Submount assembly and associated packaging method

Abstract: A submount assembly (10) includes a heat sink (16) and a submount (18) mounted upon the heat sink (16) that is formed of a thermally conductive, electrically insulating material so as to transfer heat from the various components mounted upon the submount (18) to the heat sink (16) while effectively electrically isolating the various components mounted upon the submount (18). The first surface (18a) of the submount (18) defines at least one registration feature (20), such as at least one recess, for precisely aligning components mounted thereupon. For example, the submount assembly (10) can include a microresonator cavity (22) including an active gain medium (14) disposed within a respective recess (20) defined by the submount (18). The submount assembly (10) can also include a laser diode (12) mounted upon the submount (18) in alignment with the active gain medium (14) such that the output of the laser diode (12) pumps the active gain medium (14). The submount assembly (10) also generally includes first and second metallization layers (32, 34) disposed upon different portions of the submount (18) to thereby form a cathode and an anode, respectively. A method for packaging the submount assembly (18) is also described, which reduces the risk of damage to the components mounted upon the submount (18).

Description: The present invention relates generally to a submount assembly and associated packaging method and, more particularly, to a microlaser submount assembly and associated method for packaging the microlaser. Modern electro-optic systems are being further miniaturized such that many electro-optic components are commonly mounted upon the same platform. The platform typically includes a submount which may be mounted upon a heat sink, such as a heat pump or the like. As such, the submount is generally formed of a thermally conductive material, such as a metal or semiconductor, in order to provide a path of relatively low thermal impedance from the various electro-optic components mounted upon the submount to the underlying heat sink. In order to properly function, the various components of the electro-optic system must be precisely aligned. For example, an electro-optic system that includes a pump diode and an associated laser crystal or active gain medium requires that the pump diode be precisely aligned with the laser crystal as well as various other optical components, such as lenses, mirrors and the like. While this alignment has always been somewhat challenging, the alignment of the various components of an electro-optic system has become increasingly critical and increasingly difficult as the various components of the electro-optic systems have become even smaller. Once the various electro-optic elements have been appropriately mounted upon the submount, the submount assembly, including the submount and any underlying heat sink, is commonly mounted in an appropriate package, such as a TO-3 or TO-8 package. As is known to those skilled in the art, electro-optic packages, such as TO-3 or TO-8 packages, include a number of conductive pins which must be electrically connected to appropriate leads of the submount assembly in order to provide the necessary electrical energy to the electro-optic system. Even if the various electro-optic components are properly mounted upon the submount, the mounting of the submount assembly within the package and the establishment of the electrical connections between the conductive pins of the package and respective leads of the submount assembly can disadvantageously affect the performance of the electro-optic system. In particular, the mounting of the submount assembly within a package generally requires handling of the submount assembly which may expose the submount assembly and, more particularly, the various electro-optic components, to static electricity and other deleterious conditions. In addition, conventional packaging techniques, including the establishment of appropriate electrical connections between the conductive pins of a package and the respective leads of the submount assembly, may disadvantageously heat the various electro-optic components. Although a variety of miniaturized electro-optic systems have been developed which include submounts and associated heat sinks for providing heat dissipation for the various electro-optic components, a need still exists for improved techniques for aligning the various electro-optic components of a miniaturized electro-optic system. Moreover, a need still exists for improved packaging techniques in order to appropriately package an electro-optic system without adversely affecting the electro-optic components. EP-A-294 167 discloses an assembly comprising a submount made of an electrically insulating material. EP-A-331 331 discloses a submount comprising recesses and grooves for aligning components. It is accordingly desirable to provide an improved submount assembly which includes a submount for aligning respective components of an electro-optic system. It is also desirable to provide an improved submount assembly having a submount which is not only thermally conductive, but is also electrically insulating to therefore electrically isolate the various electro-optic components mounted upon