Patent Publication Number: US-2007115617-A1

Title: Modular assembly utilizing laser diode subassemblies with winged mounting blocks

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is a continuation-in-part of U.S. patent application Ser. No. 11/313,068, filed Dec. 20, 2005; Ser. No. 11/378,570, filed Mar. 17, 2006; Ser. No. 11/378,667, filed Mar. 17, 2006; Ser. No. 11/378,696, filed Mar. 17, 2006; and Ser. No. 11/378,697, filed Mar. 17, 2006; the disclosures of which are incorporated herein for any and all purposes. This application also claims the benefit of U.S. Provisional Patent Application Ser. No. 60/739,185, filed Nov. 22, 2005, the disclosure of which is incorporated herein by reference for any and all purposes. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to semiconductor lasers and, more particularly, to a laser assembly that permits the output from multiple laser diodes to be effectively and efficiently combined.  
     BACKGROUND OF THE INVENTION  
      High power laser diodes have been widely used in industrial, graphics, medical and defense applications. The beam divergence and the relatively low output power of such lasers have, however, limited their usefulness.  
      The output beam of a laser diode is asymmetric due to the beam having a higher angular divergence in the direction perpendicular to the diode junction of the emitter (i.e., the fast axis of the emitter) than in the direction parallel to the diode junction (i.e., the slow axis of the emitter). As a result of the differences in beam divergence, the cross section of the output beam of a laser diode has an elliptical shape, typically requiring the use of a cylindrical lens or other optics to alter the divergence characteristics and shape the output beam for its intended use. Although beam optics can be used to reformat the output of individual laser diodes, in the past the use of such optics has made it difficult to combine multiple laser diode beams into a single beam of sufficient output power to suit many applications.  
      One method of combining the output beams from multiple lasers is disclosed in U.S. Pat. No. 4,828,357. As shown, the output from each laser is directed using multiple mirrors in order to form a bundle of parallel beams or to cause the beams to converge into a relatively narrow region. The patent discloses that if greater power is required than can be generated by a single beam bundle, multiple bundles of parallel beams can be combined to form a beam bundle of even greater power. The patent does not specifically disclose the use of laser diodes nor does the patent disclose altering the beam shape of the individual laser beams prior to directing the beams into the beam bundle.  
      U.S. Pat. No. 6,075,912 discloses an alternate technique for combining the output beams from multiple lasers into a single beam. In the disclosed system the output beam of each laser impinges on a discrete facet of a multi-faceted beam deflector. By properly positioning each laser relative to the facets of the beam deflector, all of the output beams are deflected into an optical fiber. The patent discloses interposing an optical system between each laser source and the corresponding beam deflector facet in order to properly image the output beam onto the deflector facet. The patent also discloses interposing an output optical system between the beam deflector and the optical fiber, the output optical system imaging the deflected output beams as a focused group of beam images into the core of the input face of the optical fiber.  
      U.S. Pat. No. 4,716,568 discloses a laser array assembly formed from a plurality of linear laser diode array subassemblies stacked one above the other, each of the subassemblies electrically connected to the adjacent subassembly. Each linear laser diode array subassembly is made up of a plurality of individual laser emitters mounted in thermal communication with a conductive plate. Although the patent discloses several ways of stacking the subassemblies in order to form the desired 2-D laser array, the patent does not disclose any optical systems for use in combining the output beams of the individual emitters and/or subassemblies.  
      U.S. Pat. No. 5,887,096 discloses an optical system that is used to guide the output beams from a rectilinear laser diode array to form a substantially uniform radiation field or pattern. In one disclosed embodiment, the optical system utilizes a plurality of reflectors where each reflector corresponds to an individual laser diode. In a preferred embodiment, the centers of the irradiated surface areas of the individual reflectors are situated in a straight line with the distance between a reflector and the corresponding laser diode exit facet being the same for each laser diode/reflector pair.  
      U.S. Pat. No. 6,240,116 discloses a laser diode array designed to achieve high beam quality and brightness. In one embodiment, the array includes a pair of diode arrays in which the emitting surface planes of the two arrays are displaced from one another in a direction parallel to the one of the optical axes defined by the arrays. The optical axes of the two arrays are offset from each other in a direction perpendicular to one of the optical axes. Lenses are used to reduce the divergence of the output beams. In at least one embodiment, reflectors are used to reduce or eliminate the dead spaces between adjacent collimated beams.  
      Although a variety of laser diode arrays and beam combining systems have been designed, what is needed in the art is a versatile laser diode assembly which can be easily tailored to specific application needs. The present invention provides such a laser diode assembly.  
