The present disclosure relates to the subject matter disclosed in PCT application No. PCT/EP01/01130 of Feb. 2, 2001, which is incorporated herein by reference in its entirety and for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a laser amplifier system comprising a solid body having a laser-active medium, an excitation source for producing an excited state of the laser-active medium, and an amplifier radiation field which passes multiply through the solid body and from which a laser beam can be extracted. 
     Such a laser amplifier system is known, for example, from EP 0 632 551. 
     In this laser amplification system, however, the amplifier radiation field is guided, even with multiple passes through the solid body, in such a way that the individual branches are reflected back on themselves. 
     The disadvantage of an amplifier radiation field formed in this way is that a high number of passes of the amplifier radiation field through the solid body can be achieved only with great difficulty. 
     It is therefore an object of the invention to improve a laser amplifier system of the generic type in such a way that the highest possible number of passes of the amplifier radiation field through the solid body can be achieved with the simplest possible optical means. 
     SUMMARY OF THE INVENTION 
     This object is achieved according to the invention, in the case of a laser amplifier system of the type described in the introduction, by the fact that radiation field guiding optics are provided, which make the amplifier radiation field entering the solid body in the form of a plurality of incident branches with locally different trajectories and leave the solid body in the form of at least one emerging branch with a trajectory locally different from the incident branches, and that the radiation field guiding optics have at least one deviating unit which forms, from at least one of the branches emerging from the solid body, a branch which enters the solid body with a trajectory locally separate from this emerging branch. 
     The advantage of the solution according to the invention is that it provides a straightforward way of achieving a large number of passes of the amplifier radiation field through the solid body, and at the same time, since the incident branches and the emerging branches respectively have locally different trajectories from one another, optimum utilization of the excited laser-active medium in the solid body takes place. 
     It is particularly favorable for the incident branches always to enter the same solid body. 
     It is even more favorable for the incident branches always to enter the same volume region of the solid body. 
     In order not to increase the size of the cross section of the incident branches formed in turn from emerging branches by the deviating units, provision is preferably made for the radiation field guiding optics to form the incident branch from the emerging branch after intermediate focusing. 
     The intermediate focusing may in this case take place independently of the deviating unit. In order to configure the beam guiding optics according to the invention as compactly as possible, provision is preferably made for the intermediate focusing to take place in the vicinity of the deviating unit. 
     A particularly expedient solution in this case provides for an intermediate focus lying between two deviating elements of the deviating unit to be produced by the intermediate focusing, which prevents the intermediate focus from lying directly in a deviating unit. 
     It is particularly favorable in this case, in order to arrange both deviating elements as far away as possible from the intermediate focus, for the intermediate focus to lie approximately centrally between the two deviating elements. 
     Since the intensity per unit area of the cross-sectional area of the radiation field increases close to the intermediate focus, provision is preferably made for the optical path between the two deviating elements lying on either side of the intermediate focus to be greater than a spacing between an input branch entering the deviating unit and an output branch emerging from the deviating unit. 
     It is particularly favorable in this case for the optical path between the two deviating elements lying on either side of the intermediate focus to correspond at least to two times the spacing of the input branch and the output branch. 
     A particularly favorable solution provides for the deviating unit to guide the amplifier radiation field in a loop which, in relation to an input branch and an output branch of the deviating unit, has an extent in an expansion direction which is greater than the spacing between the input branch and the output branch. 
     This expansion of the radiation field in the expansion direction provides the opportunity to maintain a spacing which is as large as possible between the deviating elements lying on either side of the intermediate focus. 
     Preferably, the extent of the loop in the expansion direction is at least two times the spacing between the input branch and the output branch. 
     In the scope of the exemplary embodiments described so far, it has been assumed that the radiation field guiding optics convert at least one emerging branch into an incident branch by employing a deviating unit. 
     The solution according to the invention may, however, be refined in a particularly straightforward way if the radiation field guiding optics convert a plurality of emerging branches into a plurality of incident branches by means of at least one deviating unit. 
     In the scope of the description of the individual exemplary embodiments so far, the way in which the respective incident and emerging branches of the amplifier radiation field are intended to be formed has not been discussed in detail. 
     In principle, it would be conceivable to embody them as divergent or convergent branches, albeit with the disadvantage that the cross section of the branches would become larger as the number of passes is increased. 
     In order to be able to keep the cross section of the incident and emerging branches the same size, and therefore to be able to use a volume region of the solid body with excited laser-active medium optimally for amplifying the radiation field, provision is preferably made for the radiation field guiding optics to form an amplifier radiation field in which the branches entering the solid body and the branches emerging from the solid body are collimated branches. 
     In order respectively to form a collimated incident branch in turn from a collimated emerging branch, provision is preferably made for the radiation field guiding optics to be designed as at least singly recollimating. 
