Abstract:
The invention relates to a laser assembly with a converter designed as a fiber laser for generating a converted output laser beam, and having a pumping source that supplies a pump beam to the converter and includes a plurality of laser diodes that generate the pump beam and are formed by emitters mounted on laser bars, the pumping source including beam forming optics for forming the laser beams supplied by the emitters into the pump beam.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a laser arrangement with a converter constituted as a fiber laser for generating converted laser output radiation and with a pumping source which delivers pumping radiation for the converter. The laser arrangement comprises a plurality of laser diodes generating the pumping radiation, wherein the pumping source comprises beam-forming optics for forming the laser beams delivered from the emitters into the pumping radiation, and wherein the converted output radiation of the converter is fed via a flexible light guide to a working and/or treatment head delivering output or treatment radiation. 
         [0002]    Laser arrangements are known in principle, wherein an active fiber doped with at least one rare-earth element, i.e. doped with a laser-active material, is excited with pumping radiation, so that laser radiation with a radiation emission at one fiber end is forced in this doped active fiber between two optical elements provided at the fiber ends and acting as resonator mirrors. It is also known to surround the active fiber with densely doped cladding of a light-conducting material, i.e. glass or quartz glass, which then forms a pump cladding, which is acted upon by the pumping radiation and via which the radiation is stimulated in the active fiber or in the doped core of the converter fiber formed by the active fiber and the pump cladding. It is also known to cool the fiber ends, i.e. the connections or couplings of converter fibers, with a liquid cooling medium (U.S. Pat. No. 4,732,450) or to cool the converter fiber with a cooling medium flowing along this—fiber (RU 2031 420). 
         [0003]    Finally, diode laser arrangements are known (DE 10 2011 016 253) which generally comprise a source emitting laser light in the form of at least one laser diode stack comprising a plurality laser bars each provided on a cooler, said laser bars each comprising a plurality of emitters emitting laser light and following one another in a slow axis, as well as beam-forming optics comprising a plate spreader. 
         [0004]    The problem of the invention is to specify a laser arrangement which provides laser radiation of high quality. 
       SUMMARY OF THE INVENTION 
       [0005]    The laser arrangement according to the invention can be implemented for a power range of converted laser radiation from a few kW up to powers greater than 10 kW. Furthermore, the laser arrangement according to the invention can be used for various areas, i.e. in medicine for example, but also in the processing and/or working of workpieces, for example for the high-quality cutting of workpieces and/or sheet metals, for the heating of workpieces, for example for hardening etc. The diode laser arrangement serving as a pumping source is constituted for example such that the wavelength of the pumping laser radiation lies in the range between 900 nm and 1050 nm, the wavelength of the converted laser radiation emitted by the diode laser or by its converter then being higher, for example in the range between 1050 nm and 1100 nm. 
         [0006]    In a preferred embodiment, the pump cladding of the converter fiber is surrounded by at least one further cladding, for example made of moisture-proof and/or water-proof plastic, this cladding then being enclosed, in a particularly advantageous embodiment, by a further external cladding made of a corrosion-resistant metallic material. The effect of the further cladding surrounding the pump cladding is, amongst other things that the pumping radiation remains inside the pump cladding due to total reflection and the latter thus acts intensively on the inner active fiber. 
         [0007]    Furthermore, the converter fiber is preferably accommodated in a tubular and/or sleeve-like sheathing, through which a cooling medium, preferably a liquid cooling medium, flows. The couplings and decouplings for the laser radiation into and out of the converter fiber or corresponding light-permeable material, preferably connections or end caps produced from glass or quartz glass, preferably also each extend at least with a partial region into the interior of the cooling housing and are thus also cooled by the cooling medium. 
         [0008]    “Active layer” of the laser bar is the layer in which the emitters of the laser bar are disposed and the plane whereof is orientated normal to the fast axis in which the laser beams of the emitters have the greater divergence. 
         [0009]    The expression “essentially” or “approximately” means, in the sense that the invention, deviations from the given precise value of +/−10%, preferably of +/−5% and/or deviations in the form of changes unimportant for the function. 
