Abstract:
A diode laser structure includes multiple stripe emitters disposed next to each other, in which each of the stripe emitters is configured to emit, during operation, a laser beam having a corresponding beam parameter product with respect to a slow axis (BPP SA ), where the stripe emitters are arranged such that the corresponding BPP SA  of the laser beams successively decrease from a center of the diode laser structure towards a first edge of the diode laser structure and from the center of the diode laser structure towards a second edge of the diode laser structure, the second edge being opposite the first edge. The stripe emitters are oriented in a direction of the slow axis and are offset from one another in the direction of the slow axis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority under 35 U.S.C. §120 to PCT/EP2009/000765, filed on Feb. 5, 2009, and designating the U.S., which claims priority under 35 U.S.C. §119 to European Patent Application No. 08 002 444.1, filed on Feb. 11, 2008. The contents of the prior applications are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a diode laser structure for generating diode laser radiation. The diode laser structure includes several stripe emitters which are disposed next to each other, the slow axes of which are oriented in the same direction and are disposed offset from each other in this same direction, the beam parameter products, with respect to the slow axis, of at least two of the stripe emitters being different. 
     BACKGROUND 
     Either several single emitters (e.g., broad stripe emitters with typical widths of 50 to 500 μm) or diode bars consisting of several equally wide single emitters/broad stripe emitters (e.g., diode bars of typically 10 to 100 equally wide broad stripe emitters with a filling factor of 10 to 95%) can be used for generating efficient and high brilliance diode radiation. For example, diode bars of 30 to 50 emitters each having a width of 100 μm are often used on a bar of 1 cm with a filling factor between 30 and 50%. 
     The stripe-shaped single emitters of a diode bar each emit a laser beam which propagates in a fast axis (FA) direction in which the beam diverges fast, e.g., with a radiation angle of approximately 50°, and in a slow axis (SA) direction, which is perpendicular to the FA direction, and in which the beam diverges comparatively slowly, e.g., with a radiation angle of approximately 10°. The beam quality in the FA direction is optimum (almost diffraction-limited), whereas the beam quality in the SA direction is relatively poor. In order to combine these individual beams, they are initially collimated in the FA direction by means of a micro-optical cylindrical lens and are subsequently collimated in the SA direction by means of a cylindrical lens array. The beams are subsequently rotated through 90° by means of further micro-optics and are disposed next to each other. The combined laser beam is formed by aligned optical stacking of the laser beam strips emitted by the stripe-shaped single emitters. Since the single emitters are identical, the laser beams emitted from the single emitters each have the same beam parameter product with respect to the SA direction (BPPSA) and the same beam parameter product with respect to the FA direction (BPPFA), the beam parameter product BPP being defined as follows: BPP i =θ i *w i  (i=SA, FA), where θ is the divergence of the laser beam emitted by the emitter in the SA direction or the FA direction and w is the half width of the emitter in the SA or FA direction. 
     Aligned optical stacking of the laser beam strips with respectively identical BPP SA  and BPP FA  results in a combined laser beam that has a beam parameter product which is rectangular or square in the BPP SA -BPP FA -plane and is therefore coupled into a round fiber which has a round beam parameter product in the BPP SA -BPP FA  plane, thereby causing radiation losses or incomplete utilization of the fiber. 
     The document US 2007/0195850 A1 discloses stacking single emitters having different emitter widths in the SA direction on top of each other in the FA direction, thereby adjusting the BPP SA  of each emitter to the BPP SA  of the associated fiber area. However, handling and positioning of the individual emitters is very complex and this arrangement also requires a relatively large amount of space. 
     SUMMARY 
     The present disclosure is directed towards a diode structure that includes a diode bar with single emitters or a stack of diode bars with single emitters where the beam parameter product of the generated laser diode radiation is adjusted to the round beam parameter product of a fiber, into which the laser diode radiation is to be coupled, in a simple fashion and with compact dimensions. 
