Modular diode laser assembly

[0001] This application 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.

[0002] The present invention relates generally to semiconductor lasers and, more particularly, to a laser assembly that permits the output from multiple diode lasers to be effectively and efficiently combined.

[0003] High power diode lasers have been widely used in industrial, graphics, medical and defense applications. The beam divergence and the relatively low output power of such lasers has, however, limited their usefulness.

[0004] The output beam of a diode laser 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 diode laser 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 on individual diode lasers, in the past the use of such optics has made it difficult to combine multiple diode laser beams into a single beam of sufficient output power to suit many applications.

[0005] 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.

[0006] 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.

[0007] U.S. Pat. No. 4,716,568 discloses a laser array assembly formed from a plurality of linear diode laser array subassemblies stacked one above the other, each of the subassemblies electrically connected to the adjacent subassembly. Each linear diode laser 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.

[0008] U.S. Pat. No. 5,887,096 discloses an optical system that is used to guide the output beams from a rectilinear diode laser 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 diode laser. 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 diode laser exit facet being the same for each diode laser/reflector pair.

[0009] U.S. Pat. No. 6,240,116 discloses a diode laser 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.

[0010] Although a variety of diode laser arrays and beam combining systems have been designed, what is needed in the art is a versatile diode laser assembly which can be easily tailored to specific application needs. The present invention provides such a diode laser assembly.

[0011] The present invention provides a diode laser assembly comprised of a plurality of diode laser subassemblies mounted to a stepped cooling block. The stepped cooling block includes at least three stepped regions, the center region of which is recessed relative to the other two regions. In at least one embodiment, the stepped cooling block includes more than three stepped regions, including multiple recessed regions. Mounted to the mounting surface of each step of each recessed region is a diode laser subassembly.

[0012] Each diode laser subassembly of the diode laser assembly includes a mounting block which, during diode laser subassembly mounting, is thermally coupled to the corresponding mounting surface of the recessed cooling block region. Mounted to a surface of the mounting block is a diode laser submount. The diode laser submount can be fabricated from either an electrically insulating material or an electrically conductive material. Mounted to a surface of the diode laser submount is the diode laser. The diode laser can be either a single emitter diode laser or a multi-emitter diode laser. In at least one embodiment the diode laser submount includes a pair of contact pads that are electrically coupled to the diode laser, thus providing a means of supplying power to the individual lasers.

[0013] In at least one preferred embodiment of the invention, a pair of clamping members compresses the diode laser submount against the mounting block, and the mounting block against the mounting surface of the recessed cooling block region(s). Preferably the clamping members are held in place with bolts coupled to the cooling block regions located on either side of the recessed cooling block region(s). The clamping members preferably hold electrical interconnects against electrical contact pads located on the diode laser submount. In an alternate embodiment of the invention, at least one threaded means (e.g., bolt, all-thread and nut assembly, etc.) attaches each diode laser subassembly to a mounting surface of the recessed cooling block region(s).

[0014] In at least one preferred embodiment of the invention, a beam conditioning lens is attached, for example by bonding, to each diode laser subassembly mounting block such that the output beam(s) of the diode laser 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 diode laser subassembly mounting block such that the output beam(s) of the diode laser from a different diode laser subassembly passes through the lens. The different diode laser subassembly can be an adjacent subassembly. Alternately one or more diode laser subassemblies can be located between the second beam conditioning lens and the subassembly containing the diode laser that produces the output beam that passes through the second beam conditioning lens.

[0015] 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 bottom surface, one or more cooling block side surfaces, or both. The cooling source can be integrated within the cooling block. The cooling block can have a flat bottom surface, thus creating different separation distances between each mounting surface of the recessed cooling block region 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 recessed cooling block region and the cooling block bottom surface to be the same.

[0016] 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.

