Abstract:
An optical device is used for processing a laser beam. It includes an optical element into which at least one input beam is coupled and out of which an output beam emerges. It is proposed that the optical element include a transparent member which has two mutually opposing surfaces having an intermediate plane, which is oriented in such a way that it subtends a first angle with a first spatial axis disposed orthogonally to the longitudinal axis of the input beam, and a second angle with a second spatial axis disposed orthogonally to the longitudinal axis of the input beam and to the first spatial axis, each of these being greater than zero; an that an incoupling prism for coupling the input beam into the member is provided at the one surface, and an outcoupling prism for coupling the output beam out of the member is provided at the opposite surface of the member; viewed in the direction of the longitudinal axis of the input beam, the incoupling prism and the outcoupling prism covering different regions on the member.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an optical device for processing a beam having a flat cross section into a beam having a less flat cross section, in particular a laser beam, having at least one optical element into which at least one portion of the beam is coupled as an input beam and out of which at least one portion of the beam emerges as an output beam.  
       BACKGROUND INFORMATION  
       [0002]     The radiation from laser diodes is generally highly astigmatic. This means that, in both spatial directions, the dimension of the radiation source differs, as does the radiation angle of the light. This generally yields beam cross sections which are very substantial in width in comparison to height. For that reason, the radiation from laser diode bars in particular, is not able to be directly coupled into an optical fiber. Therefore, efforts are directed to processing the beam in a way that results in a most symmetrical possible cross section, by reducing the width and increasing the height. An ideal radiation field would be as precisely wide as it is high and have the same divergence angles in both directions. To achieve or at least come close to achieving this goal, “restacking methods” are used. These methods provide for shifting regions of the radiation field of an input beam in such a way that the output beam produced has at least a more or less desired intensity distribution.  
         [0003]     From the European Published Patent Application No. 0 731 932, it is known, for example, to arrange two mirrors in parallel to one another and at a certain distance from one another. In addition, the two mirrors are slightly offset from one another. One small region of the radiation field bypasses the two mirrors without being reflected by the same. The larger portion of the radiation field is reflected back and forth between the two mirrors until it emerges in a specific region from the interspace therebetween. The disadvantage associated with this device is the substantial requisite outlay for the mirrors and the mounting thereof and the considerable adjustment requirements.  
         [0004]     From the European Published Patent Application No. 0 863 588, a device is known which employs a plate fan. In this connection, the beam offset is utilized during passage of the beam through a plurality of plane-parallel plates. The angle between the beam direction and the surface of the plate inside and outside of the plate is dependent on the refractive index of the plate material. The higher the refractive index of the material, the greater is the deviation of the two angles. Since the plate has parallel side surfaces, the beam does not experience any change in direction after passing through the plate, rather only a parallel lateral shift. For the most part, two plate fans, which are rotated by 90° with respect to each other, are used. Here as well, the manufacturing costs are comparatively high and the precise adjustment of the plate fans is complex.  
         [0005]     Also known from the German Published Patent Application No. 199 01 500 is a beam-shaping optical system having an optical element which, on its incident side, has surfaces that are inclined towards each other and are each assigned to a parallel surface on the emergent side. The optical principle is similar to that of the plate fan described above.  
       SUMMARY OF THE INVENTION  
       [0006]     The object of the present invention is to further refine a device of the type mentioned at the outset in such a way that it will be able to be manufactured and used less expensively.  
         [0007]     This objective is achieved by a device of the type mentioned at the outset in that 
        a. the optical element includes a member that is at least transparent to the wavelengths of the beam;     b. the two opposing surfaces have an intermediate plane which is oriented in such a way that it subtends a first angle with a first spatial axis disposed orthogonally to the longitudinal axis of the input beam, and a second angle with a second spatial axis disposed orthogonally to the longitudinal axis of the input beam and to the first spatial axis, each of these being greater than zero; and     c. an incoupling prism for coupling the input beam into the member is provided at the one surface, and an outcoupling prism for coupling the output beam out of the member is provided at the opposite surface thereof;     d. viewed in the direction of the longitudinal axis of the input beam, the incoupling prism and the outcoupling prism cover different regions on the member.        
 
         [0012]     The device according to the present invention is able to be manufactured very economically since, instead of mirrors or thin and fan-shaped plates, one single transparent (for example pellucid) member is simply used, which is geometrically tilted and rotated relative to the longitudinal axis of the input beam in such a way that light beams coupled into the same are totally internally reflected by the opposing surfaces, and regions of the beam are “restacked.” Suitable prisms, which may likewise be simply manufactured, are used for coupling in and coupling out the light beam. Thus, to process the beam, at most, only three elements are still needed. Moreover, mutual and geometrically simple adjustments are able to be easily performed on these elements. Thus, the costs to be expended for beam processing, in particular when working with laser diodes, are substantially reduced by the device according to the present invention.  