the submount. Furthermore it is desirable to provide an improved method of packaging an electro-optic system in which at least some of the electro-optic components are mounted upon the submount following the placement of the submount assembly within the package and the establishment of appropriate electrical connections between the conductive pins of the package and respective leads of the submount assembly in order to reduce handling of the submount assembly subsequent to the mounting of the various electro-optic components upon the submount. According to an embodiment of a first aspect of the present invention as defined in claim 1 there is provided a submount assembly comprising: a heat sink; a submount, mounted upon the heat sink, having opposed first and second surfaces, the submount comprised of a thermally conductive, electrically insulating material, wherein an interior portion of the first surface of the submount defines at least one registration feature for aligning respective components; and first and second metallization layers disposed upon different portions of the submount to thereby form a cathode and anode, respectively, wherein: the first surface of the submount includes an upper shelf and a lower shelf, and wherein the registration feature comprises an upstanding wall extending between the upper and lower shelves. Thus, a submount assembly embodying the present invention can effectively align the various components of an electro-optic system. According to an embodiment of a second aspect of the present invention there is provided a method of packaging an electro-optic system comprising the steps of: mounting a submount assembly as defined in claim 1 including the submount upon a support surface of a package, wherein the package includes a plurality of conductive pins surrounding the support surface; establishing electrical connections between the conductive pins of the package and respective leads of the submount assembly; and mounting a component upon the submount in alignment with the registration feature defined on the first surface of the submount following the steps of mounting the submount assembly in the package and establishing electrical connections therewith. Reference will now be made, by way of example, to the accompanying drawings, in which:- Figure 1 is a perspective view of a first submount assembly; Figure 2 is a perspective view of the submount assembly in Figure 1 mounted within an electro-optic package, Figure 3 is a perspective view of a second submount assembly; Figure 4 is a perspective view of a third submount assembly; Figure 5 is an exploded perspective view of a fourth submount assembly; Figure 6 is an assembled perspective view of the submount assembly of Figure 5; Figure 7 is an exploded perspective view of a submount assembly embodying the present invention; Figure 8 is an assembled perspective view of the submount assembly of Figure 7; Figure 9 is a perspective view of a fifth submount assembly which includes a segmented submount; and Figures 10A-10C are a series of sequential views illustrating a method of packaging an electro-optic system not embodying the present invention, in which Figure 10A is a side view illustrating the mounting of a heat sink and, in turn, a submount upon the support surface of an electro-optic package, Figure 10B is a top view of the electro-optic package including a portion of the submount assembly which illustrates the establishment of appropriate electrical connections between the conductive pins of the package and respective leads of the submount assembly, and Figure 10C is a side view of the resulting package following the mounting of the electro-optic components upon the submount. The present invention will now be described more fully hereinafter with reference to che accompanying drawings, in which a preferred embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Like numbers refer to like elements throughout. Figures 1 to 6, 9 and 10, respectively show diagrams of submount assemblies which do not embody the present invention, but which are described here below in order to aid understanding of the present invention. Referring now to Figure 1, a first submount assembly 10 is illustrated. Although the submout assembly can support any of a variety of electro-optic systems, the submount assembly is particularly advantageous for supporting and aligning a microlaser system which requires precise alignment between a laser diode 12 and a laser crystal or other active gain medium 14 . As such, the submount assembly will be hereinafter described in conjunction with a microlaser system. As illustrated, the submount assembly 10 includes a heat sink 16 and a submount 18 mounted upon the heat sink. It will be understood by those having skill in the art that an element or component that is described as being "on" or "mounted upon" another element may be either mounted directly on the underlying element or may merely overlie the other element with one or more intervening layers or elements being disposed between the elements. Although the heat sink can be a passive heat sink formed of thermally conductive material, such as a silver base plated with an alloy of gold and nickel, the heat sink can also be an active heat sink or heat pump such as a Peltier heat pump or other