Abstract:
Methods, devices, and systems for elongating a beam path of a light beam, in particular of a laser beam, are provided. An example method includes coupling the light beam into an interspace between a plurality of first reflective surfaces and a plurality of second reflective surfaces facing the first reflective surfaces, multiply reflecting the light beam between the first reflective surfaces and the second reflective surfaces to elongate the beam path of the light beam, and coupling out the light beam from the interspace. The light beam undergoes the steps of coupling in, repeated reflecting and coupling out at least a first time with a first pass and a second time with a second pass, and the light beam traverses a different beam path in the interspace during the first pass in comparison with during the second pass.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of and claims priority under 35 U.S.C. §120 to PCT Application No. PCT/EP2014/067719 filed on Aug. 20, 2014. The entire contents of this priority application are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to methods, devices, and systems for elongating a beam path of a light beam, in particular of a laser beam. 
       BACKGROUND 
       [0003]    WO 2012/091786 A1 disclosed an optical delay device for elongating a beam path of a light beam. The delay device has exactly one input for coupling a light beam into an interspace and exactly one output for coupling out a light beam from the interspace. The first and second pluralities of reflective surfaces are arranged and aligned relative to one another in such a way that a superimposition of two or more optical beams at each of the optical surfaces is avoided when passing through the interspace between the input and the output. 
         [0004]    Such an optical delay device can be used for example in a driver laser arrangement for an EUV light source, as described in US 2009/0095925 A1. The driver laser arrangement described therein has a beam source for generating pulsed laser radiation and one or more optical amplifiers for amplifying the pulsed laser radiation. The beam source of the driver laser arrangement serves for generating so-called seed pulses, which are amplified to high laser powers of several kW, if appropriate of 10 kW or more, in the optical amplifier or optical amplifiers. The laser radiation amplified by the driver laser arrangement is fed via a beam guiding device to a focusing device, which focuses the laser radiation or the laser beam in a target region. A target material is provided in the target region and undergoes transition to a plasma state upon irradiation with the laser beam and emits EUV radiation in the process. 
         [0005]    In the case of the driver laser arrangement described above, the amplified laser radiation can be reflected for example at the target material, which may be present, e.g., in the form of tin droplets. The back-reflection generated at such a droplet passes back into the optical amplifier or optical amplifiers and passes through the gain medium present there, such that the back-reflection is also amplified in the optical amplifier or optical amplifiers. Even a weak back-reflection possibly suffices to generate, after amplification in the gain medium of the optical amplifier, a power that can damage optical or, if appropriate, mechanical components in the optical amplifier or in the beam path upstream of the optical amplifier, e.g., in the beam source. 
         [0006]    In order to prevent the reflected, returning pulse from entering the beam source, it is possible to arrange, for example between the beam source and one of the optical amplifiers, an optical switch or a switchable diaphragm which closes the beam path of the laser beam, such that the returning laser pulse is blocked and cannot enter the beam source. Moreover, it may be advantageous to close the beam path of the laser beam between two successive pulses propagating in the direction of the target material, in order to prevent a stable optical axis from being established at which ASE (“amplified spontaneous emission”) or parasitic self-lasing (at scattering locations) may form. 
         [0007]    Since optical switches require a certain time period to block the beam path for the laser beam after the passage of a pulse, it is advantageous to provide an optical delay device in the beam path downstream or, if appropriate, upstream of an optical switch, which optical delay device elongates the beam path of the laser beam and thus increases the travel time of the laser beam. In an optical delay device, the beam path is elongated by more than  100  m, if appropriate, which can be realized by a tight folding or by a multiplicity of reflections between the reflective surfaces. In order to ensure a sufficient elongation of the beam path, optical delay devices typically require a comparatively large structural space. 
       SUMMARY 
       [0008]    The present invention discloses a method of elongating a beam path of a light beam, an optical delay device, and a driver laser arrangement for an EUV light source, which make possible a comparatively large elongation of the beam path in conjunction with a comparatively small structural space required. 
         [0009]    One aspect of the invention features a method of elongating a beam path of a light beam, comprising: coupling the light beam into an interspace between a plurality of first reflective surfaces and a plurality of second reflective surfaces facing the first reflective surfaces; multiply reflecting the light beam between the first reflective surfaces and the second reflective surfaces to elongate the beam path of the light beam; and coupling out the light beam from the interspace. The light beam undergoes the steps of coupling in, repeated reflecting and coupling out at least a first time with a first pass and a second time with a second pass, and the light beam traverses a different beam path in the interspace during the first pass in comparison with during the second pass. 
         [0010]    The invention proposes that the light beam pass through the optical delay device at least twice along different beam paths. Within the meaning of this application, different beam paths are not understood to be the inversion of the beam direction, that is to say that the light beam passes through the interspace during the at least two passes on at least two different geometrical beam paths and not along one and the same beam path (in the opposite direction), as would be the case, e.g., for polarization separation. 
         [0011]    The passage of the light beam through the interspace at least twice makes it possible to alter the light beam in terms of its properties between the first pass and the second pass, for example to amplify the light beam or to manipulate other properties (e.g. polarization, divergence, wavelength, . . . ) of the light beam. Moreover, between the two passes the light beam can, e.g., pass through an optical diode or, if appropriate, be altered by active optical elements, e.g., by acousto-optical modulators (AOMs), electro-optical modulators (EOMs), amplifiers, or passive optical elements such as beam telescopes, optical diodes, apertures, etc., for example by the laser beam being trimmed, in particular trimmed marginally. It goes without saying that the light beam need not necessarily be altered between the first and second passes; rather, after the first pass the light beam can merely be deflected from an output of the delay device to an input of the delay device in order to pass through the latter once again. 
         [0012]    It is possible to configure the first and second reflective surfaces in the form of individual mirrors, which are typically applied on a common carrier structure. It is advantageous, however, to configure the reflective surfaces in the form of mirror facets of a facet mirror, which are typically arranged or formed on a carrier structure in the form of a common substrate. The reflective surfaces can be configured on a planar surface of the carrier structure, but it is also possible for the surface of the carrier structure on which the reflective surfaces are formed to have an e.g. concave curvature. 