         [0013]     The physical effect employed by the present invention is the total internal reflection within an “optically dense” medium, in which light, which is passing through inside of a member that has a higher refractive index than the medium (for example air) surrounding it, undergoes a total internal reflection within specific limiting angles. The rotation and tilting of the intermediate plane of the member relative to a plane disposed normally to the input beam, as well as the distance between the two mutually opposing surfaces determine the type and the extent of the restacking.  
         [0014]     In this context, the rotation of the intermediate plane about an axis disposed normally to the plane of the input beam causes regions of the radiation field to be displaced in the direction of the plane of the input beam (“slow axis”), whereas the tilting of the intermediate plane about an axis which is disposed normally to the longitudinal axis of the input beam and resides in the plane of the input beam yields a certain “thickness” of the output beam. By positioning the incoupling prism and the outcoupling prism in different regions of the optical member, viewed in the direction of the longitudinal axis of the input beam, exposed regions result on both mutually opposing surfaces of the member, so that, in these regions, the radiation coupled into the member may undergo total internal reflection, as desired by the present invention. It this context, it is understood that the different regions may also overlap.  
         [0015]     Advantageous further refinements of the present invention are delineated in the dependent claims.  
         [0016]     One first especially-preferred embodiment has the distinguishing feature that the mutually opposing surfaces of the optical member are at least essentially plane-parallel and flat. Fundamentally, the result is a flat plate or a flat square, which is especially simple to manufacture. Moreover, in the context of plane-parallel surfaces which produce the total internal reflection of the radiation, the path of the radiation is readily determinable in advance.  
         [0017]     It is especially preferred when the angle between the intermediate plane and the first spatial axis is within the range of 40° to 50°, and the angle between the intermediate plane and the second spatial axis within the range of 5° to 60°, in particular within the range of 30° to 40°, the first spatial axis residing in the plane of the input beam. Most notably, such angles make possible a very efficient processing of the input beams produced by laser diode bars, while simultaneously allowing for small dimensions of the device and easy and thus also economical producibility.  
         [0018]     Another advantageous embodiment of the optical device according to the present invention has the distinguishing feature that the incoupling prism is located in the region of a longitudinal edge of the member that is most proximate to the input beam, and the outcoupling prism in the region of a side edge of the member that is the most remote from the input beam. In this configuration, the light beam is processed using the least possible amount of total internal reflections, thereby enhancing the beam quality.  
         [0019]     The optical device may be produced by joining the incoupling prism and/or the outcoupling prism to the member using an optical cement. This enables the incoupling prism, the outcoupling prism, and the member to be manufactured as separate elements that may be assembled as a modular system in conformance with the individual usage requirements. It is conceivable, for example, to manufacture different sets of incoupling prisms, outcoupling prisms, and members and then to combine them with one another in any desired manner. This allows optimal results to be achieved under very different operating conditions and at low costs. As a cement, a material is suited whose refractive index corresponds as precisely as possible to that of the member and of the prisms. An example of such a material is a UV-hardening adhesive.  
         [0020]     Alternatively, it is possible for the incoupling prism and/or the outcoupling prism to be integrally formed in one piece with the member and preferably of the same material as the member. The thus realized integral one-piece design of the optical device facilitates handling during installation and reduces the risk of optical losses when the beam is coupled into and out of the member. In addition, the need is eliminated for separate installation elements.  
         [0021]     The optical device according to the present invention may be manufactured very economically when the member, the incoupling prism, the outcoupling prism or the one-piece unit including at least two of the former elements are, respectively is, manufactured as an injection-molded part, preferably of plastic.  
         [0022]     In addition, it is especially advantageous when a collimating device is optically connected to the incoupling prism or is integrated in the same and/or when a focusing device is optically connected to the outcoupling prism or is integrated in the same. In such a case, the optical device not only assumes the “restacking” function, but also the collimation of the fast axis, in particular, and/or the coupling of the beam into an optical fiber, for example. Thus the need is eliminated for separate devices which fulfill these tasks, thereby further reducing the manufacturing costs.  
         [0023]     To this end, one practical embodiment proposes that the focusing device include a toroidally curved emergent face on the outcoupling prism. This results in a toroidal lens that is integrally formed in one piece with the outcoupling prism and that has different focal lengths for the two spatial directions. These unequal focal lengths are essential since the divergence angles are distinctly different for the two spatial directions, especially in the context of laser radiation. A toroidally curved emergent face on the outcoupling prism may be realized simply and economically.  
