Abstract:
A diode-laser bar is mounted on water-cooled heat-sink between two ceramic sub-mounts for electrically isolating cooling-water in the heat-sink from the diode-laser bar. Mounting between the two ceramic sub-mounts also provides for balancing stresses due to differences in coefficient of thermal expansion (CTE) between the sub-mounts and the diode-laser bar. Both sub-mounts are in thermal communication with the heat-sink for providing two-sided cooling of the diode-laser bar.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to diode-laser bar packaging. The invention relates in particular to packaging diode-laser bars on a water-cooled heat-sink. 
     DISCUSSION OF BACKGROUND ART 
     The term “packaging” applied to diode-laser bars refers to mounting a diode-laser bar, or an array of diode-laser bars, on some sort of cooling-base or heat-sink. This base may be a relatively massive base providing a “conductively cooled package” (CCP). For higher power operation, the base is typically water-cooled, for example by a micro-channel arrangement. 
     A diode-laser bar includes a plurality of semiconductor layers epitaxially grown on a single-crystal substrate, with a plurality of diode-laser emitters defined in the epitaxial layers. Typically, the substrate is an n-type substrate, and layers are grown such that layers forming the “p-side” (anode-side) of the diodes are uppermost. The diode-laser bar is soldered “p-side down” either directly onto the heat-sink or via a sub-mount having a coefficient of thermal expansion (CTE) intermediate that of the substrate material and the heat-sink material, which is usually copper. 
     Electrical connection to the diode-laser bars places the heat-sink and cooling-water therein, in electrical contract with the diode-laser bar energizing circuit. In an array of diode-laser bars, the water can short-circuit electric connection to the bars, unless the electrical conductivity of the water is low. A common solution to this is to use de-ionized (DI) or high-resistance water. However, DI water is more corrosive on metals than water from conventional building supplies, and the use of DI water is expensive and inconvenient by comparison. 
     In micro-channel cooled arrangements, cooling-channels generally have internal dimensions of about 0.5 millimeters (mm) or less with water forced through the channels by high pressure at relatively high velocities. This also can lead to rapid corrosion of the copper in which the water cooling-channels are formed. This corrosion can be somewhat mitigated by plating the water cooling-channels with a metal such as gold. However, since the micro-channels are “internal” to the heat-sink the plating can only be achieved by immersion-plating usually using forced-flow plating-solutions. This results in uneven plating, with the internal nature of the channels preventing non-destructive inspection for quality assurance. There is a need for an improved water-cooled heat-sink for diode-laser bars, that will facilitate, or eliminate a need for, plating of cooling-channels, and that does not require the use of de-ionized water. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to methods and apparatus for cooling one or more diode-laser bars during operation. In one aspect, diode-laser apparatus in accordance with the present invention comprises a heat-sink including first and second cooling-members spaced apart, facing each other, and in thermal communication with the heat-sink. First and second thermally conductive, electrically insulating, sub-mounts of a ceramic material are provided, each thereof having first and second opposite surfaces. A diode-laser bar assembly is located between the first and second cooling-members. The diode-laser bar assembly includes a diode-laser bar solder-bonded between the first surfaces of the first and second sub-mounts, with the second surfaces of the first and second sub-mounts in thermal communication with respectively the first and second cooling-members. 
     In an embodiment of the present invention wherein the diode-laser bar is one of a stack of diode-laser bar assemblies mounted on the heat-sink and the heat sink is water-cooled, this diode-laser bar assembly and mounting arrangement provides for electrically isolating the cooling-water from the diode-laser bars. This eliminates the need for using de-ionized or high-resistance water for cooling. Other advantages and embodiments of the present invention will be evident to one skilled in the art from the detailed description of the invention set forth below. 
    
    
     
       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 a three dimensional view that schematically illustrates a water-cooled heat-sink for a diode-laser bar stack in accordance with the present invention including a copper cooling-unit having an integral array of cooling-members, spaced apart and parallel to each other, with the cooling-unit attached to a steel base with conduits for introducing cooling-water into and out of the heat-sink. 
