Abstract:
In order to improve a solid-state laser, in particular a solid-state disc laser, comprising a resonator ( 40 ) that defines a resonator radiation field ( 30 ) and at least one solid-state disc ( 12 ) with the resonator radiation field ( 30 ) passing through it, in such a manner that the thermal lens effect can be at least substantially compensated, it is proposed that in reflection the resonator radiation field ( 30 ) strikes at least one first adaptive mirror unit ( 50, 70 ), with which a distortion of the resonator radiation field ( 30 ) as a result of a thermal lens effect of the at least one solid-state disc ( 12 ) can be substantially compensated. An adaptive mirror unit ( 50 ) can be configured by a heated ( 58   a,    58   b ) glass sheet ( 54 ) with an HR layer ( 52 ), for example, or by a pressure-induced deformation by means a fluid ( 78 ) in a space ( 76 ), which is enclosed with the mirror ( 72, 74 ).

Description:
[0001]    This application is a continuation of international application number PCT/EP2009/050448 filed on Jan. 15, 2009 and claims the benefit of German Patent Application No. 10 2008 008 078.0 filed on Jan. 28, 2008. 
         [0002]    The present disclosure relates to the subject matter disclosed in international application number PCT/EP2009/050448 of Jan. 15, 2009 and German application number 10 2008 008 078.0 of Jan. 28, 2008, which are incorporated herein by reference in their entirety and for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The invention relates to a solid-state laser, in particular a solid-state disc laser, comprising a resonator that defines a resonator radiation field and at least one solid-state disc with the resonator radiation field passing through it. 
         [0004]    Such solid-state lasers are known from the prior art, for example, European patent applications 0 632 551 A, 0 869 591 A, 0 869 592 A, 1 453 157 A. 
         [0005]    There is the problem with these that while at low optical power levels the thermal lens effect of the solid-state disc is not of great consequence, the thermal lens effect poses problems at high power levels. 
         [0006]    Therefore, the object forming the basis of the invention is to improve a solid-state laser of the above-mentioned type in such a manner that the thermal lens effect can be at least substantially compensated. 
       SUMMARY OF THE INVENTION 
       [0007]    This object is achieved according to the invention with a solid-state laser of the above-described type in that in reflection the resonator radiation field strikes at least one first adaptive mirror unit, with which a distortion of the resonator radiation field as a result of a thermal lens effect of the at least one solid-state disc can be substantially compensated. 
         [0008]    The advantage of the solution according to the invention lies in that it is thus possible to substantially compensate the distortion of the resonator radiation field as a result of the thermal lens effect or compensate it to some degree, and to thus avert the negative effects on the resonator radiation field and the beam quality. 
         [0009]    It is particularly favourable in this case if the distortions of the resonator radiation field as a result of the thermal lens effect of a maximum of two solid-state discs can be substantially compensated with the first adaptive mirror unit. 
         [0010]    It is thus possible to keep the number of the first adaptive mirror units as low as possible. 
         [0011]    A wide variety of solutions are conceivable with respect to the configuration of the first adaptive mirror unit. 
         [0012]    A particularly advantageous solution provides that the first adaptive mirror unit is a thermally adaptive mirror unit, the reflection behaviour of which is thermally adjustable. 
         [0013]    Such a thermal adjustability of the reflection behaviour is possible in this case in a wide variety of ways. 
         [0014]    One possibility provides achieving the thermal adjustability as a result of thermal expansions in the mirror unit by means of heating coils that can be heated electrically, for example. 
         [0015]    Another possibility lies in adjusting the thermally adaptive mirror unit in such a manner that the different local thermal heating occurs as a result of an incident radiation field, e.g. infrared radiation. 
         [0016]    Such an incident radiation field can be provided with any desired patterns in accordance with known possibilities for radiation field shaping, wherein a pattern effects a varying intensity progression of the radiation field, so that any desired shapes of heated regions can be achieved that lead to corresponding desired shapes of thermally expanded regions of material and therefore forwardly curved regions of the reflector surface. 
         [0017]    It is particularly favourable in this case if non-spherical distortions of the resonator radiation field can be corrected with the first adaptive mirror unit. 
         [0018]    A further advantageous solution provides that in the resonator radiation field a second adaptive mirror unit is provided, with which distortions of the resonator radiation field occurring as a result of the thermal lens effect of the at least one solid-state disc can be compensated. 