         [0024]     The focusing device may optionally include a light concentrator which is connected to the outcoupling prism, is designed as a monolithic component, and which focuses the radiation by way of the plurality of total internal reflections at its outer limiting surfaces. Such a focusing device, also described as a “lens duct”, may likewise be manufactured very simply. The dimensions of the light concentrator must be adapted to the focusing requirements of the light beam. In most cases, it is necessary to reduce the width of the radiation field for both spatial directions. To this end, the outer surfaces of the light concentrator, where the total internal reflections take place, must be designed to be both plane as well as curved. The use of a light concentrator in the manner proposed advantageously eliminates the need for adjusting the fibers when coupling the light into an optical fiber. In the simplest case, the fibers may be adhesively bonded to the end of the light concentrator. This also economizes on the costs entailed in the manufacturing and assembling of the optical device.  
         [0025]     It is also proposed that the collimating device include an incident face on the incoupling prism that is designed as a convexly curved lens. In this manner as well, a lens is produced which collimates the radiation in the direction of the fast axis. A lens of this kind has a very large acceptance angle, so that only an aspherical surface is suited.  
         [0026]     The present invention also relates to a beam-shaping device for laser diode stacks. It is proposed that the device include a plurality of optical devices of the above type which are placed one over the other to form a stack. A beam-shaping device of this kind makes it possible for the radiation field from a stack of laser diode bars to be processed in a simple manner. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  shows a laser diode bar having an actual beam shape and a nominal beam shape.  
         [0028]      FIG. 2  shows a perspective representation of a first specific embodiment of an optical device for processing the laser beam of  FIG. 1  from an oblique rear view.  
         [0029]      FIG. 3  shows-a perspective representation of the optical device of  FIG. 2  from an oblique front view.  
         [0030]      FIG. 4  shows a perspective detailed representation of an optical member of the optical device of  FIG. 2 .  
         [0031]      FIG. 5  shows a perspective representation of a second specific embodiment of an optical device from an oblique front view.  
         [0032]      FIG. 6  shows a perspective representation of the optical device of  FIG. 5  from an oblique rear view.  
         [0033]      FIG. 7  shows a perspective representation of an optical device according to  FIG. 5 , including a light concentrator.  
         [0034]      FIG. 8  shows a schematic representation for clarifying the functional principle of the light concentrator of  FIG. 7 .  
         [0035]      FIG. 9  shows a perspective representation similar to that of  FIG. 6 , of a third specific embodiment of an optical device.  
         [0036]      FIG. 10  shows a perspective representation of a region of a fourth specific embodiment of an optical device.  
         [0037]      FIG. 11  shows a perspective representation of a plurality of optical devices from an oblique front view, in accordance with a sixth specific embodiment.  FIG. 12  shows a perspective representation of the stack from  FIG. 11 , from an oblique 
     
    
     DETAILED DESCRIPTION  
       [0038]     In  FIG. 1 , a laser diode bar is denoted as a whole by reference numeral  10 . The laser beam emitted from this laser diode bar is collimated with the aid of a cylindrical lens  12  in the direction of the fast axis. The resulting laser beam  14  has a comparatively wide and flat cross-sectional shape, respectively a highly astigmatic intensity distribution. Using a suitable device, which is discussed in detail further below and is symbolized merely by an arrow  16  in  FIG. 1 , the intention is to symmetrize laser beam  14 . This means that the output beam  18  emerging from optical device  16  is not as wide or flat as input beam  14 . It is noted at this point that here and in the following a “beam” may also be understood to be a bundle of individual rays.  
         [0039]     Optical device  16  is shown in greater detail in  FIG. 2  and  3 : It includes a base plate  20 , whose plane is disposed substantially in parallel to the plane of input beam  14 . A plate-shaped optical member  22  is mounted on base plate  20 . It does not rest perpendicularly on base plate  20 , but rather is tipped to the rear, viewed in the direction of input beam  14 . In addition, optical member  22  is also rotated about an axis that is normal to the plane of input beam  14 ; thus it rests obliquely across base plate  20 . The precise geometric orientation of optical member  22  is described further below.  
         [0040]     Also mounted on base plate  20  is an incoupling prism  24 , which has approximately the basic shape of a right-angled triangle and is laid flat on base plate  20 . A hypotenuse face  26  of incoupling prism  24  ( FIG. 3 ) is joined by an optical cement (not shown) to surface  28  of optical member  22  facing input beam  14  where it covers a region  27 . In this context, incoupling prism  24  rests flat against front surface  28  of optical member  22  and, to be precise, in the area of the lower longitudinal edge thereof that is most proximate to input beam  14  in  FIG. 2  and  3 . Overall therefore, an incident face  30  of incoupling prism  24  that is formed by a cathetus surface is disposed normally to input beam  14 .  