thermoelectric cooler. As used herein, the term "heat sink" therefore includes not only heat sinks which cool a component by accepting waste heat, but also heat pumps which generate additional heat in order to warm a component. The submount 18 is formed of a thermally conductive material for providing a path of low thermal impedance to the heat sink 16, this material also being electrically insulating so as to electrically isolate the various electro-optic components mounted upon the submount. For example, the submount effectively electrically isolates the laser diode 12 and the active gain medium 14 as described below. Although the submount can be formed of a variety of thermally conductive, electrically insulating materials, it is preferably formed of beryllium oxide, aluminum oxide or aluminum nitride which are both thermally conductive and electrically insulating. The submount 18 has opposed first and second surfaces 18a,18b, the first surface of the submount defining at least one registration feature 20 for aligning the various electro-optic components supported by the submount. Although the first surface of the submount can define a variety of registration features as described hereinbelow, the submount of one example defines at least one and, more particularly, a plurality of recesses or slots for receiving respective components of the electro-optic system supported by the submount. In the first submount assembly 10 illustrated in Figure 1, the first surface 18a of the submount 18 defines a slot-like recess 20 having a relatively flat bottom and opposed upwardly extending sidewalls for receiving one of more electro-optic components, and a microresonator cavity 22 disposed within and adjacent one end of the slot. The microresonator cavity 22 preferably includes an active gain medium 14 and a saturable absorber 24 sandwiched between a pair of mirrors that define the resonant cavity. In this regard, one example of a suitable microresonator cavity is described by U.S. Patent No. 5,394,413 to John J. Zayhowski which issued on February 28, 1995, the contents of which are incorporated in their entirety herein. The microresonator cavity includes an active gain medium formed of neodymium-doped yttrium aluminum garnet (YAG) and the saturable absorber formed of tetravalent chrome-doped YAG. Although the active gain medium therein is doped with approximately 1.6 atomic percent of neodymium, the active gain medium and the saturable absorber can include different dopant percentages. The microresonator cavity of this example will emit laser pulses having a wavelength of 1.06 nanometers. As will be apparent to those skilled in the art, however, the active gain medium 14 and the saturable absorber 24 can be constructed of different materials in order to provide laser outputs having different properties, such as different wavelengths. The submount assembly 10 of Figure 1 also includes a laser diode 12 mounted upon the first surface 18a of the submount 18 in alignment with the microresonator cavity 22 and, more particularly, the active gain medium 14 of the microresonator cavity. As such, the output of the laser diode pumps the active gain medium such that the microresonator cavity emits a series of laser pulses. As described below, the laser diode is preferably spaced somewhat, such as 20 ± 5 microns, from the microresonator cavity. In addition, although the submount assembly can include a variety of laser diodes, the laser diode is preferably formed of gallium arsenide and provides 1.2 watts of pump power. As shown in Figure 1, the laser diode 12 can be mounted upon a thermal spreader 25 formed of a thermally conductive material which, in turn, is mounted upon the first surface 18a of the submount 18 For example, the thermal spreader may be formed of gold metallized diamond. In this regard, the diamond is typically a synthetic diamond that has been grown by a chemical vapor deposition process. Once grown, the diamond is generally polished and a gold coating is evaporatively deposited upon the diamond. The thermal spreader also generally has a larger footprint than the laser diode so as to effectively spread the heat generated by the laser diode over a broader area of the submount. The submount assembly 10 can also include a variety of other electro-optic components downstream of the microresonator cavity 22 for appropriately processing the laser pulses emitted by the microresonator cavity. In the first submount assembly illustrated in Figure 1, for example, the submount assembly also includes a frequency doubling crystal 26 , typically formed of KTiOPO 4 , i.e., KTP. In addition, the submount assembly of Figure 1 can include an anti-reflection coated fold prism 28 and a silver-coated right angle prism 30 for folding the output of the microresonator cavity and for thereafter directing the output of the microresonator cavity in a direction generally perpendicular to the submount 18 . For a submount disposed in a horizontal position, the output of the electro-optic system can therefore be directed vertically as shown in Figure 1. More particularly, for a submount assembly which includes a microresonator cavity having an active gain medium 16 formed of neodymium-doped YAG and a saturable absorber 24 formed of tetravalent chrome-doped YAG which emits