         [0019]    It is particularly favourable in this case if the distortions as a result of thermal lens effect of a maximum of two solid-state discs can be compensated with the second adaptive mirror unit. 
         [0020]    In particular, it is expedient in this case if spherical distortions of the laser radiation field can be compensated with the second adaptive mirror unit. 
         [0021]    In principle, the second adaptive mirror unit could likewise be configured as a thermally adaptive mirror unit. 
         [0022]    However, spherical distortions can be compensated in a particularly simple manner if the second adaptive mirror unit is configured as a hydrostatically adaptive mirror. 
         [0023]    In such a hydrostatically configured mirror the curvature of a plate is variably adjustable, wherein the curvature is adjusted by means of a hydrostatic pressure of a fluid arranged on one side of the plate. 
         [0024]    No precise details have been given so far with respect to the arrangement of the solid-state disc. 
         [0025]    Thus, an advantageous solution provides that the solid-state disc is arranged on a reflector and in reflection has the resonator radiation field passing through it. 
         [0026]    It is particularly favourable in this case if the at least one solid-state disc is arranged with the reflector on a cooling body. 
         [0027]    To enable a particular compensation to occur, the at least one solid-state disc is combined with at least one adaptive mirror unit to form an amplification module, which can be positioned in the resonator as a unit. 
         [0028]    Such an amplification module can thus be optimally adjusted with respect to the compensation and then adjusted in relation to the resonator. 
         [0029]    A particularly favourable solution provides that the at least one solid-state disc is combined with a first adaptive mirror unit for the correction of non-spherical distortions and with a second adaptive mirror unit for the correction of spherical distortions to form an amplification module, wherein in each amplification module, there are compensated the distortions of the resonator radiation field by the at least one solid-state disc provided in the amplification module. 
         [0030]    It is particularly expedient if two solid-state discs are combined with a first adaptive mirror unit and a second adaptive mirror unit to form an amplification unit. 
         [0031]    A laser according to the invention can be assembled in a particularly advantageous manner, preferably for high power levels, if the resonator radiation field passes through a plurality of such amplification modules. 
         [0032]    No precise details have been given so far with respect to the configuration of the resonator. 
         [0033]    Thus, it would be conceivable, for example, to provide a stable resonator. 
         [0034]    However, a stable resonator has disadvantages with respect to the size of the structure and with respect to decoupling at high power levels, since decoupling over a partially reflective reflector is problematic because of the heating thereof. 
         [0035]    For this reason an advantageous solution provides that the resonator is configured as an unstable resonator, since an unstable resonator provides the possibility of decoupling a radially outwardly located sub-region of the resonator radiation field by means of complete reflectors or complete transmission. 
         [0036]    It is particularly favourable in this case if the resonator is a confocal unstable resonator, since this has particularly advantageous optical properties and a particularly advantageous coupling of the modes. 
         [0037]    Moreover, it is advantageous, particularly in association with the solid-state disc, if the resonator is configured rotationally symmetrically to an optical axis of the resonator radiation field. 
         [0038]    In this case, decoupling preferably occurs on the basis that a region of the resonator radiation field located radially outwardly in relation to the optical axis can be decoupled from the unstable resonator. 
         [0039]    In this case, the radially outwardly located region preferably has the shape of an annular segment relative to the optical axis. 
         [0040]    In this case, the annular segment could merely cover a small angle range. 
         [0041]    An advantageous solution provides that the ring segment extends over approximately 180°. 
         [0042]    It is particularly favourable in this case if the decoupled radiation field is decoupled by means of a reflective output mirror. 