[0017] FIG. 1 is a perspective view of the primary components of a diode laser subassembly in accordance with the invention;

[0018] FIG. 2 is a perspective view of the assembled diode laser subassembly of FIG. 1, minus the second conditioning lens;

[0019] FIG. 3 is a perspective view of the assembled diode laser subassembly of FIG. 1, including the second conditioning lens associated with another (not shown) diode laser subassembly;

[0020] FIG. 4 illustrates the relationship between the second conditioning lens and a specific diode laser subassembly;

[0021] FIG. 5 illustrates the relationship between the second conditioning lens and a specific diode laser subassembly different from that shown in FIG. 4;

[0022] FIG. 6 is an illustration of a diode laser subassembly similar to that shown in FIGS. 1-3, utilizing a three-stripe diode laser rather than a single stripe diode laser;

[0023] FIG. 7 is an illustration of a cooling block for use with a diode laser subassembly such as those shown in FIGS. 1-6;

[0024] FIG. 8 is an illustration of a cooling block in which multiple diode laser subassemblies are clamped in place;

[0025] FIG. 9 illustrates a cooling block with an inclined cooling plane;

[0026] FIG. 10 illustrates a cooling block configured to accommodate two rows of diode laser subassemblies;

[0027] FIG. 11 illustrates portions of a diode laser subassembly that utilizes an electrically conductive submount;

[0028] FIG. 12 illustrates portions of a diode laser subassembly that utilizes a pair of attachment bolts;

[0029] FIG. 13 illustrates portions of a diode laser subassembly that utilizes a single attachment bolt; and

[0030] FIG. 14 is a cross-sectional view of a cooling block/mounting block that illustrates an alternate subassembly mounting arrangement.

[0031] The present invention provides the system designer with the means to tailor a diode laser assembly to the specific needs of a particular application. In order to provide this versatility, the system utilizes a diode laser subassembly that can be mounted in a variety of configurations.

[0032] FIGS. 1-3 illustrate a diode laser subassembly utilizing a single emitter diode laser 101. In addition to the diode laser 101, the primary components associated with the diode laser 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 diode laser mounted to another diode laser subassembly that is not shown in FIGS. 1-3.

[0033] Subassembly mounting block 103 serves several functions. First, it provides an efficient thermal path between diode laser 101 and the cooling block which is shown in later figures. Second, it provides a convenient means for registering the various components of the diode laser assembly, thereby lowering the manufacturing costs associated with the overall assembly. Third, it provides a convenient means for registering the individual diode laser subassemblies within the cooling block as shown below. In order to provide the desired diode laser cooling, preferably subassembly mounting block 103 is fabricated from a material with a high coefficient of thermal conductivity (e.g., copper).

[0034] Diode laser 101 is not attached directly to subassembly mounting block 103, rather it is mounted to submount 105. 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 diode laser 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 diode laser 101 in order to prevent de-bonding during operation or damage to the laser. In at least one embodiment of the invention, submount 105 is soldered to mounting block 103 using indium solder.

[0035] 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. 1-3, submount 105 is fabricated from an electrically insulating ceramic. The material used to bond diode laser 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 diode laser 101. In the illustrated embodiment, electrically conductive contact pads 111/113 are deposited or otherwise formed on the top surface of submount 105. Contact pads 111/113 can be formed, for example, of gold over nickel plating while a gold-tin bonding material can be used to bond diode laser 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 diode laser bonding material.

[0036] In a preferred embodiment of the invention, one contact (e.g., anode) of diode laser 101 is on its bottom surface, thus allowing one diode contact to be made by bonding the diode laser to one of the contact pads (e.g., pad 113) using an electrically conductive material. A wire bond or ribbon bond 115 is then used to electrically couple the second contact (e.g., cathode) of each diode laser to the second contact pad 111. It will be appreciated that the invention is not limited to this contact arrangement. For example, a pair of wire or ribbon bonds can be used to couple the diode laser to a pair of contact pads.

[0037] First conditioning lens 107, which in at least one embodiment is a cylindrical lens, is properly positioned relative to diode laser 101 using the extended arm portions 117 and 119 of mounting block 103. Typically lens 107 is located immediately adjacent to the exit facet of diode laser 101. Once lens 107 is properly positioned, it is bonded into place. The purpose of conditioning lens 107 is to reduce the divergence of diode laser 101 in the fast axis, preferably to a value that is the same as or less than the divergence in the slow axis.