         [0013]    A group of first and second reflective surfaces, the number of which is more than 50%, preferably more than 80%, of the total number of reflective first and second surfaces, can have the same alignment, that is to say that the normal directions of this group of the first and second reflective surfaces correspond. The first reflective surfaces and the second reflective surfaces of this group can be arranged in particular along two parallel planes that run perpendicular to the respectively identical normal directions. The first and second reflective surfaces can have a typically concave or, if appropriate, convex (typically spherical) curvature. In this case, the normal directions run through center points or the vertices of the reflective surfaces. 
         [0014]    In one variant, the light beam is reflected at (or by) at least one of the first or of the second reflective surfaces both during the first pass and during the second pass. In this case, the light beam passes through one, typically a plurality, of the first and/or second reflective surfaces doubly, i.e., both during the first pass and during the second pass, that is to say that said light beam is reflected at the respective surfaces. With the aid of reflective surfaces through which the light beam passes doubly, the number of reflective surfaces required for elongating the beam path can be reduced and structural space can thus be saved. 
         [0015]    Alternatively, it is also possible for the first and second reflective surfaces through which the beam path of the light beam passes during the first pass in the optical delay device to differ from the first and second reflective surfaces through which the beam path of the light beam passes during the second pass, that is to say that in this case the light beam passes doubly through none of the reflective surfaces. In this case, too, it is possible to realize a delay device which occupies only a slightly larger structural space as a result of the double pass, but nevertheless enables a considerable elongation of the beam path. In both cases, even with a double pass, the outlay for the mounting and adjustment mechanisms is incurred only once, and so the delay device can be realized more compactly than would be the case with two separate delay devices. 
         [0016]    In a further variant, at least one of the reflective first surfaces is imaged onto a further reflective first surface during the reflection of the light beam at one of the reflective second surfaces (and vice versa). By multiply imaging the light beam onto itself, it is possible to prevent the diameter of the light beam from increasing within the interspace or within the delay device. For the purpose of imaging, the first reflective surfaces can be concavely curved. The second reflective surfaces can likewise be concavely or, if appropriate, convexly curved. For details regarding the curvature of the reflective surfaces for realizing multiple imaging, reference is made to WO 2012/091786 A1 cited in the introduction, which is incorporated by reference in the content of this application with regard to this aspect. 
         [0017]    In one advantageous variant, the plurality of first reflective surfaces and the plurality of second reflective surfaces are arranged in a first grid and in a second grid, in which the light beam passes preferably in a meandering fashion. The two grids in each case form a regular arrangement of reflective surfaces which are arranged along the grid or lattice network, that is to say that the center points of the individual reflective surfaces are arranged at identical distances from one another. It goes without saying that such a grid need not be continuous; rather, individual reflective surfaces can be omitted particularly at the edge of the grid. By way of example, in the case of a square perimeter geometry of the reflective surfaces, the center points of the surfaces can form a square grid, in the case of which a respective center point is at the same distance from four center points arranged in a respectively adjacent row and/or column. In the case of first and second reflective surfaces having a hexagonal perimeter geometry, the reflective surfaces are typically arranged in a hexagonal grid. The geometry of the grid may depend on the perimeter geometry of the first and second reflective surfaces, as described above, but this is not necessarily the case. In this regard, by way of example, even in the case of an (approximately) rectangular perimeter geometry of the individual reflective surfaces, an arrangement of the reflective surfaces in a hexagonal grid may be advantageous. 
         [0018]    The light beam passes through the grid preferably in a meandering fashion, that is to say that the light beam passes through the reflective surfaces along a predefined direction that is maintained during a pass of the light beam. Only at the edge of the grid is the light beam deflected out of the predefined direction in order to pass through the grid once again in a laterally offset manner along the predefined direction (with an inverted beam direction). The meandering pass enables the light beam to pass through as many of the first and second reflective surfaces as possible, ideally all of said surfaces, during a pass. 
         [0019]    In order to simplify the passage of the light beam along the first and second grids, it has proved to be advantageous if the first and second grids and thus the center points of the first and second reflective surfaces are arranged offset with respect to one another in a direction perpendicular to the common normal directions (see above). The offset between the two grids is typically an integer fraction, generally half, of the distance between adjacent center points of the reflective surfaces. The direction along which the offset is effected generally runs along a row or a column of a respective grid. 
         [0020]    In one development, a direction along which the light beam passes through the first and second reflective surfaces of the first and second grids in a meandering fashion during the first pass differs from a direction along which the light beam passes through the first and second reflective surfaces of the first and second grids in a meandering fashion during the second pass. The directions along which the at least two meandering passes take place are rotated with respect to one another and may form for example an angle of 90° or of 60° with respect to one another. The rotated meandering pass makes it possible, in particular, to align the plurality of the first and second reflective surfaces parallel or with the same normal direction with respect to one another and nevertheless to enable a multiple pass in which the beam paths do not correspond. 
         [0021]    In one variant, during the first pass, the light beam is reflected from one of the first reflective surfaces to one of the second reflective surfaces and back from the latter to a further, adjacent first reflective surface, and wherein, during the second pass, the light beam is reflected from one of the first reflective surfaces to the same second reflective surface and back from the latter to a further, non-adjacent first reflective surface. It goes without saying that the second pass need not necessarily follow the first pass temporally, that is to say that the temporal order of the first and second passes is arbitrary. During the first pass, the light beam passes through the first and second reflective surfaces of a grid typically in rows or in columns (in particular in a meandering fashion), that is to say that the reflections take place between adjacent reflective optical surfaces (with minimum distance between the (surface) center points). During the second pass, by contrast, the reflections typically take place between non-adjacent first and second reflective surfaces, that is to say that the light beam typically passes through the first and second reflective surfaces at an angle with respect to the rows or columns of the grid along which the reflective surfaces are at a minimum distance from one another. 