     The BPP SA  of the stripe emitters decreases starting from the center of the diode laser structure towards both edges of the diode laser structure, in particular in a mirror-symmetrical fashion with respect to the center of the diode laser structure. In an alternative and/or supplementary fashion, the stripe emitters can be offset from each other in their beam propagation direction. 
     At least some of the stripe emitters disposed next to each other in the SA direction have different BPP SA , which are adjusted in each case to the BPP SA  of the associated fiber areas for optimum coupling into a round fiber. Towards this end, the BPP SA  of the stripe emitters can be selected in such a fashion that they can be optically combined into a beam parameter product which is approximately round in the BPP SA -BPP FA  plane. In other words, the output radiation of the stripe emitters is adjusted with respect to the BPP of the fiber in the SA direction such that the laser diode radiation can be coupled into the fiber with high efficiency. The output power and/or efficiency can be increased by varying the beam parameter products of the stripe emitters. This approach can also be transferred to other fiber cross-sections such as, e.g., elliptical fiber cross-sections. 
     The stripe emitters can be formed using monochromatic emitters, (e.g., monochromatic multimode emitters) each having the same laser wavelength. Within the scope of the application, the term monochromatic emitters, i.e., emitters with one wavelength, also includes emitters which emit over the same bandwidth. 
     The beam parameter product of a stripe emitter can be adjusted by its width and by the divergence of the emitted laser beam. In some embodiments, at least some of the stripe emitters have different widths in the direction of their slow axes, whereas in other embodiments, at least some of the stripe emitters have different divergences with respect to the slow axis. 
     One or more of the stripe emitters can each be formed from a single emitter such that the diode laser structure has single emitters of different widths, each of which can have the same or different divergence with respect to the slow axis. 
     In some embodiments, one or more of the stripe emitters can each be formed from several single emitters that have the same or different widths and can have the same or different divergence with respect to the slow axis. Stripe emitters having different widths can each be formed using a different number of identical single emitters. Stripe emitters formed in this manner have the same divergence with respect to the slow axis. 
     For coupling the laser diode radiation into a fiber, the stripe emitters can have downstream optics which image the laser beams emitted by the stripe emitters in the form of parallel laser beam strips optically disposed next to each other in the direction of their fast axes, and onto a round fiber, where the beam parameter product, with respect to the slow axis, of each laser beam strip is adjusted to the beam parameter product, with respect to the slow axis, of an associated fiber area, i.e., the laser beam strip is adjusted with respect to the BPP of the associated fiber area in the SA direction. 
     Further advantages of the invention can be extracted from the claims, the description and the drawing. The features mentioned above and below can be used individually or collectively in arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but have exemplary character for describing the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic that shows a diode laser bar with a first diode laser structure. 
         FIGS. 2   a  to  2   c  are schematics that show different embodiments of two stripe emitters of different width corresponding to the region II of  FIG. 1 . 
         FIG. 3  is a schematic that shows a diode laser bar with a second diode laser structure. 
         FIG. 4  is a schematic that shows an optical path of laser beams emitted by a diode laser structure and coupled into a fiber by means of optics. 
         FIG. 5  shows the beam parameter products of laser beam strips emitted by stripe emitters and stacked on top of each other, in a BPP SA -BPP FA  plane. 
         FIGS. 6   a  to  6   e  are schematics that show additional bar structures of a diode laser structure. 
