Abstract:
An optical assembly that can be utilized as a transmitter in a high data rate optical transceiver system is presented. The optical assembly allows a laser driver to be mounted near the laser and allows the laser driver and the laser to utilize a common heat sink. Further, assembly can be performed reliably and quickly to reduce the cost of production of the optical assembly.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is directed towards an optical assembly and, in particular, an optical assembly capable of high speed data transmission.  
           [0003]    2. Discussion of Related Art  
           [0004]    As data rates increase, the need for components that can accommodate those data rates also increase. Further, there is great interest in providing low-cost transceiver components that support high data rates in small form-factor packages.  
           [0005]    Coaxially arranged optical assemblies, such as the TO-56 packages for example, are common standard form-factors for housing optical network components. The TO-56 package allows for coupling, with an optical coupler, to an optical fiber communication line. This coaxial style packaging for optical coupling with optical fiber provides a cost effective solution for many transceiver applications. However, as data rates increase (especially beyond the 2.5 Gbps range), a new solution is needed to achieve high performance. As the higher performance is attained, however, the cost of producing high-performance optoelectronic packages can increase dramatically.  
           [0006]    Several problems arise when high performance optoelectronic devices are assembled in small form-factor packages. For example, the thermal properties of the device become more problematic as high performance devices may generate more heat than is comfortably dissipated by a small package. Further, due to impedance mismatches and other electronic effects, high speed data signals may be degraded between, for example, a laser driver and a laser. Optical alignments also become more critical at higher data rates because loss of the small tolerances associated with high bandwidth optical transmission can become more of a problem. All of these issues can make it difficult to manufacture high performance optoelectronic devices at low cost.  
           [0007]    Therefore, there is a need for transceiver components in small coaxial style package that both perform well at high data rates (for example above about 2.5 Gbps) and that are manufacturable at low cost.  
         SUMMARY  
         [0008]    In accordance with the present invention, a high performance coaxial style packaged transceiver assembly with low manufacturing cost is presented. In some embodiments the transceiver assembly includes a laser driver and a laser submount, wherein the laser driver IC is mounted on an interface board and then mounted on the same heatsink material as is the laser submount. In some embodiments, the assembly provides for a high performance transmission system in a coaxial style packaging. An optical assembly according to the present invention can include a feed-through assembly, the feed-through assembly including an access for a laser driver and a heat sink, and a laser assembly mounted on the heat sink, wherein the access for the laser driver and the laser assembly are both thermally coupled to the heat sink.  
           [0009]    In some embodiments of the invention, the output impedance of the laser driver is matched to the combined impedance of the laser and matching assembly to lower power consumption of the transceiver package. Further, in some embodiments the integrated resistor and laser submount are directly mounted on a heatsink, thereby improving high temperature performance by thermally conducting heat through the heat sink rather than through the electrical leads. In some embodiments, the thermal paths of the laser driver and the laser are sufficiently isolated to allow for efficient thermal dissipation without interference from the laser driver. In other words, in some embodiments, the thermal paths and proximity of the laser driver and the laser are balanced. In some embodiments, this balance can be achieved by sacrificing lowest-possible laser driver temperatures in order to achieve the lowest laser temperatures while maintaining acceptable electrical performance.  
           [0010]    In some embodiments, the transceiver assembly includes a thin film spiral inductor placed with the laser die on the submount to minimize the stub length seen by the high speed signal at the laser input, achieving high signal performance.  
           [0011]    In some embodiments, final optical alignment is obtained by typing the optical assembly according to the assembled location of the laser emission with respect to a nominal laser emitter location within the optical assembly. A lens cap, which includes a ball lens, can then be positioned with respect to the laser assembly in response to the type of optical assembly. This process results in a higher yield of high performance devices.  
           [0012]    Since a laser driver can be positioned very close to the laser, and since impedance matching to the combination of resistor and laser can be accomplished with an inductor positioned close to the laser, signal distortion between the laser driver and the laser can be much reduced. Further, in some embodiments, electrical traces instead of individual pins carry signals throughout the optical assembly. The electrical characteristics of electrical traces can be better controlled than pins and contributes to the high performance of optical assemblies according to the present invention. The reduction in signal distortion allows for high performance optical assemblies to be constructed in a small standard package.  
           [0013]    A method of producing an optical assembly according to the present invention can include forming a feed-through assembly, the feed-through assembly including an access for a laser driver wherein the laser driver will be thermally coupled to a heat sink; forming a laser assembly; and mounting the laser assembly on the feed-through assembly such that the laser assembly is thermally coupled to the heat sink.  
           [0014]    These and other embodiments are further described below with respect to the following figures. 