         [0043]    Further features and advantages of the invention are the subject of the following description and also the drawing representing some exemplary embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]      FIG. 1  is a schematic representation of a first exemplary embodiment of a solid-state laser according to the invention; 
           [0045]      FIG. 2  is a schematic sectional view through a first adaptive mirror unit; 
           [0046]      FIG. 3  is a plan view in the direction of arrow A in  FIG. 2 ; 
           [0047]      FIG. 4  is a sectional view similar to  FIG. 2  through a second exemplary embodiment of a first adaptive mirror unit; 
           [0048]      FIG. 5  is a plan view in the direction of arrow B in  FIG. 4 ; 
           [0049]      FIG. 6  is a sectional view similar to  FIG. 2  through a second adaptive mirror unit; 
           [0050]      FIG. 7  is a plan view in the direction of arrow C in  FIG. 6 ; 
           [0051]      FIG. 8  is a schematic representation of a second exemplary embodiment of a solid-state laser according to the invention; and 
           [0052]      FIG. 9  is a schematic representation of a third exemplary embodiment of a solid-state laser according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0053]    A first exemplary embodiment of a solid-state laser according to the invention shown in  FIG. 1  comprises an amplification element, given the overall reference  10 , which has a solid-state disc  12 , which is in turn arranged on a reflector  14 , e.g. a reflector layer  14 , wherein the reflector layer  14  is in turn arranged on a cooling body  16 , which is connected to the reflector layer  14  in a thermally conductive manner. The reflector layer  14  in turn allows a conduction of heat between the solid-state disc  12  and the cooling element, so that the solid-state disc  12  is cooled over its surface. 
         [0054]    Such an amplification unit  10  is described in detail, for example, in European patent applications 0 632 551 A, 0 869 591 A, 0 869 592 A, 1 453 157 A. 
         [0055]    The solid-state disc  12  constitutes a laser active medium, preferably a material that is also described, for example, in European patent applications 0 632 551 A, 0 869 591 A, 0 869 592 A, 1 453 157 A. 
         [0056]    The laser active medium in the solid-state disc  12  is preferably stimulated by pumped light exiting from a pumped light source  20  that strikes the solid-state disc  12  as pumped light radiation field  22 , i.e. from a side opposite the reflector layer  14 , and penetrates into the solid-state disc  12  and thus optically excites the laser active medium, wherein the reflector layer  14  preferably also simultaneously causes a reflection of the pumped light radiation field  22 , so that a multipass of the pumped light radiation field  22  through the solid-state disc and therefore an improved optical excitation thereof occurs, wherein the multipass of the pumped light radiation field  22  is not restricted to a double pass, but can have a higher number of passes, wherein a suitable guidance of the pumped light radiation field  22  is necessary for this, which is also described in European patent applications 0 632 551 A, 0 869 591 A, 0 869 592 A, 1 453 157 A. 
         [0057]    A resonator radiation field  30  strikes such an amplification unit  10  that is preferably configured rotationally symmetrically to an optical axis  32 , so that the optical axis  32  meets the solid-state disc  12  at an acute angle φ relative to a normal  34  on said solid-state disc, penetrates into the solid-state disc  12  and is reflected by means of the reflector layer  14 , and the resonator radiation field  30  exits again from the solid-state disc  12  with a path of the optical axis  32  at an angle −φ to the normal  34 . 
         [0058]    In this case, the resonator radiation field  30  is defined by a resonator with resonator mirrors  42  and  44 , wherein the resonator  40  is configured as an unstable resonator and the resonator mirrors  42  therefore constitute concave mirrors. 
         [0059]    The unstable resonator  40  is preferably configured as a confocal unstable resonator  40 . 
         [0060]    In the case of this confocal resonator  40  either one of the resonator mirrors  42  and  44  is configured as a sub-mirror that does not reflect the resonator radiation field  30  in an outer sub-region  48  representing a ring segment, but is transparent for this, as described in European patent application EP 1 566 866 in association with  FIGS. 1 and 2 , for example, or a so-called scraper mirror  46  is provided, which, as likewise described in European patent application EP 1 566 866 in association with  FIGS. 3 and 4 , decouples the ring segment from the unstable resonator  40  as outer sub-region  48  in relation to the optical axis  32 . 
         [0061]    As described in European patent applications 0 632 551 A, 0 869 591 A, 0 869 592 A, 1 453 157 A, the disc-shaped solid body  12  preferably produces only a small thermal lens, which at low power levels leads to a slight distortion of the resonator radiation field  30 , but is considerable at high optical power levels. 
         [0062]    To compensate such distortions, directly following the amplifier unit  10  there is provided a first adaptive mirror unit  50  that bears a highly reflective coating  52 , which reflects a branch  62  of the resonator radiation field  30  leading to the resonator mirror  42  and in reflection couples with a branch  64  of the resonator radiation field that is incident into the solid-state disc  12  at the angle φ to the normal  34  and is reflected on the reflector layer  14  generating an emergent branch  66  coupled thereto, which strikes a second adaptive mirror unit  70  with a highly reflective coating  72 , wherein the highly reflective coating  72  couples the branch  66  of the resonator radiation field  30  with a branch  68  of the resonator radiation field  20  that strikes the resonator mirror  44 , for example. 