[0038] In order to properly condition the output beam of diode laser 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 123 of respective arm portions 117 and 119, is not used to condition the beam from the illustrated diode laser 101. Rather the illustrated conditioning lens 109 is used to condition the output beam from an adjacent diode laser subassembly (e.g., beam 401 in FIG. 4), or the output beam from a diode laser 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 117 and 119 is dependent on which diode laser output beam is intended to pass through which second conditioning lens (i.e., the number of diode laser subassemblies separating the second conditioning lens from the diode laser source).

[0039] Although the diode laser subassembly shown in FIGS. 1-5 utilizes a single emitter, the present invention is equally applicable to multi-emitter diode lasers. For example, FIG. 6 is an illustration of a diode laser subassembly utilizing a three-stripe diode laser 601. Due to the size of diode laser 601, the contact pads 603/605 on submount 607 are typically of a different size than those on submount 105 used with the single emitter diode laser. Additionally the second conditioning lens (i.e., lens 609) is multi-faceted (i.e., facets 611-613) in order to properly condition the individual output beams of diode laser 601.

[0040] FIG. 7 is a perspective view of a preferred cooling block 700 for use with the previously described diode laser subassemblies. As shown, cooling block 700 includes a series of steps, thus allowing the output beams from a plurality of diode laser subassemblies to exit the assembly unimpeded. Preferably central section 701 of the cooling block is recessed as shown, although other cooling block/diode laser subassembly configurations can be used. For example, rather than being recessed, central section 701 can raised relative to one or both adjacent side cooling block portions. Additionally subassembly mounting section 701 can be coplanar with the adjacent cooling block portions.

[0041] Fitting the diode laser subassemblies within the recessed portion of the cooling block as illustrated in FIG. 8 provides both a means for registering the laser subassemblies within the cooling block and relative to one another. In one embodiment only the bottom surface of each of the individual subassembly mounting blocks (e.g., block 103) is in good thermal contact with cooling block 700, thus providing thermal repeatability for the assembly. It will be appreciated, however, that the assembly can be configured such that both the side surfaces and the bottom surface of each mounting block are in thermal contact with the cooling block.

[0042] In the embodiment illustrated in FIG. 8, five diode laser subassemblies 801-805 are positioned within the central recess (e.g., recess 701 shown in FIG. 7) of cooling block 807. In the illustrated embodiment, the second conditioning lens for each subassembly is located on the arm portions of the adjacent subassembly mounting block. It will be appreciated that the uppermost subassembly, i.e., subassembly 801, does not include a second conditioning lens and that the second conditioning lens for the lowermost subassembly, i.e., subassembly 805, is simply mounted to a stand alone lens carrier 809. Carrier 809 can either be integral to cooling block 807, i.e., machined from the same material, or it can be an independent carrier that is mounted to cooling block 807.

[0043] Although any of a variety of means can be used to mount the subassemblies to the cooling block, in the illustrated embodiment each diode laser subassembly is clamped to the cooling block with a pair of clamp members 811, the clamp members preferably being bolted to the cooling block. Clamp members 811 serve three purposes. First, they hold the diode laser subassemblies in place. Second, by firmly pressing the subassemblies into place, they insure that good thermal contact is made between subassembly mounting block 103 and cooling block 807. Third, clamp members 811 provide a convenient means of electrically contacting the two contact pads 111/113 (or 603/605), either through direct contact or by pressing an electrical contact against the pads, the electrical interconnect being interposed between the clamping member and the contact pad on the submount. During use, preferably either the cooling block is thermally coupled to a cooling source (e.g., thermoelectric cooler), or the cooling source is integrated within the cooling block (e.g., integral liquid coolant conduits coupled to a suitable coolant pump).

[0044] As the cooling block (e.g., block 807) is comprised of a series of steps onto which the diode laser 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. In order to achieve greater subassembly operating temperature variations, preferably the cooling source is coupled only to the bottom surface of the cooling block. In order to minimize subassembly operating temperature variations, preferably the cooling source is coupled only to the side surfaces of the cooling block. By utilizing both bottom surface and side surface cooling, a specific temperature distribution within the assembly can be achieved.

