of the fifth example, and further, another advantage is that the adjustment of the optical path can be performed by adjusting the reflecting mirror 42 which can be easily adjusted while the reflecting mirror 38 whose position and position are difficult to adjust is fixed, and the design and the orientation of the laser beam amplifier can be simpler.In the above examples, such as the optical system for multi-pass amplifying the laser beam in the amplifying region, the combination of two concave mirrors or the combination of the planar mirror and the concave mirror is used. However, the present invention is not limited to these examples, and any optical system having an optical element with focusing capability may be used. For example, a combination of a concave mirror and a convex mirror or a combination of a convex lens and a plane mirror may be used.(Laser Apparatus)Next, a laser device according to several embodiments of the present invention will be explained. The laser apparatus according to the embodiment of the present invention is used as a driving laser apparatus for irradiating a target material with a laser beam to convert the target material into a plasma in an LPP type EUV light source apparatus. The driving laser apparatus uses the laser beam amplifier according to the first or second embodiment of the present invention, and can efficiently amplify a seed laser beam at a high amplification factor and focus the amplified laser beam on a plasma emission point to generate EUV light with high efficiency.(Embodiment 3)FIG. 23 shows a configuration of an EUV light source apparatus using a driving laser apparatus according to the third embodiment of the invention. The driving laser apparatus according to the third embodiment uses a plate type laser beam amplifier as a preamplifier and a fast axial flow type laser beam amplifier as a main amplifier, and supplies a driving laser beam to the EUV light source apparatus.The driving laser device according to the third embodiment includes a master oscillator (MO) 51 for generating a seed laser beam, a spatial filter 52 for spatially filtering a spatial mode of the seed laser beam, a preamplifier 20, HR mirrors 53, 54 and 58, relay optics 55 and 57, and a main amplifier 60. Off-axis parabolic mirror 59 focuses the amplified laser beam onto a target within EUV chamber 70.The master oscillator 51 outputs, as a seed laser beam, for example, a pulsed carbon dioxide (CO 2) - laser beam having a spatial mode of small order at a repetition rate of 100 kHz and a pulse duration of about 20 nanoseconds. When the seed laser beam passes through the spatial filter 52, in which higher order components are removed from the spatial mode and the spatial mode becomes a single mode, the seed laser beam enters the input window of the preamplifier 20.The preamplifier 20 includes a plate-type laser beam amplifier according to the first embodiment. As already explained in detail in the first embodiment, the seed laser beam enters the plate type laser beam amplifier, is efficiently amplified in multiple passes by multiple reflection between two reflecting mirrors disposed opposite to each other with the amplification region therebetween, and is output as an amplified laser beam. Preamplifier 20 focuses the transfer image of the beam incident at incident beam position (first point) 34 in the path of the incident beam to second point 35 in the path of the outgoing beam. Accordingly, even if the beam path of the incident beam has some error from the reference beam path, the error of the beam path of the outgoing beam relative to the reference beam path is substantially equal to the error of the beam path of the incoming beam relative to the reference beam path, and the optical axis of the laser beam is extremely stable.The path of the laser beam output from the preamplifier 20 is changed by the HR mirrors 53 and 54 and the laser beam enters the relay optics 55, The relay optics 55 adjusts the size and the spread angle of the laser beam so that the laser beam can efficiently pass through the amplification region of the main amplifier 60 and causes the laser beam to fall on a first reflecting surface of a right-angle prism 61 in the main amplifier 60 at an incident angle slightly larger than 45 degrees.The main amplifier 60 includes the fast axial flow type laser beam amplifier according to the second embodiment. The reflecting surfaces of the right-angle prism 61 are highly reflecting surfaces. The laser beam amplified by the preamplifier is reflected by the first reflecting surface of the right-angle prism 61 and the reflected laser beam enters the amplification region 65 as a seed laser beam. The seed laser beam is amplified by the amplification region 65, incident on the concave HR mirror 64 at an incident angle slightly larger than 0 degrees, and reflected, enters the amplification region 65 again, and is amplified, incident on the second reflecting surface of the rectangular prism 61 at an incident angle slightly larger than 45 degrees, and is output from the main amplifier 60.In the main amplifier 60, an incident beam position (first point) 62 is defined on the first reflecting surface of the rectangular prism 61 and an incident beam transfer image position (second point) 63 is defined on the second reflecting surface of the rectangular prism 61. The optical system of the main amplifier 60 is adapted