         [0063]    Therefore, with the two adaptive mirror units  50  and  70  there is the possibility of compensating distortions of the resonator radiation field  30  that occur as a result of a thermal lens forming in the solid-state disc  12 . 
         [0064]    It is particularly advantageous for compensation of the distortions of the resonator radiation field  30  if the length of the branches  64  and  66  between the adaptive mirror units  50 ,  70  is as small as possible in relation to the branch  62  between the resonator mirror  42 , close to which decoupling of the region  48  of the resonator radiation field occurs, and the closest adaptive mirror unit  50  or the amplification unit  10 . 
         [0065]    The branches  64 ,  66  of the resonator radiation field  30  are preferably shorter than branch  62  by a factor of more than three, better a factor of more than five and even better a factor of more than ten. 
         [0066]    In addition, the amplification unit  10  and the adaptive mirror units  50 ,  70  are expediently arranged close to the resonator mirror  44 , at which no decoupling occurs, so that the branch  68  between this resonator mirror  44  and the closest adaptive mirror unit  70  or the amplifier unit is significantly shorter than branch  62  of the resonator radiation field. 
         [0067]    For example, the first adaptive mirror unit  50  comprises a plate that bears the highly reflective coating  52  and is composed of a thermally expanding material, e.g. a glass plate  54 , which is arranged on a cooled support  56 , wherein the glass plate  54  can be heated by heating elements  58   a  and  58   b  that are annular around a centre point M, wherein ring-shaped heated regions  59   a  and  59   b  in the plate  54  occur as a result of this, which in the state of thermal equilibrium cause a forward curvature of the plate  54  in these areas  59   a  and  59   b  because of the thermal expansion of the material, so that the highly reflective coating  52  in these regions  59   a  and  59   b  deviates from a plane form and in the regions  59   a  and  59   b  that are annular around the centre point M, for example, is raised in relation to the remaining surface. With these forwardly curved reflective regions  59   a  and  59   b  there is the possibility of compensating non-spherical distortions of the resonator radiation field  30 . 
         [0068]    Alternatively to the first adaptive mirror unit  50 , a further exemplary embodiment of a first adaptive mirror unit  50 ′ also provides a plate  54  with a highly reflective coating  52  that is arranged on the cooled support  56 . However, a locally varying heating of the plate  54  in the regions  59   a  and  59   b  occurs to make this raised as a result of a radiation field  58 , which has such a wavelength, e.g. in the infrared, that this can pass through the highly reflective coating  52  for the resonator radiation field  30  and is absorbed by the plate  54  in its interior, wherein the radiation field  58  has local variations in intensity, so that a locally different heating of the plate  54  also occurs, for example, in the regions  59   a  and  59   b , i.e. preferably in annular shape, so that the highly reflective coating in the regions  59   a  and  59   b  behaves such that it deviates from a plane surface in the same way and therefore likewise serves to compensate non-spherical distortions of the resonator radiation field  30  triggered by the solid-state disc  12 . 
         [0069]    As shown in  FIGS. 6 and 7 , the second adaptive mirror unit  70  is likewise provided with a highly reflective coating  72 , which is arranged on a plate  74 , e.g. also a glass plate, wherein the plate  74  encloses a space  76 , in which a fluid  78  is present. For example, the space  76  is delimited by a housing  80 , in which the plate  74  is held and which extends on one side of the plate  74 . As a result of a pressure change of the fluid  78  there is the possibility of curving the plate  74  concavely or convexly, so that the highly reflective coating  72  thus extends in an either slightly-convex or slightly concave surface, preferably also rotationally symmetrically to a centre point M, and thus provides the possibility of correcting spherical distortions of the resonator radiation field  30 . 
         [0070]    Moreover, such a second adaptive mirror unit  70  has the advantage that corrections are possible on a larger scale than with the first adaptive mirror units  50  or  50 ′, although only insofar as these corrections lead to spherical distortions of the resonator radiation field  30 . 
