Abstract:
A diode-laser bar package includes two diode-laser bars mounted on a heat sink with slow-axes of the diode-laser bars aligned. Three spaced apart mounting bosses extend from the front of the heat sink. The package includes two cylindrical lens assemblies. Each lens assembly has an elongated cylindrical lens bonded to a rectangular mounting block. The cylindrical lenses are aligned with the diode-laser bars. Lateral faces of the mounting block are epoxy-bonded to lateral faces of the mounting bosses. Bonding is effected by injecting liquid epoxy between the faces to be bonded though holes extending through the mounting blocks from front to the lateral faces of the mounting blocks. The liquid epoxy is then cured to complete the attachment.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates in general to packaging of diode-laser bars. The invention related in particular to method of mounting a cylindrical fast-axis collimating lens in a diode-laser bar package. 
   DISCUSSION OF BACKGROUND ART 
   A diode-laser (edge-emitting semiconductor laser) bar usually includes a plurality of individual diode-lasers (emitters) distributed along a “bar” comprising a plurality of semiconductor layers epitaxially grown on an electrically conductive semiconductor substrate. Such a bar usually has a length of about 10 millimeters (mm), a width of between about 1 mm and 1.5 mm, and a thickness of between about 100 micrometers (μm) and 300 μm. The emitters (diode-lasers) of the bar are formed in the epitaxial layers. 
   In a diode-laser bar configured to deliver near infrared radiation with a power of about 1 Watt (W) per emitter or more, the width of the emitters is typically between about 50 μm and 200 μm. Usually, the wider the emitter the higher the power output of an individual emitter. The number of emitters in a bar is determined by the length of the bar, the width of the emitters, and the spacing therebetween. Twenty emitters per bar is not an uncommon number of emitters per bar. 
   The emitters are aligned in the bar along an axis generally designated the slow-axis of the bar. This axis is so named because the beam emitted by an emitter has a relatively low divergence in this axis, for example about 10°. An axis perpendicular to the slow axis is designated the fast axis, as in this axis the emitted beams have a divergence of about 35° or even greater. In most applications of a diode laser bar it is necessary to collimate the emitted beams in the fast-axis. In a diode-laser bar package this is typically done by aligning a positive cylindrical lens, having a length about equal to the length of the bar, with the slow axis of the emitters at about a focal length of the lens, usually less than 1 millimeter (mm) from the emitters. A diode-laser bar package usually includes a heat-sink to which the bar is thermally connected, the fast-axis collimating lens fixed to the package in some way, and electrical arrangements for connecting electrical current to the emitters of the diode-laser bar. All of these components are assembled with an assortment of clamps, solders, and adhesives. 
   It is essentially impossible to have all of these components, clamps, solders, adhesives matched for thermal expansion coefficient. This is particularly true of the cylindrical lens and mounting arrangements thereof. As a result of this, the cylindrical lens in most commercial diode-laser bar packages is very vulnerable to misalignment due to thermal cycling. A few micrometers misalignment of the cylindrical lens in the fast axis can cause problematic changes in beam pointing. Most commercially available diode-laser bar packages will experience fatal lens misalignment in less than 50 thermal cycles between −55° C. and 85° C. Improving thermal-cycle lifetime of the cylindrical lens mounting in diode-laser bar packages presents a continuing challenge to manufacturers of such packages. 
   SUMMARY OF THE INVENTION 
   In one aspect of the present invention apparatus in accordance with the invention comprises a heat-sink having an elongated diode-laser bar in thermal communication therewith. The diode-laser bar has a slow-axis in the length direction of the diode-laser bar, a fast axis perpendicular to the slow-axis, and an emitting-axis perpendicular to the slow-axis and the fast axis. Two bosses extend from a front face of the heat-sink. Each of the bosses has a lateral face transverse to the slow axis of the diode-laser. The lateral faces of the bosses are spaced apart, facing each other in a direction parallel to the slow axis of the diode-laser bar. A lens assembly includes a cylindrical lens bonded to an elongated mounting slab. The mounting slab has first and second lateral faces at each end thereof. The lens assembly is positioned on the heat sink with the length of the cylindrical lens aligned with the slow axis of the diode-laser bar and with the lateral faces of the mounting slab of the lens assembly attached by an adhesive to corresponding ones of the lateral faces of the mounting bosses. 
   In a preferred embodiment of the invention, each of the mounting bosses has a hole extending therethrough from a front face thereof to the lateral face thereof. The length of the mounting slab of the lens assembly and the spacing between the lateral faces of the mounting bosses of the heat sink are selected such that there is a relatively narrow gap between each lateral face of the mounting slab and the corresponding lateral face of the mounting boss. The adhesive is injected in liquid form into the gaps via the corresponding holes in the mounting bosses and cured to harden the adhesive and complete attachment of the lens assembly to the diode-laser package. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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. 
       FIG. 1  is an exploded three-dimensional view schematically illustrating one preferred embodiment of a diode-laser bar package in accordance with the present invention before a fast-axis collimating lens is added, the package includes two slow-axis-aligned diode-laser bars mounted on a heat-sink via a thermally conductive dielectric sub-mount, and a high current interface unit arranged to be supported on the heat-sink via a block of insulating material. 