[0045] Since the operating wavelength of a diode laser is temperature dependent, the inventors have found that the operating temperature variations between subassemblies that arise due to the stepped cooling block and the use of a bottom mounted cooling source, alone or in conjunction with a side mounting cooling source, can be used to match diode laser subassembly wavelengths. Accordingly in at least one embodiment of the invention, the output wavelength of each subassembly is determined based on the subassembly'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.

[0046] Although the figures described above illustrate at least one preferred embodiment of the invention, it will be appreciated that there are numerous minor variations that are clearly envisioned by the inventors. For example, FIG. 9 illustrates an alternate embodiment of a cooling block in which the bottom surface 901 of cooling block 900 is inclined. As a result of this configuration, each mounting surface 903 is the same distance from the bottom surface 901, thus maintaining the same cooling rate for each mounted diode laser subassembly (not shown) even when thermally coupling the cooling source to the bottom surface (i.e., surface 901) of the cooling block.

[0047] In the above figures, the illustrated exemplary configurations include only a single row of subassemblies. It should be appreciated, however, that assemblies in accordance with the invention can include either more or less than the five diode laser subassemblies shown in FIG. 8. Additionally, assemblies that contain more than a single row of diode laser subassemblies can be fabricated using the present invention. For example, FIG. 10 illustrates a cooling block 1000 that can be used with a total of twelve diode laser subassemblies, six per row. The output beam from each row can either be combined using known optical techniques, or the assembly can be used to produce two separate output beams.

[0048] As previously noted, the diode laser subassemblies of the present invention can utilize either electrically insulating or electrically conducting submounts as well as any of a variety of different diode laser contacting arrangements. Thus the submount/contact arrangement shown in FIGS. 1-3 and 6 is only an exemplary configuration and should not be viewed as a limitation of the present invention. For example, FIG. 11 illustrates portions of a diode laser subassembly that utilizes an electrically conductive submount 1101. In this embodiment diode laser 101 is attached to submount 1101 with an electrically and thermally conductive solder or bonding material. As a consequence, one contact to diode laser 101 is made via electrically conductive submount 1101, directly or via subassembly mounting block 103 and/or the cooling block (not shown). Of course if electrical contact is made via subassembly mounting block 103, then an electrically conductive solder or bonding material must be used to attach submount 1101 to the subassembly mounting block. Similarly if electrical contact is made via the cooling block (not shown), then an electrically conductive solder or bonding material must be used both to attach submount 1101 to the subassembly mounting block and to attach the subassembly mounting block to the cooling block. The second contact to the diode laser is made via a contact pad 1103, each diode laser being connected to contact pad 1103 via a wire bond or ribbon bond 1105.

[0049] Although FIG. 8 shows the use of clamping members to hold the subassemblies within the cooling block, as previously noted there are numerous other techniques that can be used to mount the subassemblies. For example, FIG. 12 shows a partial diode laser subassembly 1200 in which a first bolt 1201 holds contact pad 1203 and submount 1205 as well as mounting block 1207 to the cooling block (not shown) while a second bolt 1209 holds contact pad 1211 and submount 1205 as well as mounting block 1207 to the cooling block (not shown). Preferably in this configuration submount 1205 is fabricated from an electrically insulating material.

[0050] In addition to the above-described approaches, it will be appreciated that there are numerous mounting techniques that can be used to mount the diode laser subassemblies to the cooling block, these techniques using various arrangements of clamping members, bolts and/or bonding materials (e.g., solder, adhesive). For example in the embodiment illustrated in FIG. 13, a single bolt 1301 is used to attach mounting block 1303 to the underlying cooling block (not shown). Submount 1305 is attached to mounting block 1303 by solder or an adhesive. Preferably a pair of contact pads 1307/1309 is used to electrically couple to diode laser O1. In an alternate embodiment illustrated in FIG. 14, at least one bolt 1401 attaches each mounting block 1403 to cooling block 1405, the bolts passing through the bottom of the cooling block and being screwed into the bottom of mounting blocks 1403.

[0051] 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.