         FIG. 1A  is an exploded view from above of the heat-sink of  FIG. 1  schematically illustrating an integral manifold arrangement in the steel base cooperative with the conduits of  FIG. 1 , and cooperative with an integral array of fins cooperative with the array of cooling-members in the copper cooling-unit for flowing water through the cooling-members. 
         FIG. 1B  is an exploded view from below of the heat-sink of  FIG. 1  schematically illustrating a plurality of slots extending into the cooling-members of the cooling-unit, the slots being equal in number and spacing to the number and spacing of the cooling-members. 
         FIG. 1C  is a cross-section view seen generally in the direction  1 C- 1 C of  FIG. 1 , schematically depicting a longitudinal aspect of a cooling macro-channel formed by insertion of a fin of the base of  FIG. 1A  in a slot of the cooling-unit of  FIG. 1B . 
         FIG. 1D  is a cross-section view seen generally in the direction  1 D- 1 D of  FIG. 1C  schematically depicting a lateral aspect of the cooling-member of the cooling-unit, macro-channels therein, and a diode-laser bar bonded between two ceramic sub-mounts between adjacent ones of the sub-members, with one of the sub-mounts bonded to one of the adjacent ones of the cooling-members. 
         FIG. 2  is a fragmentary plan view from above of the cooling-unit of  FIG. 1  schematically illustrating three diode-laser bars mounted between ceramic sub-mounts, mounted in-turn between cooling-members of the cooling-unit, with the diode-laser bars connected electrically in series. 
         FIG. 3  is a three-dimensional view, schematically illustrating a preferred embodiment of a water-cooled heat-sink in accordance with the present invention for a diode-laser bar, the heat-sink including a copper cooling-unit on which the diode-laser bar is mounted, the copper cooling-unit being attached to a steel base with conduits for introducing cooling-water into and out of the heat-sink, with the base and cooling-unit being configured such that, when assembled together, a plurality of spaced-apart parallel macro-channels if formed through which water flows for cooling the cooling-unit. 
         FIG. 3A  is an enlarged three-dimensional view schematically illustrating details of the cooling-unit of  FIG. 3  including the diode-laser bar sandwiched between ceramic (insulating) sub-mounts in the manner depicted in  FIG. 2 . 
         FIG. 3B  is an exploded enlarged three-dimensional view schematically illustrating further details of the cooling-unit of  FIG. 3  including components for isolating cooling-water from electrical connections to the diode-laser bar. 
         FIG. 3C  is a three-dimensional view from below illustrating a recess formed in the cooling-unit of  FIG. 3 , the recess including spaced apart grooves which form the macro-cooling-channels when the cooling-unit is assembled on the base. 
         FIG. 3D  is an enlarged three-dimensional view of the base of the heat-sink of  FIG. 3  schematically illustrating a mating-block which inserts into the cooling-unit recess of  FIG. 3C  for closing the grooves to form the macro-channels, and illustrating a system of plenums and conduits for leading water to and from the macro-channels. 
         FIG. 3E  is a hypothetical three-dimensional view of the system of conduits, plenums, and macro-channels of  FIG. 3D , with surrounding parts of the base and cooling-unit removed to reveal details of the system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1  schematically illustrates a water-cooled heat-sink  20  for a diode-laser bar stack in accordance with the present invention. Heat-sink  20  includes a copper cooling-unit  22  including elongated rectangular cooling-members (mounting-members)  24  spaced apart and parallel to each other, extending upward from the cooling-unit in a width-direction of the cooling-members, as illustrated. Diode-laser bars, not shown in this view, are mounted between the cooling-members. Preferably, the cooling-members  24  are an integral part of cooling-unit  22 , i.e., the cooling-unit including the cooling-members is machined from a single piece of copper. Cooling-unit  22  is attached by screws  26  to a base  28  including conduits  30  for introducing cooling-water into and out of the heat-sink. The choice of which conduit is an input conduit and which conduit is an output conduit, here, is somewhat arbitrary. Base  28  is preferably made from a material which is easily machined. One preferred material is stainless steel. 
     In operation of heat-sink  20 , cooling-water flows into base  28  through a selected one of conduits  30 , through each of cooling-members  24  (in parallel) in cooling-unit  22  and out of the other conduit  30 . A description of a preferred arrangement for this cooling-water flow is set forth below with reference to  FIG. 1A ,  FIG. 1B , and  FIG. 1C . Fittings or coupling units for connecting to conduits  30  to supply water and disposal hoses or tubes are not shown for simplicity of illustration. Such fittings can be selected from several well-known and commercially available fittings. 