     SUMMARY OF THE INVENTION  
      The present invention provides a laser diode assembly comprised of a plurality of laser diode subassemblies mounted to a stepped cooling block. Each laser diode subassembly of the laser diode assembly includes a mounting block which, during laser diode subassembly mounting, is coupled to the corresponding mounting surface of the stepped cooling block. Although the laser diode subassemblies are coupled to the cooling block, liquid coolant does not flow through the subassembly mounting blocks, rather the mounting blocks are merely in thermal contact with the cooling block.  
      Mounted to a surface of each subassembly mounting block is a laser diode submount. The laser diode submount can be fabricated from either an electrically insulating material or an electrically conductive material. Mounted to a surface of the laser diode submount is the laser diode. The laser diode can be either a single emitter laser diode or a multi-emitter laser diode. In at least one embodiment the laser diode submount includes a pair of contact pads that are electrically coupled to the laser diode, thus providing a means of supplying power to the individual lasers.  
      Each subassembly mounting block includes at least one, and preferably two, extensions, also referred to as mounting block wing portions, which are used in combination with clamping members to hold the mounting blocks against the cooling block. Preferably the bottom surface of each of the mounting blocks is shaped, thus providing a means of registering the mounting block subassemblies to the cooling block. Preferably tensioned electrical interconnects are used to contact the electrical contact pads located on the laser diode submounts, thus providing a means of contacting the laser diodes without putting undue stress on the laser diode submounts. In an alternate embodiment of the invention, at least one threaded means (e.g., bolt, all-thread and nut assembly, etc.) attaches each laser diode subassembly to the corresponding cooling block mounting surface. In yet another alternate embodiment of the invention, solder or other bonding material attaches each laser diode subassembly to the corresponding cooling block mounting surface.  
      In at least one preferred embodiment of the invention, a beam conditioning lens is attached, for example by bonding, to each laser diode subassembly mounting block such that the output beam(s) of the laser diode passes through the lens. In one embodiment the beam conditioning lens is a cylindrical lens. Preferably a second beam conditioning lens is also attached, for example by bonding, to each laser diode subassembly mounting block such that the output beam(s) of the laser diode from a different laser diode subassembly passes through the lens. The different laser diode subassembly can be an adjacent subassembly. Alternately one or more laser diode subassemblies can be located between the second beam conditioning lens and the subassembly containing the laser diode that produces the output beam that passes through the second beam conditioning lens.  
      In at least one embodiment of the invention, a cooling source is coupled to the cooling block. The cooling source can be coupled to the cooling block or integrated within the cooling block, for example using cooling liquid channels. The cooling block can have a flat bottom surface, thus creating different separation distances between each mounting surface of the stepped cooling block and the cooling block bottom surface. Alternately the cooling block can have an inclined bottom surface, thus causing the separation distances between each mounting surface of the stepped cooling block and the cooling block bottom surface to be the same.  
      A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of the primary components of a laser diode subassembly in accordance with the invention;  
       FIG. 2  is a perspective view of the assembled laser diode subassembly of  FIG. 1 , minus the second conditioning lens;  
       FIG. 3  is a perspective view of the assembled laser diode subassembly of  FIG. 1 , including the second conditioning lens associated with another (not shown) laser diode subassembly;  
       FIG. 4  illustrates the relationship between the second conditioning lens and a specific laser diode subassembly;  
       FIG. 5  illustrates the relationship between the second conditioning lens and a specific laser diode subassembly different from that shown in  FIG. 4 ;  
       FIG. 6  is a perspective view of the bottom surface of the subassembly mounting block shown in  FIG. 1 ;  
       FIG. 7  is a perspective view of an alternate bottom surface shape;  
       FIG. 8  is an illustration of a cooling block for use with a laser diode subassembly such as those shown in  FIGS. 1-3  and  6 ;  
       FIG. 9  is an illustration of the cooling block of  FIG. 8  populated with two rows of laser diode subassemblies;  
       FIG. 10  is an illustration of the assembly of  FIG. 9  with clamping members to hold the laser diode subassemblies in place;  
       FIG. 11  is an illustration of the assembly of  FIG. 10  with interconnects in place;  
       FIG. 12  is a cross-sectional view of an uncompressed interconnect;  
       FIG. 13  is a cross-sectional view of the interconnect assembly shown in  FIG. 12  making contact with a laser diode contact pad;  
       FIG. 14  is an illustration of an alternate means of mounting the subassemblies to the cooling block;  
       FIG. 15  is an illustration of a cooling block similar to that shown in  FIG. 8 , except for the use of an inclined cooling plane; and  
       FIG. 16  is an illustration of an assembly similar to that shown in  FIG. 11 , except that the assembly includes only a single row of laser diode subassemblies. 