to transfer an image of a seed laser beam at the first point 62 to the second point 63 to focus the transfer image. Therefore, the variation of the optical axis of the incident beam in the main amplifier 60 is converted to the variation of the optical axis of the outgoing beam without amplification, and the variation of the optical axis of the laser beam can be sufficiently suppressed. Further, the efficiency of the laser beam in the preamplifier 20 can be improved by performing double-pass amplification.The laser beam amplified by the main amplifier 60 enters the relay optics 57, the relay optics 57 adjusts the size of the laser beam so that the laser beam can be focused by the off-axis parabolic mirror 59 and converts the wavefront of the laser beam into a planar wave. The laser beam that has passed through the relay optics 57 is reflected by the high reflectivity HR mirror 58 and is incident on the off-axis parabolic mirror 59. the laser beam reflected by the off-axis parabolic mirror 59 passes through a window 74 into the EUV chamber 70 and is focused on a target at the plasma emission point 71 to generate a plasma. An EUV collection mirror 72, which has a reflective spheroidal surface, collects the EUV light emitted from the generated plasma at an intermediate focal point (IF 73). The EUV light collected at the intermediate focal point 73 is provided to an illumination unit.According to the third embodiment, although the optical path length within the driver laser device is long, the optical axes in the preamplifier 20 and the main amplifier 60 are stable, and thereby the alignment adjustment of the entire driver laser device is simple and the optical axis of the outgoing laser beam is extremely stable. As a result, the position and shape of the focal point of the laser beam is stable, and the energy stability of the EUV light is improved.In the third embodiment, in the main amplifier 60, the high-speed axial-flow type laser beam amplifier having a tubular shape is used. However, the present invention is not limited thereto, but a fast axial flow type laser beam amplifier having an elliptical, oval, rectangular, or similar shape, a rotational axial orthogonal type laser beam amplifier, or a plate type laser beam amplifier may be used. Further, a fast axial flow type laser beam amplifier or the like may be used in the preamplifier 20.(Embodiment 4)FIG. 24 shows a configuration of an EUV light source apparatus using a driving laser apparatus according to the fourth embodiment of the present invention. As shown in FIG. 24, the driving laser apparatus according to the fourth embodiment includes a master oscillator 51 for generating a seed laser beam, a spatial filter 52 for spatially filtering a spatial mode of the seed laser beam, a preamplifier 20 including a plurality of stages of m laser beam amplifiers, HR mirrors 53, 54 and 58, relay optics 55 and 57, and a main amplifier 60 including a plurality of stages of n laser beam amplifiers. Here, "m" and "n" are natural numbers greater than or equal to 2. The driver laser device amplifies the seed laser beam to the required light intensity and outputs the amplified laser beam to an off-axis parabolic mirror 59. Off-axis parabolic mirror 50 focuses the amplified laser beam onto a target. The focal point of the amplified laser beam coincides with the plasma emission point 71 within the EUV chamber 70.The driving laser device according to the fourth embodiment is different from the driving laser device according to the third embodiment, as shown in FIG. 23, in that the preamplifier 20 includes a plurality of stages of plate-type laser beam amplifiers and the main amplifier 60 includes a plurality of stages of fast axial flow type laser beam amplifiers. The other elements do not differ, and the multiple stages of the laser beam amplifier will be explained below.The preamplifier 20 in the driving laser device according to the fourth embodiment includes m stages of plate-type laser beam amplifiers according to the first embodiment connected in series. In each plate-type laser beam amplifier 20k, the optical system is arranged so that a transfer image of the image of the incident beam is focused at the first point 43k located adjacent to the entrance window to the second point 35k located adjacent to the output window. Accordingly, the error of the beam path of the outgoing beam relative to the reference beam path is at most equal to the error of the beam path at the incident beam position.Fig. 24 shows the case where the m stages of the laser beam amplifier are connected in series in the preamplifier 20 of the driver laser device. However, relay optics may be provided between the upstream laser beam amplifier 20k and the subsequent laser beam amplifier 20(k+1), as required. The relay optics transfer an image of the outgoing beam at the second point 35k of the upstream laser beam amplifier 20k to the first point 34(k+1) of the subsequent laser beam amplifier 20(k+1) to focus a transfer image. In this way, the plurality of stages of the laser beam amplifier are provided by adjusting the optical axis so that the optical axis of the outgoing laser beam of the laser beam amplifier 20k passes through the first point 34(k+1) of the subsequent laser beam amplifier 20(k+1), and thereby the amplification factor of the preamplifier can be increased while suppressing the displacement of