         [0022]    The adjacent first reflective surfaces can be arranged, in particular, in the same row (or column) of the first grid as the second reflective surface of the second grid. The non-adjacent first surfaces of the first grid can be arranged in two rows (or columns) of the first grid which are arranged adjacent to the row (or column) of the second grid with the second reflective surface and which lie for example directly above or directly below the row of the second grid in which the second reflective surface is arranged. 
         [0023]    In particular, during a third pass, the light beam can be reflected from one of the first reflective surfaces to the same second reflective surface and back from the latter to a non-adjacent first reflective surface, wherein the first reflective surface and the further first reflective surface during the third pass do not correspond to the first reflective surface and the further first reflective surface during the second pass. In this case, the light beam passes through the reflective surfaces along a different direction during the third pass compared with during the second pass. In this way, it is possible to realize a triple pass of the light beam through the optical delay device with three respectively different beam paths. 
         [0024]    Another aspect of the invention features an optical delay device for elongating a beam path of a light beam, comprising: a plurality of first reflective surfaces; a plurality of second reflective surfaces facing the first reflective surfaces; at least one input for coupling the light beam into an interspace formed between the first reflective surfaces and the second reflective surfaces; and at least one output for coupling out the light beam from the interspace after the elongating of the beam path of the light beam by multiple reflections at the first reflective surfaces and at the second reflective surfaces. The optical delay device is configured to guide the light beam between the at least one input and the at least one output on at least two different (geometrical) beam paths through the interspace between the first and second reflective surfaces. As was described further above, the realization of two or more geometrical beam paths makes it possible to reduce the structural space of the optical delay device that is required for the elongation of the beam path. 
         [0025]    In one advantageous embodiment, a group of first and second reflective surfaces, the number of which is more than 50%, preferably more than 80%, of the total number of reflective first and second surfaces, has an identical normal direction, that is to say that the first and second reflective surfaces of this group are aligned parallel to one another. The light beam can be reflected back and forth between the parallel-aligned first and second reflective surfaces of the group at in each case identical angles of incidence and reflection. The identical alignment of the first and second reflective surfaces simplifies the production thereof, particularly if they are manufactured as facets on a common substrate. 
         [0026]    Preferably, the plurality of first reflective surfaces and the plurality of second reflective surfaces are arranged in a first grid and in a second grid. The arrangement in a grid is advantageous in order to reflect the light beam on at least one of the first or of the second reflective surfaces both during a first pass and during a second pass, as was described further above. 
         [0027]    In one development, the first grid and the second grid are aligned parallel and arranged offset with respect to one another, wherein the offset is preferably an integer fraction, in particular half, of a distance between the center points (or the vertices) of adjacent reflective surfaces of the first and second grids. In this case, the first grid and the second grid, to put it more precisely the center points or the vertices of the first and second reflective surfaces, are arranged in parallel planes. The two grids aligned parallel, which typically have first and second reflective surfaces having an identical geometry and having an identical surface area, are arranged offset with respect to one another by an integer fraction of the distance between the center points or the vertices of the first and/or second reflective surfaces in order to simplify the multiple reflection between the first and second reflective surfaces. In particular, in this way it is possible to simplify the multiple reflection between first and second surfaces of the first grid and second grid that are aligned parallel to one another, which fosters meandering passage through the reflective surfaces of the respective grid. Individual reflective surfaces, in particular arranged at the edge of the grid, can be aligned in a tilted fashion relative to the grid plane or relative to the parallel-aligned first and second reflective surfaces, in order to bring about a deflection of the light beam at the edge of the grid. The center points of said reflective surfaces also lie on the grid, that is to say that the center points of said surfaces are not offset relative to the respective grid despite the tilting. The direction along which the two grids are offset generally runs along a row or a column of a respective grid or along a direction that runs through the center points of at least two adjacent reflective surfaces. 
         [0028]    In a further embodiment, the delay device is configured to guide the light beam on the first beam path via the first and second reflective surfaces of the first and second grids in a meandering fashion along a first direction, and it is configured to guide the light beam on the second beam path via the first and second reflective surfaces of the first and second grids in a meandering fashion along a second direction, which is different than the first direction. As was described further above in connection with the method, the light beam can pass repeatedly through the optical delay device particularly effectively in this way. In particular, the first and second reflective surfaces at which the reflection takes place along the first and second direction, respectively, can be aligned parallel to one another. It is only at the edges of the respective grid that typically first and second reflective surfaces are present which are arranged in a tilted manner relative to the parallel-aligned reflective surfaces in order to bring about a deflection of the light beam and a lateral offset in order that said light beam can pass through the grid once again along the first and respectively second direction (with an opposite beam direction). 
         [0029]    In order to couple the light beam into the optical delay device at a suitable angle or with a suitable beam direction and at a suitable location, such that said light beam can pass through both the first beam path and the second beam path, a single input and a single output possibly suffice. In general, it is more advantageous to provide in each case separate inputs and outputs for coupling into and out of the first beam path and for coupling into and out of the second beam path. 
         [0030]    In a further embodiment, the plurality of first reflective surfaces has a first group of first reflective surfaces and a second group of first reflective surfaces, and the plurality of second reflective surfaces has a first group of second reflective surfaces and a second group of second reflective surfaces, wherein the reflective first and second surfaces of the first group and the reflective first and second surfaces of the second group are arranged and aligned in such a way that the light beam runs along a first beam path only between reflective first and second surfaces of the first group and the light beam runs along a second beam path only between reflective first and second surfaces of the second group. In this embodiment, the light beam passes through the first and second surfaces of the respective group only once for both beam paths. 