    
    
     DESCRIPTION OF THE SPECIFIC EMBODIMENTS  
      The present invention provides the system designer with the means to tailor a laser diode assembly to the specific needs of a particular application. In order to provide this versatility, the system utilizes a laser diode subassembly that can be mounted in a variety of configurations.  
       FIGS. 1-3  illustrate a laser diode subassembly utilizing a single emitter laser diode  101 . In addition to the laser diode  101 , the primary components associated with the laser diode subassembly are the subassembly mounting block  103 , submount  105 , first conditioning lens  107  and second conditioning lens  109 . As described in detail below, although second conditioning lens  109  is mounted to subassembly mounting block  103 , it is used with the output beam of a laser diode mounted to another laser diode subassembly that is not shown in  FIGS. 1-3 .  
      Subassembly mounting block  103  provides a convenient means for registering the various components of the laser diode subassembly as well as a means of registering the individual laser diode subassemblies within the cooling block as described further below. Additionally, mounting block  103  provides sufficient heat dissipation to allow individual laser diode subassemblies to be tested in either pulsed or CW mode prior to mounting the subassemblies within the cooling block. With respect to the laser diode subassembly, in the preferred embodiment laser diode  101  is not attached directly to subassembly mounting block  103 , rather it is mounted to submount  105  which, in turn, is mounted to surface  111  of mounting block  103 . Preferably submount  105  as well as the means used to attach submount  105  to mounting block  103  are both materials with a high coefficient of thermal conductivity, thus insuring that the heat produced by laser diode  101  is efficiently coupled to mounting block  103 . Additionally the coefficient of thermal expansion for the material selected for submount  105  is matched, to the degree possible, to laser diode  101  in order to prevent de-bonding or damage to the laser diode during operation. In at least one embodiment of the invention, submount  105  is soldered to mounting block  103  using indium solder.  
      Submount  105  can be fabricated from either an electrically conductive (e.g., copper, copper tungsten, etc.) or an electrically insulative (e.g., aluminum nitride, beryllium oxide, CVD diamond, silicon carbide, etc.) material. In the embodiment illustrated in  FIGS. 1 and 2 , submount  105  is fabricated from an electrically insulating ceramic. The material used to bond laser diode  101  to submount  105  is selected, at least in part, on the composition of submount  105  and/or the composition of any layers (e.g., contact pads) interposed between submount  105  and laser diode  101 . In the illustrated embodiment, electrically conductive contact pads  113 / 115  are deposited or otherwise formed on the top surface of submount  105 . Contact pads  113 / 115  can be formed, for example, of gold over nickel plating while a gold-tin bonding material can be used to bond laser diode  101  to contact pad  113 . It will be appreciated that there are a variety of materials well known in the industry that are suitable for use as contact pads as well as laser diode bonding material.  
      In a preferred embodiment of the invention, one contact (e.g., anode) of laser diode  101  is on its bottom surface, thus allowing one diode contact to be made by bonding the laser diode to one of the contact pads (e.g., pad  113 ) using an electrically conductive material. A wire or ribbon bond  117 , or alternately another type of contacting member (not shown), is then used to electrically couple the second contact (e.g., cathode) of each laser diode to the second contact pad  115 . It will be appreciated that the invention is not limited to this contact arrangement. For example, both the anode and the cathode can be coupled to a pair of contact pads using multiple wire/ribbon bonds.  
      First conditioning lens  107 , which in at least one embodiment is a cylindrical lens, is preferably located between extended arm portions  119 / 120  of mounting block  103 , respectively. Typically lens  107  is located immediately adjacent to the exit facet of laser diode  101 . Once lens  107  is properly positioned, it is bonded into place. The purpose of conditioning lens  107  is to reduce the divergence of laser diode  101  in the fast axis, preferably to a value that is the same as or less than the divergence in the slow axis.  