         [0041]     Situated at a rear surface  32  of optical member  22  on base plate  20  is an outcoupling prism  34 . It is designed as an irregular octagonal block. The side, top, and bottom surfaces of the outcoupling prism that are not provided with reference numbers are oriented on the whole in parallel to the axis of input beam  14  and also of output beam  18 . A contact surface  36  facing optical member  22  has an oblique, respectively tipped form in the two spatial directions such that, following application of an optical cement, it rests, at least in some areas, flat on rear surface  32  of optical member  22  where it covers a region  37 . An emergent face  38  of outcoupling prism  34  opposing contact surface  36 , in turn, is disposed normally to the axis of output beam  18 .  
         [0042]     Incoupling prism  24 , outcoupling prism  34 , as well as plate-shaped optical member  22  are each fabricated as separate parts out of glass. The purpose of incoupling prism  24  is to couple input beam  14  into optical member  22 . Analogously, outcoupling prism  34  has the function of coupling output beam  18  out of optical member  22 . The actual processing or restacking of the laser radiation in a process involving a multiplicity of total internal reflections, is carried out in optical member  22 . This is described with reference to  FIG. 4 : Of input beam  14 ,  FIG. 4  shows only two outer regions having reference numerals  14   a  and  14   b . One longitudinal axis of input beam  14  is indicated by a dot-dash line denoted by reference numeral  40 . The plane of input beam  14  is marked by a dot-dash line and is denoted by  42 . A first spatial axis X orthogonal to longitudinal axis  40  of input beam  14  is denoted by  44  and resides in plane  42  of the input beam. A second Y-axis disposed orthogonally to longitudinal axis  40  of input beam  14  is oriented normal to plane  42  of input beam  14  and is denoted by  46 . Defined between the two surfaces  28  and  32  of optical member  22  is an intermediate plane  48  indicated by a dot-dash line in  FIG. 4 . As mentioned at the outset, this plane is tilted back by an angle A relative to Y-axis  46 . It is also rotated by an angle B relative to X-axis  44 . In the present exemplary embodiment, angle A is approximately 35°, angle B 45°. A thickness D of plate-shaped optical member  22  is uniform throughout and, in the present exemplary embodiment, is approximately 0.7 mm.  
         [0043]     Use is made of the total internal reflection principle for beam processing within optical member  22 . This means that light that is passing through inside of optical member  22 , whose material has a higher refractive index that the medium (generally air) surrounding it, undergoes a total reflection at the exposed regions of surfaces  28  and  32 , within specific limiting angles. On the other hand, at the unexposed regions of surfaces  28  and  32  of optical member  22 , namely at region  27  covered by hypotenuse face  26  of incoupling prism  24  and at region  37  covered by contact surface  36  of outcoupling prism  34  (compare  FIG. 2  and  3 ), no total internal reflection takes place, since incoupling prism  24 , outcoupling prism  34 , as well as optical member  22  are fabricated from the same material having the same refractive index.  
         [0044]     One first considers the path of rays of partial beam  14   a  of input beam  14 : This path of rays is coupled through incoupling prism  24  (not shown in  FIG. 4 ) into optical member  22 . However, regions  27  and  37  overlap in the area of a position  50 . Thus, at rear surface  32  of optical member  22 , input beam  41  a is not reflected, but rather coupled immediately via outcoupling prism  34  out of optical member  22 . It emerges as partial beam  18   b  out of optical member  22  and finally out of outcoupling prism  34 . In this context, partial output beam  18   a  has the same direction and position as partial input beam  14   a.    
         [0045]     Partial input beam  14   b  is likewise coupled via incoupling prism  24  into optical member  22 . However, this occurs at a position  52  where rear surface  32  is exposed. Due to the inclined position of optical member  22  and thus also of rear surface  32 , partial input beam  14   b  undergoes total reflection at a position  54   a  at the exposed rear surface  32 . Due to the rotation of intermediate plane  48  and, as a result, also of two surfaces  28  and  32  by angle B about Y-axis  46 , input beam  14   b  does not impinge normally on rear surface  32 , but rather obliquely, and is therefore reflected laterally. Due to the tilting of intermediate plane  48  and, as a result, also of rear surface  32  by angle A about X-axis  44 , partial input beam  14   b  is moreover reflected obliquely upwards relative to intermediate plane  48  at point of reflection  54   a.    