the optical axis.The main amplifier 60 in the driving laser apparatus according to the fourth embodiment has n stages of fast axial flow type laser beam amplifiers according to the second embodiment as shown in FIG. 5, which are connected in series. In each tubular laser beam amplifier 60i, the optical system is arranged to focus a transfer image of the image of the incident beam at the first point 62i set on the first reflecting surface of the right-angle prism 61i to the second point 63i on the second reflecting surface of the right-angle prism 61i. Accordingly, even if the optical axis of the incident beam is shifted from the reference optical axis to some extent, the shift of the outgoing beam at the position of the transfer image of the outgoing beam (second point) 63iis at most equal to the error of the optical path at the incident beam position (first point) 62i.Relay optics 55 adjusts the laser beam reflected from HR mirror 54 to provide a laser beam that is incident on the first point of the first laser beam amplifier. Subsequently, the image of the incident beam at the first point 62i is transferred to the second point 63i in each laser beam amplifier 60i to focus a transfer image. The plurality of stages of laser beam amplifiers are provided by adjusting the optical axis so that the image of the outgoing laser beam at the second point 63i of the laser beam amplifier 60i is transferred to the first point 62(i+1) of the subsequent laser beam amplifier 60(i+1) through the relay optics 66i, whereby the amplification factor of the main amplifier can be increased and the output power of the main amplifier can be increased, suppressing the shift of the optical axis.According to the fourth embodiment, since the multiple stages of laser beam amplifiers are connected in series, the optical axes in the preamplifier 20 and the main amplifier are stable although the optical path length inside the driver laser device is extremely long, whereby the alignment adjustment of the entire driver laser device is easy and the optical axis of the outgoing laser beam is extremely stable. As a result, the position and shape of the focal point of the laser beam is stable, and the energy stability of the EUV light is improved.In the fourth embodiment, fast axial flow type laser beam amplifiers having a tubular shape are used in the main amplifier 60. However, the present invention is not limited thereto, but fast axial flow type laser beam amplifiers having an elliptical, oval, rectangular or similar shape, triaxial orthogonal type laser beam amplifiers, or plate type laser beam amplifiers may be used. Further, in the preamplifier 20, fast axial flow type laser beam amplifiers or the like can be used.(Embodiment 5)FIG. 25 shows a configuration of an EUV light source apparatus using a driving laser apparatus according to the fifth embodiment of the present invention. As shown in FIG. 25, the driving laser apparatus according to the fifth embodiment includes a master oscillator 51, a spatial filter 52, a relay optics 80, a preamplifier including multiple stages of n laser beam amplifiers, HR mirrors 53, 54, and 58, relay optics 55 and 57, and a main amplifier 60 including multiple stages of n laser beam amplifiers.The single mode of the seed laser beam output from the master oscillator 51 is filtered by the spatial filter 52 in which a pinhole 81 is formed, and the image of the laser beam in the pinhole 81 of the spatial filter 52 is transferred to the first point of the first-stage laser beam amplifier of the preamplifier 20 through the relay optics 80 and a transfer image is focused. The operation of the preamplifier 20 is the same as that in the fourth embodiment as shown in Fig. 24.The image of the laser beam at the second point of the last stage laser beam amplifier in preamplifier 20 is transferred to the first point of the first stage laser beam amplifier of main amplifier 60 through relay optics 65. The operation of the main amplifier 60 is the same as that in the fourth embodiment as shown in Fig. 24. The laser beam amplified by the main amplifier 60 reaches the relay optics 57.The driver laser device according to the fifth embodiment is different from the driver laser device according to the fourth embodiment, as shown in FIG. 24, in that the relay optics are inserted at some locations to form transfer optics. As shown in FIG. 25, the relay optics 80 inserted between the spatial filter 52 and the preamplifier 20 transfers the image of the laser beam in the aperture hole 81 of the spatial filter 52 to the first point of the first stage laser beam amplifier in the preamplifier 20 to focus a transfer image.Then, the relay optics 56 inserted between the multistage preamplifier 20 and the multistage main amplifier 60 transfers the image of the laser beam at the second point on the last stage laser beam amplifier in the preamplifier 20 to the first point set on the reflecting surface of the right angle prism of the first stage laser beam amplifier in the main amplifier 60 to focus a transfer image.According to the fifth embodiment, since the relay optics are added, the laser beam can be reliably incident on the first points of the preamplifier and the main amplifier, and variations in the amplification efficiency of the preamplifier and the main amplifier are suppressed, and stable amplification can be performed in the entire laser device.