       FIG. 1A  is an exploded view from above of heat-sink  20  of  FIG. 1  schematically illustrating an integral manifold arrangement  31  in the base  28  cooperative with conduits  30 , and cooperative with cooling-members  24  in cooling-unit  22 . Manifold  31  includes elongated plenums  36  machined into surface  32  of base  28 . Here, the input plenum is designated as plenum  36 A and the output plenum is designated as plenum  36 B, corresponding to the designation of the input and output conduits with which the plenums connect. Screws  26  for attaching unit  22  to base  28  are not shown in this view. The screws extend through holes  27 , here countersunk, in unit  22 , and are received by threaded holes  29  in base  28 . 
     Between plenums  36 A and  36 B is an array of boat-shaped fins  34  (fins with quarter-rounded ends). The fins are spaced apart and parallel to each other with a center-to-center spacing equal to the center-to-center spacing of cooling-members  24  in unit  22 . Surrounding the manifold arrangement of plenums and fins is a trench or groove  40  configured for accommodating a water-seal (not shown in this view) such as an elastomer ring or the like. It is preferable that the base, including plenums, fins, and the water-seal groove are machined from a single piece of material. 
       FIG. 1B  is an exploded view from below schematically depicting slots  42  extending into cooling-members  24  of cooling-unit  22 . Also depicted are threaded holes  46  in base  28 . These are provided for attaching the heat-sink unit to a base, walls, or a support structure of a housing in which the inventive heat-sink will be used. 
       FIG. 1C  is a cross-section view seen generally in the direction  1 C- 1 C of  FIG. 1 . This view depicts a longitudinal aspect of slots  42  in unit  22 . Here, the slots have a bathtub-like longitudinal shape, cooperative with the boat-shape of fins  34  such that, when each fin is inserted into a corresponding slot, a macro-channel  50 , having a height H, is formed in each cooling-member  24 . Height H, of course results from a difference in the depth of the slot and the height of the fin being less than the depth of the slot). 
     Macro-channel  50  has rounded corners resulting from the longitudinal shape selection of the fins and slots. The ends of channel  50  align with plenums  36 A and  36 B in base  28 . The plenums are in fluid communication with conduits  30  via ducts  46 A and  46 B in base  28 .  FIG. 1D  is a cross-section view seen generally in the direction  1 D- 1 D of  FIG. 1C  schematically depicting a lateral aspect of cooling-members  24  of cooling-unit  22  with macro-channels  50  having a width W. 
     A particular advantage of this inventive, two-piece construction for providing cooling macro-channels  50  is that surfaces that form the channels can be plated, for example gold-plated, by conventional electroplating methods. The plating can be inspected before heat-sink  22  is assembled. Preferably, at least those surfaces provided by copper cooling-unit  22  should be plated. Surfaces of the channels provided by base  28  may be plated if the selected base material is not inherently corrosion resistant. 
     Regarding dimensions of macro-channels  50 , for a cooling-member  24  having a width of about 1.0 mm, each macro-channel preferably has a height H of between about 3.7 mm and about 4.0 mm, and a width W of about 0.5 mm. These dimensions are provided for guidance only and should not be considered limiting. 
     The shape of the rounded corners of the macro-channels is not critical, but is provided to ensure that there is free flow of cooling-water as depicted, avoiding any sharp corners or recesses in which water could be trapped. Suitable channel-dimensions and corner-shape can be readily determined, by trial and error, for any predetermined range of pressure difference between inlet and outlet, using commercially available thermal-analysis software such as SolidWorks, from Dassault Systèmes of Vélizy-Villacoublay, France. The channel-width should bring cooling-water close enough to the surface of the cooling-members to optimize cooling while still leaving the cooling-member sufficiently rigid to support bonding operations for diode-laser bars. 