     In this case, “recollimating” means the conversion of a collimated radiation field via intermediate focusing into a collimated radiation field. 
     It is even better for the radiation field guiding optics to be designed as multiply recollimating, so that a plurality of collimated emerging branches can in turn be converted into a collimated incident branch. 
     During the formation of the incident branches and of the emerging branches, it is particularly favorable in terms of the formation of the amplifier radiation field for an intermediate-focused branch to be formed between the collimated emerging branch and the collimated incident branch by the radiation field guiding optics during each recollimation, that is to say when converting a collimated emerging branch into a collimated incident branch. This makes it possible to preserve the optical beam cross section in a particularly favorable way. 
     In terms of the interaction of the recollimation with the deviating unit, no detailed indications have been given so far. For instance, a particularly advantageous solution provides for the intermediate-focused branch required during the recollimation to pass respectively through a deviating unit according to the invention. 
     In principle, it would be conceivable to provide separate recollimating optics of the radiation field guiding optics for each recollimation. 
     Expediently, provision is made in this case for the intermediate-focused branch to travel along an optical path which corresponds to two times the focal length of the recollimation. 
     In terms of the design of the various recollimating optics, it would be conceivable to carry out different recollimations with different focal lengths. It is particularly favorable for all the recollimating optics to have the same focal length. 
     It is particularly favorable for a plurality of recollimating optics to be combined to form a radiation field shaping element. 
     A radiation field shaping element according to the invention, which causes at least one recollimation, may be designed as an element through which the amplifier radiation field passes, for example a lens system or, in the simplest case, a single lens. 
     As an alternative to this, it is also conceivable, however, to design the radiation field shaping element as a reflecting element. 
     In the simplest case, the radiation field shaping elements designed as a reflecting element is designed as a concave mirror. 
     Such a beam shaping element must, according to the invention, have a focusing element and a collimating element for each recollimation, so that conversion of a collimated emerging branch into the intermediate-focused branch and then conversion of the intermediate-focused branch in turn into a collimated incident branch is possible. 
     A solution in which a plurality of focusing elements and a plurality of collimating elements are combined to form a radiation field shaping element is particularly favorable. 
     It is particularly favorable in this case for this one radiation field shaping element to form collimating and focusing elements with different regions. 
     A particularly favorable embodiment of a radiation field shaping element provides for the radiation field shaping element to be designed rotationally symmetrically with respect to a mid-axis running through the solid body. 
     Such a rotationally symmetric design provides either a lens system which is designed and arranged rotationally symmetrically with respect to the mid-axis or a mirror system designed and acting rotationally symmetrically with respect to the mid-axis. 
     Such a mirror system is, in the simplest case, designed in such a way that the collimating and focusing elements are regions of a concave mirror designed rotationally symmetrically with respect to the mid-axis. 
     Such a concave mirror may, for example, be a parabolic mirror. It is also conceivable, however, to design this concave mirror as a toric mirror. 
     In terms of the number of deviating units, no detailed indications have been given in connection with the explanation of the individual exemplary embodiments so far. 
     For instance, an advantageous exemplary embodiment provides for the radiation field guiding optics to comprise at least two deviating units, each of these deviating units forming, from an input branch of the amplifier radiation field which is formed from one of the emerging branches, an output branch with a trajectory locally separate therefrom, from which the corresponding incident branch is formed. 
     In principle, it would be conceivable to provide a separate deviating unit for each incident branch to be formed from an emerging branch. 
     A solution designed in a particularly favorable way provides, however, for at least one of the deviating units to form, from at least two input branches formed from branches emerging from the solid body, at least two output branches from which the corresponding branches entering the solid body are formed, so that the number of deviating units can advantageously be reduced to two. 
     Furthermore, it is particularly favorable for the radiation field guiding optics to comprise two deviating units, and for an output branch of each of the deviating units to lead to the formation of a branch which enters the solid body, from which in turn, after is has passed through the solid body, an emerging branch is produced, from which an input branch of the respective other deviating unit is formed. 
     Such a solution permits, in a particularly favorable way, the two deviating units to be joined together, so that a particularly compact optical solution is obtained. 
     Further advantageous radiation field guiding optics according to the invention provide for them to comprise a first and a second deviating unit, and for the two deviating units, respectively by deviating the amplifier radiation field relative to a single deviating symmetry plane assigned to the respective deviating unit, to convert at least three input branches, formed from at least three different emerging branches of the amplifier radiation field, into at least three output branches which have trajectories correspondingly locally separate from the input branches and from which at least three incident branches are produced. 
     A further advantageous embodiment of the radiation field guiding optics according to the invention provides for the radiation field guiding optics to have at least one deviating unit, and for the deviating unit to form, from at least one input branch, an output branch which is offset in relation to a mid-axis of the radiation field guiding optics by an angular spacing such that at least one further input branch lies in the angle range between this input branch and the output branch formed therefrom. 