       
    
    
     DESCRIPTION OF THE FIGURES  
       [0015]    [0015]FIG. 1A shows an embodiment of an optical assembly according to the present invention.  
         [0016]    [0016]FIG. 1B shows an optical and electrical block diagram of the optical assembly shown in FIG. 1A.  
         [0017]    [0017]FIGS. 2A through 2D show mounting of laser diode and photodiode assemblies of an embodiment of an optical assembly according to the present invention.  
         [0018]    [0018]FIG. 3A shows an embodiment of a feed-through assembly of an optical assembly according to the present invention.  
         [0019]    [0019]FIGS. 3B through 3R illustrate an embodiment of the feed-through shown in FIG. 3A.  
         [0020]    [0020]FIGS. 4A through 4I illustrate layout and assembly of an embodiment of optical assembly  100  according to the present invention.  
         [0021]    [0021]FIGS. 5A through 5E illustrate assembly of a particular embodiment of a photodiode assembly compatible with the particular embodiment of feed-through assemblies shown in FIGS. 3B through 3O.  
         [0022]    [0022]FIGS. 6A through 6F illustrate assembly of a particular embodiment of a laser assembly compatible with the particular embodiment of feed-through assembly shown in FIGS. 3B through 3O.  
         [0023]    [0023]FIGS. 7A through 7I illustrate an embodiment of an assembly method for production of an optical assembly according to the present invention. 
     
    
       [0024]    Elements having the same designation in the figures have the same or similar functions.  
       DETAILED DESCRIPTION  
       [0025]    [0025]FIG. 1A illustrates an optical assembly  100  according to the present invention. Optical assembly  100  includes a feed-through assembly  101  and a lens cap  102 . Lens cap  102  can, in some embodiments, be attached with feed-through assembly  101  to form a hermetic seal. In some embodiments, a photo diode assembly  105  and laser assembly  104  are mounted to feed-through assembly  101  and enclosed with lens cap  102 . Lens cap  102  further includes a lens  110 , which in some embodiments is a ball lens, in order to couple light from laser assembly  104  into an optical fiber. In some embodiments, further components can be included within and without the area sealed by feed-through assembly  101  and lens cap  102 . For example, an impedance matching inductor  103  can be mounted to feed-through assembly  101  within the area sealed by feed-through assembly  101  and lens cap  102 . Further, feed-through assembly  101  can include a mounting access  129  for a laser driver  106 . Laser driver  106 , in some embodiments, can receive digital data signals and drive a laser of laser assembly  104  to produce a corresponding optical signal.  
         [0026]    In some embodiments, laser assembly  104  can be mounted directly on a heat sink  125 . Heat sink  125  is part of feed-through assembly  101 . In some embodiments, laser assembly  104  is mounted on ceramic layers of feed-through assembly  101 , as shown in FIG. 1A, and thermally coupled to heat sink  125 .  
         [0027]    During assembly, optical assembly  100  can be “typed” according to the offset of the laser emission laser assembly  104  after laser assembly  104  is attached to feed-through  101 . Cap  102  can then be positioned and attached to feed-through  101  in accordance with the type of optical assembly  100 .  
         [0028]    In some embodiments, brackets  107  and  108  can be provided to mechanically attach feed-through  101  to a ceramic layer (not shown). Further, brackets  107  and  108  can be electrically coupled to conducting traces formed on feed-through  101  to electrically couple optical assembly  100  to a ceramic layer (not shown).  
         [0029]    [0029]FIG. 1B shows an example transceiver system  130  that can utilize one or more devices such as optical assembly  100  illustrated in FIG. 1A. Transceiver system  130  includes transceivers  131  and  132  optically coupled through optical fiber  137 - 1  through  137 -(N+M). Each of optical fibers  137 - 1  through  137 -(N+M) include connectors that are optically coupled to transceivers  131  and  132 . Transceiver  131  includes transmitters  133 - 1  through  133 -N, each of which is optically coupled to a corresponding one of receivers  135 - 1  through  135 -N of transceiver  132 . Further, transceiver  131  includes receivers  134 - 1  through  134 -M, each of which is optically coupled to a corresponding one of transmitters  136 - 1  through  136 -M of transceiver  132 . Transceiver  131  can include any number of transmitters N and any number of receivers M. Typically, for many commercial systems, the number of transmitters N and the number of receivers M are both  1 . One or more of transmitters  133 - 1  through  133 -N and  136 - 1  through  136 -M include an optical assembly  100  according to the present invention. The receiver portions of transceiver system  130 , i.e. receivers  134 - 1  through  134 -N and  136 - 1  through  136 -M, includes a photodetector system and electrical filter/driver for receiving optical signals from an optical fiber.  
