Abstract:
To improve a laser system comprising at least one externally stabilizable semiconductor laser, from the laser active zone of which a laser radiation field can be coupled oute, and a feedback element, disposed externally in the laser radiation field, which couples out, from the laser radiation field, a feedback radiation field having a defined wavelength and bandwidth, and couples back same into the active laser zone for determining the wavelength and bandwidth of the laser radiation field, in such a way that the wavelength stabilization may be achieved more cost-effectively, it is proposed that the feedback element is a resonant waveguide grating which reflects back a portion of the laser radiation field lying within an angular acceptance range.

Description:
[0001]    This application is a continuation of International application No. PCT/EP2011/054377 filed on Mar. 22, 2011 and claims the benefit of German application number 10 2010 003 227.1 filed on Mar. 24, 2010, the teachings and disclosure of which are hereby incorporated in their entirety by reference thereto. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a laser system comprising at least one externally stabilizable semiconductor laser, from the laser active zone of which a laser radiation field can be coupled out, and a feedback element, disposed externally in the laser radiation field, which couples out, from the laser radiation field, a feedback radiation field having a defined wavelength and bandwidth, and couples back the same into the active laser zone for determining the wavelength and bandwidth of the laser radiation field. 
         [0003]    These types of laser systems are known from B. L. Voladin et al., Optics Letters, Aug. 15, 2004, Vol. 29, No. 16, pp. 1891-1893. The volume Bragg grating used therein is very expensive due to the specialized materials used. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the invention, therefore, to improve a laser system of the generic kind in such a way that the wavelength stabilization may be achieved more cost-effectively. 
         [0005]    For a laser system of the type described at the outset, this object is achieved according to the invention in that the feedback element is a resonant waveguide grating which reflects back a portion of the laser radiation field lying within an angular acceptance range. 
         [0006]    The advantage of this type of resonant waveguide grating is that, due to the fact that it reflects resonantly, it is very narrow-band and in particular may be implemented very cost-effectively, and thus represents an ideal option for determining the wavelength of the laser radiation field. 
         [0007]    In the context of the solution according to the invention, externally stabilizable semiconductor lasers have a back-reflection of less than 30% at the exit side. Semiconductor lasers having a back-reflection of less than 10%, preferably less than 1%, at the exit side, are externally stabilizable in a particularly advantageous manner. 
         [0008]    It is particularly advantageous if the feedback element is a resonant waveguide grating having multiple periodicity, i.e., a resonant waveguide grating whose structures have a multiple periodicity instead of a single periodicity, since the angular acceptance range is enlarged for this type of waveguide grating by suitable design. 
         [0009]    It is particularly advantageous if the resonant waveguide grating has an angular acceptance range of greater than ±1°, and it is even more advantageous if the resonant waveguide grating has an angular acceptance range of greater than ±2°. 
         [0010]    Furthermore, it is preferably provided that the resonant back-reflection of the waveguide grating is polarization-dependent, i.e., only one polarization direction is resonantly reflected back, and the polarization direction oriented perpendicularly thereto passes through the resonant waveguide grating during transmission. 
         [0011]    The resonant waveguide grating is preferably formed in such a way that it has a resonant back-reflection direction which extends parallel to the direction of incidence of the laser radiation field, but in the opposite direction to same. 
         [0012]    Moreover, it is preferably provided that the resonant waveguide grating operates in a substantially absorption-free manner in the resonance wavelength range. 
         [0013]    Operation in a substantially absorption-free manner is understood to mean that less than 5%, preferably less than 1%, and particularly preferably less than 0.5%, of the incident intensity is absorbed. 
         [0014]    The exemplary embodiments mentioned before have provided no further detailed information concerning the structure of the resonant waveguide grating. 
         [0015]    An advantageous implementation form provides that the resonant waveguide grating has waveguide layers applied to a substrate. 
         [0016]    This type of substrate may be made of quartz glass, crystalline quartz, YAG, sapphire, or diamond, for example. 
         [0017]    With regard to the plurality of waveguide layers, the manner in which the grating structure is to be specified has not been defined previously in greater detail. 
         [0018]    One approach provides that a grating structure is specified by a substrate surface which carries the waveguide layers. 
         [0019]    To this end, another alternative approach provides that the grating structure is specified by a topmost waveguide layer facing away from the substrate. 
         [0020]    The number of waveguide layers in conjunction with the above-described exemplary embodiments has not been specified previously in greater detail. 
         [0021]    An advantageous approach provides that the resonant waveguide grating is composed of at least three waveguide layers: a topmost waveguide layer having a high index of refraction, a waveguide layer therebeneath having a lower index of refraction, and a waveguide layer, situated opposite from the topmost waveguide layer, having a high index of refraction. 
         [0022]    The waveguide layers with a high index of refraction have an index of refraction in the range from 1.7 to 3.5. 
         [0023]    The waveguide layer with a low index of refraction has an index of refraction in the range from 1.3 to 1.6. 
         [0024]    In particular, it is provided that the difference in the indices of refraction between the waveguide layer having a high index of refraction and the waveguide layer having a low index of refraction is at least 0.2. 