laser pulses having a wavelength of 1.06 nanometers, the first submount assembly illustrated in Figure 1 will produce linearly polarized pulses having a wavelength of 532 nanometers. The submount assembly 10 also includes first and second metallization layers 32,34 disposed upon different portions of the submount 18 to thereby form a cathode and an anode, respectively. Although the metallization layers can be formed of a variety of electrically conductive materials, the first and second metallization layers are preferably formed of diffusion bonded copper. As also shown in Figure 1, the first and second metallization layers can be disposed within respective recesses 20 defined by the first surface 18a of the submount. Once the laser diode 12 has been mounted upon the submount 18 , appropriate electrical connections can be established between the cathode, the anode and the laser diode as will be described in more detail hereinafter in conjunction with a fabrication method. In this regard, the laser diode and, more particularly, the thermal spreader 25 upon which the laser diode is mounted in the first submount assembly can be mounted upon one of the metallization layers 32,34. Since the thermal spreader is metallized, the mounting of the thermal spreader upon the metallization layer establishes a electrical connection between the underlying metallization layer and the laser diode. One or more wire bonds or ball bonds can also be formed between an upper surface of the laser diode and the other metallization layer, as illustrated in Figure 1. By applying appropriate voltage and current to the anode and cathode, the electro-optic system carried by the first submount assembly 10 will provide the desired laser output. As shown in Figure 2, for example, the first submount assembly 10 is generally mounted within an electro-optic package 36 , such as TO-8 package or a TO-3 package, as will be described in more detail hereinafter. As shown in Figure 2, however, the package generally includes a plurality of conductive pins 38 surrounding the submount assembly. By establishing appropriate electrical connections between the conductive pins of the package and the respective leads of the first submount assembly, including the anode, the cathode and any electrical leads associated with the heat sink 16 , the electro-optic system can be operated by appropriately driving the conductive pins of the electro-optic package. As shown in Figures 3-6 (relating to second, third and fourth submount assemblies not embodying the present invention) and Figures 7-8 (relating to a submount assembly embodying the present invention) a submount 18 can support a variety of electro-optic systems. With reference to Figure 3 (showing a second submount assembly), for example, the electro-optic system can include a fourth harmonic generator 40 , typically formed of BBO, disposed downstream of the frequency doubling crystal 26 in order to generate a fourth order output. Accordingly, for a submount assembly 10 which includes a microresonator cavity 22 having an active gain medium 16 formed of neodymium-doped YAG and a saturable absorber 24 formed of tetravalent chrome-doped YAG which emits laser pulses having a wavelength of 1.06 nanometers, the second submount assembly illustrated in Figure 3 will produce linearly polarized pulses having a wavelength of 266 nanometers. In addition, the electro-optic system can include an energy monitor 42 downstream of the fourth harmonic generator 40 in order to measure the energy of the laser output (as shown in Fig. 3). As such, the electro-optic system also includes a beam splitter 44, positioned downstream of the fourth harmonic generator, for providing the energy monitor with a fraction of the laser output while directing the remainder of the laser output in a direction generally perpendicular to the submount 18 . As also shown in Figure 3, the second submount assembly 10 can also include the temperature sensor 46 mounted upon the first surface 18a of the submount for monitoring the temperature of the submount. The temperature sensor is electrically connected to the thermoelectric cooler 16 such that the amount of cooling provided by the heat pump can be adjusted based upon the temperature of the submount. Preferably, the temperature sensor and the thermoelectric cooler are designed such that the heat pump maintains the submount at a relatively constant temperature. Figure 4 illustrates a third submount assembly 10. As shown, the submount 18 therein defines a slot 20 in which a microresonator cavity 22 and an electro-optically tuned waveguide or one or more non-linear optic crystals 48 are disposed. As described above, the third submount assembly also includes a laser diode 12 mounted in alignment with the microresonator cavity such that the output of the laser diode pumps the microresonator cavity and, more particularly, the active gain medium 14 of the microresonator cavity. As illustrated in Figure 4, the submount 18 can be mounted upon the passive heat sink 16 , such as a silver base that is plated with an alloy of gold and nickel. In contrast to the packaged design of Figure 2, the submount can be disposed within a housing 50 having a plurality of conductive pins such that by establishing appropriate electrical connections between the