         FIG. 7  is a schematic that shows a diode stack of several diode laser bars each having the first diode laser structure shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The diode laser bar  1  shown in  FIG. 1  has a diode laser structure  2  with eight stripe emitters  31  to  38  which are disposed parallel next to each other, the slow axes of which are oriented in the same direction (which is horizontal in  FIG. 1 ) and which are each disposed at an offset from each other in this horizontal direction. Some of the stripe emitters  31  to  38  have different widths w 1  to w 8  in the SA direction, where the width of the stripe emitters  31  to  38  decreases starting from the center of the diode laser structure  2  towards the two edges of the diode laser structure  2  in a mirror-symmetrical fashion. The following applies: w 4 =w 5 &gt;w 3 =w 6 &gt;w 2 =w 7 &gt;w 1 =w 8 . The width of the central emitters  34 ,  35  is, e.g., 50 to 500 μm. In a particular example, the width of the central emitters  34 ,  35  is between about 100 to 200 μm. The emitter width decreases towards the edges by up to 50%. For example, the emitter width can decrease toward the edges by up to 30% from emitter to emitter. The decrease in emitter width from emitter to emitter can be identical or different both in relative and absolute terms. The laser beams  41  to  48  emitted by the stripe emitters  31  to  38  can have the same divergence θ SA  in the SA direction. The beam parameter product, BPP SA (i)=w SA (i)*θ SA (i)=w SA (i)*θ SA  (i=1, . . . , 8) with respect to the SA direction, is maximum in the center of the diode laser structure  2  and decreases towards the two edges of the diode laser structure  2 . That is, the following applies: BPP SA ( 4 )=BPP SA ( 5 )&gt;BPP SA ( 3 )=BPP SA ( 6 )&gt;BPP SA ( 2 )=BPP SA ( 7 )&gt;BPP SA ( 1 )=BPP SA ( 8 ). In contrast, the stripe emitters  31  to  38  can have the same height in the FA direction and their emitted laser beams  41  to  48  can have the same divergence in the FA direction. As is described in more detail below with reference to  FIG. 5 , the beam parameter products BPP SA ( 1 ) to BPP SA ( 8 ) with respect to the SA axis of the stripe emitters  31  to  38  can be selected such that they are optically combined into a beam parameter product which is approximately round in the BPP SA -BPP FA -plane. 
     The stripe emitters  31  to  38  can be formed by monochromatic emitters, such as multimode emitters, each having the same laser wavelength. Within the scope of the present specification, the term monochromatic emitters, i.e., emitters with one wavelength, also includes emitters emitting over the same bandwidth. 
       FIG. 2  shows different embodiments for the stripe emitters  33 ,  34  of different widths shown in  FIG. 1 . In  FIG. 2   a , the stripe emitters  33 ,  34  of different widths are each formed by one single emitter  53 ,  54 , respectively. As an alternative to the use of single emitters of different widths, the single emitters can also be reproduced by groups with a different number of emitters. In  FIG. 2   b , the stripe emitters  33 ,  34  of different widths are each formed by a different number of identical, and therefore equally wide single emitters  6 , thus ensuring identical threshold currents for each of the stripe emitters. The wider stripe emitter  34  in  FIG. 2   c  is formed by two single emitters  7 ,  8  having different widths, which is suitable for stripe emitters to be presented having a width of more than 200 μm. 
     The diode laser bar  1  shown in  FIG. 3  differs from the diode laser bar of  FIG. 1  in that some of the stripe emitters  31  to  38  which are disposed next to each other, have a different divergence θ SA ( 1 ) to θ SA ( 8 ) with respect to the slow axis in each case, and each of the stripe emitters  31  to  38  have the same width w. The divergence of the stripe emitters  31  to  38  decreases starting from the center of the diode laser structure  2  towards both edges of the diode laser structure  2 , for example, in a mirror-symmetrical fashion. The following applies: θ SA ( 4 )=θ SA ( 5 )&gt;θ SA ( 3 )=θ SA ( 6 )&gt;θ SA ( 2 )=θ SA ( 7 )&gt;θ SA ( 1 )=θ SA ( 8 ). The beam parameter product BPP SA (i)=w(i)*θ SA (i)=w*θ SA (i) (i=1, . . . , 8), with respect to the SA direction, is maximum in the center of the diode laser structure  2  and decreases towards the two edges of the diode laser structure  2 , for example, in a mirror-symmetrical fashion. In other words, the following applies: BPP SA ( 4 )=BPP SA ( 5 )&gt;BPP SA ( 3 )=BPP SA ( 6 )&gt;BPP SA ( 2 )=BPP SA ( 7 )&gt;BPP SA ( 1 )=BPP SA ( 8 ). As is described in more detail below with respect to  FIG. 5 , the beam parameter products BPP SA ( 1 ) to BPP SA ( 8 ) of the stripe emitters  31  to  38 , with respect to the slow axis, are selected in such a fashion that they can be optically combined into a beam parameter product which is approximately round in the BPP SA -BPP FA  plane. 