         [0025]    The indices of refraction of the topmost waveguide layer and of the waveguide layer situated opposite from the topmost waveguide layer are preferably approximately identical, in particular identical. 
         [0026]    Furthermore, no information has been provided concerning the configuration of the waveguide layers in conjunction with the explanation previously of the resonant waveguide grating. 
         [0027]    An advantageous approach provides that the waveguide layers of the resonant waveguide grating extend in planes which run parallel to a transverse plane extending transversely with respect to the direction of incidence of the laser radiation field. 
         [0028]    It is preferably provided that the waveguide layers of the waveguide grating extend in planes which are oriented parallel to a transverse plane extending perpendicularly with respect to the direction of incidence. 
         [0029]    In particular, it is provided that the resonant waveguide grating extends, on a side facing the incident laser radiation, transversely with respect to a direction of incidence, preferably perpendicularly with respect to the direction of incidence. 
         [0030]    Previously, no further detailed information has been provided concerning the properties of the resonant waveguide grating, aside from the reflection thereof. 
         [0031]    An advantageous embodiment of a resonant waveguide grating provides that the resonant waveguide grating partially back-reflects an incident laser radiation field, offset in a wavelength propagation direction. 
         [0032]    Such a wavelength propagation direction is in particular the row direction in which elevations of the resonant waveguide grating are situated one after the other, the elevations in particular extending transversely with respect to the row direction. 
         [0033]    It is preferably provided that the resonant waveguide grating has a wavelength propagation direction which extends parallel to a strip direction in which the resonant waveguide grating extends. 
         [0034]    Previously, no further detailed information has been provided concerning a support for the resonant waveguide grating. 
         [0035]    An advantageous approach provides that the resonant waveguide grating is situated on a substrate that is substantially transparent to the laser radiation field. 
         [0036]    In principle, the substrate for the waveguide grating may be any desired substrate. 
         [0037]    It is preferably provided that the resonant waveguide grating is coolable in order to avoid undesired heating of the resonant waveguide grating. 
         [0038]    It has proven to be particularly advantageous for the resonant waveguide grating to be situated on a heat sink that is transparent to the laser radiation field. 
         [0039]    This type of transparent heat sink may, for example, be a support for the substrate, or also a support for the resonant waveguide grating itself. 
         [0040]    Alternatively, it is likewise advantageous if the transparent heat sink is made of one or more of materials such as crystalline quartz, YAG, sapphire, and diamond. 
         [0041]    No information has been provided concerning the arrangement of the resonant waveguide grating in conjunction with the explanation previously of the individual embodiments. 
         [0042]    For example, it would be conceivable to locate the resonant waveguide grating in a divergent region of the laser radiation field. 
         [0043]    In this case, however, due to the limited angular acceptance range in the divergent region, only an arrangement of the resonant waveguide grating at a very small distance from an aperture of the semiconductor laser would be meaningful. 
         [0044]    For this reason, it is preferably provided that the resonant waveguide grating is situated in an at least partially collimated region of the laser radiation field, so that when the waveguide is correctly oriented, the position of the resonant waveguide grating relative to the laser radiation field does not have a dominant influence on the functioning of the laser system. 
         [0045]    Alternatively, however, it is also conceivable for the resonant waveguide grating to be situated in a focused portion of the laser radiation field with correct imaging, so that the arrangement of the resonant waveguide grating is less sensitive to the angle. 
         [0046]    Furthermore, from a purely theoretical standpoint, the resonant waveguide grating could extend over the entire radiation cross-section of the laser radiation field, in which case the intensity reflected back in the feedback radiation field must be limited by suitable selection of the polarization, or suitable divergence or convergence of the laser radiation field. 
         [0047]    For this reason, a particularly advantageous exemplary embodiment of a laser system according to the invention provides that the resonant waveguide grating is situated in a subarea of the radiation cross-section of the laser radiation field, so that by limiting the subarea of the radiation cross-section, it is possible to limit the intensity of the feedback radiation field. 
         [0048]    Previously, no further detailed information has been provided concerning the configuration of the resonant waveguide grating in the radiation cross-section. 
         [0049]    An advantageous approach provides that the resonant waveguide grating extends in a strip-like manner in a radiation cross-section of the laser radiation field. 
         [0050]    In this regard, the resonant waveguide grating could be situated completely within the radiation cross-section. 
         [0051]    A particularly advantageous approach which in particular allows the coupling of a plurality of laser radiation fields, and thus, also of a plurality of semiconductor lasers, provides that the resonant waveguide grating extends in a strip-like manner within at least two adjacently situated radiation cross-sections of adjacently situated laser radiation fields. 
         [0052]    As an alternative to providing a continuous waveguide grating, preferably in the form of a strip or some other shape within the radiation cross-section, another advantageous approach provides that the resonant waveguide grating has grating patches distributed within the particular radiation cross-section. 
         [0053]    These types of grating patches are preferably noncontiguous individual waveguide grating parts, situated at various locations in the radiation cross-section, whose overall action creates the feedback radiation field, so that the exiting radiation field has a plurality of areas that are radiation-free, but have smaller surface areas, which are then negligible in the far field. 
         [0054]    These types of grating patches may, for example, be statistically distributed over the radiation cross-section. 