     Continuing with reference to  FIG. 1D , a lateral aspect of a particularly preferred mounting scheme in accordance with the present invention for a diode-laser bar in a space  25  between adjacent cooling-members  24  is depicted. Here, a diode-laser bar  60  is solder-bonded between metallized surfaces  64 A and  64 B of two ceramic (insulating) sub-mounts  62 A and  62 B. The ceramic material of the sub-mounts is preferably relatively highly thermally conductive. 
     Another factor influencing the choice of ceramic material is the CTE, which should be compatible with substrate material of the diode-laser bar, the solder used for the bonding and the diode-laser bar substrate material. For gallium arsenide (GaAs) substrates, suitable ceramic materials include beryllium oxide (BeO) and aluminum nitride (AlN). 
     These materials permit that a hard solder such as gold/tin (Au/Sn) solder can be used to bond the diode-laser bar to the sub-mount without inducing intolerable stress on the diode-laser bar due to thermal cycling (on and off operation) during normal use. One advantage of bonding the diode-laser bar between two ceramic sub-mounts is that whatever stress is produced is balanced, thereby minimizing distortion of the diode-laser bar and alignment of emitters thereof. Slow axis misalignment of emitters in a diode-laser bar is whimsically termed ‘smile” by practitioners of the art. 
     One of the sub-mounts, between which diode-laser bar  60  is bonded, has a metallized surface  66  solder-bonded to one of the cooling-members. In  FIG. 1D  sub-mount  62 A is bonded to the cooling-member. Preferably a thermally conductive packing  68 , such as a shim or plated solder material, is inserted between the “un-bonded” sub-mount (here, sub-mount  62 B) and the cooling-member to put the sub-mount in thermal communication with a cooling-member. Clearly, a better thermal communication is established between the bonded sub-mount and the cooling-member to which it is bonded. Accordingly, it is preferable that the epitaxial-layers side (p-side or anode-side) of the diode-layer bar is bonded to the “bonded sub-mount”. However, with a sufficiently thin sub-mount, for example less than about 0.4 mm thick, and the inclusion of shim  68 , there is still effective cooling of the n-side of the epitaxial layers of the diode-laser bar. 
       FIG. 2  is a fragmentary plan view from above of the cooling-unit of  FIG. 1  schematically illustrating three diode-laser bars mounted between ceramic sub-mounts mounted in-turn between cooling-members of the cooling-unit as discussed above with reference to  FIG. 1D . It should be noted in particular that the cooling-members are sufficiently long that the entire length of a diode-laser bar can be in communication with the straight portion (between rounded corners) of the cooling-channels in the cooling-members. 
     Further, ceramic sub-mounts  62 A and  62 B are sufficiently long to permit a partial overlap of a length equal to or greater than the length of the diode-laser bar. The partial overlapping is done with the non-overlapped portions of the sub-mounts at opposite ends of the diode-laser bar, and in this instance, the overlapping is sequentially alternated between adjacent pairs of sub-mounts. 
     This alternate partial overlapping arrangement of the metallized sub-mounts permits convenient connection of the diode-laser bars in series. In this arrangement, reading from left to right, a positive (Pos) lead is connected to the non-overlapped part of sub-mount  62 A at one end of diode-laser bar  60  and a negative (Neg) lead is connected to the non-overlapped part of sub-mount  62 B at the other end of the diode-laser bar. The negative lead from sub-mount  62 A is connected to a positive lead attached to sub-mount  62 A of the next-diode-laser bar, and so on. 
     The connecting leads (sheet or strip electrodes) are made sufficiently rigid that the shape of the electrodes is retained in normal use, and cannot accidentally come into contact with an exposed part of a cooling-member. Because of this, and because of there being an electrically insulating sub-mount on each side of the diode-laser bars, cooling-water in the heat-sink is electrically isolated from the diode-laser bars. 
     For convenience of illustration, optical axes (well-known fast- and slow-axes) of the diode-laser bars are shown inset in  FIG. 2 . The emission direction of the emitters of the diode-laser bars is as indicated, i.e., perpendicular to the plane of the drawing, in the extension-direction of the cooling members. A plurality of diode-laser bars arranged in this manner is typically referred to as a vertical-stack or fast-axis stack of diode-laser bars. 