conductive pins and the respective leads of the third submount assembly, including the anode and the cathode, the laser diode can be appropriately driven. In one example of the third submount assembly, the housing is formed of oxygen free, high conductivity copper that is sealably mounted to the underlying heat sink by means of thermal grease, indium foil or the like so as to minimize the thermal impedance of the seal between the housing and the heat sink. The housing also generally includes an anti-reflection coated window 52 aligned with the KTP waveguide through which the linearly polarized TEM oo laser pulses are emitted. A fourth submount assembly 10 is illustrated in Figures 5 and 6. As shown, the fourth submount assembly includes a submount 18 having a first surface 18a which defines a first groove 54 . The first groove opens through the first surface and has a bottom 54a and a pair of opposed sidewalls 54b. The fourth submount assembly also includes a laser diode 12 mounted upon one sidewall of the first groove. In this regard, the laser diode is generally mounted upon a thermal spreader 25 which, in turn, is mounted to a sidewall of the first groove. The laser diode is oriented such that the output of the laser diode is directed toward the opening of the first groove through the first surface of the submount. In the fourth submount assembly illustrated in Figure 6, the output of the laser diode is directed in an upward direction. The fourth submount assembly 10 also includes an active gain medium 14 and, more typically, a microresonator cavity 22 including an active gain medium and an associated saturable absorber 24 . The microresonator cavity is supported by the first surface 18a of the submount 18 so as to overlie at least that portion of the first groove 54 within which the laser diode 12 is disposed. As such, the output of the laser diode will pump the active gain medium so as to produce a series of laser output pulses. Although the active gain medium can be supported by the first surface of the submount in a variety of fashions, the first surface of the submount in this example preferably defines a plurality of ledges 55 opening into the first groove. In this case, edge portions of the microresonator cavity can be supported by the plurality of ledges so as to bridge the first groove and overlie the laser diode. Preferably, the first surface of the submount defines ledges adjacent each of the opposed sidewalls 54b of the first groove for supporting the four corners of the microresonator cavity as illustrated in Figures 5 and 6. The fourth submount assembly 10 is preferably formed such that the laser diode 12 is appropriately spaced from the microresonator cavity 22 . Although different spacings can be employed, the laser diode is generally spaced from the microresonator cavity by 20 ± 5 microns. In order to provide the desired spacing, the laser diode is preferably mounted upon the thermal spreader 25 proximate one end thereof. By appropriately sizing the thermal spreader relative to the depth of the first groove 54 , the laser diode can be appropriately spaced from the microresonator cavity by mounting the thermal spreader to the sidewall of the first groove such that the end of the thermal spreader opposite the laser diode contacts the bottom 54a of the first groove. As shown in Figures 5 and 6, the first surface 18a of the submount 18 also preferably defines a second groove 58 extending between one edge of the submount and the first groove 54 . The second groove preferably opens into the first groove at a position generally aligned with the laser diode 12 such that the laser diode is exposed via the second groove. As illustrated by Figures 5 and 6, the first and second grooves are preferably orthogonal. As such, the second groove facilitates the mounting of the laser diode upon the sidewall 54b of the first groove by permitting access to the sidewall of the first groove in a direction generally perpendicular to the sidewall. Although not essential, the cross-sectional shapes and dimensions of the first and second grooves are generally the same. As further illustrated in Figures 5 and 6, the fourth submount assembly 10 also generally includes first and second metallization layers 32,34 disposed upon different portions of the submount 18 . Although the first and second metallization layers can be disposed within respective recesses 20 defined by the first surface 18a of the submount as described above in conjunction with the first submount assembly shown in Figure 1, the first and second metallization layers of the fourth submount assembly are disposed upon opposite sidewalls 54b of the first groove 54 . By metallizing the thermal spreader 25 upon which the laser diode 12 is mounted, electrical contact is established between the laser diode and the metallization layer coating the sidewall upon which the laser diode is mounted. As shown, wire bonds or ball bonds can be established between the laser diode and the metallization layer coating the other sidewall of the first groove. By applying appropriate voltage and current to the metallization layers, the laser diode can be driven so as to produce an output which pumps the microresonator cavity 22 overlying the laser diode. As described previously, the submount 18 in the fourth