       FIG. 2  is a three-dimensional view schematically illustrating the package of  FIG. 1  in assembled form. 
       FIG. 3  is an exploded view schematically illustrating the package of  FIG. 2  and two fast-axis-collimating lens assemblies, each thereof including an elongated cylindrical fast-axis collimating lens bonded to a thermal-coefficient-of-expansion-matched mounting and stiffening tab, and each thereof attachable via this tab to two bosses integral with and extending from a front-face of the heat-sink. 
       FIG. 3A  is a fragmentary three-dimensional view schematically illustrating details of the bosses on the front face of the heat-sink of  FIG. 3 . 
       FIG. 4  is a three-dimensional view schematically illustrating the package of  FIG. 3  in assembled form. 
       FIG. 4A  is a fragmentary three-dimensional view schematically illustrating details of one of the cylindrical-lens assemblies mounted between two of the bosses on the front face of the heat-sink of  FIG. 3 . 
       FIG. 5  is a graph schematically illustrating beam size and relative beam pointing for two examples of practical diode-laser packages in accordance with the embodiment of the present invention depicted in  FIG. 4 . 
       FIG. 6  is three-dimensional view schematically illustrating another embodiment of the present invention similar to the embodiment of  FIGS. 1-4  but wherein the lens-assembly mounting-bosses are part of a separate unit attached to the heat-sink by screws. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1 , and  FIG. 2  schematically illustrate a preferred embodiment  20  of a diode-laser bar package in accordance with the present invention before a cylindrical lens assembly is added to the package.  FIG. 1  is an exploded view depicting certain separate components.  FIG. 2  depicts the components in assembled form. Package  20  includes two diode-laser bars  22 , each thereof soldered to a thermally conductive dielectric sub-mount  26 . Sub-mounts  26  are soldered to a raised portion  25  of a metal heat-sink  24 . The heat sink is preferably made from copper (Cu) or a copper-tungsten (Cu—W) alloy. Sub-mount  26  is preferably made from beryllium oxide. This selection of materials, however, should not be considered as limiting the present invention. The front edge of the diode-laser bar (not numerically designated) preferably overhangs the sub-mount by a few microns and is parallel to front face  48  of heat-sink  24 . 
   A high current interface unit  30  includes power-supply connection pads  32  and bridge pad  33  electrically isolated from each other. The pads are laminated on a plate  34  of a dielectric (fiberglass epoxy) material. Pads  32  are for connecting assembly to  22  to a high current power supply, and bridge pad  33  facilitates series connection of one diode-laser bar with the other. Intermediate electrode strips and wire bonds (not shown) connect the diode-laser bars to connection pads  32  and bridge pad  31 . Methods of connecting diode-laser bars via intermediate strips and wire bonds are well known to those skilled in the art and a description thereof is not necessary for understanding principles of the present invention. 
   High current interface unit  30  is assembled on to a rectangular block  36  of a dielectric material by screws (not shown) extending through apertures  35  in plate  34  into threaded holes  37  in block  36 . Block  36  may be formed from glass. Block  36  is attached to heat sink  24  by screws (not shown) extending upwards through apertures  44  in heat sink  24  into threaded apertures (not shown) in block  36 . Electrical leads (not shown) from a power supply (not shown) are clamped to pads  32  via screws (not shown) extending downward through apertures  40  in high current interface unit  30  into threaded holes  41  in glass block  36 . The orientations of the fast-axis, slow-axis and propagation-axis of diode-laser bar  22  are indicated in  FIG. 1  as the X-axis, Y-axis, and Z-axis respectively 
   Referring in particular to  FIG. 2 , extending from front face  48  of heat sink  24  are two end-bosses  50  and a central boss  52 . In a preferred arrangement these bosses are integral parts heat sink  24 , i.e., the heat sink, including raised portion  25  thereof and bosses  50  and  52 , is preferably machined from a single metal block. The bosses include faces  51  transverse to the slow-axis of the diode-laser bar, and preferably perpendicular to the slow-axis of the diode-laser bar as depicted in  FIGS. 1 and 2 . These faces are provided for lens assembly attachment. Bosses  50  each have a hole  54  extending therethrough from a front surface thereof (here, sloping surface  55 ) to the transverse face  51  thereof. Central boss  52  includes two such holes. An aperture  56  in face  55  functions as an entrance aperture of the hole and an aperture  58  in face  51  functions as an exit aperture of the hole. Depending on the dimensions of the structure, the entrance aperture could be located on the side face or the top face of the boss. 
     FIG. 3 ,  FIG. 3A ,  FIG. 4 , and  FIG. 4A  schematically depict attachment of two cylindrical-lens assemblies  60  to package  20 . Here, the lens assemblies, not being of symmetrical construction, are designated as a right-hand assembly  60 A and a left-hand assembly  60 B.  FIG. 3  is an exploded view depicting assemblies  60 A and  60 B before attachment.  FIG. 4  depicts the lens assemblies assembled into package  20 .  FIG. 3A  is a fragmentary enlarged view of  FIG. 3  providing an enlarged view of central boss  52  and the right-hand boss  50 .  FIG. 4A  is a fragmentary enlarged view of  FIG. 4  depicting lens assembly  60 A between bosses  52  and  50 . 
