Abstract:
The invention relates to a device for converting laser radiation ( 21 ) into laser radiation having an M profile, comprising separating means ( 34 ), which can separate the laser radiation ( 21 ) into at least two partial beams ( 22, 23 ) which, at least in some sections or partially, move in different directions or are arranged offset from one another, and optics means ( 38 ), which can introduce the at least two partial beams ( 22, 23 ) in a working plane and/or can, at least in some sections, superimpose the at least two partial beams ( 22, 23 ) in the working plane, wherein the separating means ( 34 ) comprise a lens array ( 39, 41 ) having at least two lenses ( 40, 42 ).

Description:
This is an application filed under 35 USC §371 of PCT/EP2011/072203, filed on Dec. 8, 2011, claiming priority to DE 10 2010 053781.0, filed on Dec. 8, 2010. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a device for converting laser radiation into laser radiation having an M-profile according to the preamble the claim  1 . 
     DEFINITIONS 
     In the propagation direction of the laser radiation refers to the average propagation direction of the laser radiation, in particular when the laser radiation is not a plane wave or is at least partially divergent. Laser beam, light beam, partial beam or beam, unless expressly stated otherwise, is not an idealized beam of the geometric optics, but a real light beam, for example a laser beam which does not have an infinitesimally small beam cross-section, but has an extended beam cross-section. M-profile denotes an intensity profile of laser radiation which has a lower intensity in the center of the cross-section than in one or more regions distal from the center. 
     A device the aforementioned type is disclosed, for example, in WO 93/14430 A1. In the device described therein, an optical fiber is terminated in a conical end section serving as separating means. The laser radiation exiting from this conical end section has an annular intensity distribution, which can be referred to as an M-profile, a short distance behind the optical fiber in a working plane perpendicular to cone axis. 
     Disadvantageously, the beam quality in the working plane is poor, in particular when the laser radiation coupled into the optical fiber is produced from a laser diode bar or a plurality of laser diode bars. 
     The underlying problem of the present invention is to provide a device of the aforedescribed type capable of generating laser radiation with an M-profile and with better beam quality. 
     BRIEF SUMMARY OF THE INVENTION 
     This is attained according to the invention with a device of the aforedescribed type having a separation device comprising at least one lens array with at least two lenses. The dependent claims recite preferred embodiments of the invention. 
     According to claim  1 , the separating means include at least one lens array with at least two lenses. With the at least one lens array, at least two partial beams propagating in different directions can be produced. In this way, a region of lower intensity or a hole is created in the cross-section the laser radiation, in particular in the center. This region of lower intensity or this hole can be transferred to a working plane or to the entrance face of an optical fiber. This produces in the working plane or at the output of the optical fiber laser radiation with an M-profile and with good beam quality. 
     The separating means may include at least one substrate that is at least partially transparent and which has an entrance face and an exit face for the laser radiation, wherein the at least one lens array is arranged on the entrance face and/or the exit face. 
     According to a simple embodiment, the at least one lens array may have concave lenses and may be arranged on the exit face of the at least one substrate, whereas a single convex lens may be provided on the entrance face of the at least one substrate. With this embodiment, for example, the laser radiation from one laser diode bar can be introduced into an optical fiber, with laser radiation with an M-profile then exiting at the output of the optical fiber. 
     According to another embodiment, the separating means may include at least one first lens array and at least one second lens array, each having at least two lenses, wherein the at least one first lens array is arranged on the entrance face of the at least one substrate and the at least one second lens array is arranged on the exit face the at least one substrate. This embodiment is particularly beneficial when the laser radiation from a plurality of laser diode bars is to be converted. 
     According to one embodiment, a Galilean telescope or a plurality of Galilean telescopes may be formed by the entrance face and the exit face of the at least one substrate. In this way, a plurality of partial beams, which have the same divergence as the entering laser radiation, can be produced with a suitably selected demagnification of the telescope. 
     According to another embodiment, the lenses of the at least one lens arrays may be cylindrical lenses. 
     Furthermore, the entrance face and/or the exit face may have at least two different segments, wherein the cylinder axes of the lenses in a first of the segments are aligned differently from the cylinder axes of the lenses in a second of the segments. With the different segments having cylinder axes with different orientations, a better fill factor of, for example, an optical fiber can be attained. 