         [0010]    Developments, advantages and possible applications of the invention also emerge from the following description of examples of embodiment and from the figures. All the described and/or diagrammatically represented features in themselves alone or in any combination are in principle the subject-matter of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention is explained in greater detail below with the aid of the figures using examples of embodiment. 
           [0012]    In the figures: 
           [0013]      FIG. 1  shows, in a simplified representation, a laser arrangement with an actively cooled converter fiber according to the invention; 
           [0014]      FIG. 2, 3  show, in a diagrammatic representation, a pumping source of the laser arrangement of  FIG. 1  constituted by a diode laser arrangement; 
           [0015]      FIG. 4  shows, in a simplified representation, the embodiment of the laser beams of two laser bars before the conversion, after the spreading and after the recombination; 
           [0016]      FIG. 5, 6  show cross-sections of the converter fiber in different embodiments; 
           [0017]      FIG. 7  shows, in an enlarged diagrammatic representation and in cross-section, the cooled converter fiber of the laser arrangement of  FIG. 1 ; 
           [0018]      FIG. 8  shows a cross-section corresponding to line I-I of  FIG. 6 ; 
           [0019]      FIG. 9  shows, in a representation as in  FIG. 1 , a laser arrangement according to the invention with the pumping source and the converter as well as a treatment or working head and a flexible light guide in the form of a light guide cable for the transmission of the converted laser radiation to the treatment or working head; 
           [0020]      FIG. 10  shows a representation similar to  FIG. 9 , wherein the light guide is constituted by the converter fiber, in a further embodiment; and 
           [0021]      FIG. 11  shows, in a partial representation, a laser arrangement according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    For a simpler understanding, three spatial axes orientated normal to one another are each denoted by X, Y and Z in  FIGS. 2 and 3 . The laser arrangement according to the invention generally denoted by  1  in  FIGS. 1-9  essentially comprises a converter  2  in the form of a fiber laser, a diode laser arrangement  3  serving as a pumping source for converter  2  and focusing optics for the converted laser radiation decoupled from converter  2 . In the embodiment represented, the aforementioned components have the structure described below: 
       Diode Laser Arrangement  3   
       [0023]    In the represented embodiment, diode laser arrangement  3  comprises two parallel laser diode stacks  5 , which are mutually offset in the direction of the Y axis and which each comprise a plurality of laser bars  6  disposed upon one another in a stacked manner in the direction of the Z axis, which laser bars are each provided with a cooling body and comprise a plurality of emitters emitting laser light. The latter are provided following one another on the given laser bar  6  in the direction of their slow axis (Y axis) and therefore normal to the stack axis or Z axis and emit the laser light in the direction of the X axis, which is orientated normal to the slow axis and fast axis of the emitters and, in the represented embodiment, is the optical axis of diode laser arrangement  3 . Furthermore, the arrangement is made such that each laser bar  6  of a stack  5  lies at the same level as laser bar  6  of the other stack  5 . 
         [0024]    Located in the beam path of the laser radiation emitted by laser bars  6  in the form of a beam bundle of single beams  7  is a fast axis collimator (not represented in  FIGS. 2 and 3 ), which is constituted for example by a cylindrical lens lying with its axis in the Y axis and which brings about a collimation of laser beams or single beams  7  in the fast axis, i.e. in the Z axis normal to the active layer of laser bars  6 , in which (fast axis) the radiation of the emitters of each laser bar  6  has the greatest divergence. Following the fast axis collimator, the laser radiation essentially exists as a narrow-band beam bundle of single beams  7 , as is represented in  FIG. 4  in the position a). The fast axis collimator is followed in the beam path by an optical device  8 , which is constituted as a plate spreader and serves for the further formation of the laser beam bundle, and more precisely in such a way that the laser beam bundle is first split or spread into beams  7 . 1  in different planes parallel to the XY plane, wherein beams  7 . 1  are also mutually offset in the Y axis from plane to plane, as is represented in  FIG. 4  in position b). In a further optical device  9  also constituted as a plate spreader, single beams  7 . 1  of a plurality of laser bars  6  are then pushed on top of one another diagonally in the manner represented in  FIG. 4  in position c), so that a beam bundle  7 . 2  results. Laser beams  7 ,  7 . 1  and  7 . 2  from two laser bars  6  of a stack  5  are reproduced in  FIG. 4 . To allow a better distinction to be made, the laser beams of one laser bar  6  are shaded and those of the other laser bar  6  are not shaded. 
