Abstract:
A diode-laser assembly having an electrically isolated diode-laser bar on a cooled base-element is disclosed. The diode-laser bar is electrically isolated from the base-element and electrically isolated from any coolant water, thereby eliminating the need for de-ionized water and mitigating corrosion due to galvanic action. Multiple such diode-laser assemblies are stackable, with small bar-to-bar pitch, enabling a high-current and high-brightness diode-laser stack.

Description:
PRIORITY 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 62/268,327, filed Dec. 16, 2015, the entire disclosure of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates in general to diode-laser bar packaging. The invention relates in particular to packaging diode-laser bars on water-cooled heat-sinks. 
       DISCUSSION OF BACKGROUND ART 
       [0003]    Diode-lasers are efficient devices for converting electrical power into coherent optical power. An edge-emitting diode-laser has a diode-laser emitter, which is a type of waveguide laser-resonator, grown on a single-crystal substrate. The diode-laser emitter emits laser-radiation through an end facet in an emission direction. For high-power applications, a diode-laser bar having a plurality of diode-laser emitters provides a convenient way to scale the optical power available from a single diode-laser emitter. A diode-laser bar has typically between 10 and 60 such diode-laser emitters spaced apart and arranged in a “horizontal” linear array thereof. For further power scaling, a plurality of diode-laser bars can be stacked “vertically” to make a two-dimensional array of diode-laser emitters. Diode-laser bars arranged in this manner are typically referred to as a “vertical stack”. 
         [0004]    A diode-laser bar includes a plurality of semiconductor layers epitaxially grown on the substrate, with the 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. 
         [0005]    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. The base is usually made of copper. In a “conductively cooled package” (CCP) the base has sufficient mass to remove waste heat from the diode-laser bar. For higher power operation, the base is typically water-cooled, for example through a micro-channel arrangement. The diode-laser bar is soldered “p-side down” either directly onto the base or via a submount. The submount is made of a material having a coefficient of thermal expansion (CTE) between that of the substrate material and the base material, generally a material having a CTE close to that of the substrate material. 
         [0006]    Electrical connection to the diode-laser bars places the base, 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. The use of DI water is also expensive and inconvenient. 
         [0007]    Even small “stray” currents through the cooling water, between metal elements at different electric potentials, can cause metal corrosion through galvanic action. In addition to erosion of metal elements, particles produced by galvanic action can block cooling-channels in micro-channel cooled arrangements, which have typical internal dimensions of about 0.5 millimeters (mm) or less. Plating the cooling-water channels with a metal such as gold can mitigate such corrosion. However, plating internal channels by immersion-plating (usually using forced-flow plating solutions) results in uneven plating that is difficult to inspect for quality assurance. 
         [0008]    There is a need for an improved diode-laser bar assembly, having the cooling-water electrically isolated from both the n-side and p-side electrical potentials. Such an assembly should preferably not require the use of de-ionized water. 
       SUMMARY OF THE INVENTION 
       [0009]    In one aspect, electro-optical apparatus in accordance with the present invention comprises a base-element and an electrically-insulating submount. The submount has first and second opposite surfaces. The first surface is metallized. The second surface is bonded to the base-element. A diode-laser bar is provided having a p-side and an n-side. The p-side is bonded to the metallized first surface of the submount, leaving a portion of the metalized first surface exposed. First and second electrical contacts are provided. Each electrical contact is on an electrical insulator attached to the base-element and spaced apart from the submount. A first flexible electrical connector is provided, extending between the first electrical contact and the exposed metalized layer on the submount. A second flexible electrical connector is provided, extending between the second electrical contact and the n-side of the diode-laser bar. 
         [0010]    In another aspect, electro-optical apparatus in accordance with the present invention comprises a base-element and an electrically-insulating submount. The submount has first and second opposite surfaces. The first surface is metallized. The second surface is bonded to the base-element. First and second diode-laser bars are provided. Each diode-laser bar has a p-side and an n-side. The p-sides are bonded to the metallized first surface of the submount, leaving a portion of the metalized first surface exposed. First, second, and third electrical contacts are provided. Each electrical contact is on an electrical insulator attached to the base-element and spaced apart from the submount. A first flexible electrical connector is provided, extending between the first electrical contact and the exposed metalized layer on the submount. A second flexible electrical connector is provided, extending between the second electrical contact and the n-side of the first diode-laser bar. A third flexible electrical connector is provided, extending between the third electrical contact and the n-side of the second diode-laser bar. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    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. 
           [0012]      FIG. 1A  is a side-elevation view schematically illustrating one preferred embodiment of diode-laser assembly in accordance with the present invention. 