     Additional features and advantages of the present invention will become clear based the following description of preferred embodiments with reference to the appended drawings, which show in: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  a comparison of an exemplary profile of a laser beam before and after passage through a device according to the invention, wherein the intensity is plotted against the radius in arbitrary units; 
         FIG. 2  an exemplary diagram of an intensity distribution of a meridional beam in an optical fiber; 
         FIG. 3  an exemplary diagram of an intensity distribution of a sagittal beam in an optical fiber; 
         FIG. 4  an exemplary diagram of an intensity distribution of another sagittal beam in an optical fiber; 
         FIG. 5  a three-dimensional diagram of a typical M-profile of a laser beam, wherein the intensity is plotted vertically against the radius in two mutual orthogonal directions; 
         FIG. 6  a side view of a first embodiment of separating means of a device according to the invention; 
         FIG. 7  schematically, the splitting of a laser beam by the separating means according to  FIG. 6 ; 
         FIG. 8  a side view of a first embodiment of a device according to the invention with the separating means according to  FIG. 6 ; 
         FIG. 9  the intensity distribution generated on the entrance face of an optical fiber by the device according to  FIG. 8 ; 
         FIG. 10  the intensity distribution according to  FIG. 9  after passage of the laser light through the optical fiber; 
         FIG. 11  a side view of a second embodiment of a device according to the invention with exemplary beam paths; 
         FIG. 12  a schematic view according to the arrow XII in  FIG. 11 ; 
         FIG. 13  a view according to  FIG. 12  onto a third embodiment of a device according to the invention; 
         FIG. 14  schematically, the splitting of the laser beam by the third embodiment of a device according to the invention; 
         FIG. 15  a detail of the splitting shown in  FIG. 14 ; 
         FIG. 16  the superposition of the intensity profiles of the portions of the laser radiation incident on the entrance face of the optical fiber after the splitting. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Identical components and functionally identical components, beams or arrows in the Figures have identical reference symbols. 
     The continuous curve  1  in  FIG. 1  shows an exemplary intensity profile of laser radiation emanating, for example, from a laser diode bar and collimated with a typical optical system. Such an intensity profile has in the center of the laser radiation a maximum  2 , with the intensity decreasing from the center to the edges. 
     Conversely, the dashed curve  3  in  FIG. 1  shows an intensity profile, which can be generated from the laser radiation according to the continuous curve  1  after passage through a device according to the invention. The intensity profile represented by the dashed curve  3  is an example for an M-profile. The M-profile has a local minimum  4  in the center the beam, whereas maxima  5  of the intensity occur outside the center. For example, such an M-profile is rotationally symmetric with respect to the propagation direction of the laser beam or the laser radiation. 
       FIG. 2  illustrates a so-called meridional beam or a so-called meridional mode, respectively, in an optical fiber. A meridional mode has a distinct maximum intensity  6  on the longitudinal axis of the optical fiber.  FIG. 3  and  FIG. 4  show so-called sagittal beams or so-called sagittal modes, respectively, which each have an intensity minimum  7  in the region of the longitudinal axis of the optical fiber. 
     When a laser beam with an M-profile is to exit at the exit of optical fiber, the beam(s) entering the optical fiber should have only sagittal modes, if possible, or should be able to transform possible meridional modes into sagittal modes.  FIG. 5  shows an exemplary laser beam with an M-profile at the exit of an optical fiber. The deep local minimum  8  in the center of the laser beam is clearly visible. 
       FIG. 6  shows a first embodiment of separating means  9  of a device according to the invention. The separating means  9  are formed by a transparent substrate  10  which has an entrance face  11  and an exit face  12  for the laser beam or the laser radiation. 
     A convex lens  13 , in particular a convex cylindrical lens with a cylinder axis extending into the drawing plane of  FIG. 6 , is formed on the entrance face  11 . The lens  13  is in the form of a circular arc having a radius indicated by the arrow  14  and an origin indicated by the point  15 . 
     A lens array  16  with two concave lenses  17   a ,  17   b , in particular two concave cylindrical lenses with cylinder axes extending into the drawing plane of  FIG. 6 , is formed on the exit face  12 . The lenses  17   a ,  17   b  are each formed by a circular arc having radii indicated by the arrows  18   a ,  18   b  and origins indicated by the points  19   a ,  19   b . The origins of the circular arcs for the lenses  17   a ,  17   b  are spaced from each other in a direction perpendicular to the optical axis  20 . 