         [0055]    However, one practical approach provides that the grating patches are situated in row directions extending at a spacing from one another, so that the grating patches are present at defined locations within the radiation cross-section. 
         [0056]    The grating patches may be statistically arranged. Another approach provides for a periodic arrangement of the grating patches. 
         [0057]    Previously, no further detailed information has likewise been provided concerning the arrangement of the semiconductor lasers. 
         [0058]    An advantageous embodiment provides that the laser system has a plurality of externally stabilizable semiconductor lasers situated one after the other in a row direction in a semiconductor laser bar. 
         [0059]    These types of semiconductor lasers are preferably arranged in such a way that the laser radiation fields have low divergence (slow axis) in a first direction parallel to the row direction. 
         [0060]    In addition, the semiconductor lasers are preferably arranged in the semiconductor laser bar in such a way that the laser radiation fields have high divergence (fast axis) in a second direction extending transversely with respect to the row direction. 
         [0061]    For these types of semiconductor laser bars, it is preferably provided that the semiconductor lasers situated one after the other in the row direction generate laser radiation fields which have radiation cross-sections situated one after the other in a direction extending parallel to the row direction. 
         [0062]    In addition, if coupling of the laser radiation fields is to take place, it is advantageously provided that the strip-shaped resonant waveguide grating extends parallel to the row direction within at least two radiation cross-sections, so that the semiconductor lasers can be coupled with one another. 
         [0063]    The strip-shaped resonant waveguide grating could extend only from an edge region of one radiation cross-section to the edge region of the other radiation cross-section, or in an overlapping area of the radiation cross-sections. 
         [0064]    However, it is also conceivable for the strip-shaped waveguide grating to extend over more than one-half of each of the at least two radiation cross-sections. 
         [0065]    A particularly preferred approach provides that the resonant waveguide grating forms a strip that extends through all laser radiation fields of the semiconductor laser bar in the direction parallel to the row direction. 
         [0066]    When semiconductor lasers which are situated in semiconductor laser bars are provided, in order to increase the number of semiconductor lasers by a multiple, it is preferably provided that the semiconductor laser bars are arranged stacked one on top of the other in a stacking direction, so that apertures in the semiconductor lasers are situated in a two-dimensional surface. 
         [0067]    Thus, even more semiconductor lasers are available, so that the available power may be increased. 
         [0068]    Previously, no further detailed information has been provided concerning the production of the semiconductor lasers. 
         [0069]    An advantageous approach provides that the semiconductor lasers are made of the same material in each of the semiconductor laser bars. 
         [0070]    Such an approach opens up the possibility of producing the semiconductor lasers from the same material, within the semiconductor laser bar. 
         [0071]    If all semiconductor lasers are to operate in the same wavelength range, all semiconductor laser bars may also be made of the same material. 
         [0072]    Alternatively, however, it is possible for the semiconductor lasers to be made of different materials in different semiconductor laser bars, so that, for example, based on the type of material used for the production, the wavelength range in which the semiconductor lasers are stabilizable may also be varied. 
         [0073]    Numerous approaches are conceivable with regard to the wavelength at which the semiconductor lasers are stabilized. 
         [0074]    An advantageous approach provides that all semiconductor lasers are stabilized at the same wavelength. 
         [0075]    For other applications, however, it may also be advantageous to externally stabilize the semiconductor lasers at different wavelengths. 
         [0076]    That is, it is possible, for example, to divide the semiconductor lasers into groups, each of the groups being externally stabilized at a particular wavelength which differs from the wavelength at which the respective other groups are stabilized. 
         [0077]    According to the invention, however, it is also possible for each semiconductor laser of the laser system according to the invention to be stabilized at a wavelength that is different from the other semiconductor lasers. 
         [0078]    In this case, the concept of the different wavelengths is thus maintained, which has the advantage that in this case the laser radiation fields generated by the respective semiconductor lasers may be superimposed in a particularly easy manner. 
         [0079]    If one considers the above-described concepts using different wavelengths in conjunction with semiconductor laser bars, the semiconductor lasers are preferably externally stabilized at different wavelengths in the respective semiconductor laser bars. 
         [0080]    This concept may be continued even further, in that the semiconductor lasers are externally stabilized in different semiconductor laser bars at wavelengths which differ from those of the other semiconductor laser bars. 