           [0013]      FIG. 1B  is an end-elevation view schematically illustrating the diode-laser assembly of  FIG. 1A . 
           [0014]      FIG. 1C  is a plan-view schematically illustrating the diode-laser assembly of  FIG. 1A . 
           [0015]      FIG. 2  is a side-elevation view, partially in cross-section, schematically illustrating another preferred embodiment of diode-laser assembly in accordance with the present invention, similar to the embodiment of  FIG. 1A , but further including a spacer for vertical stacking. 
           [0016]      FIG. 3  is a side-elevation view, partially in cross-section, schematically illustrating a preferred embodiment of diode-laser stack in accordance with the present invention having coolant water flowing therein. 
           [0017]      FIG. 4  is a side-elevation view schematically illustrating electrical current flow through the diode-laser stack of  FIG. 3 . 
           [0018]      FIG. 5  is an end-elevation view schematically illustrating another preferred embodiment of diode-laser assembly in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Referring now to the drawings, wherein like components are designated by like numerals,  FIG. 1A  is a side-elevation view schematically illustrating one preferred embodiment of a diode-laser assembly  10  in accordance with the present invention. Diode-laser assembly  10  comprises a rectangular diode-laser bar  20  having an n-side  30 , an opposite p-side  40 , and a perpendicular emitting face  50 . Diode-laser bar  20  generates laser-radiation that propagates in an emission direction  60  when energized by driving an electrical current between n-side  30  and p-side  40 . Diode-laser bar  20  is bonded onto a larger rectangular electrically-insulating submount  70 , at one end thereof. Diode-laser bar  20  and submount  70  together are bonded onto an even larger rectangular base-element  80 , at one end thereof. Emitting face  50  is thereby located at one end of diode-laser assembly  10 , as depicted. 
         [0020]    Submount  70  has a metalized surface  90  that is electrically conducting and an opposite surface  100 . Metalized surface  90  is preferably made by plating the whole surface with a layer of copper metal. Thick-film metalization technology may be necessary to fabricate a metal layer having sufficient thickness to withstand the electrical current required to energize diode-laser bar  20 . Thick-film metallization service is commercially available, for example from Remtec Inc. of Norward Mass. Alternatively, metalized surface  90  may be fabricated using direct-bonded-copper (DBC) technology. DBC bonding service is also commercially available, for example from Rogers Corporation of Rogers Connecticut. Metalized surface  90  has a preferred thickness of between about 25 micrometers (μm) and 125 μm. Surface  100  is also metalized to enable soldering, as described below. 
         [0021]    P-side  40  of diode-laser bar  20  is hard soldered onto metalized surface  90  of submount  70 , thereby forming a thin hard-solder layer  110 . Diode-laser bar  20  only covers a portion of metalized surface  90 , exposing the remaining portion of metalized surface  90  to facilitate electrical connection to p-side  40 . Surface  100  of submount  70  is soft soldered onto base-element  80 , thereby forming a thin soft-solder layer  120 . 
         [0022]    Submount  70  is made of an electrically-insulating material having a coefficient of thermal expansion (CTE) that matches diode-laser bar  20 . By way of example, if diode-laser bar  20  is made of gallium arsenide (GaAs), submount  70  is preferably made of beryllium oxide (BeO). By way of another example, if diode-laser bar  20  is made of indium phosphide (InP), submount  70  is preferably made of aluminum nitride (AlN). Beryllium oxide and aluminum nitride are both ceramic materials that have relatively-high thermal conductivity for electrical insulators. 
         [0023]    Base-element  80  is usually made of copper and has plated surfaces to facilitate soldering and for corrosion protection. Gold over nickel is a preferred plating for copper. Hard-solder layer  110  is preferably made of a gold-tin (AuSn) alloy. Soft-solder layer  120  is preferably made of an indium-silver (InAg) alloy, but other soft-solder alloys may be used, such as a tin-silver-copper (SnAgCu) alloy. The preferred materials above protect diode-laser bar  20  from mechanical stress and conduct waste heat from diode-laser bar  20  to base-element  80 , while also electrically isolating n-side  30  from base-element  80 . 
         [0024]    Diode-laser assembly  10  further includes an electrical contact  130  that is attached to a rectangular electrical insulator  140 . Insulator  140  is attached to base-element  80  and spaced apart from submount  70 . Electrical contact  130  is electrically isolated from base-element  80 . A flexible electrical connector  150  extends between electrical contact  130  and metalized surface  90 . 
         [0025]      FIG. 1B  is an end-elevation view schematically illustrating further details of diode-laser assembly  10 . Emitting face  50  includes a plurality of diode-laser emitters  160  that are electrically connected internally to n-side  30  and p-side  40  of diode-laser bar  20 . Although eleven diode-laser emitters  160  are depicted, the actual number of diode-laser emitters would be specific to the diode-laser bar selected for a particular application. 