     A further particularly favorable solution provides for the radiation field guiding optics to have a first deviating unit, which deviates the amplifier radiation field relative to a first deviating symmetry plane, and to have a second deviating unit, which deviates the amplifier radiation field relative to a second deviating symmetry plane, and for the deviating symmetry planes to run at an angle with respect to one another, which preferably corresponds to 360° divided by the sum of the incident and emerging branches arising during a pass of the amplifier radiation field through the radiation field guiding optics and the solid body. 
     The term “pass of the amplifier radiation field” is in this case intended to mean propagation of the amplifier radiation field through the radiation field guiding optics, during which the propagation direction is preserved. 
     In terms of the arrangement of the deviating symmetry plane relative to the mid-axis, no detailed indications have been given so far. A particularly favorable solution provides for the deviating symmetry plane to run parallel to the mid-axis. 
     It is particularly favorable for the deviating symmetry plane to run through the mid-axis. 
     In terms of the arrangement of the input branches and the output branches of the deviating units, no detailed indications have been given so far. For instance, it is particularly favorable for the input branches of the amplifier radiation field to have trajectories spatially separate from one another. 
     In this case, it is particularly favorable for the input branches of the amplifier radiation field to be arranged relative to one another at angular spacings around the mid-axis of the radiation field guiding optics. 
     It is furthermore advantageous for the output branches to have separate trajectories from one another. 
     It is likewise favorable in this case for the output branches to have separate trajectories from the input branches. 
     It is particularly expedient for the output branches to be arranged relative to one another and relative to the input branches at angular spacings around the mid-axis of the radiation field guiding optics. 
     It is particularly advantageous in this case for the input branches and output branches produced during a pass of the amplifier radiation field through the radiation field guiding optics to be arranged without overlap in the space around the mid-axis of the radiation field guiding optics. 
     It is even more advantageous for the input branches and output branches, as well as an incident branch of the amplifier radiation field, during a pass to be arranged without overlap in the space around the mid-axis of the radiation field guiding optics. 
     A particularly advantageous solution provides for the input branches and output branches to be respectively arranged in separate space segments around the mid-axis of the radiation field guiding optics, and to extend inside the space segments transversely with respect to their propagation direction. 
     Preferably, the space segments are arranged in such a way that they stretch over approximately the same angle range around the mid-axis. 
     Particularly advantageous space utilization is obtained when the space segments of the input branches and of the output branches, as well as the space segment of the incident branch, essentially enclose the mid-axis. 
     Further features and advantages of the solution according to the invention are the subject matter of the following description and the graphical representation of a few exemplary embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic representation of a first exemplary embodiment of the laser amplifier system according to the invention in perspective; 
     FIG. 2 shows a section along the line  2 — 2  in FIG. 1; 
     FIG. 3 shows a representation of the laser amplifier system according to the invention according to FIG. 1, a first incident branch, a first emerging branch and a first intermediate-focused branch being represented of the amplifier radiation field; 
     FIG. 4 shows a section along the line  4 — 4  in FIG. 3; 
     FIG. 5 shows a representation of the laser amplifier system according to the invention according to FIG. 1, a second incident branch, a second emerging branch and a second intermediate-focused branch being represented of the amplifier radiation field; 
     FIG. 6 shows a section along the line  6 — 6  in FIG. 5; 
     FIG. 7 shows a representation of the laser amplifier system according to FIG. 1, a third incident branch, a third emerging branch and a third intermediate-focused branch being represented of the amplifier radiation field; 
     FIG. 8 shows a section along the line  8 — 8  in FIG. 7; 
     FIG. 9 shows a representation of the laser amplifier system according to FIG. 1, a fourth incident branch, a fourth emerging branch and a fourth intermediate-focused branch being represented of the amplifier radiation field; 
     FIG. 10 shows a section along the line  10 — 10  in FIG. 9; 
     FIG. 11 shows a representation of a second exemplary embodiment of a laser amplifier system according to the invention similar to FIG. 1; 
     FIG. 12 shows a section along the line  12 — 12  in FIG. 11; 
     FIG. 13 shows a representation of the laser amplifier system according to FIG. 11, the first incident branch, the first emerging branch and the first intermediate-focused branch being represented of the amplifier radiation field; 
     FIG. 14 shows a section along the line  14 — 14  in FIG. 13; 
     FIG. 15 shows a representation of the laser amplifier system according to FIG. 11, the second incident branch, the second emerging branch and the second intermediate-focused branch being represented of the amplifier radiation field; 
     FIG. 16 shows a section along the line  16 — 16  in FIG. 15; 
     FIG. 17 shows a representation of the laser amplifier system according to FIG. 11, the third incident branch, the third emerging branch and the third intermediate-focused branch being represented of the amplifier radiation field; 