         [0030]    [0030]FIG. 1C shows an optical and electrical diagram for the circuits of optical assembly  100 . As shown in FIG. 1C, a laser driver  106  is coupled to laser assembly  104 . Laser assembly  104  includes a laser which provides optical output. The optical output from laser assembly  104  is captured by a ball lens  110  on end cap  102  (see FIG. 1A) to couple light into an optical fiber (see FIG. 1B). Photo diode assembly  105  captures light from laser assembly  104  and provides electrical feedback to laser driver  106 . Photodiode assembly  105  and laser assembly  104  are coupled to photodiode power and laser pump power through conductors provided on feed-through assembly  101  shown in FIG. 1A. Control, data, and power are also provided to laser driver  106  through conductors provided on feed-through assembly  101 .  
         [0031]    Laser assembly  104  can be any source of light that can be modulated in response to a signal from laser driver  106 . In some embodiments, laser assembly  104  can be a Mitsubishi Electric and Electronics model ML792H28 laser. In some embodiments, optical laser assembly  104  can be an uncooled InGaAsP 1310 nm DFB laser. The wavelength of light output by laser assembly  104 , in some embodiments, is nominally 1310 nm (e.g. about 1290 to about 1330 nm); however, laser assembly  104  may produce any other central wavelength.  
         [0032]    Laser driver  106  can be any circuit that converts a received digital signal to a signal appropriate for modulating the laser of laser assembly  104 . In some embodiments, laser driver  106  can be a Maxim Max3932E/D driver (Maxim Corporate Headquarters, 120 San Gabriel Drive, Sunnyvale, Calif. 94086).  
         [0033]    In some embodiments, laser driver  106  and laser assembly  104  are mounted on the same interface board of feed-through assembly  101 . In some embodiments, laser driver  106  is mounted on a ceramic substrate of assembly  101 , which is mounted on heat sink  125 , while laser assembly  104  is mounted directly on heat sink  125  (see FIG. 1A). Reducing the distance between laser driver  106  and laser assembly  104  by mounting the two components on the feed-through assembly  101  allows for higher data rates by shortening the transmission distance between laser driver  106  and laser  104 .  
         [0034]    As shown in FIG. 1C, impedance matching between the output impedance of laser driver  106  and the input impedance of laser assembly  104  is accomplished by resistor  109  and inductor  103 . In some embodiments, resistor  109  and laser assembly  104  are mounted directly on heat sink  125 , which can result in an improved high temperature performance. Thermal conduction through heat sink  125  instead of through conducting leads or traces reduces the amount of electrical interference caused by thermal effects in the leads. Further, matching the output impedance of laser driver  106  to the combined impedance of the laser of laser assembly  104  and matching resistor  109  results in lower power consumption of resistor  109 . For example, matching a 20 Ohm output impedance of laser driver  106  to resistor  109  and the input impedance of laser assembly  104  results in lower power consumption by resistor  109  than does matching the 50 Ohm output impedance of the combination of laser driver  106  and resistor  109  with the input impedance of laser assembly  104 .  
         [0035]    Inductor  103  may be a microwave spiral inductor such as that produced by US Microwave L10 62nH-20Q case 30×30 (US Microwaves, 2964-2966 Scott Blvd., Santa Clara, Calif. 95054), for example. Mounting inductor  103  adjacent to laser assembly  104  on feed-through assembly  101  and, in some embodiments directly onto laser assembly  104 , can minimize the stub length, and hence the additional impedance due to the length of the conductor coupling laser driver  106  with the input terminal of laser assembly  104 . The reduced stub length can aid in high data rate performance. Further, utilizing a thin film spiral inductor, inductor  103  can be placed within a small package such as the TO-56 standard package. In some embodiments, inductor  103  can be mounted on laser assembly  104 .  
         [0036]    [0036]FIGS. 2A through 2D illustrate the physical, optical, and electrical configuration of an embodiment of optical assembly  100  with photodiode assembly  105  and laser assembly  104  on feed-through assembly  101 . As shown, laser assembly  104 , with laser  119 , is mounted such as to provide optical coupling. Laser  119  emits in both the forward and rear directions, providing optical input to photodetector assembly  105 .  
         [0037]    Throughout this disclosure, directional references to forward and rear refer to the direction of transmission of light with the forward direction being toward an optical fiber (i.e., ball lens  110  is at the front because it couples light into the optical fiber). Further, references to top and bottom (or up and down) are relative to laser  119  and heat sink  125 , with heat sink  125  being on the bottom and laser  119  being on the top of optical assembly  100 .  