         [0071]    Therefore, in the first exemplary embodiment of the solid body of the invention according to  FIG. 1  there is the possibility of compensating non-spherical distortions of the resonator radiation field  30  with the first adaptive mirror unit  50 , on the one hand, and of compensating spherical distortions of the resonator radiation field  30  with the second adaptive mirror unit  70 , on the other. 
         [0072]    Both the first adaptive mirror unit  50  and the second adaptive mirror unit  70  can be actuated by a control means given the overall reference  90 , wherein the control means  90  enables control of the degree of compensation of the distortions of the resonator radiation field  30 , which are dependent on the power of the resonator radiation field  30 , so that in adaptation to the power of the resonator radiation field  30  the distortions triggered by the thermal lens effect in the solid-state disc  12  can be respectively corrected at least substantially, preferably completely, and therefore it is altogether possible to use the amplifier unit  10  with the solid-state disc  12 , which develops a thermal lens effect, in the unstable resonator  40  and make use of the advantages of the unstable resonator  40 , which consist of providing this with an extremely compact structure and, in particular in the case of high power levels, enabling an advantageous decoupling by means of the output mirror  46  or a partial resonator mirror  42 , which only decouples a part-segment  48  of the resonator radiation field  30 , but nevertheless is either completely transparent for this or completely reflects this, so that the problems that occur with partially transparent mirrors in the case of stable resonators can be avoided. 
         [0073]    In a second exemplary embodiment of a solid-state laser according to the invention illustrated in  FIG. 8 , two amplification units  10   a  and  10   b  provided with a first adaptive mirror unit  50  and a second adaptive mirror unit  70 , are provided in the resonator radiation field  30  of the unstable resonator  44 , wherein the first adaptive mirror unit  50 , for example, is arranged so that, in the same manner as in the first exemplary embodiment, it couples branch  62  of the resonator radiation field  30  with branch  64 , which is then incident on the solid-state disc  12   a , whereas the second adaptive mirror unit  70  couples branch  66  with branch  69  of the resonator radiation field  30 , wherein branch  69  is then incident on the solid-state disc  12   b , the reflector layer  14   b  of which couples branch  69  with branch  68  of the resonator radiation field  30 , which then itself strikes the resonator mirror  44 , for example. 
         [0074]    In this exemplary embodiment the control means  90  controls the first adaptive mirror unit  50  as well as the second adaptive mirror unit  70  in such a manner that these two jointly substantially compensate the distortions of the resonator radiation field  30 , which are generated by the thermal lens effects of the solid-state discs  12   a  and  12   b , so that the thermal lens effects of two solid-state discs  12   a  and  12   b  can be compensated overall by two adaptive mirror units, namely the first adaptive mirror unit  50  and the second adaptive mirror unit  70 . 
         [0075]    Therefore, even with the provision of only two adaptive mirror units  50  and  70  it is possible to use two amplification units  10   a  and  10   b  with solid-state discs  12   a  and  12   b  in the unstable resonator  40 . 
         [0076]    The adaptive mirror units  50  and  70  as well as the amplification units  10   a  and  10   b  are preferably combined to form a compensated amplification module  100 , which acts in a distortion-free manner on the resonator radiation field  30  because of the compensation of the thermal lens effects. 
         [0077]    In a third exemplary embodiment shown in  FIG. 9 , a plurality of such amplification modules  100  described in the second exemplary embodiment according to  FIG. 8  are provided in the resonator radiation field  30 , namely amplification modules  100   a ,  100   b  and  100   c , which overall act in a distortion-neutral manner on the resonator radiation field  30 , so that the number of amplification modules  100  in the resonator radiation field  30  can be increased without the resonator radiation field  30  being subject to such large distortions that the resonator radiation field  30  can no longer be defined by the unstable resonator  40  with the resonator mirrors  42  and  44 . 
         [0078]    There is thus the possibility of increasing the power of the solid-state laser according to the third exemplary embodiment in a scalable manner according to the first or second exemplary embodiment without the thermal lens effects of the solid-state discs  12  having a negative effect on the resonator radiation field  30 . 
         [0079]    It is particularly favourable if the amplification modules  100  are arranged close to the resonator mirror  44 , at which no decoupling of the resonator radiation field  30  occurs, so that branch  62  is significantly longer than branches  64 ,  66 ,  69  and  68 , preferably by more than a factor of three, better more than a factor of five and even better more than a factor of ten.