      In order to properly condition the output beam of laser diode  101 , preferably a second conditioning lens  109  is used. It should be understood that the specific second conditioning lens  109  shown in  FIGS. 1-3 , although mounted to the top surfaces  121  and  122  of respective arm portions  119  and  120 , is not used to condition the beam from the illustrated laser diode  101 . Rather the illustrated conditioning lens  109  is used to condition the output beam from an adjacent laser diode subassembly (e.g., beam  401  in  FIG. 4 ), or the output beam from a laser diode subassembly that is more than one subassembly removed from the subassembly (e.g., beam  501  in  FIG. 5 ). It will be appreciated that the focal length of second conditioning lens  109  as well as the height of arm portions  119  and  120  is dependent on which laser diode output beam is intended to pass through which second conditioning lens (i.e., the number of laser diode subassemblies separating the second conditioning lens from the laser diode source).  
      In the preferred embodiment of the invention, registration regions  123  and  124  are machined, or otherwise formed, within extended arm portions  119  and  120 , respectively. A third lens, for example a volume Bragg grating, can be mounted within registration regions  123  and  124 .  
      As previously noted, in addition to provide a means of registering the individual subassembly components, preferably mounting block  103  is also used to register the individual subassemblies within the cooling block. In order to better perform this function, in at least one preferred embodiment of the invention the bottom surface of mounting block  103  is shaped.  FIG. 6  illustrates a preferred contoured bottom surface  601 . It will be appreciated that the invention is not limited to this specific shape. For example,  FIG. 7  illustrates a bottom surface  701  of an alternate mounting block  700 . In this embodiment the bottom surface of the mounting block retains the same perimeter shape as the upper surface (as illustrated in  FIG. 1 ).  
       FIG. 8  is a perspective view of a preferred cooling block  800  for use with the laser diode subassemblies shown in  FIGS. 1-3  and  6 . As shown, cooling block  800  includes two rows  801  of stepped mounting surfaces  803 . By stepping the mounting surfaces the output beams from the attached laser diode subassemblies (not shown in this figure) are allowed to exit the assembly unimpeded. Each mounting surface  803  is configured to accept a laser diode assembly. More specifically, the shape of each mounting surface  803  corresponds to that of bottom surface  601  of mounting block  103 , thus providing a simple means of registering the subassemblies within the cooling block. It will be appreciated that if the bottom surface of the mounting block is differently shaped, for example as described relative to  FIG. 7 , the stepped mounting surfaces are correspondingly shaped. Preferably the mounting blocks, described in detail above, are fabricated from a material with a high coefficient of thermal conductivity (e.g., copper), thus providing an efficient thermal path between laser diode  101  and the cooling block. During use, preferably either the cooling block is thermally coupled to a cooling source (e.g., thermoelectric cooler, multi-channel cooling block), or the cooling source is integrated within the cooling block (e.g., integral liquid coolant conduits within the cooling block that are coupled to a suitable coolant pump).  
      The cooling block shown in  FIG. 8  is configured to accept two rows of laser diode subassemblies, each row containing five subassemblies. The output beams from the two rows of subassemblies can either be combined into a single beam using known optical techniques, or the assembly can be used to produce two separate output beams. It should be appreciated that the present invention is not limited to this particular configuration. For example, a device in accordance with the invention can include more or less rows of subassemblies. Similarly, each row can be configured to include more or less subassemblies than the five that cooling block  800  is configured to accept.  
      It will be appreciated that there are numerous techniques that can be used to mount the laser diode subassemblies to the cooling block, these techniques using various arrangements of clamping members, bolts and/or bonding materials (e.g., solder, adhesive).  FIGS. 9 and 10  illustrate a preferred mounting technique in which mounting block wing portions  125 / 126 , which extend beyond submount mounting surface  111  of each mounting block  103 , are used to hold the mounting blocks in place within the cooling block. There are several benefits associated with the use of wing portions  125 / 126  to clamp each subassembly within the cooling block. First, wing portions  125 / 126  provide a means of insuring that the subassemblies are held tightly in place, thus improving the transfer of heat from the laser diodes, through the mounting blocks, and into the cooling block. Second, by clamping mounting block surfaces that are removed from surface  111 , the risk of damaging submount  105  during assembly is significantly diminished.  
       FIG. 9  shows ten laser diode subassemblies  200  positioned within mounting block  800 . Note that for the sake of clarity, second conditioning lens  109  is not shown in  FIGS. 9-11 . It will be understood, however, that in normal use there is a second conditioning lens, e.g., lens  109 , associated with each laser diode subassembly. The second conditioning lens for the lowermost laser diode subassembly or subassemblies are mounted to a stand-alone lens carrier that is either integral to, or independent of, the cooling block.  