         [0038]    Electrical connections are made between photodetector assembly  105  and laser assembly  104  and the conductors mounted on feed-through assembly  101 . FIG. 2D illustrates some clearance dimensions, in millimeters, for an embodiment of feed-through assembly  101  according to the present invention. In particular, clearances for a wire bond tool  118  with respect to parts of feed-through  101  are illustrated for a particular embodiment of the invention. In some embodiments, electrical connections between photodiode assembly  105 , laser assembly  104  and the conductors of feed-through assembly  101  can be by gold wire-bond technologies, which are familiar to those skilled in the art.  
         [0039]    [0039]FIG. 3A illustrates some embodiments of feed-through assembly  101  according to the present invention. In the embodiment of feed-through assembly  101  shown in FIG. 3A, insulating ceramic layers  121 ,  122 ,  123 , and  124  are bonded with a spacer  120  and heat sink  125 . The combination is mounted within supports  127  and  126  and mounted to sealant ring  128 . Insulating ceramic layers  121 ,  122 ,  123 , and  124  provide electrical connections throughout feed-through assembly  101 . Further, access  129  is provided in ceramic layer  121  to electrically and mechanically mount laser driver  106  to ceramic layer  122 .  
         [0040]    In some embodiments, ceramic layers  121 ,  122 ,  123 , and  124  can be formed from a ceramic material with standard metallization utilized to form electrical conductors. In some embodiments, the characteristics of the metallization layers can be tailored to possess particular properties by controlling widths and material composition of the conducting traces. Resistor  109  can be formed directly on one of ceramic layers  121 ,  122 ,  123 , or  124  or, alternatively, may be formed directly on laser assembly  104 .  
         [0041]    [0041]FIG. 3I shows an embodiment of ceramic layer  121  with metallization  150 . In the embodiment shown in FIG. 3I, for example, each of the traces of metallization  150  can be 100 Ohm traces. In particular, traces to the left of the line marked A can be formed by a first layer of tin-lead solder, for example about 0.013 millimeters thick, with a second layer of copper about 0.025 millimeters thick deposited over the first layer. A third layer of a substrate film is formed over the second layer. In some embodiments, the substrate film is a polymide film, for example about an 0.025 millimeter thick layer of Kapton produced by Dow Chemical, is formed.  
         [0042]    Finally, a fourth layer of copper plating is added. This layering provides electrical connections to feed-through  101  directly. Between the lines marked A and B, however, a different layering of 100 Ohm traces can be utilized. Between lines A and B, a first layer of tin-lead solder having a thickness of about 0.013 millimeter can be added and a second layer of copper trace of about 0.025 mm thickness can be added over the first layer. Finally, a top layer of substrate film, for example about 0.025 mm of Kapton, can be added. To the right of the line marked B, a layer of substrate material, for example a 0.025 mm layer of Kapton, can be utilized in the formation of traces of metallization  150 .  
         [0043]    As is apparent, the metallization utilized on feed-through  101  must withstand the rigors of attachment to supports  126  and  127 , and sealing ring  128 . In some embodiments, supports  126  and  127  are hermetically sealed to spacer  120 , insulating plates  121  through  124 , and heat sink  125  by, for example, a gold-copper braze. In some embodiments, the hermetic seal provides a He leak rate of less than about 1×10 −8  atm-cc/sec. In formation of a hermetic seal between supports  126  and  127  and spacer  120 , insulating plates  121  through  124 , and heat sink  125 , solder material, braze material, or glass may be used.  
         [0044]    [0044]FIG. 3B shows a cross-sectional view of the embodiment of feed-through  101  shown in FIG. 3A after assembly. The dimensions and tolerances shown in FIGS. 3B through 3R are in millimeters and apply to one particular example embodiment of the invention and are not generally limiting. FIGS. 3C through 3H show various views of an assembled embodiment of feed-through  101  shown in FIG. 3B. FIG. 3I illustrates the dimensions and metallization of a particular embodiment of insulating layer  121  shown in FIG. 3B. FIG. 3J shows the dimensions of a particular embodiment of insulating layer  122  shown in FIG. 3B. FIG. 3K shows a particular embodiment of insulating layer  123  of the embodiment of feed-through  101  shown in FIG. 3B. FIG. 3L shows the dimensions of a particular embodiment of insulating layer  124  of the embodiment of feed-through  101  shown in FIG. 3B. FIG. 3M shows the dimensions of the bottom side of insulating layer  124  of the embodiment of feed-through  101  shown in FIG. 3B, and in particular shows metallization to provide external electrical connections for the embodiment of feed-through  101  shown in FIG. 3M. FIG. 3N shows the dimensions of a particular embodiment of support  126  of the embodiment of feed-through  101  shown in FIG. 3B. FIG. 3O shows the dimensions of support  127  for the particular embodiment of feed-through  101  shown in FIG. 3B. FIG. 3P shows dimensions for spacer  120  for the particular embodiment of feed-through  101  shown in FIG. 3B. FIG. 3Q shows dimensions for sealing ring  128  for the particular embodiment of feed-through  101  shown in FIG. 3B. FIG. 3R shows the dimensions for heat sink  125  for the particular embodiment of feed-through  101  shown in FIG. 3B.  