     FIG. 18 shows a section along the line  18 — 18  in FIG. 17; 
     FIG. 19 shows a representation of the laser amplifier system according to FIG. 11, the fourth incident branch, the fourth emerging branch and the fourth intermediate-focused branch being represented of the amplifier radiation field; 
     FIG. 20 shows a section along the line  20 — 20  in FIG. 19; 
     FIG. 21 shows a similar representation to FIG. 1 of a third exemplary embodiment of the laser amplifier system according to the invention; 
     FIG. 22 shows a section along the line  22 — 22  in FIG. 21; 
     FIG. 23 shows a representation of the laser amplifier system corresponding to FIG. 21, the first incident branch, the first emerging branch, the first intermediate-focused branch and the second incident branch being represented of the amplifier radiation field; 
     FIG. 24 shows a section along the line  24 — 24  in FIG. 23; 
     FIG. 25 shows a representation of the laser amplifier system according to FIG. 21, the second emerging branch, the second intermediate-focused branch and the third incident branch being represented of the amplifier radiation field; 
     FIG. 26 shows a section along the line  26 — 26  in FIG. 25; 
     FIG. 27 shows a representation of the laser amplifier system in FIG. 21, the third emerging branch, the third intermediate-focused branch, the fourth incident branch and the fourth emerging branch being represented of the amplifier radiation field and 
     FIG. 28 shows a section along the line  28 — 28  in FIG.  27 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first exemplary embodiment of a laser amplifier system according to the invention, represented overall in FIGS. 1 and 2, comprises a laser-active medium in the form of a solid body  10 . 
     Such a laser-active medium is, for example, one such as described in EP 0 632 551. 
     The solid body  10  has a frontal flat side  12  and a rear flat side  14 , and rests via the rear flat side  14  on a reflector, denoted as a whole by  16 , which is for example a layer applied to the rear flat side  14 . 
     Furthermore, cooling of the solid body  10  is carried out via the rear reflector  16  by means of a cooling device  20 , the cooling device being, for example, a cold finger as likewise described in EP 0 632 551. 
     The two flat sides  12  and  14  of the solid body  10  need not necessarily be designed as planes, but may also, for example, have a curvature. 
     Pumping of the solid body  10  is carried out via a known pump device, for example likewise via a pump radiation field which may likewise be guided, for example, as in EP 0 632 551, although it is also conceivable to guide the pump radiation field, and make it enter the solid body  10 , in accordance with the German Patent Application 198 35 107 or 198 35 108. 
     For this reason, a pump radiation source  30  is represented merely schematically, which produces a pump radiation field  32  that enters the solid body  10  and is preferably focused onto the solid body  10 . 
     For optimum extraction of the coherent radiation produced by the laser-active medium, an amplifier radiation field, denoted as a whole by  40 , is provided which comprises a radiation field shaping element  42  as well as a first deviating unit  44  and a second deviating unit  46 . 
     The radiation field shaping element  42  as well as the deviating units  44  and  46  together form overall radiation field guiding optics, denoted as a whole by  48 , which determine the formation of the amplifier radiation field  40 . 
     The radiation field shaping element is, for example, an element which is capable of converting a collimated branch into a focused branch, or vice versa, that is to say focusing or collimating. 
     Such an exemplary embodiment of a radiation field shaping element  42  is, in the first exemplary embodiment, an element through which the radiation field passes, which may be formed by a lens or a lens system. 
     The first deviating unit  44  comprises an inner deviating prism  50  with a first reflecting face  52  and a second reflecting face  54 , which run in a wedge-shaped fashion with respect to one another and point away from one another, that is to say they face away from one another. The first deviating unit  44  furthermore comprises two outer deviating prisms  56  and  58 , which respectively have reflecting faces  60  and  62 ;  64  and  66  that run in a V-shape with respect to one another and face toward one another. 
     Preferably, the reflecting faces  60  and  66  run parallel to the reflecting faces  52  and  54  of the inner deviating prism  50 , and are arranged facing toward them so that the scattering reflected by one reflector face respectively strikes the reflector face opposite it. 
     The reflector faces  62  and  64  are furthermore arranged at an angle of 90° relative to the reflector faces  60  and  66 , respectively, and therefore also at an angle of 90° with respect to one another. In this case, by the reflector faces  62  and  64 , the radiation is respectively reflected by one of these reflector faces to the other, so long as this radiation comes from the allocated reflector face  62  or  66 , respectively. 
     Similarly, the second deviating unit  46  comprises an inner deviating prism  70  with two reflector faces  72  and  74  running at an angle of 90° with respect to one another and facing away from one another. 
     Furthermore, the second deviating unit  46  comprises two outer deviating prisms  76  and  78  respectively with reflector faces  80  and  82 ;  84  and  86  that run in a V-shape with respect to one another and face toward one another, the reflector face  80  running parallel to the reflector face  72  and the reflector face  86  running parallel to the reflector face  74 , and the reflector faces respectively running parallel to one another facing toward one another. 