         [0026]    Diode-laser assembly  10  further includes an electrical contact  170 A affixed to a rectangular electrical insulator  180 A. Insulator  180 A is attached to base-element  80  on one side of submount  70  and separated apart therefrom. Electrical contact  170 A is electrically isolated from base-element  80 . A flexible electrical connector  190 A extends between electrical contact  170 A and n-side  30 . Similarly, an electrical contact  170 B affixed to an electrical insulator  180 B is attached to base-element  80  on the opposite side of submount  70 . A flexible electrical connector  190 B extends between electrical contact  170 B and n-side  30 . This arrangement having two flexible electrical connectors  190 A and  190 B provides redundant electrical connection to n-side  30  and enables higher currents to be supplied to diode-laser bar  20 . 
         [0027]    Insulators  140 ,  180 A, and  180 B may be combined into a single unitary insulator element, without departing from the spirit and scope of the present invention. It should be noted that elements  170 A,  170 B,  180 A,  180 B,  190 A, and  190 B are omitted from  FIG. 1A  for simplicity of illustration. Similarly, elements  130 ,  140 , and  150  are omitted from  FIG. 1B  for simplicity of illustration. 
         [0028]      FIG. 1C  is a plan-view schematically illustrating further details of diode-laser assembly  10 . Electrical contacts  130 ,  170 A, and  170 B each extend laterally beyond the other elements of assembly  10 , to facilitate external electrical connection to laser-diode bar  20 . Electrical connection to p-side  40  (not visible) is made through electrical contact  130 , flexible electrical connector  150 , and metalized surface  90 . Electrical connection to n-side  30  is made through electrical contacts  170 A and  170 B and through flexible electrical connectors  190 A and  190 B. Flexible electrical connectors  150 ,  190 A, and  190 B are preferably made using wire-bond or ribbon-bond technologies, whereby each connector includes a plurality of high-gauge “wires”. These flexible connector technologies are preferred because they do not induce mechanical stress on diode-laser bar  20  and they have low electrical inductance for high-speed applications. 
         [0029]    Regarding exemplary dimensions for diode-laser assembly  10 , base-element  80  has preferred dimensions of about 33 mm (length)×about 14 mm (width)×about 1.4 mm (height). Submount  70  has a preferred width of about 11.5 mm and a preferred height of about 0.77 mm. Diode-laser bar  20  has a preferred height of about 0.14 mm. 
         [0030]      FIG. 2  is a side-elevation view, partially in cross-section, schematically illustrating another preferred embodiment of diode-laser assembly  200  in accordance with the present invention. Diode-laser assembly  200  is similar to diode-laser assembly  10 , but further includes a spacer  210 , an inlet port  220 , an outlet port  230 , and a coolant channel  240 . Spacer  210  enables vertical stacking of a plurality of such diode-laser assemblies. Spacer  210  has a preferred height of between about 1.3 mm and 1.6 mm for compatibility with the exemplary dimensions above, most preferably about 1.6 mm. Inlet port  220  and outlet port  230  extend through the full thickness of base-element  80  and spacer  210 . Inlet port  220  and outlet port  230  are fluidly connected inside base-element  80  by coolant channel  240 . 
         [0031]      FIG. 3  is a side-elevation view, partially in cross-section, schematically illustrating one preferred embodiment  300  of diode-laser stack in accordance with the present invention. At least three diode-laser assemblies  200  are depicted stacked together vertically for power scaling. More or less assemblies can be stacked together as required by a particular application, without departing from the spirit and scope of the present invention. Diode-laser assemblies  200  are stacked together using spacers  210 . This arrangement minimizes mechanical stress transferred to diode-laser bars  20 . 
         [0032]    The drawing illustrates coolant water flow (dashed arrowed line) through stack  300 . Diode-laser assemblies  200  are in parallel fluid connection between inlet port  220  and outlet port  230 . Coolant water flows under pressure from an external supply (not shown) through inlet port  220 , through coolant channel  240  within each one of the diode-laser assemblies  200 , and returns through outlet port  230 . Waste heat conducted away from each one of the diode-laser bars  20  is removed by the flowing coolant water. Each one of the coolant channels  240  may incorporate a micro-channel arrangement (not depicted) to maximize contact between the coolant water and base-element  80  at the end thereof proximate to diode-laser bar  20 . 