submount assembly 10 can be mounted upon a heat sink 16, such as a Peltier heat pump or other thermoelectric cooler. In this regard, the submount is also preferably formed of a thermally conductive, electrically insulating material, such as beryllium oxide, aluminum oxide or aluminum nitride. As such, the submount will provide a path of low thermal impedance from the electro-optic components to the heat sink, while effectively electrically isolating the various electro-optic components as well as the first and second metallization layers. The fourth submount assembly 10 can also be mounted within an appropriate electro-optic package 36 , such as a TO-3 or TO-8 package, as shown in Figure 2 and as described below. In this regard, the fourth submount assembly can be packaged as described below in conjunction with Figures 10A-10C so as to further protect the various electro-optic components mounted upon the submount 18 . Although the first to fifth submount assemblies 10 (not embodying the present invention) illustrated in Figures 1-6 and described hereinabove include a submount 18 having a first surface 18a which defines one or more recesses 20 for receiving and aligning respective electro-optic components, each submount assembly and, more particularly, the submount can define various other types of registration features for aligning respective components. Figures 7 and 8 show a submount assembly embodying the present invention, which includes a submount having a first surface which includes an upper shelf 18c and a lower shelf 18d . According to this embodiment, the registration feature defined by the submount is an upstanding wall 18e extending between the upper and lower shelves. As illustrated, electro-optic components can be precisely positioned with respect to the upstanding wall such that the components are aligned. For example, the microresonator cavity 22 and a right angle prism 59 can be mounted upon the lower shelf 18d adjacent the upstanding wall 18e while a laser diode 12 can be mounted upon the upper shelf 18c in alignment with the microresonator cavity such that the output of the laser diode pumps the active gain medium 14 of the microresonator cavity. Typically, the laser diode is spaced, such as by 20 ± 5 microns, from the registration feature 20 , i.e. the upstanding wall, in order to provide the proper spacing between the emitting facet of the laser diode and the microresonator cavity. As shown in Figures 7 and 8, first and second metallization layers 32,34 are also disposed upon different portions of the submount 18 . Although not illustrated, the upper shelf of the submount can define respective recesses within which the first and second metallization layers are disposed, if so desired. Since the submount is not only thermally conductive but is also electrically insulating, the submount assembly 10 can effectively electrically isolate the various electro-optic components and the first and second metallization layers. The submount assembly 10 of Figures 7 and 8 also includes a heat sink 16 , such as a thermoelectric cooler. As illustrated in Figures 7 and 8, conventional thermoelectric coolers include one or more couples, typically formed of bismuth telluride, sandwiched between a pair of thermally conductive plates, such as aluminum oxide plates. As shown in Figures 1-6 in conjunction with the first to fourth submount assemblies, however, the submount 18 in an embodiment of the present invention can also serve as the upper plate of the thermoelectric cooler such that the submount and the heat sink 16 are an integral assembly. Although not shown, the submount assemblies of Figures 1-6 (first to fourth submount assemblies) can include other types of heat sinks, including more conventional thermoelectric coolers, if so desired. Likewise, the submount assembly of Figures 7 and 8 embodying the present invention can be integrated with a thermoelectric cooler as shown in Figures 1-6, if so desired. Although the first to fourth submount assemblies 10 illustrated in Figures 1-6, and a submount assembly embodying the present invention illustrated in Figs. 7-8, include a single heat sink 16 or, more precisely, a single Peltier heat pump, the submount assembly can be segmented and can correspondingly have a plurality of heat sinks, one of which is associated with each segment of the submount assembly. As shown in Figure 9, the submount 18 of a fifth assembly can be divided into a plurality of segments separated by relatively thin air gaps 19 . In particular, the submount can include a first segment 18' which supports the laser diode 12 , a second segment 18" which supports the microresonator cavity 22 , and a third segment 18"' which supports the components downstream of the microresonator cavity including the frequency doubling crystal 26 . Correspondingly, the fifth submount assembly can include three heat sinks and, more preferably, three Peltier heat pumps, one of which is associated with each segment of the submount. As also illustrated in Figure 9, the fifth submount assembly 10 can include a plurality of temperature sensors 46 , one of which is associated with and monitors the temperature of each respective segment of the submount 18 . Although not illustrated, a controller can receive signals from the temperature sensors