     The depth T of the substrate  10  (see  FIG. 6 ) may for, example, be 2.127 mm. The radius of the convex lens  13  may, for example, be 2.0575 mm. The radii of the concave lenses  17   a ,  17   b  may each be 1.097 mm. The spacing between the origins of the radii of the concave lenses  17   a ,  17   b  is 0.2 mm in a direction that extends in  FIG. 6  from the top to the bottom. 
       FIG. 7  shows how laser radiation  21  is split by the separating means into two divergent partial beams  22 ,  23 . 
     The separating means  9  are particularly suitable for the laser radiation emitted from a laser diode bar. 
       FIG. 8  shows a device according to the invention constructed with the separating means  9 . The device can introduce the laser radiation from a laser diode bar  24  into an optical fiber  25  having a length of, for example, 50 mm. The optical fiber  25  may also be longer or shorter. To this end, the device includes optical means  26  capable of focusing the two partial beams exiting from the separating means  9  onto the entrance face the optical fiber  25 . The optical means  26  each have crossed cylindrical lenses  27 ,  28 ,  29  operating as focusing means. Alternatively or in addition to the cylindrical lenses  27 ,  28 ,  29  which provide focusing, lens arrays operating as homogenizing means may be provided. 
       FIG. 8  shows the laser diode bar  24  and an optical system  30  for collimation and optionally rotation of the laser radiation emitted from the individual emitters of the laser diode bar. A comparable optical system is described in EP 1 006 382 A1 and is hereby incorporated in the present description by reference. 
     Two spaced-apart stripes  31  of the laser radiation are applied by the optical means  26  onto the entrance face of the optical fiber. After passage through the optical fiber  25 , a laser beam with an M-profile  32 , which clearly has an intensity minimum  33  in the center of the laser beam, is generated at the exit of the optical fiber  25 . 
     The device illustrated in  FIG. 11  is suitable for introducing into a single optical fiber laser light emitted from more than one laser diode bar, for example from five or ten laser diode bars. The device includes separating means  34  formed by a transparent substrate  35  which has an entrance face  36  and one exit face  37  for the laser radiation. The device also includes optical means  38 , in particular for homogenizing the laser radiation. 
     A first lens array  39  with concave lenses  40 , in particular concave cylindrical lenses with cylinder axes extending into the drawing plane of  FIG. 11 , is formed on the entrance face  36 . A second lens array  41  with convex lenses  42 , in particular convex cylindrical lenses with cylinder axes extending into the drawing plane of  FIG. 11 , is formed on the exit face  37 . A lens  40  of the first lens array  39  is arranged opposite a respective lens  42  of the second lens array  41  in one-to-one correspondence. 
     The depth T of the substrate  35  (see  FIG. 11 ) may be, for example, about 5 mm. More particularly, the depth T of the substrate between the entrance face and the exit face may be defined by the following equation:
 
 T=|f   1   −f   2   |·n  
 
wherein f 1  is the focal length of the lenses  40  of the first lens array  39 , f 2  is the focal length of the lenses  42  of the second lens array  41 , and n is the refractive index of the at least one substrate  35 .
 
     Overall, the individual lenses  40 ,  42  opposing each other across the depth T of the substrate  35  form a plurality of Galilean telescopes. The magnification of the light passing from left to right in  FIG. 11  is between about 0.7 and 0.9. 
     Accordingly, there is a reduction in the cross-section which causes the exemplary laser radiation  21  entering from the left in  FIG. 11  to be split into two partial beams  22   23  after passing through the separating means  34 . 
     The optical means  38  include two spaced-apart substrates  43 ,  44 , with a respective lens array  45 ,  46  formed of lenses  47 ,  48 , preferably cylindrical lenses, arranged on each of the substrates  43 ,  44 . The cylinder axes of the lenses  47 ,  48  extend into the drawing plane of  FIG. 11 . The lens arrays  45 ,  46  are spaced from each other by a distance equal to the focal length of the lenses  48  of the lens array  46 . The lens arrays  45 ,  46  operate thus as homogenizing means in a known fashion. 
     The laser radiation should be comparatively closely packed at the output of the homogenizing means formed by the lens arrays  45 ,  46 , so that as much brightness as possible can be introduced, for example, into an optical fiber. The focal length of the lenses  48  of the lens array  46  should be smaller than the quotient of the pitch (distance between centers) of the lenses  40  of the first lens array  39  and the divergence of the entering laser radiation  21 . 