         [0033]      FIG. 4  is a side-elevation view schematically illustrating conventional current flow (dashed arrowed line) through diode-laser stack  300 . Stack  300  further includes a plurality of interconnectors  310  that electrically connect electrical contact  170 B of each one of the diode-laser assemblies  200  to electrical contact  130  of the diode-laser assembly immediately above. Although not visible in the drawing, interconnectors  310  may also connect electrical contact  170 A of each diode-laser assembly  200  to electrical contact  130  of the diode-laser assembly immediately above. Interconnectors  310  thereby electrically connect the n-side of each one of the diode-laser bars  20  to the p-side of the diode-laser bar immediately above. Terms “above” and “below” are used here for convenience of description, and are not meant to imply specific spatial orientations of the stack in use. 
         [0034]    Interconnectors  310  may be soldered to the electrical contacts (as depicted) or attached to the electrical contacts using mechanical fasteners, as appropriate for a particular application. Soldering provides robust mechanical attachment, although there may be electrical resistance across each solder interface, which would cause an unwanted cumulative power loss. Fasteners provide direct contact and enable quick assembly and disassembly of the stack. For example, interconnector  310  may be threaded to accept a standard screw as a fastener. 
         [0035]    An optional anode connector  320  is depicted that enables convenient electrical connection to electrical contact  130  of the diode-laser assembly at the bottom of stack  300 . Current originates from an external current source (not shown), flows through anode connector  320 , and then flows alternately through each diode-laser assembly  200  and each interconnector  310 . Current returning to the external current source from the top of stack  300  is not depicted for simplicity of illustration. 
         [0036]    Alternatively, the stacked diode-laser assemblies may be supplied with current individually, by omitting interconnectors  310  and connecting one or more external current sources directly to electrical contacts  130 ,  170 A, and  170 B. This alternative arrangement has an advantage that individual diode-laser assemblies may be energized to control the vertical distribution of laser-radiation emitted by diode-laser stack  300 . Yet another advantage is that a fraction of the diode-laser bars may be operated at higher currents in an application that is sensitive to the wavelength of the laser-radiation, especially when ramping from low to high powers. In general, serially connecting the stacked diode-laser assemblies to a common current source is more convenient and more cost effective than energizing the diode-laser assemblies individually. 
         [0037]    Within each diode-laser assembly  200 , conventional current flows in turn through electrical contact  130 , flexible electrical connector  150 , metalized surface  90 , diode-laser bar  20 , flexible electrical connectors  190 A and  190 B, and electrical contacts  170 A and  170 B. Each base-element  80  is electrically isolated from the externally supplied current by submount  70 , insulator  140 , and insulators  180 A (not visible) and  180 B. The coolant water depicted in  FIG. 3  is thereby electrically isolated from the externally supplied current. 
         [0038]    Whether diode-laser assemblies  200  are supplied with current individually or in series, it is preferable to connect all base-elements  80  to a common ground that is electrically isolated from externally supplied current. It is also preferable to electrically connect all metal elements in contact with the coolant water to a common ground, thereby mitigating any stray currents that could cause corrosion. 
         [0039]      FIG. 5  is an end-elevation view schematically illustrating yet another preferred embodiment of diode-laser assembly  400  in accordance with the present invention. Diode-laser assembly  400  is similar to diode-laser assembly  10 , but has two diode-laser bars  410 A and  410 B instead of one diode-laser bar  20 . P-sides  40  of diode-laser bars  410 A and  410 B are hard soldered onto metalized surface  90 . N-side  30  of diode-laser bar  410 A is connected to electrical contact  170 A through flexible electrical connector  190 A. N-side  30  of diode-laser bar  410 B is connected to electrical contact  170 B through flexible electrical connector  190 B. 
         [0040]    Each diode-laser bar in diode-laser assembly  400  may be supplied with current individually by connecting independent external current sources to electrical contacts  170 A and  170 B. This arrangement enables diode-laser bars  410 A and  410 B to be energized separately to control the horizontal distribution of laser-radiation emitted by diode-laser assembly  400 . This arrangement also reduces the current required by each one of the diode-laser bars in diode-laser assembly  400  compared to the one diode-laser bar in diode-laser assembly  10 . 
         [0041]    Returning to  FIGS. 3 and 4 , diode-laser assemblies  200  in diode-laser stack  300  are preferably stacked with minimum bar-to-bar pitch, to maximize the brightness of the laser-radiation emitted from the stack. For the exemplary dimensions above and the most-preferable 1.6 mm height of spacer  210 , the bar-to-bar pitch is 3.0 mm. Although the present invention solves problems of electrical isolation and corrosion when using coolants containing water, the invention could be used with anhydrous coolants without departing from the spirit and scope of the present invention. 
         [0042]    The present invention is described above in terms of a preferred embodiment and other embodiments. The invention is not limited, however, to the embodiments described and depicted herein. Rather, the invention is limited only by the claims appended hereto.