         [0025]    In detail, optical arrangement  8  comprises two parallel plate spreaders  8 . 1 , which are constituted basically identically in the represented embodiment and each comprise a plurality of thin plates  10 . The latter are produced from a light-conducting material, for example of glass (optical glass) or quartz glass and have for example a square format. Each plate disposed with its surface sides in the XZ plane has two mutually opposite narrow plate sides for the entry and exit of the laser beams. These end faces are inclined differently from plate to plate with respect to the optical axis or X axis, so that a fan-like structure results and the spreading of single beams  7  into single beams  7 . 1  in the direction of the Z axis results. 
         [0026]    Optical device  9  also comprises a plurality of plates  11  of the light-conducting material or glass or quartz glass following one another in the form of a stack. Plates  11  are disposed with their surface sides in the XY plane and again each comprise, in the beam path of the laser beams, two parallel plane end faces for the entry and exit of the laser beams. These end faces are inclined differently from plate to plate with respect to the optical axis (X axis), so that a fan-like structure results and the pushing of single beams  7 . 1  on top of one another to form beam bundle  7 . 2  results, as is represented in  FIG. 4  in position c). 
         [0027]    Provided following optical device  9  is a slow axis collimator  12 , which corrects the divergence that the laser beams exhibit in the slow axis (Y axis) and, in the represented embodiment, is constituted by a cylindrical lens, which is curved only about an axis parallel to the Y axis. 
         [0028]    The fast axis collimator (not represented), optical devices  8  and  9  and slow axis collimator  12  constitute, in the represented embodiment, beam-forming optics  13  of diode laser arrangement  3 . 
       Converter  2   
       [0029]    Converter  2  constituted as a fiber laser comprises, amongst other things, a converter fiber  14 , which in the represented embodiment is multilayered, i.e. with an inner active fiber  15  (active core) made of a light-conducting material, preferably of glass or quartz glass. Fiber  15  is doped at least with a laser-active medium or substance, for example with erbium and/or ytterbium and/or neodym, and is surrounded by cladding  16  made of a light-conducting material, preferably of glass or quartz glass, which forms the pump cladding (pump clad) of converter fiber  14  and is made of undoped light-conducting material, preferably glass or quartz glass. Cladding  16  is surrounded by further cladding  17  made of a suitable plastic, for example water-proof plastic. Outer cladding  18 , for example made of a corrosion-resistant metallic material, serves as the outer termination of converter fiber  14 . 
         [0030]    This structure of converter fiber  14  is represented in  FIGS. 5 and 6 . As can also be seen in these figures, cladding  16  serving as the pump cladding has a cross-section diverging from the circle shape, in order in this way to optimise the coupling of the laser or pump radiation delivered by pumping source or diode laser  3  into inner active fiber  15 . In  FIG. 5 , cladding  16  has an octagonal cross-section with pronounced corners and, in  FIG. 6 , an octagonal cross-section with rounded corners. Inner fiber  15  has for example a diameter in the range between 10 μm and 20 μm and cladding  16  has a diameter of approx. 400 μm to 200 μm. Other cross-sections of converter fiber  14  suitable for this are possible, for example an eccentric arrangement of active fiber  15  in the cladding  16  which, in this case too, then preferably has a cross-section diverging from the circle shape. 
         [0031]    At its two ends, converter fiber  14  is provided in each case with an optical connection enabling the entry and exit of laser radiation into converter fiber  14  or out of this fiber, said optical connection being in the form of an end cap  19  and  20  made of light-conducting material, preferably of glass or quartz glass. Furthermore, converter fiber  14  is accommodated in the interior  21 . 3  of a sheathing  21  which extends over the entire length of this fiber and through which a cooling medium, for example a liquid cooling medium or cooling water, can flow, at the ends of which sheathing end caps  19  and  20  are also provided, which each extend, for the purpose of their cooling, with a partial length into sheathing  21  or into its interior  21 . 3 . In the represented embodiment, sheathing  21  is constituted tubular or sleeve-like, and more precisely with a cooling medium inlet  21 . 1  in the region of end cap  19  acted upon, amongst other things, by the laser radiation from diode laser  3 , and with a cooling medium outlet  21 . 2  in the region of end cap  20 , which also serves, amongst other things, for conducting out the laser radiation converted by converter  2 . The two end caps  19  and  20  are coated with an antireflection layer for the pumping radiation and the converted laser radiation and are also connected to active fiber  15 , for example by means of a splice connection. Furthermore, end caps  19  and  20  have for example a length of 10 mm to 40 mm and a diameter in the range between approx. 5 mm and 20 mm. 