     Furthermore, the reflector faces  82  and  84  run at an angle of 90° with respect to the reflector faces  80  and  86 , and furthermore at an angle of 90° relative to one another, and they are likewise oriented in such a way that one of the reflector faces  82 ,  84  reflects radiation to the other reflector face  84 ,  82 , so long as this radiation comes from the corresponding reflector face  80  or  86 , respectively, of the associated outer deviating unit  76  or  78 . 
     Overall, all the reflector faces  52 ,  54 ,  60 ,  62 ,  64 ,  66  of the first deviating unit  44  run perpendicular to a longitudinal symmetry plane  90 , and all the reflector faces  72 ,  74 ,  80 ,  82 ,  84 ,  86  of the second deviating unit  46  run perpendicular to a longitudinal symmetry plane  92  that meets the first symmetry plane  90  at an angle α which depends, as will be explained in detail later, on the number of collimated branches of the amplifier radiation field  40  and is equal to 360° divided by the number of collimated branches. 
     Furthermore, the longitudinal symmetry planes  90  and  92  run at a spacing from a mid-axis  94 , which intersects the solid body  10  and runs symmetrically with respect to the collimated branches of the amplifier radiation field  40 . 
     The reflector faces  52  and  54  of the inner deviating prism  50  are in this case mirror-symmetric with respect to a first deviating symmetry plane  96  of the first deviating unit  44 , and the reflector faces  72  and  74  are symmetric with respect to a second deviating symmetry plane  98  of the second deviating unit  46 , these deviating symmetry planes  96  and  98  running through the mid-axis  94  and intersecting in it. In this case, the deviating symmetry planes  96  and  98  likewise meet at the angle α. 
     Furthermore, the reflector faces  60  and  66  as well as  62  and  64  of the outer deviating prisms  56  and  58  are arranged symmetrically with respect to the first deviating symmetry plane  96 , so that all these reflector faces run at an angle of 45° with respect to the first deviating symmetry plane  96 . 
     Similarly, the reflector faces  80  and  86  as well as  82  and  84  of the outer deviating prisms  76  and  78  of the second deviating unit  46  are arranged symmetrically with respect to the second deviating symmetry plane  98  and therefore likewise all run at an angle of 45° with respect to it. 
     The construction of the amplifier radiation field  40  with the radiation field guiding optics  48  according to the invention is now represented in detail in FIGS. 3 to  10 . 
     The starting point for the formation of the amplifier radiation field  40  is a divergent branch  100 , which preferably runs parallel to the mid-axis  94  and is converted, by the radiation field shaping element  42 , into a first collimated incident branch  102   1  which enters the solid body  10 , specifically at an angle E with respect to the mid-axis  94  (FIG.  3 ). 
     After it has passed through the solid body  10 , a reflection takes place at the reflector  16 , so that a first collimated branch  104   1  emerging from the solid body  10  is produced, which runs at the angle A with respect to the mid-axis  94 , the angle A corresponding to the angle E. 
     This first collimated emerging branch  104   1  strikes the radiation field shaping element  42  and is converted by it into a first intermediate-focused branch  106   1 , which comprises a focused sub-branch  108   1  that, for its part, forms a so-called first input branch  142   1  by a section running parallel to the mid-axis  94 , and strikes the reflector face  52 , is reflected by it perpendicular to the deviating symmetry plane  96  onto the reflector face  60 , is reflected by the latter onto the reflector face  62  and in turn propagates perpendicular to the first deviating symmetry plane  96  in the direction of the reflector face  64  (FIG.  3 ). 
     Preferably, the radiation field shaping element  42  is in this case designed in such a way that an intermediate focus  110   1  of the focused sub-branch  108   1  of the intermediate-focused branch  106   1  lies in the deviating symmetry plane  96 , and therefore a first divergent sub-branch  112   1  propagates out from the focus  110   1  starting from the deviating symmetry plane  96 , specifically perpendicular to it, strikes the reflector face  64  and is reflected by it to the reflector face  66  and is then in turn reflected by the latter to the reflector face  54 , which deviates this divergent sub-branch  112   1  of the first intermediate-focused branch  106   1  to the radiation field shaping element  42 , specifically in such a way that it forms a section oriented parallel to the mid-axis  94 , which represents a so-called first output branch  144  and with it strikes the radiation field shaping element  42 . 
     Overall, the intermediate-focused branch  106   1  runs in a plane  114  parallel to, but at a spacing from, the longitudinal symmetry plane  90  and symmetrically with respect to the deviating symmetry plane  96  through the first deviating unit  44  (FIG.  4 ). 
