A water-cooled heat-sink for a diode-laser bar includes a copper-cooling-unit having an integral mount thereon for the diode-laser bar. The copper-cooling-unit is attached to a steel base-unit. The base-unit and the cooling-unit are cooperatively configured such that at least one cooling-channel is formed in the cooling-unit by the attachment of the base-unit to the cooling-unit. The cooling-channel is positioned to cool the mount when cooling-water flows through the cooling-channel.

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 an electrically insulating ceramic sub-mount having a high thermal conductivity and first and second opposite sides. A diode-laser bar is solder-bonded to the first side of the ceramic sub-mount. A heat-sink assembly for the diode-laser bar includes a base-unit and a copper cooling-unit, each thereof having first and second opposite sides. The cooling-unit and the base-unit are attached together with the first side of the base-unit mating with the second side of the cooling-unit. The copper cooling-unit has an integral mounting-member on the first side thereof. The second side of the ceramic sub-mount is solder-bonded to the mounting-member. The base-unit and the cooling-unit are cooperatively configured such that at least one cooling-channel having first and second opposite ends is formed in the cooling-unit by the attachment of the base-unit to the cooling-unit. The cooling-channel is arranged to cool the integral mounting-member when cooling-water flows therethrough. The base-unit includes an input-passage for directing water into the first end of the cooling-channel and an output-passage for conducting water away from the second end of the cooling-channel.

In one preferred embodiment of the invention, the apparatus is for mounting a single diode-laser on a single mounting-member in the form of a platform on the cool-unit. The cooling-channel is one of a plurality of channels formed under the platform. The diode-laser bar emits in a direction parallel to the platform.

In another preferred embodiment of the invention, the apparatus is for mounting a plurality of diode-lasers on a corresponding plurality of mounting-members spaced apart and parallel to each other extending upwards from the cooling-unit. The cooling-channel is one of a plurality of cooling-channels with one thereof in each of the mounting-members. The diode-laser bars emit in a direction parallel to the extension-direction of the mounting-members.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like components are designated by like reference numerals,FIG. 1schematically illustrates a water-cooled heat-sink20for a diode-laser bar stack in accordance with the present invention. Heat-sink20includes a copper cooling-unit22including elongated rectangular cooling-members (mounting-members)24spaced 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-members24are an integral part of cooling-unit22, i.e., the cooling-unit including the cooling-members is machined from a single piece of copper. Cooling-unit22is attached by screws26to a base28including conduits30for 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. Base28is preferably made from a material which is easily machined. One preferred material is stainless steel.

In operation of heat-sink20, cooling-water flows into base28through a selected one of conduits30, through each of cooling-members24(in parallel) in cooling-unit22and out of the other conduit30. A description of a preferred arrangement for this cooling-water flow is set forth below with reference toFIG. 1A,FIG. 1B, andFIG. 1C. Fittings or coupling units for connecting to conduits30to 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. 1Ais an exploded view from above of heat-sink20ofFIG. 1schematically illustrating an integral manifold arrangement31in the base28cooperative with conduits30, and cooperative with cooling-members24in cooling-unit22. Manifold31includes elongated plenums36machined into surface32of base28. Here, the input plenum is designated as plenum36A and the output plenum is designated as plenum36B, corresponding to the designation of the input and output conduits with which the plenums connect. Screws26for attaching unit22to base28are not shown in this view. The screws extend through holes27, here countersunk, in unit22, and are received by threaded holes29in base28.

Between plenums36A and36B is an array of boat-shaped fins34(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-members24in unit22. Surrounding the manifold arrangement of plenums and fins is a trench or groove40configured 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. 1Bis an exploded view from below schematically depicting slots42extending into cooling-members24of cooling-unit22. Also depicted are threaded holes46in base28. 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. 1Cis a cross-section view seen generally in the direction1C-1C ofFIG. 1. This view depicts a longitudinal aspect of slots42in unit22. Here, the slots have a bathtub-like longitudinal shape, cooperative with the boat-shape of fins34such that, when each fin is inserted into a corresponding slot, a macro-channel50, having a height H, is formed in each cooling-member24. 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-channel50has rounded corners resulting from the longitudinal shape selection of the fins and slots. The ends of channel50align with plenums36A and36B in base28. The plenums are in fluid communication with conduits30via ducts46A and46B in base28.FIG. 1Dis a cross-section view seen generally in the direction1D-1D ofFIG. 1Cschematically depicting a lateral aspect of cooling-members24of cooling-unit22with macro-channels50having a width W.

A particular advantage of this inventive, two-piece construction for providing cooling macro-channels50is that surfaces that form the channels can be plated, for example gold-plated, by conventional electroplating methods. The plating can be inspected before heat-sink22is assembled. Preferably, at least those surfaces provided by copper cooling-unit22should be plated. Surfaces of the channels provided by base28may be plated if the selected base material is not inherently corrosion resistant.

Regarding dimensions of macro-channels50, for a cooling-member24having 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 toFIG. 1D, a lateral aspect of a particularly preferred mounting scheme in accordance with the present invention for a diode-laser bar in a space25between adjacent cooling-members24is depicted. Here, a diode-laser bar60is solder-bonded between metallized surfaces64A and64B of two ceramic (insulating) sub-mounts62A and62B. 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 bar60is bonded, has a metallized surface66solder-bonded to one of the cooling-members. InFIG. 1Dsub-mount62A is bonded to the cooling-member. Preferably a thermally conductive packing68, such as a shim or plated solder material, is inserted between the “un-bonded” sub-mount (here, sub-mount62B) 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 shim68, there is still effective cooling of the n-side of the epitaxial layers of the diode-laser bar.