         [0081]    Further features and advantages of the solution according to the present invention are the subject matter of the following description and the illustration of several exemplary embodiments in the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0082]      FIG. 1  shows a schematic side view of a first exemplary embodiment of a laser system according to the invention; 
           [0083]      FIG. 2  shows a section along line  2 - 2  in  FIG. 1 ; 
           [0084]      FIG. 3  shows a plan view, from the top, of the first exemplary embodiment of the laser system according to the invention in the direction of the arrow A in  FIG. 1 ; 
           [0085]      FIG. 4  shows a section along line  4 - 4  in  FIG. 1 ; 
           [0086]      FIG. 5  shows a cross-section of a first variant of a resonant waveguide grating according to the invention; 
           [0087]      FIG. 6  shows an illustration of the angular acceptance of the first variant of the waveguide grating according to the invention; 
           [0088]      FIG. 7  shows an illustration of the angular acceptance of a waveguide grating according to the invention having single periodicity; 
           [0089]      FIG. 8  shows a section, similar to  FIG. 5 , through a second variant of a waveguide grating according to the invention; 
           [0090]      FIG. 9  shows an enlarged illustration of the second variant of the waveguide grating according to the invention; 
           [0091]      FIG. 10  shows a longitudinal section, similar to  FIG. 8 , through a strip-shaped waveguide grating according to the invention which is elongated in a row direction; 
           [0092]      FIG. 11  shows a perspective view of a second exemplary embodiment of a laser system according to the invention; 
           [0093]      FIG. 12  shows a section along line  12 - 12  in  FIG. 11 ; 
           [0094]      FIG. 13  shows a section along line  13 - 13  in  FIG. 11 ; 
           [0095]      FIG. 14  shows a section, similar to  FIG. 13 , through a third exemplary embodiment of a laser system according to the invention; 
           [0096]      FIG. 15  shows a partial section, similar to  FIG. 1 , through a fourth exemplary embodiment of a laser system according to the invention; 
           [0097]      FIG. 16  shows a section, similar to  FIG. 15 , through a fifth exemplary embodiment of a laser system according to the invention; 
           [0098]      FIG. 17  shows a section, similar to  FIG. 15 , through a sixth exemplary embodiment of a laser system according to the invention; 
           [0099]      FIG. 18  shows an enlarged illustration of a collimating lens in the sixth exemplary embodiment of the laser system according to the invention; 
           [0100]      FIG. 19  shows a side view, similar to  FIG. 1 , of a seventh exemplary embodiment of a laser system according to the invention; 
           [0101]      FIG. 20  shows a plan view, similar to  FIG. 3 , of the seventh exemplary embodiment of the laser system according to the invention; 
           [0102]      FIG. 21  shows a section along line  21 - 21  in  FIG. 19 ; 
           [0103]      FIG. 22  shows an illustration, similar to  FIG. 13 , of an eighth exemplary embodiment of a laser system according to the invention; 
           [0104]      FIG. 23  shows a side view, similar to  FIG. 1 , of a ninth exemplary embodiment of a laser system according to the invention; 
           [0105]      FIG. 24  shows a section along line  24 - 24  in  FIG. 23 ; 
           [0106]      FIG. 25  shows a section, similar to  FIG. 24 , through a tenth exemplary embodiment of a laser system according to the invention; and 
           [0107]      FIG. 26  shows a partial section, similar to  FIG. 15 , through an eleventh exemplary embodiment of a laser system according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0108]    An exemplary embodiment, illustrated in  FIG. 1 , of a laser system according to the invention illustrated in  FIGS. 1 to 3 , includes a semiconductor laser, denoted overall by reference numeral  10 , having a laser active zone  11  from whose emitting aperture  12  a laser radiation field  20  exits. 
         [0109]    The emitting aperture  12  extends in a first direction  14  over a width B and in a second direction  16  over a height H, the width B being a multiple of the height H, preferably at least 50 times, preferably at least 80 times, of the height H. Typical values for the decoupling aperture are a width B of 100 μm and a height H of 1 μm. 
         [0110]    The laser radiation field  20  exiting from the emitting aperture  12  has a low divergence in the direction parallel to the first direction  14 , which at 95% power capacity is less than 30°, preferably less than 15°, while in the direction parallel to the second direction  16 , the laser radiation field  20  has a divergence which at 95% power capacity is greater than 90°. 
         [0111]    This divergent laser radiation field  20  is collimated, in the direction parallel to the first direction  14  as well as in the direction parallel to the second direction  16 , with respect to its divergence in the direction parallel to the second direction  16 , by a collimating lens  30 , so that a radiation field  40  which is collimated in the second direction, and thus partially collimated overall, exits from the collimating lens, the radiation field  40  having the same divergence as the laser radiation field  20  in the first direction  14 , as illustrated in  FIG. 3 . 
         [0112]    To stabilize the laser radiation field  20  exiting from the semiconductor laser  10 , as well as the partially collimated region  40  of the radiation field  20  formed therefrom, at a particular wavelength or in a particular wavelength range, a resonant waveguide grating  50  which extends in a transverse plane  48  is provided which, as illustrated in  FIG. 3 , extends, in the transverse plane  48  parallel to the first direction, with an extent WG 1 , which corresponds to at least 50% of an extent SQ 1  of a radiation cross-section SQ in the transverse plane  48  parallel to the first direction  14 . 
         [0113]    The extent WG 1  is preferably in the range between approximately 50% and approximately 80% of the extent SQ 1  of the radiation cross-section SQ. 
         [0114]    In addition, as illustrated in  FIG. 1 , the resonant waveguide grating  50  extends with an extent WG 2 , in the transverse plane  48  parallel to the second direction  16 , which corresponds to at least 5%, at most 30%, of the extent SQ 2  of the radiation cross-section SQ in the transverse plane  48  parallel to the second direction  16 . 
         [0115]    Furthermore, with regard to an optical axis  52  extending perpendicularly with respect to the transverse plane  48 , the resonant waveguide grating  50  is arranged parallel to the first direction  14  and parallel to the second direction  16 , in a symmetrical manner with respect to this optical axis  52 , which represents a center axis for each of the laser radiation fields  20 ,  40 . 