Claims

A laser apparatus, comprising: - a master oscillator (51) for generating a pulsed laser beam; - a preamplifier (20) for amplifying the pulsed laser beam provided by the master oscillator, comprising a laser beam amplifier, comprising: - a container for accommodating the laser medium; - a pair of electrodes (22, 22b, 23, 23b) for performing a discharge in the laser medium to form an amplification region (30, 30a, 30b) for a laser beam in the laser medium; and an optical system (37, 37a, 38, 38a, 42, 43) for forming an optical path between a first point (34) at which the laser beam is incident and a second point (35) from which the amplified laser beam is output such that the amplification region in the optical path is between the first point and the second point, the first point and the second point being conjugate with each other, and the laser beam incident at the first point being amplified during at least twice passage through the amplification region and transferred to the second point; a first relay optical system (55) for adjusting a magnitude and an expansion angle of the pulsed laser beam output from the preamplifier; a main amplifier (60) for amplifying the pulsed laser beam provided from the preamplifier via the first relay optical system; a second relay optical system (57) for coordinating the pulsed laser beam output from the main amplifier to output parallel light.The laser apparatus according to claim 1, wherein the container is provided with a first window (33) through which the laser beam is incident and a second window (36) through which the laser beam is output, and the first point is located in the vicinity of the first window and the second point is located in the vicinity of the second window.The laser apparatus according to claim 1 or 2, wherein: the container has a chamber (21) for accommodating the laser medium; the pair of electrodes has a plate electrode pair (22, 23) disposed in the chamber with the laser medium therebetween for generating a discharge in the laser medium when supplied with a radio frequency voltage, thereby exciting the laser medium to amplify the laser beam; and the laser beam amplifier forms a plate type laser beam amplifier.The laser device according to any one of claims 1 to 3, wherein the optical system includes one of a combination of a planar mirror (42, 43) and a concave mirror (37, 38), a combination of two concave mirrors (37, 38), and a combination of a concave mirror and a convex mirror.The laser apparatus according to any one of claims 1 to 4, wherein the optical system focuses a transfer image of the laser beam incident at the first point in the optical path between the first point and the second point at least once, and then re-focuses the transfer image at the second point.The laser device according to any one of claims 1 to 5, further comprising: a saturable absorber (31) disposed in the optical path between the first point and the second point.The laser apparatus according to claim 1 or 2, wherein the container has a discharge tube (25, 25a, 25b) in which a path for the laser beam is formed; the pair of electrodes generate a discharge in the laser medium flowing in the path of the discharge tube when supplied with a radio frequency voltage, thereby exciting the laser medium to amplify the laser beam; and the laser beam amplifier forms a fast axial flow type laser beam amplifier.The laser apparatus according to any one of claims 1 to 7, wherein a transfer magnification of the optical system is substantially "1".The laser device of any one of claims 1 to 8, wherein the laser medium comprises a CO 2- laser gas comprising carbon dioxide (CO 2).The laser apparatus according to any preceding claim, wherein the main amplifier comprises: a container for accommodating a laser medium; a pair of electrodes for generating a discharge in the laser medium to generate an amplification region (65) for a laser beam in the laser medium; and an optical system (61, 65) for forming an optical path between a third point where the laser beam is incident and a fourth point from which the laser beam is outputted such that the amplification region in the optical path is between the third point and the fourth point, wherein the third point and the fourth point are conjugate with each other, and the laser beam incident in the third point is amplified during at least two times of passage through the amplification region and then transferred to the fourth point.The laser device according to the preceding claim, wherein the first relay optics transfer the pulsed laser beam output from the second point of the preamplifier to the third point of the main amplifier.The laser device of any preceding claim, wherein the preamplifier comprises a plurality of laser beam amplifiers connected in series.The laser beam apparatus according to any preceding claim, wherein the main amplifier comprises a plurality of laser beam amplifiers connected in series.

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