     The radiation field shaping element  42  then forms, from the first intermediate-focused branch  106   1 , a second collimated incident branch  102   2  which strikes the solid body  10  and passes through it, so that a second collimated emerging branch  104   2  is formed by the reflector  16  (FIG.  5 ). 
     This collimated emerging branch  104   2  strikes the radiation field shaping element  42  and is converted by it into a second intermediate-focused branch  106   2  which, with its second input branch  142   2 , runs parallel to the mid-axis  94  and in this case strikes the reflection face  72  as a focused sub-branch  108   2 , which is reflected by the reflection face  72 , by the reflection face  80  and by the reflection face  82 , and forms an intermediate focus  110   2  which lies in the deviating symmetry plane  98  of the second deviating unit  46 . Starting from the intermediate focus  110   2 , the intermediate-focused branch  106   2  propagates as a divergent sub-branch  112   2  in the direction of the reflection face  84 , is reflected by it to the reflection face  86  and then by the latter to the reflection face  74 , so that the divergent sub-branch  112   2  in turn strikes the beam shaping element  42  as a second output branch  144   2  parallel to the mid-axis  94 . 
     In this case, the second intermediate-focused branch runs overall in a plane  116 , which is parallel to but at a spacing from the longitudinal symmetry plane  92 , through the second deviating unit  46  and is furthermore symmetric with respect to the deviating symmetry plane  98  (FIG.  6 ). 
     From this second output branch  1442 , the radiation field shaping element  42  in turn forms a third collimated incident branch  102   3 , which enters the solid body  10  and from which, by reflection at the reflector  16 , the third collimated emerging branch  104   3  is formed which in turn strikes the radiation field shaping element  42 . The radiation field shaping element  42  forms, from the third collimated emerging branch  104   3 , a third intermediate-focused branch  106   3  which, as a third input branch  142   3  and as a focused sub-branch  108   3 , strikes the reflector face  54 , is reflected by it to the reflector face  66  and by the reflector face  66  to the reflector face  64 , and propagates as far as an intermediate focus  110   3  which in turn lies in the deviating symmetry plane  96 , as represented in FIG.  7 . 
     Starting from the intermediate focus  110   3 , a divergent sub-branch  110   3  then propagates in the direction of the reflector face  62 , and from the latter in the direction of the reflector face  60 , and it then strikes the reflector face  52  and is in turn reflected by the latter parallel to the mid-axis  94  and forms the third output branch  144   3 . 
     The third intermediate-focused branch  106   3  also runs in a plane  118 , which is parallel to but at a spacing from the longitudinal symmetry plane  90 , and therefore also parallel to the plane  114 , although it is not congruent with the plane  114  (FIG.  8 ). 
     Furthermore, the third intermediate-focused branch  106   3  likewise runs symmetrically with respect to the deviating symmetry plane  96 . 
     Preferably, the planes  114  and  118  are symmetric with respect to the longitudinal symmetry plane  90 . 
     From the third intermediate-focused branch  106   3 , the radiation field shaping element  42  forms, as represented in FIG. 9, the fourth collimated incident branch  102   4  which enters the solid body  10  and from which, by the reflector  16 , the fourth collimated emerging branch  104   4  is formed which in turn strikes the radiation field shaping element  42  and from which the radiation field shaping element  42  forms a fourth intermediate-focused branch  106   4  which, as represented in FIGS. 9 and 10, firstly strikes the reflector face  74  in the form of a focused sub-branch  108   4 , and is reflected by it to the reflector face  86  and then to the reflector face  84 , with a focus  110   4  in turn lying in the deviating symmetry plane  98 . 
     Starting from the focus  110   4 , a divergent branch  112   4  is formed which strikes the reflector face  82 , the reflector face  80  and then the reflector face  72 . Therefore, the fourth intermediate-focused branch  106   4  likewise runs overall in a plane  120 , which runs parallel to the longitudinal symmetry plane  92  and therefore also parallel to the plane  116 , but does not coincide with the plane  116  (FIG.  10 ). 
     Preferably, the planes  116  and  120  are symmetric with respect to the longitudinal symmetry plane  92  (FIG.  2 ). 
     The fourth intermediate-focused branch  106   4 , starting from its fourth input branch  142   4 , likewise runs essentially symmetrically with respect to the deviating symmetry plane  98 , although, departing from complete symmetry, not as far as the radiation field shaping element  42 , but rather it strikes with the divergent sub-branch  112   4 , with its fourth output branch  144   4  running parallel to the mid-axis  94 , an extraction mirror  129  which deviates the divergent branch  112   4  transversely with respect to the mid-axis  94  and makes it emerge from the radiation field guiding optics  48  as an extracted branch, as represented in FIGS. 9 and 10; this may also, for example, enter a further laser amplifier system as a divergent branch. Upon reaching the extraction mirror  129 , a pass of the amplifier radiation field  40  through the radiation field guiding optics  48  is completed. 