     Exemplary dimensions in the arrangement of  FIG. 2  are as follows. The length of cooling-members  24  is about 20 mm; the thickness of the cooling-members is about 1 mm; the width (“thickness”) of spaces  25  between the cooling-members is about 1 mm. Here again, these dimensions are provided for guidance only, and should not be considered as limiting the present invention. 
     Principles of the invention described above in the context of cooling a fast-axis stack of diode-laser bars are equally applicable to cooling a single diode-laser bar. By way of example,  FIG. 3  schematically illustrates a preferred embodiment of a diode-laser bar package  70  including water-cooled heat-sink in accordance with the present invention. The diode-laser bar is in a “sandwich”  90  between metallized, ceramic, electrically insulating sub-mounts as described above for the inventive fast-axis stack arrangement. The diode-laser bar axes are shown inset in  FIG. 3 . The heat-sink of package  70  includes a copper cooling-unit  72  on which the diode-laser bar sandwich is mounted. The copper cooling-unit is attached to a steel base  74  with a conduit  76  for introducing water into the package and a conduit  77  for delivering water from the package. Details of the conduit arrangements (not shown) within base  74  and cooling-channels or macro-channels (also not shown) are discussed in detail further hereinbelow. 
     Continuing with reference to  FIG. 3 , and with reference in addition to  FIG. 3A , and  FIG. 3B , diode-laser bar sandwich  90 , comprising diode-laser bar  60  bonded between ceramic sub-mounts  62 A and  62 B is bonded to cooling-unit in the form of an integral mounting-platform  73  of cooling-unit  72 . The emission direction of the diode-laser bar is parallel to surface  73 A of the mounting platform. 
     The diode-laser bar is bonded epitaxial-side (p-side or anode-side) down on sub-mount  62 A, which is the sub-mount in contact with platform  73 . A separate cathode-side cooling-block (cooling-unit)  80  is bonded to ceramic sub-mount  62 B. A thermally conductive packing or shim  92  of solder material, such as indium (In) or the like, places cathode-side cooling-block in thermal communication with a raised portion  75  of the cooling-unit (see  FIG. 3A ). Extended end-portions  80 A (see  FIG. 3B ) of the cathode-side cooling-block are provided for mounting collimating optics (not shown) for the diode-laser bar. Cooling-water flows in contact with platform  73  and part of raised portion  75  as outlined in phantom in  FIG. 3B . 
     Terminal blocks  82  and  84  (anode and cathode respectively) are attached to raised portion  75  of cooling-unit by screws and insulating bushings (not shown), with insulating pads  86  placed between the blocks and the raised portion of the cooling-unit. This is important in preventing any electrical contract between the terminal blocks and the cooling-unit. 
     Electrical contact with the diode-laser is made from electrical leads clamped at one end thereof between terminal blocks  82  and  84  and corresponding insulators  86 , and bonded the opposite end thereof to the (metallized) diode-laser sides of ceramic sub-mounts  62 A and  62 B. In  FIG. 3A , a cathode lead is depicted symbolically as a wire-lead  94 . In practice, this is a sheet electrode (for current carrying capacity) but is not depicted as such in  FIG. 3A  to avoid obscuring other details of the heat-sink-assembly. In  FIG. 3B , examples  87 A and  87 B of such sheet-electrodes are depicted. Dashed lines indicate the connection of electrodes  87 A and  87 B to sub-mounts  62 A and  62 B, respectively. Here again, this method of electrical connection to the diode-laser by metallized sides of the ceramic sub-mounts is for avoiding any electrical contact between the diode-laser bar and the heat-sink. 
     Details of cooling-arrangements for the inventive heat-sink are next described with reference to  FIG. 3C ,  FIG. 3D  and  FIG. 3E .  FIG. 3C  is a three-dimensional view from below illustrating a recess  96  formed in cooling-unit  72 . The recess includes spaced-apart grooves  98  which form macro-cooling-channels when cooling-unit  72  is assembled onto the base of the heat-sink. Ridges  100  separate grooves  98  (except for end ones  98 ′ thereof). The grooves terminate in raised (less deep) portions  102  at each end of the groves (only one visible in  FIG. 3C ). 