         [0031]    Preferably, in this embodiment, the first and second reflective surfaces of the first group are arranged in a first, inner ring region and the first and second reflective surfaces of the second group are arranged in a second, outer ring region. The center points of the reflective surfaces of the first group and the center points of the reflective surfaces of the second group in this case are typically arranged along two concentric circles. By arranging the two groups of reflective surfaces in two ring regions, despite dispensing with multiple utilization of the reflective surfaces it is possible to realize an elongation of the beam path with only slightly larger structural space. Typically, the light beam passes through adjacent first and second reflective surfaces of a respective ring region successively, that is to say that the light beam passes through the entire first or second ring region either in the clockwise direction or in the counterclockwise direction. The light beam can pass through the first ring region and the second ring region either in the same direction or in opposite directions. A ring-shaped arrangement of the reflective surfaces has proved to be advantageous particularly when the optical delay device is used in an optical amplifier. It goes without saying that a third, fourth, . . . ring region can also be provided, which is/are arranged concentrically with respect to the first and second ring regions. 
         [0032]    In one development, a shield, in particular in the form of a hollow cylinder, for preventing radiation from crossing from the first beam path into the second beam path, or vice versa, is arranged between the first group of reflective first and second surfaces and the second group of reflective first and second surfaces. If separate inputs and outputs are in each case provided for the first beam path and the second beam path, the shield can completely separate or shield the two beam paths from one another. If only one input and one output are provided in the delay device, it is typically necessary to provide a passage for the laser beam in the shield. 
         [0033]    In a further embodiment, the delay device is configured to image at least one of the reflective first surfaces onto a further reflective first surface during the reflection of the light beam at one of the reflective second surfaces. As was described further above in connection with the method, it is advantageous to realize multiple imaging in the case of multiple reflection of the light beam in order to prevent the beam diameter of the light beam from increasing during the multiple reflection. 
         [0034]    In a further embodiment, the first reflective surfaces are concavely curved. The concave curvature of the reflective surfaces makes it possible to realize multiple imaging. The second reflective surfaces, too, are typically either concavely or, if appropriate, convexly curved in order to realize multiple imaging. 
         [0035]    In a further embodiment, the plurality of first reflective surfaces is applied on a first carrier structure, and the plurality of second reflective surfaces is applied on a second carrier structure. The surfaces of the first and second carrier structures on which the first and second reflective surfaces are applied are typically configured as planar surfaces, that is to say that the center points or the vertices of the first and second reflective surfaces are arranged in each case in a common plane. The first and second carrier structures, to put it more precisely the mutually facing surfaces of the first and second carrier structures, on which the first and second reflective surfaces are applied, are typically aligned parallel to one another. 
         [0036]    The first and second reflective surfaces can be formed on the carrier structures in the form of mirror facets of a facet mirror, that is to say that the carrier structures form a substrate on which the reflective surfaces are produced e.g. by diamond turning. The reflective surfaces can also be configured as individual mirrors that are fixed, e.g. screwed, on the carrier structure. In order to increase the reflectivity, the individual mirrors can also be provided with a dielectric coating, as is described in WO 2012/091786 A1 cited in the introduction, which is incorporated by reference in the content of this application with regard to this aspect. The input or the inputs and the output or the outputs can be configured, e.g., in the form of openings on the carrier structures. However, it is also possible, if appropriate, to realize the input or the output by means of one or more mirrors introduced into the interspace between the first and second carrier structures. In this case, the light beam is radiated onto the mirror serving as input and is deflected e.g. by 90° onto one of the first or of the second reflective surfaces. Accordingly, the light beam can be deflected at a further mirror and be coupled out from the interspace. 
         [0037]    In one development, the delay device comprises spacers for connecting the carrier structures whilst maintaining a predefined distance from one another. In order to enable the multiple reflection between the first and second reflective surfaces, it is necessary for the carrier structures, which are typically aligned parallel to one another, to be arranged at a predefined distance from one another. 
         [0038]    A further aspect of the invention relates to a driver laser arrangement for an EUV (extreme ultraviolet) light source (or an EUV laser driver), comprising: at least one optical delay device as described above. In this case, the optical delay device typically serves for elongating the travel path of a laser beam, for example of a CO 2  laser beam, which is generated by a beam source. The optical delay device can be arranged in the beam path of the laser beam downstream of an optical switch, for example in the form of an acousto-optical modulator, in order to elongate the travel path of the laser beam and in this way to obtain enough time for switching over the optical switch, such that a back-reflection generated at a target material can be blocked by the optical switch. In this case, the laser beam can pass through the two beam paths of the delay device directly successively. However, it is also possible, between passing through the first beam path of the delay device and passing through the second beam path of the delay device, for the laser beam to be guided through one or more optical components in which the properties of the laser beam are altered. By way of example, the laser beam can be amplified in an optical amplifier between the first pass and the second pass. The optical delay device can also be used in an optical amplifier of the driver laser arrangement. In this case, a typically gaseous amplifier medium, for example in the form of CO 2  gas, is introduced into the interspace between the first and second reflective surfaces. 
         [0039]    Further advantages of the invention are evident from the description and the drawing. Likewise, the features mentioned above and those that will further be presented can be used in each case by themselves or as a plurality in any desired combinations. The embodiments shown and described should not be understood to be an exhaustive enumeration, but rather are of exemplary character for outlining the invention. 
         [0040]    DESCRIPTION OF DRAWINGS 
         [0041]      FIGS. 1A and 1B  show schematic illustrations of two exemplary embodiments of a driver laser arrangement for an EUV light source, comprising two and one optical delay device(s), respectively. 
         [0042]      FIG. 2  shows a schematic illustration of a first exemplary embodiment of an optical delay device with first and second reflective surfaces arranged in a first grid and a second grid, offset with respect to the first grid. 
         [0043]      FIGS. 3A and 3B  show schematic illustrations of a first and second square grid having first and second reflective surfaces ( FIG. 3A ) and of two meandering beam paths along two different directions at the second grid ( FIG. 3B ). 
         [0044]      FIGS. 4A and 4B  show schematic illustrations of the two grids from  FIG. 3A  arranged one above the other and having two and respectively three meandering beam paths along two and respectively three directions rotated with respect to one another. 