         [0032]    Furthermore, converter  2  comprises two resonator mirrors  22  and  23 , whereof resonator mirror  22  lies, amongst other things, in the beam path of the laser radiation delivered by diode laser arrangement  3  and brings about focusing of this radiation onto end cap  19  or onto converter fiber  14  and, for this purpose, is curved in the manner of a convex lens at its side facing away from end cap  19 . At the side facing end cap  19 , resonator mirror  22  is curved convex in the manner of a concave mirror for its resonator mirror function. Furthermore, resonator mirror  22  is constituted such that, at its coupling side facing away from end cap  19 , it has no or essentially no reflection properties, i.e. is constituted as an antireflection mirror for the pumping radiation, and, at its side facing end cap  19 , has a high reflection, i.e. is constituted as a high-reflection mirror for the radiation exiting from converter fiber  14 . Resonator mirrors  22  and  23  are preferably constituted adjustable. 
         [0033]    A further resonator mirror  23  is provided in the beam path following end cap  20 . In order to generate parallel or essentially parallel output laser radiation, this resonator mirror  23  is again convex or in the manner of a convex lens at its side facing away from end cap  20  and concave or in the manner of a concave mirror at its side facing end cap  20 . Furthermore, resonator mirror  23  is constituted such that it acts at its concave side as a high-reflection mirror for the pumping radiation and as an antireflection mirror for the converted output radiation of the converter, i.e. enables a passage of the converted output radiation or laser radiation without reflection or essentially without reflection. 
         [0034]      FIGS. 7 and 8  show, again in an enlarged representation, converter fiber  14  with sheathing  21  and with annular cooling channel  21 . 3  formed by said sheathing and surrounding converter fiber  14  over its entire circumference. In order to ensure that converter fiber  14  is always spaced apart from the inner surface of sheathing  21  over its entire length and that cooling channel  21 . 3  is thus constituted in the optimum manner over the entire length of converter fiber  14 , converter fiber  14  is repeatedly held by, in each case, one support element  24 . The latter essentially comprises a housing  25 , which forms a housing interior  26  which is closed to the exterior and in which converter fiber  14  passed through housing interior  26  is held between an upper and a lower fiber holder  27 . Provided at two sides of housing  25  lying opposite one another is, in each case, a flange-like connection  28  through which converter fiber  14  is passed and which is used for the connection of a hose piece or tube piece  29 , which is part of sheathing  21 . In the represented embodiment, therefore, the latter comprises holding elements  24  and hose or tube pieces  29 . 
         [0035]    In laser arrangement  1 , as  FIG. 9  shows, the converted laser radiation focused by focusing optics  4  is coupled into a light guide cable  30  and transmitted via this cable to a treatment or working head  31 , in which the laser radiation is converted by beam-forming optics  32  into an output and/or treatment beam  33  focused at a treatment point or focus. Beam-forming optics  32  comprises for example a collimation lens  32 . 1  and a focusing lens  32 . 2  following the latter in the beam path. During the treatment or for the treatment for example, treatment head  31  is moved and/or adjusted in space, e.g. with a manipulator, and more precisely for example in three spatial axes orientated normal to one another, whereas the pumping source constituted by diode laser arrangement  3  and also converter or fiber laser  2  and focusing optics  4  are stationary during the treatment, i.e. form a stationary unit  34  during the treatment. Treatment head  31  can however also be disposed stationary during the treatment. 