     Overall, as represented in FIG. 2, all the collimated branches  102  and  104  of the amplifier radiation field  40  lie respectively in individual space segments  130   1  to  130   8  around the mid-axis  94 , with all the space segments  130  stretching over the same angular spacing around the mid-axis  94 . 
     Furthermore, the collimated branches propagating in the space segments  130   2  to  130   4  interact with the second deviating unit  46 , while the collimated branches propagating in the space segments  130   5  to  130   8  interact with the first deviating unit  44 . 
     Both the first deviating unit  44  and the second deviating unit  46  lead, in the case of each intermediate-focused branch  106 , to the formation of a loop  140  whose input branch  142  and whose output branch  144  have a spacing AB, while the loop  140  has an extent AU in at least one expansion direction EX which is greater than the spacing AB, preferably equal to at least two times the spacing AB. 
     The effect achieved by this is that the mirror faces lying on either side of the respective intermediate focus  110  of the intermediate-focused branch  106 , for example the mirror faces  62  and  64  or the mirror faces  82  and  84 , have a mutual spacing corresponding roughly to the extent AU of the loop  140 , the spacing preferably being equal to half of the extent AU, so that the reflection faces  62  and  64 ;  82  and  84  arranged closest to the respective focus  110  are placed as far as possible away from the focus, in order to obtain a beam cross section which is as large as possible, and therefore an intensity per unit area of the beam cross section which is as small as possible, on the respective reflector faces  62  and  64 ;  82  and  84 , so that it is possible to avoid damage to the reflector faces  62  and  64 ;  82  and  84  due to excessive intensity per unit area of the beam cross section. 
     Furthermore, the deviating units  44  and  46  are designed in such a way that, starting from the radiation field shaping element  42 , the optical path in each of the loops  140   1  to  140   3 , which in turn respectively lead back to the radiation field shaping element  42 , is of equal size so that, in the simplest case, the radiation field shaping element  42  can convert one of the collimated branches  102 ,  104  into one of the intermediate-focused branches  106 , or vice versa, in all the space segments  130   1  to  130   8  with the same focal length. 
     If parasitic modes are intended to be avoided, then space filters, for example in the form of shutter diaphragms, will preferably be allocated to one or more intermediate foci  110 . 
     In a second exemplary embodiment of a laser amplifier system according to the invention, represented in FIGS. 11 and 12, those elements which are identical to the ones in the first exemplary embodiment are provided with the same reference numbers, so that comprehensive reference can be made to the comments relating to the first exemplary embodiment. 
     In particular, the deviating units  44  and  46  are arranged in the same way relative to the radiation field shaping element  42  as in the first exemplary embodiment. 
     In contrast to the first exemplary embodiment, the amplifier radiation field does not pass through the radiation field shaping element  42 , but rather the radiation field shaping element  42  is designed as a reflecting element, for example as a concave mirror, which may have either parabolic reflection faces or toric reflection faces in cross section. 
     Therefore, the loops  140   1  to  140   3  passing through the deviating units lie on the same side of the radiation field shaping element  42  as the individual collimated branches  102  and  104 . 
     The consequence of this is that the inner deviating prisms  50 ′ and  70 ′ are provided, in relation to the mid-axis  94 , with a circular recess  51  and  71 ′, respectively, which permit unimpeded through-passage of the collimated branches  102 ,  104 , the recesses  51  and  71  furthermore being dimensioned in such a way that total reflection of the sections of the intermediate-focused branches  106  propagating parallel to the mid-axis  94  always takes place at the reflection faces  52  and  54 ;  72  and  74 . 
     This means that the radius of the recesses  51  and  71  in relation to the mid-axis is smaller than the spacing, from the mid-axis  94 , of the sections of the intermediate-focused branches  106  running parallel to the mid-axis  94 . 
     In other regards, the construction of the amplifier radiation field  40  in the second exemplary embodiment takes place in the same way as in the first exemplary embodiment, as can be seen from FIGS. 13 to  20 . 
     In a third exemplary embodiment of the laser amplifier system according to the invention, represented in FIGS. 21 and 22, the solid body  10  is not provided with a reflector, but rather it is arranged in the amplifier radiation field  40 ′ in such a way that this can pass through the solid body  10 . 
     Furthermore, in the through-radiation direction of the solid body  10 , radiation field shaping elements  42   a  and  42   b  are arranged on either side thereof, each of which is capable of converting a collimated branch  102  or  104  into an intermediate-focused branch  106 , and vice versa. 
     In the simplest case, the radiation field shaping elements  42   a  and  42   b  are designed as identical concave mirrors. 
     Furthermore, the first deviating unit  44  is arranged on one side of the solid body  10  and is used to expand those intermediate-focused branches  106  which are produced by the radiation field shaping element  42   b , while the second deviating unit  46  is arranged on the opposite side of the solid body  10  and is used to expand the intermediate-focused branches  106  produced by the radiation field shaping element  42   a.    