         [0045]      FIGS. 5A and 5B  show schematic illustrations of a first and second hexagonal grid having first and second reflective surfaces ( FIG. 5A ) and of two meandering beam paths along two mutually perpendicular directions at the second grid ( FIG. 5B ). 
         [0046]      FIG. 6  shows schematic illustrations of the two grids from  FIG. 5A  arranged one above the other and having two meandering beam paths along two mutually perpendicular directions. 
         [0047]      FIG. 7  shows a schematic illustration of a second exemplary embodiment of an optical delay device with first and second reflective surfaces arranged in a first, inner ring region and in a second, outer ring region. 
     
    
     DETAILED DESCRIPTION 
       [0048]    In the following description of the drawings, identical reference signs are used for identical or functionally identical component parts. 
         [0049]      FIG. 1A  shows highly schematically an EUV light source  1  comprising a beam source  2 , an amplifier arrangement  3  having three optical amplifiers  4   a,    4   b,    4   c  or amplifier stages, a beam guiding device  5  (not illustrated in more specific detail) and a focusing device in the form of a focusing lens  6 . The focusing lens  6  serves to focus a pulsed light beam in the form of a laser beam  7 , which light beam is generated by the beam source  2  and amplified by the amplifier arrangement  3 , at a target region or at a target position T, at which a target material  8  is introduced. Upon irradiation with the laser beam  7 , the target material  8  undergoes transition to a plasma state and emits EUV radiation in the process, said radiation being focused by means of a collector mirror  9 . 
         [0050]    In the example shown in  FIG. 1A , the collector mirror  9  has an opening for the passage of the laser beam  7  and the focusing lens  6  separates a vacuum chamber  10 , in which the target material  8  is arranged, from the beam guiding device  5 . In the example shown, the beam source  2  comprises a CO 2  laser to generate in short temporal succession a first pulse  11   a  (pre-pulse) and a second pulse  11   b  (main pulse), which are jointly amplified in the amplifier arrangement  3  and focused onto the target material  8  or into the region of the target position T. The beam source  2  together with the amplifier arrangement  3  forms a driver laser arrangement  12  of the EUV light source  1 . 
         [0051]    The wavelength λ L  of the laser beam  7  which is generated by the beam source  2  is constant and is approximately 10.6 μm in the example shown of a beam source  2  in the form of a CO 2  laser. As can be discerned on the basis of the pulse heights in  FIG. 1A , the two pulses  11   a,    11   b  are generated by the beam source  2  with different powers and are amplified in the three optical amplifiers  4   a - c  of the amplifier arrangement  3 . To prevent a situation in which a back-reflection of the amplified laser beam  7 , said back-reflection being generated at the target material  8 , enters the beam source  2  in the opposite direction after passing through the amplifier arrangement  3 , an acousto-optical modulator  15  is arranged in the beam path of the laser beam  7  between the beam source  2  and the first amplifier  4   a  of the amplifier arrangement  3 . The acousto-optical modulator  15  can serve as an optical switch and block the beam path of the laser beam  7  propagating back to the beam source  2 . For this purpose, the acousto-optical modulator  15  can be configured to be switched over within a switchover time from a first switching position, in which the pulsed laser beam  7  is transmitted, to a second switching position, in which the pulsed laser beam  7  is blocked. Said switchover time can be shorter than the time period required by the laser beam  7  from the acousto-optical modulator  15  to the target material  8  and back. In order to elongate the beam path of the laser beam  7 , an optical delay device  16  is arranged in the beam path downstream of the acousto-optical modulator  15  in the case of the EUV light source  1  shown in  FIG. 1A . 
         [0052]      FIG. 1B  illustrates a further example of an EUV light source  1  which differs from the EUV light source  1  from  FIG. 1A  in that two further acousto-optical modulators  15   a,    15   b  are arranged between the first optical amplifier  4   a  and the second optical amplifier  4   b.  In the example shown in  FIG. 1B , the laser beam  7  passes through the optical delay device  16  a first time and a second time, where the laser beam  7  is amplified in the first optical amplifier  4   a  between the first and second passes through the delay device  16 . In contrast to the illustration shown in  FIG. 1A , the second pass of the laser beam  7  through the optical delay device  16  can be effected between the two acousto-optical modulators  15   a,    15   b.    
         [0053]      FIG. 2  shows the optical delay device  16  from  FIGS. 1A and 1B  in a three-dimensional view. The optical delay device  16  comprises two carrier structures  17   a,b  in the form of carrier plates. A plurality of first reflective surfaces  18  arranged in a first square grid R 1  are formed on the first carrier structure  17   a.  Correspondingly, a plurality of second reflective surfaces  19  arranged in a second square grid R 2  are formed on the second carrier structure  17   b.  In the example shown, the first and second reflective surfaces  18 ,  19  are configured as individual mirrors having a circular geometry which are fixed on the planar surfaces of the carrier structures  17   a,b,  e.g., with the aid of screws or the like. 
         [0054]    An interspace  20  is formed between the two carrier structures  17   a,b  and thus between the first and second reflective surfaces  18 ,  19 , into which interspace the laser beam  7  is coupled at an input  21  in the form of a through opening formed in the first carrier structure  17   a  and is coupled out via an output  22 , likewise configured in the form of a through opening, said output being formed at the second carrier structure  17   b  opposite the first carrier structure. In the example shown, the two carrier structures  17   a,b  are connected to one another via four rod-shaped spacers  23   a - d  aligned parallel to one another, in order to keep the two carrier structures  17   a,b  at a desired distance A from one another. 
         [0055]    The center points M of adjacent circular first and second reflective surfaces  18 ,  19  are arranged at a distance L from one another both in the horizontal direction (X-direction) and in the vertical direction (Y-direction), said distance corresponding to the grid spacing or the lattice spacing of the first and second grids R 1 , R 2  in the X-direction and in the Y-direction, respectively. The second grid R 2  is arranged offset in the X-direction relative to the first grid R 1 , where the offset corresponds to half of the distance L, i.e., L/ 2 , between adjacent first and second reflective surfaces  18 ,  19  of the first and second grids R 1 , R 2 , as can readily be discerned in particular in  FIG. 3A , which shows a detail of a first grid R 1  and a detail of an opposite second grid R 2  in each case in a plane view. 