that are representative of the temperature of each of the various segments of the submount. The controller can then apply appropriate voltages to the leads of the Peltier heat pumps in order to individually control the temperature of each segment of the submount. For example, since the laser diode 12 generally generates more heat than the other components, the heat pump 16' associated with the first submount segment 18' upon which the laser diode is mounted can cool the first submount segment more than the second and third submount segments. In order to maintain the wavelength of the signals emitted by the laser diode within the central portion of the pump band of the active gain medium 14 , the first submount segment is preferably cooled such that the temperature of the laser diode remains relatively constant since the wavelength of the signals emitted by the laser will otherwise vary if the temperature of the laser diode fluctuates. Additionally, the heat pumps associated with the second and third submount segments can cool or heat the respective submount segments such that the microresonator cavity 22 and the other components, such as the frequency doubling crystal 26 , are maintained at a temperature that enhances their operating efficiency. A method for packaging the electro-optic system not embodying the present invention will now be described in conjunction with the embodiment of the submount assembly 10 illustrated in Figures 7 and 8. In this method the submount assembly, including the submount 18 , is initially mounted upon a support surface of a package 36 , such as a TO-3 or TO-8 package. As shown in Figure 10A, the heat sink 16, such as a Peltier heat pump, can be initially mounted upon the support surface of the package and the submount can thereafter be mounted upon the heat sink. Although the laser diode 12 and the metallization layers 32,34 are generally disposed upon the submount prior to its mounting to the heat sink, one or more of the electro-optic components, including the microresonator cavity 22 , are not mounted upon the submount at the time of its mounting to the heat sink. Although the submount assembly can be mounted in a variety of fashions, the heat sink is typically adhered to the support surface of the package and the second surface 18b of the submount is typically adhered to the heat sink by means of a thermally matched epoxy, such as aluminum oxide filled or silver filled epoxies. The package 36 includes a plurality of conductive pins 38 which extend through the base 60 of the package and which surround the support surface of the package. Once the heat sink 16 and the submount 18 have been mounted, electrical connections are then established between the conductive pins of the package and respective leads of the submount assembly 10 , as illustrated in Figure 10B. For example, electrical connections are established between the conductive pins of the package and the anode, the cathode and respective leads of the thermoelectric cooler. Once the submount assembly 10 has been mounted upon the support surface of the package 36 and appropriate electrical connections have been established, the remainder of the electro-optic components, including the microresonator cavity 22 , can be mounted upon the submount 18 in alignment with the registration features 20 , such as the recesses, defined by the first surface 18a of the submount. For example, the microresonator cavity and the right angle prism 59 are preferably mounted upon the submount as described above in conjunction with Figures 7 and 8. Although the components can be mounted in a variety of fashions, the electro-optic components are preferably mounted upon the submount by means of a pick and place machine utilizing vacuum pick-up in order to increase the efficiency of the fabrication process. As shown in Figure 10C, the lid 62 is mounted to the base of the package 36 so as to enclose the submount assembly. As shown in Figure 2, the lid includes a window 64 , however, such that by applying appropriate voltages to the conductive pins 38 , the electro-optic system supported by the submount can emit laser pulses through the window. By mounting at least some of the components including the microresonator cavity 22 upon the submount 18 after the submount assembly 10 has already been mounted upon the support surface of the package 36, the method described above provides a number of advantages. In particular, such a fabrication method can reduce the handling of the various components, thereby reducing the exposure of the components to static electricity and other deleterious conditions. In addition, the packaging technique described above can limit heating of a number of electro-optic components, including the microresonator cavity, that could deleteriously affect the operation and/or lifetime of the other electro-optic components. As described above, a submount assembly 10 embodying the present invention includes a submount 18 for precisely aligning the various components of an electro-optic system, including a laser diode 12 and an associated microresonator cavity 22 . In addition, the submount is not only thermally conductive, but is also electrically insulating in order to effectively electrically isolate the various electro-optic components mounted thereupon.