     Regarding exemplary dimensions in  FIG. 3C , the grooves (between portions  102 ) are preferably about 0.6 mm deep (as defined by the height or depth difference between the grooves and ridges  100 ). The grooves are preferably about 1.2 mm wide. The length of recess  96  including the grooves is preferably long enough to extend along most of the length of platform  73  of cooling-unit  72 , and wide enough to extend partially under raised portion  75  of the cooling-unit, as can be seen in the phantom outline in  FIG. 3B . The total depth of recess  96  (at the groves) is preferably selected such that grooves  98  are within about 0.3 mm of the surface of platform  73  (see  FIG. 3B ) of the cooling-unit. 
       FIG. 3D  is an enlarged three-dimensional view of the base  74  of the inventive heat-sink. A footprint of cooling-unit  72  is depicted in phantom. Water conduits within the base are also depicted in phantom. Continuing reference is made to  FIG. 3C . 
     Base  74 , here, is assumed to be machined from a single piece of metal such as stainless-steel. A mating-block portion  104  of the base is configured to engage raised portions  100  in recess  96  of  FIG. 3C  for closing grooves  98  to form macro-channels. In that regard, mating-block  104  is a close fit in the length of the recess and a close fit between raised portions  102  in the recess. On opposite sides of block  104  are machined elongated plenums  106 A and  106  B. The block and plenums are surrounded by a machined groove  108  for accommodation a sealing ring. 
     Plenums correspond in position to raised (channel-terminating) portions  102  in recess  96  of  FIG. 3C . Plenum  106 A is in fluid communication with a straight portion  76 A of inlet conduit  76 . Plenum  106 B is in fluid communication with a straight portion  77 A of outlet conduit  77 . The selection of conduit  76  for inlet, and conduit  77  for outlet, is somewhat arbitrary, and should not be considered as limiting the present invention. 
       FIG. 3E  is a hypothetical three-dimensional view of the system ( 100 ) of conduits, plenums, and grooves of  FIGS. 3C and 3D , with surrounding parts of the base and cooling-unit removed to reveal details of the system. Here it can be seen that the engagement of block  104  (in the base) of  FIG. 3D  with the recess (in the cooling-unit) of  FIG. 3C , when the base and cooling-unit are assembled together, causes the grooves to become macro-channels (macro-conduits)  102 , which link inlet and outlet plenums  106 A and  106 B, respectively, and corresponding inlet and outlet conduits  76  and  77  respectively. Flow though the macro-channels is as indicated. 
     The present invention is described above in terms of two embodiments. In one aspect of the invention a heat-sink includes a single-piece copper cooling-unit, and a single-piece base-unit, which, when assembled together, form macro-channels in the cooling-unit through which water can be circulated. The term “macro-channel” as used herein implies that the channel preferably has a minimum dimension not less than about 0.2 mm. 
     One embodiment of the inventive heat-sink is configured for mounting a fast-axis stack of diode-laser bars. The other is configured for mounting a single diode-laser bar, however, a slow-axis (horizontal) array of bars could utilize a plurality of these heat-sinks on a common platform. In either embodiment, the two-piece construction allows corrosion-resistant plating of the copper portions of the macro-channels before the heat-sink is assembled. 
     In another aspect of the invention, a diode-bar is solder-bonded between an overlapping region of two metallized, ceramic sub-mounts before being mounted on the cooling-unit of the heat-sink. Each of the ceramic sub-mounts is in thermal communication with the cooling-unit for cooling the diode-laser bar. Electrical connection is made to non-overlapping portions of the metallized ceramic sub-mounts for making electrical connection to the diode-laser bar. This arrangement has an advantage that the diode-laser bar, and electrical connections thereto, are electrically isolated from the cooling-unit and the cooling-water therein, for resisting corrosion of macro-channels in the cooling-unit by the water. 
     The arrangement also has an advantage that stresses induced in the diode-laser bar due to CTE mismatch between the material of the diode-laser bar and the ceramic are balanced out, minimizing slow-axis misalignment of emitters in the diode-laser bar and providing for increased reliability under temperature cycling. This latter advantage can be enjoyed even with heat-sink arrangements that are not water-cooled, i.e., passively or conductively cooled. 
     The present invention is not limited to the above-described embodiments. Rather the invention is limited only by the claims appended hereto.