         [0056]    The first and second reflective surfaces  18 ,  19  of the first and second grids R 1 , R 2  in  FIG. 3A  differ from the first and second reflective surfaces  18 ,  19  in  FIG. 2  in that these have a square geometry and are arranged (approximately) directly adjacent to one another. The first and second reflective surfaces  18 ,  19  shown in  FIG. 3A  are mirror facets formed on a common substrate (not shown in  FIG. 3A ) that is applied on a carrier structure  17   a,b.  The substrate can be fixed on the carrier structure  17   a,b  like a single large reflective surface or like an individual mirror by means of a single fixing/adjustment method, e.g., with the aid of screws. The mirror facets  18 ,  19  can be produced on the substrate, which may include copper, for example, by means of surface processing, for example by means of diamond turning. If appropriate, a reflection-enhancing coating can be applied on the first and second reflective surfaces  18 ,  19 . For the production of the first and second reflective surfaces  18 ,  19  in the form of mirror facets, it has proved to be advantageous if the largest possible number of first and second reflective surfaces  18 ,  19  have an identical alignment, e.g., the same normal direction, which in the example shown corresponds to the Z-direction along which the two parallel-aligned grids R 1 , R 2  or the two carrier structures  17   a,b  are arranged at a distance from one another. In the case of the grids R 1 , R 2  shown in  FIG. 3A , all parallel-aligned first and second surfaces  18 ,  19  form a group G. In  FIG. 3A , surfaces  18 ,  19  associated with the group G are clarified by the fact that they are illustrated as non-hatched surfaces. The first and second surfaces  18 ,  19  associated with the group G form a proportion of more than 50%, in the example shown more than 80%, of the total number of first and second reflective surfaces  18 ,  19  of the respective grids R 1 , R 2 . The only reflective surfaces which do not belong to the group G with identical alignment are first and second reflective surfaces  18 ,  19 —illustrated in a hatched manner in  FIG. 3A —which are arranged at the edge of the respective grid R 1 , R 2  and which serve for deflecting the laser beam  7 , as is described in greater detail further below. 
         [0057]    The optical delay device  16  shown in  FIG. 2  and in  FIG. 3A  is configured in such a way that the laser beam  7  is guided in the interspace  20  between the input  21  and the output  22  on a first beam path Si and in the interspace  20  (if appropriate between a further input and a further output) also on a second geometrical beam path S 2 , which is different than the first beam path. For guiding the laser beam  7  along two different geometrical beam paths S 1 , S 2 , besides the possibly required provision of a further input and output on the optical delay device  16 , which can be realized, e.g., by introducing further through openings in the carrier structures  17   a,b,  a suitable alignment of the lateral first and second reflective surfaces  18 ,  19  (illustrated in a hatched manner) relative to the XY plane and also an offset of the first and second grids R 1 , R 2  are required. 
         [0058]    As is indicated with the aid of dashed lines in  FIG. 3A , in the first beam path S 1 , the laser beam  7  is reflected from a first reflective surface  18   a  of the first grid R 1  to a second reflective surface  19   a  of the second grid R 2  and is reflected back from the latter to a surface  18   b  of the first grid R 1  that is adjacent in the X-direction. Such a reflection between adjacent first surfaces  18   a,b  of the first grid R 1  (and analogously between adjacent second surfaces  19   a,b  of the second grid R 2 ) typically takes place, in the first beam path S 1 , row by row (i.e., in the X-direction) between all reflective first and second surfaces  18 ,  19  which belong to the group G of identically aligned reflective surfaces  18 ,  19 . 
         [0059]    As is indicated in  FIG. 3A , the first beam path Si at the edge of the first grid R 1  and respectively the second grid R 2  does not run between adjacent first and respective second reflective surfaces  18 ,  19 , but rather between first and second reflective surfaces  18 ,  19  which are not directly adjacent in the square grid and which are arranged in different rows of the grid R 1 , R 2 . In this way, the laser beam  7  can pass through the respective grid R 1 , R 2  in a meandering fashion along the first beam path S  1 , as is indicated in  FIG. 3B . 
         [0060]    In contrast to the first beam path S 1 , the laser beam  7  passes through the first grid R 1  and respectively the second grid R 2  along the second beam path S 2  not row by row in the X-direction, but rather along a direction U rotated by 60° with respect thereto (cf.  FIG. 3B ). The second beam path S 2  likewise runs in a meandering fashion along the direction U rotated relative to the respective grid R 1 , R 2 , i.e., not along the rows and columns of the respective grid R 1 , R 2 , as is the case for the first beam path S 1 . Therefore, the second beam path S 2  runs not only at the edges of the respective grid R 1 , R 2  between non-adjacent first and second reflective surfaces  18 ,  19 , but also between those of the first and second reflective surfaces  18 ,  19  which belong to the group G and are thus aligned identically. 
         [0061]    By way of example,  FIG. 3A  shows part of the second beam path S 2  from a first reflective surface  18   c  of the first grid R 1 , from which surface the laser beam  7  is guided along the second beam path S 2  to the same second reflective surface  19   a  as in the case of the first beam path S 1 . The laser beam  7  is reflected back from this second reflective surface  19   a  along the second beam path S 2  to a further, non-adjacent first reflective surface  18   d.  As is evident in  FIG. 3B , for the second meandering beam path S 2  at the respective upper and lower edges of the grids R 1 , R 2  two rows (not illustrated in  FIG. 3B ) having first and second reflective surfaces  18 ,  19  are additionally required, through which typically only the second beam path S 2  passes, but not the first beam path S 1 . By contrast, the laser beam  7  in both beam paths S  1 , S 2  passes through a portion of the reflective surfaces of the three rows of the respective grids R 1 , R 2  illustrated in  FIG. 3B . 