         [0116]    The resonant waveguide grating  50  is used to reflect back a portion of the intensity of the laser radiation field  20  in the range of 5% to 30%, preferably in the range of 10% to 20%, as a feedback radiation field  42  in the direction of the semiconductor laser diode  10  in order to stabilize the laser conditions in the laser active zone  11  of the semiconductor laser diode  10  at a defined wavelength or in a defined wavelength range, only a small portion of the back-reflected intensity reaching the laser active zone  11  of the semiconductor laser diode  10 . 
         [0117]    That is, the extent of the resonant waveguide grating  50 , assuming that it reflects back approximately 100% of the intensity in its area, should be dimensioned in such a way that it reflects the above-mentioned portion of the intensity back into the laser active zone  11  of the semiconductor laser  10 , while the remaining intensity of the partially collimated region  40  of the laser radiation field  20 , which is not back-reflected by the resonant waveguide grating  50 , is available in the form of an exiting radiation field  60 , the exiting radiation field  60  having, as a result of the back-reflection of the intensity in the region of the resonant waveguide grating  50 , an intensity-free region  62 , at least directly downstream from the resonant waveguide grating  50 , which, however, is less noticeable in the far field due to diffraction effects at an outer contour of the resonant waveguide grating  50 . 
         [0118]    In the first exemplary embodiment of the laser system according to the invention, the resonant waveguide grating  50 , as illustrated in  FIG. 4 , is strip-shaped and extends in a longitudinal direction  54 , while a grating structure  56  has structural elements  58  which are oriented transversely with respect to the longitudinal direction  54 . 
         [0119]    As illustrated in  FIG. 5 , for example, a first variant of the resonant waveguide grating  50  includes a substrate  70  having a surface structure  72  which has elevations  78  above a base surface  74  that follow one after the other in a row direction  76 , an extent E of the elevations  78  in the row direction  76  as well as a distance A between the elevations  78  having a multiple periodicity, i.e., at least a double periodicity, and the elevations  78  extending, with the same cross-sectional area, transversely with respect to the row direction  76 . 
         [0120]    In the first exemplary embodiment, the row direction  76  extends parallel to the longitudinal direction  54 , so that the elevations  78  extend, with their constant cross-section, transversely with respect to the longitudinal direction  54 . 
         [0121]    In the first exemplary embodiment of the resonant waveguide grating  50 , a layer  82  having a higher index of refraction, and on same a layer  84  having a lower index of refraction, and on layer  84 , a layer  86  once again having a higher index of refraction, each having a defined thickness, are applied to the surface structure  72  of the substrate  70 , so that the surface structure  72  of the substrate continues through the layers  82 ,  84 , and  86 , and so that the layers  82 ,  84 , and  86  form the resonant waveguide grating having multiple periodicity. As illustrated in  FIG. 6 , this type of resonant waveguide grating  50  having multiple periodicity has an expanded angular acceptance range WA m , in a plane parallel to the row direction  76 , for incident radiation which deviates from a normal  80 , in contrast to an angular acceptance range WA e  of a resonant waveguide grating having single periodicity, illustrated in  FIG. 7 . 
         [0122]    The angular acceptance range is the angular range within which 95% of the reflected intensity is present. 
         [0123]    The layers  82  and  86  having a higher index of refraction and the layer  84  having a lower index of refraction extend parallel to a plane  48 , extending perpendicularly with respect to the normal  80 , in which the resonant waveguide grating  50  is situated. 
         [0124]    The expanded angular acceptance range WA m  illustrated in  FIG. 6  means that the resonant waveguide grating  50  is able to completely reflect beams which deviate by ±2° from the normal  80  to the resonant waveguide grating  50 . 
         [0125]    As an alternative to the first variant of the resonant waveguide grating  50  illustrated in  FIG. 5 ,  FIG. 8  illustrates a second variant of the resonant waveguide grating  50 ′ in which the substrate  70  is provided with a restructured surface which extends parallel to the plane  48 , and on which the layer  82 ′ having a higher index of refraction, the layer  84 ′ having a lower index of refraction, and the layer  86 ′ once again having a higher index of refraction are situated, the layer  86 ′ being provided with a surface structure  92  which likewise has a base surface  94 , and elevations  98  which rise above the base surface  94  in a row direction  96  and which as a whole follow one after the other in the row direction  96  and have a constant cross-sectional surface transverse to the row direction  26 . 
         [0126]    The elevations  98  are likewise provided with an extent E in the direction of the row direction  96 , and distances A are present between the elevations  98 , so that the surface structure  92  likewise results in a multiple periodicity, preferably a double periodicity, which likewise results in the expanded angular acceptance relative to the normal  80 , explained in  FIG. 6  in conjunction with the first variant. 
         [0127]    The functional principle of this type of resonant waveguide grating is described by A. Avrutskii et al. in the publication Sov. I. Quantum Electron. 16(8) August 1986, pp. 1063-1065. 
         [0128]      FIG. 9  shows by way of example the parameters provided for a wavelength of λ=969 nm, which represents the resonance wavelength at which the entire intensity is reflected from the resonant waveguide grating  50 ′. 