FIG. 2is a fragmentary plan view from above of the cooling-unit ofFIG. 1schematically 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 toFIG. 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-mounts62A and62B 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-mount62A at one end of diode-laser bar60and a negative (Neg) lead is connected to the non-overlapped part of sub-mount62B at the other end of the diode-laser bar. The negative lead from sub-mount62A is connected to a positive lead attached to sub-mount62A 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 inFIG. 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 ofFIG. 2are as follows. The length of cooling-members24is about 20 mm; the thickness of the cooling-members is about 1 mm; the width (“thickness”) of spaces25between 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. 3schematically illustrates a preferred embodiment of a diode-laser bar package70including water-cooled heat-sink in accordance with the present invention. The diode-laser bar is in a “sandwich”90between metallized, ceramic, electrically insulating sub-mounts as described above for the inventive fast-axis stack arrangement. The diode-laser bar axes are shown inset inFIG. 3. The heat-sink of package70includes a copper cooling-unit72on which the diode-laser bar sandwich is mounted. The copper cooling-unit is attached to a steel base74with a conduit76for introducing water into the package and a conduit77for delivering water from the package. Details of the conduit arrangements (not shown) within base74and cooling-channels or macro-channels (also not shown) are discussed in detail further hereinbelow.

Continuing with reference toFIG. 3, and with reference in addition toFIG. 3A, andFIG. 3B, diode-laser bar sandwich90, comprising diode-laser bar60bonded between ceramic sub-mounts62A and62B is bonded to cooling-unit in the form of an integral mounting-platform73of cooling-unit72. The emission direction of the diode-laser bar is parallel to surface73A of the mounting platform.

The diode-laser bar is bonded epitaxial-side (p-side or anode-side) down on sub-mount62A, which is the sub-mount in contact with platform73. A separate cathode-side cooling-block (cooling-unit)80is bonded to ceramic sub-mount62B. A thermally conductive packing or shim92of solder material, such as indium (In) or the like, places cathode-side cooling-block in thermal communication with a raised portion75of the cooling-unit (seeFIG. 3A). Extended end-portions80A (seeFIG. 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 platform73and part of raised portion75as outlined in phantom inFIG. 3B. Terminal blocks82and84(anode and cathode respectively) are attached to raised portion75of cooling-unit by screws and insulating bushings (not shown), with insulating pads86placed 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 blocks82and84and corresponding insulators86, and bonded the opposite end thereof to the (metallized) diode-laser sides of ceramic sub-mounts62A and62B. InFIG. 3A, a cathode lead is depicted symbolically as a wire-lead94. In practice, this is a sheet electrode (for current carrying capacity) but is not depicted as such inFIG. 3Ato avoid obscuring other details of the heat-sink-assembly. InFIG. 3B, examples87A and87B of such sheet-electrodes are depicted. Dashed lines indicate the connection of electrodes87A and87B to sub-mounts62A and62B, 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 toFIG. 3C,FIG. 3DandFIG. 3E.FIG. 3Cis a three-dimensional view from below illustrating a recess96formed in cooling-unit72. The recess includes spaced-apart grooves98which form macro-cooling-channels when cooling-unit72is assembled onto the base of the heat-sink. Ridges100separate grooves98(except for end ones98′ thereof). The grooves terminate in raised (less deep) portions102at each end of the groves (only one visible inFIG. 3C).

Regarding exemplary dimensions inFIG. 3C, the grooves (between portions102) are preferably about 0.6 mm deep (as defined by the height or depth difference between the grooves and ridges100). The grooves are preferably about 1.2 mm wide. The length of recess96including the grooves is preferably long enough to extend along most of the length of platform73of cooling-unit72, and wide enough to extend partially under raised portion75of the cooling-unit, as can be seen in the phantom outline inFIG. 3B. The total depth of recess96(at the groves) is preferably selected such that grooves98are within about 0.3 mm of the surface of platform73(seeFIG. 3B) of the cooling-unit.

FIG. 3Dis an enlarged three-dimensional view of the base74of the inventive heat-sink. A footprint of cooling-unit72is depicted in phantom. Water conduits within the base are also depicted in phantom. Continuing reference is made toFIG. 3C.

Base74, here, is assumed to be machined from a single piece of metal such as stainless-steel. A mating-block portion104of the base is configured to engage raised portions100in recess96ofFIG. 3Cfor closing grooves98to form macro-channels. In that regard, mating-block104is a close fit in the length of the recess and a close fit between raised portions102in the recess. On opposite sides of block104are machined elongated plenums106A and106B. The block and plenums are surrounded by a machined groove108for accommodation a sealing ring.

Plenums correspond in position to raised (channel-terminating) portions102in recess96ofFIG. 3C. Plenum106A is in fluid communication with a straight portion76A of inlet conduit76. Plenum106B is in fluid communication with a straight portion77A of outlet conduit77. The selection of conduit76for inlet, and conduit77for outlet, is somewhat arbitrary, and should not be considered as limiting the present invention.

FIG. 3Eis a hypothetical three-dimensional view of the system (100) of conduits, plenums, and grooves ofFIGS. 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 block104(in the base) ofFIG. 3Dwith the recess (in the cooling-unit) ofFIG. 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 plenums106A and106B, respectively, and corresponding inlet and outlet conduits76and77respectively. 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.