         [0062]      FIGS. 4A and 4B  show an excerpt from the first and second grids R 1 , R 2 —illustrated one above the other—from  FIGS. 3A and 3B  with a first and second beam path S 1 , S 2  (cf.  FIG. 4A ) and with a third beam path S 3  (cf.  FIG. 4B ). The third beam path S 3  passes through the first and second reflective surfaces  18 ,  19  in a meandering fashion along a third direction V, which is rotated by 120° relative to the direction X of the first beam path Si and by 60° relative to the direction U of the second beam path S 2 . In the case of the arrangement indicated in  FIG. 4A , not all of the first and second reflective surfaces  18 ,  19  of the bottommost row and of the topmost row (not illustrated in  FIG. 4A ) of the respective grids R 1 , R 2  can be utilized for the second beam path S 2 , and so these can, if appropriate, be omitted or be utilized for the third beam path S 3 . In the case of the arrangement illustrated in  FIG. 4B , by contrast, the second beam path S 2  and the third beam path S 3  respectively pass through in each case half of the first and second reflective surfaces  18 ,  19 , such that these contribute to the elongation of the respective second and third beam paths S 2 , S 3  of the laser beam  7 . 
         [0063]    The respective directions X, U, V of the meandering beam paths S 1 , S 2 , S 3  need not necessarily be aligned at 60° with respect to one another; it is for example also possible, depending on the geometry of the respective grids R 1 , R 2 , to choose the directions X, U, V of the meandering beam paths S 1 , S 2 , S 3  to be perpendicular to one another, as is described below on the basis of two hexagonal grids R 1 , R 2  shown in  FIGS. 5A and 5B . 
         [0064]    The two hexagonal grids R 1 , R 2  from  FIGS. 5A and 5B , analogously to the square grids R 1 , R 2  shown in  FIGS. 3A and 3B , are displaced in the X-direction by half of the distance L (i.e., L/ 2 ) between the center points M of adjacent first and second reflective surfaces  18 ,  19 . As was described further above in connection with  FIGS. 3A and 3B , the laser beam  7  passes through the first and second grids R 1 , R 2  on the first beam path S 1  in a meandering fashion along the X-direction, i.e., along the rows of the respective grids R 1 , R 2 . The second beam path S 2  runs along the first and second reflective surfaces  18 ,  19  in a manner rotated by 90°, i.e., along the Y-direction, as can be discerned in  FIG. 5B . As a result of the perpendicular alignment shown in  FIGS. 5A and 5B , it is possible to realize a strict separation of the meandering beam paths S 1 , S 2  and thus to avoid crosstalk from one beam path Si into the other beam path S 2 . In this case, the input  21  and the output  22  of the delay device  16  can be arranged in a manner separated far from one another spatially and in the beam propagation direction, without further optical elements having to be provided in the delay device  16  for this purpose. 
         [0065]    As in  FIGS. 3A and 3B , in  FIGS. 5A and 5B , too, the laser beam  7  along the first beam path S 1 , during the first pass, is reflected from one of the first reflective surfaces  18   a  to one of the second reflective surfaces  19   a  which belongs to the same row of the second grid R 2  as the first reflective surface  18   a.  The laser beam  7  is reflected from the second reflective surface  19   a  to an adjacent first reflective surface  18   b  of the first grid R 1  that is arranged in the same row. As in the case of the square grids R 1 , R 2  from  FIGS. 3A and 3B , in the case of the hexagonal grids R 1 , R 2  from  FIGS. 5A and 5B , too, during the second pass along the second beam path S 2  the laser beam  7  is reflected from a further one of the first reflective surfaces  18   c  to the same second reflective surface  19   a  of the second grid R 2  and back from the latter to a further, non-adjacent first reflective surface  18   d.  Both in the example illustrated in  FIGS. 3A and 3B  and in the example illustrated in  FIGS. 5A and 5B , the non-adjacent first reflective surfaces  18   c,    18   d  are arranged in two rows lying above and respectively below that row in which is arranged the second reflective surface  19   a  to and from which the laser beam  7  is reflected. 
         [0066]    As in the example shown in  FIGS. 3A and 3B , in the example shown in  FIGS. 5A and 5B , too, the first and second reflective surfaces  18 ,  19  which belong to the group G having identically aligned first and second reflective surfaces  18 ,  19  are illustrated in a non-hatched manner, while those reflective surfaces  18 ,  19  which are tilted to the plane of the drawing or with respect to the XY plane are illustrated in a hatched manner. A deflection or reflection between different rows of the respective grids R 1 , R 2  takes place at the first and second surfaces  18 ,  19  illustrated in a hatched manner. 
         [0067]      FIG. 6  illustrates the first and second reflective surfaces  18 ,  19  of a lower left partial region of the two hexagonal grids R 1 , R 2  one above the other. As can readily be discerned in  FIG. 6 , the first and second surfaces  18 ,  19  along the lateral edges of the two hexagonal grids R 1 , R 2  are tilted relative to the XY plane in order to bring about the meandering deflection of the respective beam path S 1 , S 2 . 
         [0068]    Finally,  FIG. 7  shows an optical delay device  16   a  that differs from the delay device  16  shown in  FIG. 2  essentially in that the first and second reflective surfaces  18 ,  19  are not arranged in a first and second grid R 1 , R 2 , but rather in a first and second ring region B 1 , B 2  in each case on a common substrate  25   a,b.  The first, inner ring region B 1  and the second, outer ring region B 2  are arranged concentrically with respect to one another. The plurality of first reflective surfaces  18  is subdivided by the ring regions B 1 , B 2  into a first group G 1  of first reflective surfaces  18   a  which belong to the first ring region B  1 , and into a second group G 2  of reflective surfaces  18   b  which belong to the second ring region B 2 . Correspondingly, the second reflective surfaces  19  are subdivided into a first group G 1  of second reflective surfaces  19   a  which belong to the first ring region B 1 , and into a second group G 2  of second reflective surfaces  19   b  which belong to the second ring region B 2 . 
