Abstract:
Optical apparatus includes a diode-laser bar stack having N fast-axis stacked diode-laser bars cooperative with a parallel sided transparent stacking plate. The stacking plate receives N original beams from the N diode-laser bars and converts the N beams to 2N fast-axis stacked beams having one-half of a width the original beams and one-half of a fast-axis spacing between the original beams.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to two dimensional arrays of diode-lasers. The invention relates in particular to vertical stacks of one-dimension arrays of diode-lasers (diode-laser bars). 
       DISCUSSION OF BACKGROUND ART 
       [0002]    Vertical diode-laser bar stacks are now used for providing optical pump radiation for high-power (1 KW) fiber lasers. One such vertical diode-laser bar stack is schematically depicted in  FIG. 1A  and  FIG. 1B . Details of one-example of the stack construction are depicted in  FIG. 1B . Here, each diode-laser bar  17  is mounted on the front of a corresponding heat sink member  19 . The heat-sink members are clamped together between clamping and mounting blocks  23 A and  23 B. Each heat-sink member has a forward-extending portion  21  to which a fast-axis collimating (FAC) lens or a module including a FAC lens and a slow-axis collimating (SAC) lens array can be attached. 
         [0003]    A 26-bar stack such as stack  18 , with nineteen emitters per bar, can deliver radiation having a total power of about 1.4 kW. Such diode-laser bars are designated by practitioners of the art as having a slow-axis (low divergence axis) aligned with the length of the diode-laser bar; a fast-axis (high divergence axis) perpendicular to the slow-axis, and a propagation-axis perpendicular to both the fast and slow axes. The slow-axis, fast-axis, and propagation-axis are alternatively designated as the x-axis, y-axis, and z-axis by practitioners of the art. 
         [0004]    Referring to  FIG. 1A , each of the diode-laser bars has a dedicated cylindrical fast-axis collimating (FAC) lens  20 , which, as the name suggests, collimates light from each emitter in the bar in the highly divergent fast-axis direction. In this example, there are twenty-six lenses  20 . Spaced apart from each FAC lens in the z-axis direction is an array  22  of cylindrical slow-axis collimating (SAC) lenses  24 . The number of lenses  24  in each array  22  corresponds with the number of spaced-apart emitters (diode-lasers) in each of the diode-laser bars. Here, it is assumed that there are nineteen ( 19 ) emitters in each bar. Each SAC lens is aligned with a corresponding emitter. The FAC lenses and SAC lens-arrays are held in alignment with each other by brackets  26  (shown on only one side in  FIG. 1A  for convenience of illustration). Assemblies of FAC and SAC lenses are available from several commercial suppliers. 
         [0005]    In such a diode-laser bar stack the vertical (fast-axis separation) of beams from adjacent diode-laser bars is limited by the thickness of the diode-laser bar substrates and the thickness of water cooled sub-mounts for the diode-laser bars. The fast-axis brightness of all combined beams from the diode-laser bar stack is limited by the fast-axis separation of the beams. The amount of the combined radiation that can be focused into an optical fiber at any particular numerical aperture is directly dependent on the total power and brightness of the radiation from the bar stack and the beam parameter product (BPP) of the focused radiation. 
         [0006]    Generally, the approach to optimizing (minimizing) the focused BPP is to limit the height of the stack, and limit the fill-factor (the ratio of total of emitter widths to the total length of the bar). The height of the stack can be limited, for any given pitch of the diode-laser bars in the stack, simply by limiting the number of diode-laser bars in the stack. A convenient compromise has been found to be a stack of 13 bars, each 10 mm long and with a fill factor of 18%, with a pitch of about 3.3 millimeters (mm). In the fast-axis, the etendue can be reduced by reducing the height of the stack, for example, by limiting the number of the vertically stacked bars. 
         [0007]    Various approaches have been suggested for limiting the fast-axis extent of a combined radiation output from a diode-laser bar stack using optical arrangements to reduce the fast-axis spacing between beams from a fixed number of diode-laser bars. A number of such approaches is described in detail in U.S. Pat. No. 6,993,059, assigned to the assignee of the present invention and the complete disclosure of which is hereby incorporated herein by reference. 