      In the preferred embodiment, and as illustrated in  FIG. 10 , each laser diode subassembly  200  is held in place with a pair of clamping members (e.g., members  1001 ,  1002 ,  1003 ). In the illustrated embodiment, due to the use of multiple rows of laser diode subassemblies, preferably between rows a single clamping member  1002  is used to clamp two different subassemblies. It should be understood, however, that even with multiple rows of subassemblies, each subassembly can utilize two independent clamping members, i.e., one on either wing portion. Other clamping variations are clearly envisioned by the inventors, for example using stepped clamping members that can be used to hold more than one subassembly in place (for example, combining several members  1003  into a single member). It will also be appreciated that other variations are possible depending upon the heights of the wing portions relative to the cooling block. In the preferred embodiment, bolts  1005  secure each clamping member to the cooling. Alternately, all-thread/nut assemblies or other threaded means can be used to secure the clamping members, and thus the subassemblies, in place.  
      It will be appreciated that there are countless methods of contacting the individual laser diodes  101 , typically using contact pads such as pads  113 / 115  described relative to the earlier figures. In general, the means used to contact the individual laser diodes depends upon whether the intent is to connect the laser diodes in series, or in parallel, or in groups (for example, in groups of laser diodes emitting specific wavelengths), or individually thereby providing individual laser diode addressability.  FIG. 11  illustrates a preferred interconnect configuration that serially couples the laser diodes together. In the illustrated configuration, interconnect  1101  is electrically coupled to anode contact pad  1103  of laser diode  1105  while interconnect  1107  is electrically coupled to cathode contact pad  1109  of laser diode  1111 . Other interconnects (e.g., interconnects  1113 ) are used to serially couple together the other laser diodes of the stack. Note that in the illustrated configuration, the polarity of the second and fourth rows of laser diodes is reversed relative to the polarity of the first, third and fifth rows of laser diodes. It will be appreciated that in the illustrated embodiment, if the cooling block is electrically conductive, the electrical interconnects must be electrically isolated from the cooling block (for example, using electrically insulating sleeves/washers).  
      Although it will be appreciated that there are numerous methods of electrically coupling to the laser diodes, in a preferred embodiment shown in  FIGS. 12 and 13 , the interconnects are fabricated from a relatively thin electrically conductive material, or materials, that have sufficient springiness to provide a simple means of contacting the contacting pads.  FIG. 12  is a cross-sectional view of an exemplary interconnect such as interconnect  1107 . In the illustrated embodiment, interconnect  1107  is prevented from shorting, via bolt  1005 , using electrically insulating sleeve member  1201  and electrically insulating washers  1203 . As shown in  FIG. 13 , when bolt  1005  holds the interconnect in place against clamping member  1001 , the end portion  1205  of interconnect  1107  is held in tension against contact pad  115 .  
      As previously noted there are countless minor variations of the invention utilizing different electrical interconnects, clamping assemblies and cooling blocks that are clearly envisioned by the inventors. For example,  FIG. 14  illustrates an embodiment similar to that shown in  FIG. 9  except that each mounting block wing portion  125  and  126  is directly bolted to cooling block  1401  with a bolt  1403 . As with  FIG. 9 , for the sake of illustration clarity neither the second conditioning lens nor the electrical interconnects for each subassembly are shown.  
      As the cooling block (e.g., cooling block  800  and  1401 ) is comprised of a series of steps onto which the laser diode subassemblies are mounted, the cooling rate and thus the operating temperature of the individual laser subassemblies varies depending upon the distance between the cooling source coupled to the cooling block and the individual subassemblies. Since the operating wavelength of a laser diode is temperature dependent, the inventors have found that the operating temperature variations between subassemblies that arise due to the stepped cooling block can be used to match laser diode subassembly wavelengths. Accordingly in at least one embodiment of the invention, the output wavelength of each subassembly is determined based on the subassembly&#39;s position within the cooling block. Then each subassembly is positioned within the cooling block to provide the closest possible match to the desired output wavelength of the entire assembly. In an alternate embodiment of a cooling block shown in  FIG. 15 , the bottom surface  1501  of the cooling block is inclined. As a result of this configuration, each mounting surface  1503  is the same distance from the bottom surface  1501 , thus maintaining the same cooling rate for each mounted laser diode subassembly (not shown) even when thermally coupling the cooling source to the bottom surface (i.e., surface  1501 ) of the cooling block.  
      In the above figures, the illustrated exemplary configurations include two rows of subassemblies. As previously noted, however, the invention is not limited to two rows nor is the invention limited to a specific number of subassemblies per row. For example,  FIG. 16  is an illustration of a laser diode assembly utilizing a single row of laser diode subassemblies.  
      As will be understood by those familiar with the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.