         [0046]     At  54   b , partial input beam  14   b  again impinges on front surface  28  of optical member  22 . This position is located outside of region  27  covered by hypotenuse face  26  of incoupling prism  24  on front surface  28  of optical member  22 . Thus, partial input beam  14   b  is reflected, in turn, at position  54   b  in the direction of original axis  40  and then impinges again at  54   c  on rear surface  32  of optical member  22 . Input beam  14   b  continues to be reflected back and forth in this manner within optical member  22  until it arrives in region  37  of rear surface  32  of optical member  22  covered by contact surface  36  of outcoupling prism  34 . In this region, input beam  14   b  is coupled out of optical member  22  at position  58  and arrives in outcoupling prism  34 . There, it emerges as partial output beam  18   b  from emergent face  38 .  
         [0047]     As is apparent from  FIG. 2  through  4 , the extreme right region  14   b  of input beam  14  is “restacked” as a result of the total internal reflection within optical member  22 , so that it emerges from optical device  16  as partial output beam  18   b  above partial region  18   a . The broad and flat input beam  14  is reshaped by optical device  16 , which includes incoupling prism  24 , optical member  22 , and outcoupling prism  34 , into a less broad and, therefore, distinctly thicker output beam  18   b . It is understood that, in reality, input beam  14  does not have any discrete partial beams. This is not the case for output beam  18 : It is actually made up of a stack of partial output beams  18   a ,  18   b , . . . . The number of partial output beams and the spacing between them is set by plate thickness D, as well as by angles A and B.  
         [0048]     In the following, other specific embodiments of optical devices  16  are described. In this context, those elements and regions, whose functions are equivalent to those of elements and regions of previously described exemplary embodiments, are denoted by the same reference numerals. They are generally not explained in detail again.  
         [0049]     In the case of optical device  16  shown in  FIG. 5  and  6 , optical member  22 , incoupling prism  24 , as well as outcoupling prism  34  are designed as a one-piece monolithic unit. A base plate is not present in this specific embodiment. Optical device  16  shown in  FIG. 5  and  6  is manufactured as a plastic injection molded part.  
         [0050]     In addition to the restacking function, optical device  16  may also assume other functions, such as coupling the output beam into an optical fiber  60 , in accordance with  FIG. 7 . To this end, fastened to emergent face  38  of outcoupling prism  34  is a focusing device  62 , which, in the exemplary embodiment shown in  FIG. 7 , is designed as a light concentrator, also described as “lens duct.” The radiation is focused in the same by way of a plurality of total internal reflections at its exposed lateral surfaces. Optical fiber  60  is simply adhesively bonded to the end of light concentrator  62 . The principle of such a light concentrator is shown in  FIG. 8 . An arrow  63  denotes the beam direction. The dimensions of light concentrator  62  must be adapted to the individual requirements of the particular operational case. In most cases, it is necessary to reduce the width of the radiation field for both spatial directions. The outside surfaces of the light concentrator shown in  FIG. 8  have a straight-line design. However, they may also be curvilinear.  
         [0051]     In the specific embodiment shown in  FIG. 9 , focusing device  62  is designed as a toroidal lens that is devised as a suitably curved form of emergent face  38  on outcoupling prism  34 . In this manner, different focal lengths may be realized for both spatial directions. Such unequal focal lengths are essential, since the divergence angles of output beam  18  may be distinctly different for the two spatial directions.  
         [0052]     Another task that may be additionally assumed by the optical device is the collimation of the fast axis of input beam  14 . To this end, incident face  30  on incoupling prism  24  is designed as an aspherical lens  66  in that it is convexly curved, as is apparent from  FIG. 10 .  
         [0053]     To achieve high power densities, laser diode bars are also stacked in the manner of a laser diode stack. In the specific embodiment shown in  FIG. 11  and  12 , five laser diode bars  10   a  through e are stacked as a laser diode stack  68 . Therefore, in the embodiment shown in  FIG. 11  and  12 , five optical devices  16   a - 16   e  are stacked one over another for purposes of beam shaping. It is clearly discernible that optical member  22  in optical devices  16  shown in  FIG. 11  and  12  is distinctly lower in height than, for example, in the embodiment shown in  FIG. 2  and  3 . An oblong, rectangular spacer block  70  is set on each of incoupling prisms  24  to permit the individual optical devices  16   a  through  16   e  to be stacked with axial precision and in parallel. The configuration shown in  FIG. 11  and  12  has the effect of dividing the radiation field of each laser diode bar  10   a  through  10   e  and of thereby stacking the individual radiation fields one over the other.