         [0008]    The simplest of these approaches uses a parallel-sided glass block in front of a diode-laser bar stack and titled away from the stack in the fast-axis. The lower part of the block on a first side thereof closest the diode-laser bar stack has an array of spaced-apart reflective strips aligned with the fast-axis of the diode-laser bars. The number of reflective strips is one-half the number of diode-laser bars and the pitch of the strips is equal to the pitch of the diode-laser bars. On the opposite (second) side of the block, above the strips in the fast-axis direction, is a reflectively coated area. Beams from the lower bars in the stack pass between the strips on the first side of the block and under the coating on the second of the bloc. Beams from the upper bars in the stack pass over the array of strip through the first side of the block are reflected by the coated area on the second side and onto the reflective strips; and are reflected by the reflective strips interspersed between beams transmitted through the strips. While this approach provides a simple means of increasing fast-axis brightness, the approach does not provide for increasing the BPP of the focused combined beams. 
       SUMMARY OF THE INVENTION 
       [0009]    In one aspect, optical apparatus in accordance with the present invention comprises a plurality N of diode laser-bars characterized as having a slow-axis in a length direction, a fast-axis perpendicular to the slow-axis, and a propagation-axis perpendicular to the slow-axis and the fast-axis. The diode-laser bars are stacked one above another in the fast-axis direction with a predetermined pitch P therebetween. A plurality N of fast-axis collimating lenses is provided one for each of the diode-laser bars and a plurality N of slow-axis collimating lens arrays is provided, one for each of the diode-laser bars, the diode-lasers bars. The fast-axis collimating lenses, and slow-axis collimating lenses provide a plurality N of combined-radiation beams propagating one above another in the fast-axis direction parallel to the propagation-axis direction. Each beam has a width W in the slow-axis direction. A transparent plate is located in the path of the combined radiation beams. The transparent plate has a thickness and first and second opposite surfaces parallel to each other. The surface are inclined to the fast-axis direction at a first angle, and inclined to the slow-axis direction at a second angle. The first surface of the plate faces the diode-laser bar stack. First and second internally reflective coatings partly cover respectively the first and second surfaces of the plate. The first and second internally reflective coatings are configured and the thickness of the plate and the first and second angles are selected such that the plate transmits 2N combined beams propagating one above another in the fast-axis direction parallel to each other in the propagation-axis direction, with each beam having a width less than W in the slow-axis direction, and with the beams spaced apart in the fast-axis direction by a distance of about P/2. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention. 
           [0011]      FIG. 1A  is an isometric three-dimensional view schematically illustrating one aspect of a prior-art vertical diode-laser bar stack. 
           [0012]      FIG. 1B  is an isometric three-dimensional view schematically illustrating another aspect of the prior-art vertical diode-laser bar stack of  FIG. 1 . 
           [0013]      FIG. 2  is a three dimensional view schematically illustrating a preferred embodiment of optical apparatus in accordance with the present invention including a fast-axis diode-laser bar stack and a stacking plate allowed to create two fast-axis stacked beams from a combined beam emitted by each of the diode-laser bars, with the created beams having about one-half of a width the original beams and having a fast-axis separation about one-half of a fast-axis separation of the original beams. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 2  schematically illustrates one-preferred embodiment of optical apparatus  40  in accordance with the present invention apparatus in accordance with the present invention including a vertical stack  42  of diode-laser bars and an inventive beam-processing plate  50  for increasing the fast-axis brightness of combined beams  44  from the diode-laser bar stack. The diode-laser bar stack is mounted on a base  41  can be considered as a simple version of the above described diode-laser bar stack with only 13 diode-laser bars stacked. Only sufficient detail of the diode-laser bar-stack is shown in  FIG. 2  for understanding principles of the present invention. 
         [0015]    Only two beams  44  from the diode-laser bar stack are shown, for simplicity of illustration. Beams from the diode-laser bars are collimated in both the fast-axis and the slow-axis as described above with reference to  FIG. 1A  and  FIG. 1B . All fast-axis collimating lenses  20  are depicted in  FIG. 2 , but only one slow-axis collimating lens array  24  is shown, again for convenience of illustration. The collimating lens-array has a number of individual collimating lenses corresponding to the number of emitters (not shown) in the diode-laser bar. Beams  44  have a width W between bounding rays depicted by bold lines. The beams have a fast-axis spacing (pitch) P indicated in  FIG. 2  as the fast-axis distance between apexes of adjacent fast-axis collimating lenses. 