         [0036]      FIG. 10  shows a further embodiment of a laser arrangement  1   a , which differs from laser arrangement  1  in that converter fiber  14  accommodated in sheathing  21  reaches up to treatment head  31   a , which for example can again be moved and/or adjusted in space during the treatment or for the treatment. In this embodiment, therefore, converter fiber  14  accommodated in sheathing  21  forms the connection or the light guide between unit  34  of laser arrangement  1   a  stationary during the treatment and treatment head  31   a . Furthermore, the converter fiber and sheathing  21  form a flexible converter fiber/cooling hose combination  35 . In this embodiment, the pumping source constituted by diode laser arrangement  3 , optical element  22  and the end of converter fiber/cooling hose combination  35  comprising end cap  19  are provided in stationary unit  34 . Optical element  23  and the end of converter fiber/cooling hose combination  35  constituting end cap  20  are provided in treatment head  31   a . Optical element  23  is part of beam-forming optics  32   a  of treatment head  31   a , which in the represented example of embodiment comprises, in addition to optical element  23  and following the latter in the beam path, focusing lens  32   a . 2  corresponding to focusing lens  32 . 2 , and more precisely for forming focused output and/or treatment beam  33 . Treatment head  31   a  can also be disposed stationary during the treatment. 
         [0037]      FIG. 11  shows a further embodiment of a laser arrangement  1   b , which differs from laser arrangement  1  in that the converted laser radiation is decoupled into light guide cable  30  not at resonator mirror  23 , but rather between pumping source or diode laser arrangement  3  and resonator mirror  22 , i.e. at this resonator mirror, and more precisely by a dichroitic mirror  36 , which is inclined with respect to the axial direction of the pumping radiation of diode laser arrangement  3 , i.e. with respect to the X axis, and the surface sides whereof form with the Y-Z plane an angle less than 90°, for example an angle of 45° or essentially of 45°. Mirror  36  separates the laser radiation of diode laser arrangement  3  from the converted laser radiation of converter fiber  14 . Possible combinations of the properties of mirror  36  are:
       Mirror  30  is permeable for the pumping radiation, but highly reflecting for the converted radiation, or   Mirror  30  is highly reflecting for the pumping radiation, but permeable for the converted radiation.       
 
         [0040]    Irrespective of the given embodiment, the wavelength of the laser radiation delivered by diode laser arrangement  3  lies for example in the range between 900 nm and 1500 nm. The wavelength of the converted laser radiation lies above the wavelength of the pumping radiation, for example at 1500 nm to 1100 nm. The pumping radiation delivered by diode laser arrangement  3  lies for example in a power range from several kW up to powers greater than 10 kW. For this reason, too, optimum cooling of converter fiber  14  is required to carry away power losses, which arise amongst other things during the coupling of the pumping radiation into inner fiber  15 . 
         [0041]    To increase the pumping power and therefore also the output power and/or the power density, it is expedient to operate diode laser arrangement  3  in such a way that the pumping radiation comprises wavelength multiplexing of a plurality of wavelengths from a wavelength range of the pumping radiation, for example from the wavelength range between 900 nm and 1060 mm. 
         [0042]    The invention has been described above using examples of embodiment. It goes without saying that numerous changes and modifications are possible, without thereby departing from the inventive idea underlying the invention. 
       LIST OF REFERENCE NUMBERS 
       [0000]    
       
           1 ,  1   a ,  1   b  laser arrangement 
           2  converter 
           3  diode laser arrangement (pumping source) 
           4  focusing optics 
           5  stack 
           6  laser bars 
           7 ,  7 . 1   7 . 2  single beams 
           8 ,  9  optical element 
           8 . 1  plate spreader 
           10 ,  11  plates 
           12  slow axis collimator 
           13  beam-forming optics 
           14  converter fiber 
           15  inner active fiber or active core 
           16 - 18  cladding 
           19 ,  20  end cap 
           21  sheathing 
           21 . 1 ,  21 . 2  connection 
           21 . 3  cooling channel 
           22 ,  23  resonator mirror 
           24  holding device 
           25  housing 
           26  housing interior 
           27  fiber holder 
           28  connection 
           29  tube piece or hose piece 
           30  light guide cable 
           31 ,  31   a  treatment head 
           32 ,  32   a  beam-forming optics 
           32 . 1  collimator lens 
           32 . 2 ,  32   a . 2  focusing lens 
           33  focused output and/or treatment beam 
           34  stationary part 
           35  converter fiber/cooling hose combination 
           35  mirror 
         X, Y, Z spatial axes