       FIG. 4  schematically shows the optical path of the laser beams  41  to  48  which are emitted by the inventive diode laser structure  2 . The laser beams  41  to  48  are coupled into a round fiber  11  by means of an optics  10 . The optics  10  transforms the individual laser beams  41  to  48  into a common laser beam in which the laser beams  41  to  48  emitted by the stripe emitters  31  to  38  are imaged onto the round fiber  11  as parallel laser beam strips, which are stacked on top of each other in the direction of their fast axes. 
       FIG. 5  shows in the BPP SA -BPP FA  plane the beam parameter products BPP( 1 ) to BPP( 8 ) of the laser beam strips which are emitted by the stripe emitters  31  to  38  and stacked on top of each other. The beam parameter products, BPP( 1 ) to BPP( 8 ), are adjusted to the round beam parameter product BPP(F) of the round fiber  11 . Towards this end, the beam parameter products BPP( 1 ) to BPP( 8 ) in the slow axis are adjusted to the associated beam parameter product of the associated fiber area. In other words, the beam parameter products BPP SA ( 1 ) to BPP SA ( 8 ) of the stripe emitters  31  to  38  are selected such that they can be optically combined into the beam parameter product BPP(F) of the fiber  11 , which is round in the BPP SA -BPP FA  plane. 
     In the diode laser structures  2  shown in  FIGS. 1 and 3 , the decoupling surfaces of the stripe emitters  31  to  38  lie in a common plane that is perpendicular to the direction of propagation of the laser beams  41  to  48  emitted by the stripe emitters  31  to  38 .  FIGS. 6   a  to  6   e  show additional diode laser structures  2  of diode laser bars  1 , in which the decoupling surfaces of the stripe emitters  31  to  38  do not lie in a common plane that is perpendicular to the direction of propagation of the laser beams  41  to  48  emitted by the stripe emitters  31  to  38 . 
     In  FIGS. 6   a  and  6   b , the decoupling surfaces of the stripe emitters  31  to  38  are disposed such that their ends facing away from the decoupling surfaces are in a common plane but have different lengths in the direction perpendicular to their SA and FA directions. The length of the stripe emitters  31  to  38  increases ( FIG. 6   a ) or decreases ( FIG. 6   b ) starting from the center of the diode laser structure  2  towards both edges of the diode laser structure  2 . The decoupling surfaces of the stripe emitters  31  to  38  are offset from each other in the form of steps starting from the center of the diode laser structure  2  on both sides in the SA direction. 
     In  FIGS. 6   c  and  6   d , the stripe emitters  31  to  38  have the same length in the direction perpendicular to their SA and FA directions and are disposed offset from each other in the form of steps on each side in the SA direction. The decoupling surfaces of the stripe emitters  31  to  38  are offset from each other in the form of steps on both sides in the SA direction starting from the center of the diode laser structure  2 . While in  FIG. 6   c , the diode laser bar  1  is designed in the form of steps on the side facing away from the decoupling surfaces of the stripe emitters  31  to  38 , the diode laser bar  1  in  FIG. 6   d  is not designed in the form of steps on the side facing away from the decoupling surfaces of the stripe emitters  31  to  38 . In  FIG. 6   e , identical stripe emitters  31  to  38  are disposed offset from each other in the form of steps in the SA direction. In  FIG. 7 , several diode laser bars  1  with the diode structure  2  shown in  FIG. 1  are combined into a diode stack  20 , in which each diode laser bar  1  is disposed between two heat sinks  21 . 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.