         [0129]    Further information concerning this type of resonant waveguide grating is found in Sentenac et al., JOSA A, Vol. 22, p. 475, 2005. 
         [0130]    Materials such as Ta 2 O 5 , HfO 2 , Nb 2 O 5 , or also TiO 2  or Si 3 N 4 , are preferably provided for the layers  82  and  84 . 
         [0131]    Materials such as SiO 2  are preferred for the layer  86 . 
         [0132]    In summary, the basic principle of this type of resonant waveguide grating  50 ,  50 ′ is that radiation fields IF impinging on a top side  100  facing away from the substrate  70 , in a direction of incidence  104 , with an electrical vector  102  of the electrical field which defines the polarization direction, are completely reflected by the layer system  90  or  90 ′ in a back-reflection direction  106 , when the radiation field is in the resonance wavelength range RW of the resonant waveguide grating  50 ,  50 ′ and the vector  102  of the electrical field extends parallel to the row direction  76 ,  96 , whereby, as illustrated in  FIG. 10 , a direct reflection results so that the reflected radiation field RF is generated, and wave guiding in the row direction  76 ,  96  also takes place in the layer system  90  or  90 ′, and, offset from the row direction  76 ,  96 , reflected radiation fields CRF which are coupled with the reflected radiation field RF and which propagate in the back-reflection direction  106  are coupled out. 
         [0133]    In a second exemplary embodiment, illustrated in  FIG. 11  and  FIG. 12 , a semiconductor laser bar denoted overall by reference numeral  110  includes a multiplicity of semiconductor lasers  10   1  to  10   n  which are all arranged in a row at a distance from one another in a row direction  112  parallel to the first direction  14 , and which, as illustrated in  FIG. 11 , generate partially collimated areas  40   1  to  40   n  of the laser radiation fields  20   1  to  20   n  having radiation cross-sections SQ, which are likewise situated in a plane parallel to the plane  48  in the direction  114  parallel to the row direction  112 , but which overlap one another due to the divergence in the first direction  14  which is not compensated for by the collimating lens  30 , since the extent SQ 1  of the radiation cross-sections SQ in the direction  114  parallel to the first direction is greater than the distance between successive emitting apertures  12   1  to  12   n  in the row direction  112  of the semiconductor laser bar  110 . 
         [0134]    Also situated in the plane  48  is a strip-shaped resonant waveguide grating  120  which is provided with base structures corresponding to one of the waveguide gratings  50 ,  50 ′, and which extends continuously, parallel to the direction  114 , and passes through all radiation field cross-sections SQ 1  to SQ n . 
         [0135]    The resonant waveguide grating  120  has the same structure as the resonant waveguide grating  50 ′, but in the row direction  96 ′, extends through all radiation cross-sections SQ. 
         [0136]    In addition, the strip-shaped resonant waveguide grating  120  has an extent WG 2 ′ transverse to the direction  114  which corresponds to the extent WG 2  of the first exemplary embodiment, while the extent WG 1 ′ of the strip-shaped resonant waveguide grating  120  corresponds to a multiple of the extent SQ 1  of the radiation field cross-sections SQ in the first direction  14 . 
         [0137]    The resonant waveguide grating  120  now brings about a back-reflection at the specified resonance wavelength in each of the collimated laser radiation fields  20 , as described in conjunction with the first exemplary embodiment. 
         [0138]    Furthermore, as a result of the limited wave guiding at the resonance wavelength RW in the row direction  96 , and the back-reflected radiation fields RF and CRF, the laser radiation fields  40  of the respective adjacent semiconductor lasers  10  are coupled with one another, so that as a whole, the semiconductor lasers  10   1  to  10   n  of the laser bar  110  are thus coupled with one another with regard to the wavelength at which they operate due to the resonant waveguide grating  120 , as illustrated in  FIG. 13 . 
         [0139]    In a third exemplary embodiment of a laser system according to the invention illustrated in  FIG. 14 , the resonant waveguide grating  130 , similarly as described for the first exemplary embodiment in conjunction with  FIG. 10 , includes regions  132  having the surface structure  92  and regions  134  without the surface structure  92 , whereby no input, emission, or back-reflection of the radiation field occurs from regions  134 ; rather, coupling in as well as coupling out and back-reflection occur only in the regions  132  having the surface structure  92 . 
         [0140]    As a result, coupling between the regions  132  occurs, for example due to input of laser radiation into the region  132   x , coupling with the regions  132   x+1  and  132   x−1  occurs, from which emission with back-reflection of the laser radiation once again occurs. If the regions  132   x  are now situated in such a way that in each case approximately centrally with respect to the corresponding region  40  of the laser radiation field  20 , for example with respect to the region  40   x , laser radiation is coupled in, then by wave guiding, the laser radiation is conducted to the regions  40   x+1  and  40   x−1 , where it is reflectively coupled out. 