     For example, the divergence of the entering laser radiation  21  may be equal to about 0.01 rad. Furthermore, the pitch of the lenses  40  of the first lens array  39  may be equal to about 1 mm. The focal length of the lenses  48  of the lens array  46  should then be between about 70 mm and 100 mm. 
     The spacing between the substrate  35  and the substrate  43  may be about twice the pitch, i.e. about 1 mm to 3 mm. 
     Furthermore, the optical means  38  may include focusing means arranged after the lens arrays  45 ,  46  and operating as homogenizing means, or also operating as homogenizing means in lieu of the lens arrays  45 ,  46 , such as the cylindrical lenses  27 ,  28 ,  29  according to  FIG. 8  which operate as homogenizing means. 
     The entrance face  36  of the separating means  34  may be constructed as indicated in  FIG. 12 . The incident laser radiation  21  is indicated here by a small rectangle. 
     However, according to another embodiment of the present invention, the entrance and the exit face of the separating means may also be segmented, as shown in  FIG. 13 . 
       FIG. 13  shows an embodiment wherein the entrance face  49  and the unillustrated exit face of the separating means  50  are divided into eight segments. A corresponding lens array  51   a ,  51   b ,  51   c ,  51   d ,  51   e ,  51   f ,  51   g ,  51   h  of lenses  52   a ,  52   b ,  52   c ,  52   d ,  52   e ,  52   f ,  52   g ,  52   h  is arranged on each of these segments of the entrance face  49  and the exit face, wherein the lenses are preferably each formed as cylindrical lenses. The lenses  52   a ,  52   b ,  52   c ,  52   d ,  52   e ,  52   f ,  52   g ,  52   h  of the entrance face and the unillustrated lenses of the exit face may be formed like those in  FIG. 11  and may be spaced apart from each other. 
     The cylinder axes of neighboring lenses  52   a ,  52   b ,  52   c ,  52   d ,  52   e ,  52   f ,  52   g ,  52   h  hereby enclose with each other an angle of α=45°. By employing these segmented separating means  50 , the laser radiation of a larger number of laser diode bars can be introduced into one and the same optical fiber. The incident laser radiation  21  is also indicated in  FIG. 13  by a square. The sides of this square each enclose with the cylinder axes of the lenses  52   a ,  52   b ,  52   c ,  52   d ,  52   e ,  52   f ,  52   g ,  52   h  an angle of β=67.5°. 
     It would also be possible to provide more or fewer than eight segments. The angle β between the sides of the square, which corresponds to the incident laser radiation  21  and the cylinder axes of the lenses  52   a ,  52   b ,  52   c ,  52   d ,  52   e ,  52   f ,  52   g ,  52   h  may also have a different value. For example, the angle β could also be 0° and/or 45° and/or 90°. 
       FIG. 14  illustrates schematically the generation of a plurality of partial beams  22   a ,  22   b ,  22   c ,  22   d ,  22   e ,  22   f ,  22   g ,  22   h ,  23   a ,  23   b ,  23   c ,  23   d ,  23   e ,  23   f ,  23   g ,  23   h  through segmentation after the separating means  50 .  FIG. 15  shows the partial beams  22   a ,  22   b  formed by the lens arrays  51   a  of a segment and incident on the entrance face  53  of an optical fiber. 
       FIG. 16  illustrates the superposition of all partial beams  22   a ,  22   b ,  22   c ,  22   d ,  22   e ,  22   f ,  22   g ,  22   h ,  23   a ,  23   b ,  23   c ,  23   d ,  23   e ,  23   f ,  23   g ,  23   h  at the entrance face  53  of the optical fiber. The outside part of  FIG. 16  shows how the partial beams  22   a ,  22   b ,  22   c ,  22   d ,  22   e ,  22   f ,  22   g ,  22   h ,  23   a ,  23   b ,  23   c ,  23   d ,  23   e ,  23   f ,  23   g ,  23   h  contribute to the intensity distribution in the individual regions  53   a ,  53   b ,  53   c ,  53   d ,  53   e ,  53   f ,  53   g ,  53   h  of the entrance face  53 . It is clearly apparent that an intensity distribution equivalent to an M-profile is already produced at the entrance face  53  of the optical fiber. In particular, the minimum  54  in the center of the entrance face  53  is clearly visible. 
     This M-profile can be even more homogeneous after passage through the optical fiber.