Claims: A submount assembly (10) comprising: a heat sink (16); a submount (18), mounted upon said heat sink (16), having opposed first and second surfaces (18a, 18b), said submount (18) comprised of a thermally conductive, electrically insulating material, wherein an interior portion of the first surface (18a) of said submount (18) defines at least one registration feature for aligning respective components; and first and second metallization layers (32, 34) disposed upon different portions of said submount (18) to thereby form a cathode and anode, respectively, wherein: the first surface (18a) of said submount (18) includes an upper shelf (18c) and a lower shelf (18d), and wherein the registration feature comprises an upstanding wall (18e) extending between the upper and lower shelves (18c, 18d). A submount assembly (10) according to claim 1, further comprising: an active gain medium (14) disposed upon the lower shelf (18d) adjacent the upstanding wall (18e); and a laser diode (12) mounted upon the upper shelf (18c) in alignment with said active gain medium (14) such that an output of said laser diode (12) pumps said active gain medium (14). A submount assembly (10) according to claim 1 or 2, wherein said first and second metallisation layers (32, 34) are disposed within respective recesses defined by the first surface (18a) of said submount (18). A submount assembly (10) according to any preceding claim, wherein the submount (18) is formed of a material selected from the group consisting of beryllium oxide, aluminium oxide and aluminium nitride. A submount assembly (10) according to any preceding claim, wherein said submount (18) comprises a plurality of submount segments (18', 18", 18'''), and wherein said heat sink (16) comprises a plurality of heat sinks (16', 16'', 16''') associated with respective ones of said submount segments (18', 18", 18'''). A method of packaging an electro-optic system comprising the steps of: mounting a submount assembly (10) according to claim 1 upon a support surface of a package (36), wherein the package (36) includes a plurality of conductive pins (38) surrounding the support surface; establishing electrical connections between the conductive pins (38) of the package (36) and respective leads of the submount assembly (10); and mounting a component upon the submount (18) in alignment with the registration feature defined on the first surface (18a) of the submount (18) following said steps of mounting the submount assembly (10) in the package (36) and establishing electrical connections therewith. A method according to claim 6, wherein the mounting step comprises the step of disposing an active gain medium (14) upon the lower shelf (18d) adjacent the upstanding wall (18e) and in alignment with a laser diode (12) that is mounted upon the upper shelf (18c) such that an output of the laser diode (12) pumps the active gain medium (14). A method according to claim 6 or 7, further comprising the step of forming first and second metallization layers (32, 34) within respective recesses defined by the first surface (18a) of the submount (18) prior to mounting the submount assembly (10) upon the support surface of the package (36).

Inventors: PETERSON BRIAN LEE, PETERSON, BRIAN LEE
Applicants: LITTON SYSTEMS INC, LITTON SYSTEMS, INC.
Application year: 1999
Main classification: H01S3
IPC codes: H01S5, H01L21, H01S3