         [0141]    In a fourth exemplary embodiment of a laser system according to the invention illustrated in  FIG. 15 , a multiplicity of laser bars  110   1  to  110   n  are situated one above the other in the direction of the second direction  16 , and a separate collimating lens  30   1  to  30   n  is provided for each of the laser bars  110 , so that the partially collimated regions  40  of the laser radiation fields are all partially collimated in the second direction  16 , and impinge on waveguide gratings  120   1  to  120   n  which are provided for each of the laser bars  110  and which are situated, for example, on a shared substrate  70 ″ which in turn is situated on a rear side  142  of a focusing lens  140  shared by the partially collimated regions  40 ″ of the laser radiation fields  20 , so that the substrate  70 ″ is held by the lens  140 . 
         [0142]    Thus, in each of the laser bars  110 , all individual semiconductor lasers  10   1  to  10   n  are coupled by the respective resonant waveguide grating  120 , but there is no coupling of the partially collimated regions  40  with a laser bar  110   x+1  or  110   x−1  situated thereabove or therebeneath, respectively. 
         [0143]    As an alternative to the fourth exemplary embodiment, in a fifth exemplary embodiment illustrated in  FIG. 16 , the substrate  70  is not provided on the rear side  142  of the lens  140 ; instead, an optically transparent heat sink  150  made of sapphire, for example, is located on the rear side, the heat sink, on its side facing the laser bar  110 , carrying the substrate  70 ″ having the resonant waveguide gratings  120 . 
         [0144]    By means of the heat sink  150 , it is easily possible to optimally dissipate the heat generated in the resonant waveguide gratings  120 , and in particular to maintain the waveguide gratings  120  at a constant operating temperature, since the operating temperature also has an effect on the resonance wavelength. 
         [0145]    In a sixth exemplary embodiment, illustrated in  FIGS. 17 and 18 , the collimating lenses  30 ′ 1  to  30 ′ n  are formed from a Fresnel lens  152  which is situated on one side of an optically transparent heat sink  154 , and the waveguide grating  120 ′ 1  to  120 ′ n  associated with the respective semiconductor laser bar  110   1  to  110   n  is situated on a side of the heat sink  154  opposite from the Fresnel lens  152 , since the collimated region  40 ″′ is already present on this side of the heat sink  154 . 
         [0146]    The respective waveguide grating  120 ′ 1  to  120 ′ n  has a design corresponding to the second, third, or fourth exemplary embodiment. 
         [0147]    In contrast to the preceding exemplary embodiments, in which in each case it was assumed that a continuous strip-shaped resonant waveguide grating  50 ,  120  is used, a seventh embodiment illustrated in  FIGS. 19 to 21  provides that the waveguide grating  50 ″, which acts reflectively on the partially collimated region  40 , is composed of a multiplicity of grating patches  162 ,  164 ,  166 , whereby the grating patches  162 ,  164 , and  166  may be statistically distributed within the radiation cross-section SQ of the radiation field  40  or are uniformly arranged, for example, as illustrated in  FIG. 21 , in each case two grating patches  162  being situated in a row direction  172  parallel to the first direction  14 , grating patches  164  being situated in a row direction  174  parallel to the first direction  14 , and grating patches  166  being situated in a row direction  176  parallel to the first direction  14 , and the grating patches  162 ,  164 , and  166  being situated at a distance from one another along the respective row direction  172 ,  174 , and  176 . 
         [0148]    To include the highest possible number of transversal modes of the laser radiation field  20  in the partially collimated region  40 , the grating patches  162 ,  164 , and  166  are all arranged in such a way that the grating patches  162 ,  164 , and  166  receive and reflect back areas of the radiation cross-section SQ, preferably in a distribution over an extent SQ 1  of the radiation cross-section SQ parallel to the first direction  14 . 
         [0149]    An advantageous arrangement provides that a projection of the grating patches  162 ,  164 , and  166  on one of the row directions  172 ,  174  or  176 , as illustrated in  FIG. 20 , results in the grating patches  162 ,  164 , and  166  being distributed parallel to the first direction over at least 80% of the extent SQ 1  of the radiation cross-section SQ parallel to the first direction  14 , and thus, within this area of 80% of the extent SQ 1 , being able to reflect transversal modes of the laser radiation field  20  in the partially collimated region  40 . 
         [0150]    The grating patches  162 ,  164 , and  166  are preferably arranged in such a way that, when projected on one of the row directions  172 ,  174 , and  176 , they are either situated at small distances from one another, abut one another, or even overlap in the respective row direction  172 ,  174 , or  176 . 
         [0151]    The advantage of this exemplary embodiment is that the intensity-free regions of the laser radiation field  40  which result from the resonant waveguide grating may thus be obtained not as contiguous regions, but as distributed regions, so that the overall beam quality in the far field is impaired as little as possible. 
         [0152]    The grating patches  162 ,  164 , and  166  are preferably provided as resonant waveguide gratings  50 ′ which are designed and constructed as described in conjunction with the first exemplary embodiment, for example. 
         [0153]    In an eighth exemplary embodiment of a laser system according to the invention, illustrated in  FIG. 22 , a resonant waveguide grating  50 ′″ is provided for each of the collimated laser radiation fields  40  and, in contrast to the preceding exemplary embodiments, extends with its respective longitudinal direction  54  parallel to the second direction  16 , and thus, transversely or preferably perpendicularly with respect to the first direction  14 . 