   Each lens assembly includes an elongated cylindrical lens  62  attached via an epoxy adhesive or the like to a rectangular mounting block  64 . Preferably, mounting block  64  has a coefficient of expansion matched to that of the cylindrical lens, and, most preferably, the mounting block is made from the same material as that of the cylindrical lens. This, combined with the stiffness of mounting block  64  and the matching expansion coefficients of the lens and the block, provides that there is no bending moment on the lens in the fast-axis direction of the diode-laser during thermal cycling. 
   Mounting block  64  has lateral faces  66 , here, perpendicular to the longitudinal axis of the lens, i.e., perpendicular the slow-axis of the diode-laser bar when the lens assembly is correctly mounted in the package. The length of the mounting block is preferably selected such that when the block is located between faces  51  of bosses  50  and  52 , there can be a gap  70  between each boss-face  51  and the corresponding lateral face  66  of mounting block. Preferably the gap has a width less than about 0.01 inches. A particularly preferred gap width is about 0.005 inches. 
   A lens assembly  60  is mounted in package  20  by locating mounting block  64  thereof between faces  51  of a boss  52  and a boss  50 . With diode-laser bar  22  operating to provide an alignment beam, the lens assembly is manipulated by suitable tooling (not shown) attached to the mounting block until the cylindrical lens is optimally aligned with the diode-laser bar, essentially parallel to the slow-axis of the diode-laser bar. Once the cylindrical lens is optimally aligned, the lens is held in position by the tooling, and a measured quantity of a UV and thermally curable epoxy is injected, via a hypodermic needle inserted into aperture  56  of a hole  54 , through aperture  58  of the hole, into a gap  70 . This procedure is then repeated for the other gap  70 . The injected epoxy over the faces  51  and  66 , and surface tension effects and viscosity of the liquid epoxy retain the epoxy in the gap between the faces. When the epoxy is cured (hardened) the tooling can be removed and the lens remains aligned. In a preferred curing method, liquid epoxy is initially UV cured, alignment (manipulation) tooling is removed, and the package is transferred to an oven to complete the epoxy curing thermally. A preferred UV and thermal curing epoxy is “Optocast 3410” manufactured by Electronic Materials Inc. (EMI) of Breckenridge, Colo. 
   Because of this inventive mounting arrangement for the lens assembly, the only significant forces acting on the lens assembly will be those due to any dimension change of the adhesive and those due to thermal coefficient of expansion mismatch between the metal of the bosses and the material of the mounting block of the lens assembly. These forces (free body forces) will be in opposition parallel to the slow-axis of the diode-laser bar as indicated in  FIG. 4A  by arrows T. Any net force will be directed along the slow-axis of the diode-laser bar and will not tend to misalign in the lens in the fast-axis direction of the diode-laser bar. The fast-axis direction is the critical alignment direction lens  52 . Although the a copper heat-sink  24  and a glass mounting block  64  would have a large CTE mismatch (17 PPM/° C. and 9 PPM/° C., respectively) the opposing free-body forces depicted in  FIG. 4A  imply that, even with plastic deformation of the epoxy, the lens will only move in the non-critical (slow-axis) direction and the lens performance parameters will not be affected. 
   This implication is supported by results of thermal-cycling experiments performed on two different samples of diode-laser bar packages in accordance with the present invention.  FIG. 5  graphically schematically illustrates measured relative beam-pointing for the two packages (dotted curve and solid curve) as a function of thermal cycling. Each of the thermal cycles was from −55° C. to +85° C. There was no significant change in pointing after as many as 500 such thermal cycles. The variation in each of the curves of  FIG. 5  is within the measurement accuracy. Lenses attached by prior art methods will usually fail (become fatally misaligned) after no more than 50 such thermal cycles. 
     FIG. 6  is an exploded three-dimensional view schematically illustrating another embodiment  20 A of a diode-laser bar package in accordance with the present invention. Packer  20 A is similar to above-described package  20  with an exception that heat-sink-integral lens-assembly-mounting bosses  50  and  52  of package  20  are replaced with similarly configured bosses  50 A and  52 A that are integrated into a separate unit  76 . Unit  76  is attached to front face  48  of heat sink  24  by socket screws  78 . Unit  78  may be made from the same material as the heat sink or from a different material having a thermal coefficient of expansion similar to that of the heat-sink. By way of example, a unit  78  made from titanium can be used with a heat-sink  24  made from copper. When unit  76  is attached to heat sink  24 , lens assemblies  60 A and  60 B can be attached to the package as described above with reference to package  20  of  FIGS. 1-4 . 
   It should be noted that while the present invention has been described in the context of a diode-laser bar package including two diode-laser bars. The invention is equally applicable to a package including only one diode-laser bar or a package including three or more diode-laser bars. The invention is not limited to the embodiments described and depicted herein. Rather, the invention is limited only by the claims appended hereto.