         [0069]    The first and second reflective surfaces  18 ,  19  of the first and second ring regions B 1 , B 2  are arranged and aligned in such a way that the laser beam  7  is reflected back and forth only between the first and second reflective surfaces  18   a,    19   a  of the first ring region B 1  or only between the first and second reflective surfaces  18   a,    19   a  of the second ring region B 2 . In the example shown in  FIG. 7 , therefore, the laser beam  7  passes through none of the first and second reflective surfaces  18 ,  19  of the two ring regions B 1 , B 2  on more than one beam path S 1 , S 2 . In the example shown in  FIG. 7 , the reflective surfaces  18 ,  19  are formed in the form of mirror facets on the respective substrate  25   a,b  composed of copper. The reflective surfaces  18 ,  19  form circle sectors of the respective ring regions B 1 , B 2  and can be produced for example by diamond processing of the substrate  25   a,b,  as described above. 
         [0070]    As can be discerned in  FIG. 7 , two inputs  21   a,b  in the form of through openings are configured on the first carrier structure  17   a.  Analogously, two outputs  22   a,b  in the form of two through openings are configured on the second carrier structure  17   b.  The first input  21   a  and the first output  22   a  replace one of the mirror facets of the reflective surfaces  18   a,    19   a  of the first ring region B 1 . Correspondingly, the second input  21   b  and the second output  22   b  replace one of the mirror facets of the reflective surfaces  18   b,    19   b  of the second ring region B 2 . In this case, the first and the second beam paths S 1  and S 2 , respectively, run between the first and second inputs  21   a  and  21   b,  respectively, and the first and second outputs  22   a  and  22   b,  respectively. This is advantageous particularly if a shield  24  in the form of a hollow cylinder is fitted in the interspace  20 , which shield separates the first ring region B 1  from the second ring region B 2  in a substantially light-tight manner, as is indicated by dashed lines in  FIG. 7 . Alternatively, if appropriate, provision may be made of just a single input for coupling the two beam paths S 1 , S 2  into the interspace  20  of the optical delay device  16   a  and a single output for coupling out the two beam paths S 1 , S 2  from the interspace  20  of the optical delay device  16   a.    
         [0071]    In the example shown, the laser beam  7  passes through the reflective first and second surfaces  18   a,    19   a  of the first ring region B 1  along the first beam path Si in the counterclockwise direction and the first and second reflective surfaces  18   b,    19   b  of the second beam path S 2  in the clockwise direction. It goes without saying that the laser beam  7  can alternatively also pass through the reflective surfaces  18   a,    19   a,    18   b,    19   b  of the first and second ring regions B 1 , B 2  in the same direction along the first and second beam paths S 1 , S 2 . The position of the respective first and second reflective surfaces  18   a,    19   a,    18   b,    19   b  in the circumferential direction at which the laser beam  7  is coupled in and coupled out can correspond for the first and second beam paths S 1 , S 2  in the circumferential direction. Alternatively, the positions in the circumferential direction at which the coupling in and the coupling out take place can be chosen differently for the first and second beam paths S 1 , S 2 , in order in this way to be able to geometrically separate the two beam paths S 1 , S 2  even more simply, as is illustrated in  FIG. 7 . 
         [0072]    The optical delay device  16   a  shown in  FIG. 7  can be integrated in particular into one of the optical amplifiers  4   a - c  from  FIGS. 1A and 1B , for example into the first optical amplifier  4   a  (cf.  FIG. 1A ). In this case, a typically gaseous gain medium, in the form of CO 2  gas in the example shown, is introduced in the interspace  20 . It goes without saying that instead of first and second reflective surfaces  18 ,  19  in the form of individual mirrors, the first and second reflective surfaces  18 ,  19  can be configured in the form of mirror facets in the case of the delay device  16   a  shown in  FIG. 7  as well. 
         [0073]    In the examples described here, typically at least the first reflective surfaces  18  or at least the second reflective surfaces  19  are concavely curved in order, upon reflection at a respective opposite second or first reflective surface  19 ,  18 , to image the laser beam  7  onto a further reflective first or second reflective surface  18 ,  19 . In the example described in  FIG. 2 , where the first and second reflective surfaces  18 ,  19  are arranged in each case in a grid R 1 , R 2 , typically both the first and the second reflective surfaces  18 ,  19  are concavely curved. In the example shown in  FIG. 7  by contrast, generally the second reflective surfaces  19  are convexly curved. It goes without saying that—apart from the curvature—there is no fundamental difference between the first reflective surfaces  18  and the second reflective surfaces  19 . 
         [0074]    Instead of applying the first and second reflective surfaces  18 ,  19  on the planar surfaces of carrier structures  17   a,b,  alternatively it is also possible, if appropriate, to provide the carrier structures  17   a,b  themselves with a concave or, if appropriate, convex curvature. In general, for the multiple imaging in the case of the application described here, however, it is necessary for the first and second reflective surfaces  18 ,  19  to have a dedicated curvature which is greater than a possibly present curvature of the respective carrier structures  17   a,b  in order in this way to be able to maintain the beam diameter of the laser beam  7  in the course of the elongation of the beam path S 1 , S 2 . 
         [0075]    In the manner described further above, a laser beam can pass through one and the same delay device  16 ,  16   a  in at least a first and a second pass along different geometrical beam paths S 1 , S 2 , . . . , as a result of which the structural space of the optical delay device  16 ,  16   a  that is required for the elongation of the beam path S 1 , S 2 , . . . is comparatively small. The alignment of the first and second reflective surfaces  18 ,  19  in the optical delay device  16 ,  16   a  is typically fixedly predefined, that is to say that generally it is not possible to alter the alignment of the first and second reflective surfaces  18 ,  19  during the operation of the optical delay device  16 ,  16   a.    
         [0076]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.