         [0154]    To be able to reflect back a sufficient number of transversal modes, the extent WG 1 ′ of the resonant waveguide grating  50 ″′ is large enough that it is at least 20% of the extent SQ 1  of the radiation cross-section SQ in the first direction  16 . 
         [0155]    In addition, the resonant waveguide grating  50 ″′ has an extent WG 2  in the second direction  16  which is at least 80% of the extent SQ 2 ′ of the extent of the radiation cross-section SQ′ in the second direction. 
         [0156]    A resonant waveguide grating  50 ″ arranged in this way must thus back-reflect polarized laser radiation fields  20 ′ parallel to the second direction  16  and also include a sufficiently large number of transversal modes. 
         [0157]    In other respects, the eighth exemplary embodiment of the laser amplifier system according to the invention operates similarly as in the preceding exemplary embodiments, so that full reference may be made to the statements regarding same. 
         [0158]    In a ninth exemplary embodiment of a laser amplifier system according to the invention, illustrated in  FIG. 23  and  FIG. 24 , the partially collimated region  40  of the radiation field  20  is focused on an intermediate focus area  184  by a focusing lens  180  having a longer focal length than the collimating lens  30 , and situated within this intermediate focus area  184  is a resonant waveguide grating  50 ′″ whose grating structure  56 ″′ has structural elements  54 ′″ oriented transversely with respect to the second direction  16 , similarly as for the eighth exemplary embodiment. 
         [0159]    For example, the resonant waveguide grating  50 ′″ extends, with its the extent WG 1 , in the first direction approximately over the same distance as in the eight exemplary embodiment, i.e., over more than 20%, for example, but with its extent WG 2  extends in the second direction  16  over at most 30% of a radiation cross-section SQ′ in the focus area  184 . 
         [0160]    In this exemplary embodiment, due to the focused incidence of the laser radiation field  20  on the resonant waveguide grating  50 ″′, only the portion of the incident laser radiation field  20  situated within the region of the angular acceptance of ±2°, explained in conjunction with the first exemplary embodiment, is reflected, while the remaining portion of the laser radiation field passes through the resonant waveguide grating  50 ′″, so that, due to the direction of incidence of the laser radiation field  20 , only a fraction of the intensity of the incident laser radiation field  20  is reflected by the resonant waveguide grating  50 ″′. 
         [0161]    Therefore, in a tenth exemplary embodiment it is also possible to design the waveguide grating  50 ″′ to be large enough that the entire radiation cross-section SQ″ in the focus area impinges on the waveguide grating  50 ″′, as illustrated in  FIG. 25 . 
         [0162]    In other respects, the tenth exemplary embodiment corresponds to the ninth exemplary embodiment, to which reference is made with regard to the remaining features. 
         [0163]    In all exemplary embodiments subsequent to the first exemplary embodiment, elements which are identical to those of one of the preceding exemplary embodiments are provided with the same reference numerals, so that with regard to these elements, in each case reference is made to the preceding exemplary embodiments. 
         [0164]    In an eleventh exemplary embodiment of a laser system according to the invention, illustrated in  FIG. 26 , similarly as for the fourth exemplary embodiment, a multiplicity of laser bars  110   1  to  110   n  is stacked one on top of the other in the direction of the second direction  16 , and, for example, a separate collimating lens  30   1  to  30   n  is provided for each of the laser bars  110 , so that the partially collimated regions  40  of the laser radiation fields are all partially collimated in the second direction  16 , and impinge on a waveguide grating  50   1   1  to  50   n   n  , provided for each individual semiconductor laser  10  of the laser bars  110 , which bars are situated, for example, on a shared substrate  70 ″ which in turn is situated on a rear side  142  of a focusing lens  140  shared by the partially collimated regions  40 ″ of the laser radiation fields  20 , so that the substrate  70 ″ is held by the lens  140 . 
         [0165]    Thus, in each of the laser bars  110 , all individual semiconductor lasers  10   1  to  10   n  are individually externally stabilizable by the respective resonant waveguide grating  50   1   x  to  50   n   x , and there is no coupling of the partially collimated regions  40  with a semiconductor laser  10  situated to the side thereof, thereabove, or therebeneath. 
         [0166]    It is thus possible to externally stabilize each individual semiconductor laser  10  at a selected wavelength. 
         [0167]    For example, the semiconductor lasers  10  of a semiconductor laser bar  110  could be externally stabilized at the same wavelength, and the wavelength could be varied from the semiconductor laser bar  110   x  to the semiconductor laser bar  110   x+1  or  110   x−1 . 
         [0168]    A particularly advantageous solution provides that at a different wavelength, each of the semiconductor lasers  10   x   x  is externally stabilized by the corresponding resonant waveguide grating  50   x   x  at a wavelength that is different from the wavelength of the other semiconductor lasers  10   x+y   x+y . 
         [0169]    The semiconductor lasers  10   1   x  to  10   n   x  of each of the semiconductor laser bars  110   x  are preferably made of the same material, but in each case are externally stabilized at a different wavelength, and the various semiconductor bars  110  may be made of different materials, so that the semiconductor lasers  10  in these semiconductor laser bars  110  may be stabilized in different wavelength ranges.