         [0016]    Beam processing plate  50  has parallel faces  50 A and  50 B. A base  50 C of plate  50  is bonded to slightly wedge-shaped mounting block  43  attached to base  41 . On face  50 A of plate  50  is a parallel array of strips  60  which are highly reflective for the diode-laser radiation, at least (internally) on the side facing into the plate. The array of strips has a pitch P corresponding to the pitch of the diode-laser bars in the stack. In the illustrated embodiment, each strip is as long as the beams  44  are wide. The height of strips  60  is sufficient to completely intercept the fast-axis height of a beam  44 . Spaces between strips  60  are wide enough to allow the fast-axis height of a beam  44  to pass between adjacent strips. The parallel array of strips  60  is aligned parallel to the x-z plane of the diode-laser bars. 
         [0017]    Plate  50  is tilted (tipped) toward the fast-axis of the diode-laser bars by an angle θ, and rotated away from the slow-axis of the diode-laser bars by an angle φ. On face  50 B of plate  50  is coating  66 , here rectangular in shape and at least internally reflective. Coating  66  has a straight edge  68  aligned parallel to the fast-axis of the diode-laser bars. Edge  68  is aligned about centrally in the width of beams  44  within the plate. Coating  66  in the slow-axis direction has a width greater than half of the width of beams  44 . Coating  66  has a length in the fast-axis direction of the diode laser bars at least sufficient to intercept all beams  44  within plate  60 . It is recommended that portions of faces  50 A and  50 B not having reflective coatings  66  or  60  thereon are anti-reflection coated for the wavelength of radiation from the diode-laser bars. 
         [0018]    The function of plate  50  can be followed by following the progress of a beam  44  from the uppermost diode-laser into, through and out thereof. One half-portion of the beam-width is intercepted by reflective coating  66  allowing the other half portion  44 A to be transmitted through face  50 B of the plate in the propagation-axis direction. 
         [0019]    The half-portion  44 B intercepted by coating  66  is reflected downwards and laterally onto the uppermost reflective strip under the transmitted portion  44 A. The strip  60  reflects beam-portion  44 B in the propagation-axis direction such that beam-portion  44 B leaves plate  60  under transmitted beam portion  44 A at a level below the level of beam-portion  44 A in the z-axis direction. 
         [0020]    The processing of a beam  44  from any other than the uppermost will require that the beam pass between two adjacent strips  60  as illustrated, but will otherwise be the same. Beam cross-sections are indicated by elongated dashed rectangles to assist in following the beam progress described above. 
         [0021]    In an example of stacking plate  50  for a diode-laser bar stack having a pitch P of about 3.3 mm, the plate is a fused silica plate having a thickness of about 12 mm. Angle θ is about 5.9 degrees and angle φ is about 17.3 degrees. The reflective coatings are preferably multilayer dielectric coatings. 
         [0022]    The effect of processing (stacking) plate  50  is to take the original number of beams from the diode-laser bar stack and create therefrom twice as many beams half as wide (W/2) as the original beam, with a separation P/2 therebetween, i.e., half of the pitch (P) of bars in stack  42 . The slow-axis divergence of the two beams obtained from each original beam will be essentially the same as that of the original beam. 
         [0023]    As the slow-axis etendue of the beams stacked by the plate will be essentially half of the etendue of the original beams, this can provide for a reduced BPP of the focused beams in the slow-axis direction. The BPP in the fast-axis direction will not change appreciably, since the total width of the beam in the fast-axis direction will only increase from N times the pitch to N+½ times the pitch, where n is the number of bars. 
         [0024]    Alternatively, each of the stacked beams can be made to have the slow-axis etendue of the original beams by increasing, i.e., doubling, the fill factor of the diode-laser bars, say from the above-discussed 18% to 36%. This can about double the total power in the beams without any reduction in BPP. In other words, the  13 -bar diode-laser bar stack of  FIG. 2  will have about the same power-output as the prior art diode-laser bar stack of  FIGS. 1A and 1B . 
         [0025]    It should be noted here, that while it may be preferable to have all beam portions  44 A and  44 B aligned one above the other in the fast-axis direction, the  44 A beams and the  44 B beams may be slightly displaced one from another in the slow-axis direction without significantly adversely affecting any of the above discussed advantages of the arrangement of diode-laser bar stack and inventive stacking plate  50 . Those skilled in the art will also recognize that coating  66  could be an array of parallel strips similar to strips  60  with the array staggered such that strips of coating  66  intercepted the beams passing between or over strips  60 . 
         [0026]    In summary, the present invention is described above with reference to preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather the invention is limited only by the claims appended hereto.