     In principle, however, the construction of the amplifier radiation field takes place in the same way as in the second exemplary embodiment, although with the difference that radiation respectively takes place through the solid body. 
     The construction of the amplifier radiation field  40 ′ is represented in detail in FIGS. 23 to  28 . 
     For instance, the formation of the first collimated incident branch  102   1 , which enters the solid body  10  and passes through it, from the incident branch  100  takes place by means of the radiation field shaping element  42   a . The first collimated emerging branch  104   1 , propagating from the solid body  10  in the same direction as the incident branch  102   1 , in this case strikes the radiation field shaping element  42   b  that forms the first intermediate-focused branch  106   1 , which strikes the reflecting face  52  of the inner deviating prism  50 ′, and is deviated by it to the reflecting face  60  and then to the reflecting face  62  of the first deviating unit  44  of the outer deviating prism  56 , the focused sub-branch  108   1  forming the focus  110   1 , starting from which the diverging sub-branch  112   1  of the intermediate-focused branch  106   1  propagates in the direction of the outer deviating prism  58  of the first deviating unit  44 , and then is reflected by the reflecting faces  64  and  66  in such a way that it in turn strikes the reflecting face  54 , which in turn deviates the intermediate-focused branch  106   1  in the direction of the radiation field shaping element  42   b  which, for its part, in turn forms the second collimated incident branch  102   2  by reflection (FIGS. 23,  24 ). 
     After transmission through the solid body  10 , the second collimated emerging branch  104   2  is formed, as represented in FIGS. 25,  26 , which strikes the radiation field shaping element  42   a  that, for its part, in turn forms the second intermediate-collimated branch  106   2 , which strikes the reflecting face  72  of the inner deviating prism  70  of the second deviating unit  46 , is reflected by it to the reflecting face  80  and then to the reflecting face  82  of the outer deviating prism  76 , so that the focused sub-branch  108   2  finally forms the intermediate focus  110   2 , starting from which the divergent sub-branch  112   2  propagates in the direction of the outer deviating prism  78 , is reflected by the reflection faces  84  and  86  and finally strikes the reflection face  74  of the inner deviating prism  70 ′, in order to be deviated by it in the direction of the radiation field shaping element  42   a.    
     The inner deviating prism  70 ′ produces the third incident branch  102   3 , which in turn gives rise to the third collimated emerging branch  104   3  that strikes the radiation field shaping element  42   b , which in turn produces the third intermediate-focused branch  106   3  that, after reflection at the reflection face  66  and the reflection face  64 , produces the intermediate focus  110   3  with the focused sub-branch  108   3 , so that the in turn resulting divergent sub-branch  112   3  strikes the outer deviating unit  56  and, after reflection at the reflection face  62  and the reflection face  60 , in turn strikes the reflection face  52  of the inner deviating prism  50 ′, which deviates the third intermediate-focused branch  106   3  onto the radiation field shaping element  42   b  that, from this third focused branch, as represented in FIGS. 27,  28 , produces the fourth collimated incident branch  102   4  which becomes the fourth collimated emerging branch  104   4  after having passed through the solid body  10 . This does not then strike the radiation field shaping element  42   a , but rather it can be directly deviated through an extraction mirror  130 ′ and form the extracted beam  132 ′. 
     It would, however, also be possible to make the fourth emerging branch  104   4  strike the radiation field shaping element  42   a , and to extract it after the formation of a fourth intermediate-focused branch  106   4 . 
     In the third exemplary embodiment as well, guiding of the intermediate-focused branches  106  in loops  140  and expansion thereof relative to the respective deviating symmetry plane  96  or  98 , takes place in the deviating units  44  and  46  in the same way as in the first and second exemplary embodiments. 
     Wherever the same reference numbers are used in the second and third exemplary embodiments, and no other description of the various elements is given, comprehensive reference is made to the description relating to the first exemplary embodiment.
The invention relates to a laser amplifier system consisting of a solid body, which comprises a laser-active medium, of an excitation source for producing an excited state of the laser-active medium, and of an amplifier radiation field, which repeatedly permeates the solid body and out of which a laser beam can be decoupled. The aim of the invention is to improve a laser-amplifier system of this type so that the highest number of passages of the amplifier radiation field through the solid body can be attained using optical means that are provided in the most simple possible form. To this end, the invention provides radiation field guiding optics which enable the amplifier radiation field to enter the solid body in the form of a number of incident branches with locally different trajectories, and which enable the amplifier radiation field to exit the solid body in the form of at least one emerging branch with a trajectory that differs locally from those of the incident branches. In addition, the radiation field guiding optics comprise at least one deviating unit which, out of at least one of the branches emerging from the solid body, forms a branch which enters the solid body and which has a trajectory that differs locally from that of said emerging branch.