         [0045]    [0045]FIG. 4A illustrates assembly of an embodiment of optical assembly  100  according to the present invention. Photodiode assembly  105  and laser assembly  104  are constructed and mounted on feed-through assembly  101 . Cap  102 , with ball lens  110 , are then mounted to sealant ring  128 . As shown in the embodiment of optical assembly  100  shown in FIG. 4A, lens assembly  104  can be mounted directly onto heat sink  125  of feed-through assembly  101 . Photodiode assembly  105  can be mounted on insulating board  121  of feed-through assembly  101 .  
         [0046]    Photodiode assembly  105  includes photodiode submount  401  and photodiode  402  mounted on photodiode submount  401 . Submount  401  provides structure for photodiode  402  and electrical contacts to electrically couple photodiode  402  with conducting traces on feed-through assembly  101 . Photodiode  402  can be any device that produces an electrical signal in response to an optical signal. FIGS. 5A through 5E illustrate a particular embodiment of submount  401 . Submount  401  is an “L-shaped” mount that is formed from an insulating material appropriate for holding photodiode  402 . Submount  401 , for example, can be formed of alumina, alumina nitrate, or other suitable material. Metallization of conducting leads  501  can be formed on submount  401  in order to electrically couple photodiode  402 , when mounted on submount  401 , to conducting traces in feed-through assembly  101 . Conducting leads  501  should be capable of withstanding the conditions of mounting photodiode assembly  105  onto feed-through assembly  101 . In some embodiments, conducting leads  501  can be formed from electroless Ni and electroless Au about 1.5 micron thick.  
         [0047]    [0047]FIG. 5A shows a planar view of a particular embodiment of a first surface of submount  401  with conducting leads  501 . FIG. 5B shows a planar view of a particular embodiment of a second surface oriented perpendicularly to the first surface of submount  401 . FIG. 5C shows a planar view of a third surface oriented opposite the first surface of submount  401 . FIG. 5E shows a view of submount  401  along the directions T-T indicated in FIG. 5C. FIG. 5D shows a view of submount  401  rotated 12.5° clockwise from the view U-U shown in FIG. 5E.  
         [0048]    Photodiode  402 , as shown in FIG. 4A, can be mounted to a conducting surface such as surface  502  as shown in FIG. 5D. A second electrical connection can be made to conducting surface  503  of FIG. 5D by wire bonding. Electrical contact, then, can be made between photodiode  402  and electrical traces on submount  401 .  
         [0049]    As shown in FIG. 4A, laser assembly  104  includes laser subassembly  404 . Laser  119  is mounted onto laser subassembly  404 . In the embodiment shown in FIG. 4A, inductor  103  is also mounted on laser assembly  104 . In some embodiments, laser  119  can be an InGaAsP 1310 nm DFB laser, which can be purchased as model ML792H28 from Mitsubishi Electric. Inductor  103  can be purchased from US Microwave as an L10 62nH-20Q case 30×30 inductor.  
         [0050]    [0050]FIGS. 6A through 6F illustrate formation of an embodiment of laser submount assembly  404  according to the present invention. Although a particular embodiment is described in FIGS. 6A through 6F, one skilled in the art will recognize that the present invention is not limited to this embodiment.  
         [0051]    [0051]FIG. 6A shows an embodiment of laser assembly submount  404 . Submount  404  includes a top portion  601  and a bottom portion  602 . Both top portion  601  and bottom portion  602  can be formed from aluminum nitride, for example material AN271 purchased from Kyocera. Other manufacturers, including Dupont, Alfa, and Tokuyama, produce similar materials. Conducting traces can be deposited onto top portion  601  and bottom portion  602  to provide both electrical connections and thermal transport paths.  
         [0052]    [0052]FIG. 6B shows a top surface of top portion  601 , with metallization. Top portion  601  also includes vias  606  which, when filled with a conducting material, provide electrical and thermal connections through top portion  601 . Further, a thin film resistor can be deposited in resistor region  605 . Resistor  109  of FIG. 1C, then, can be deposited in region  605 . In some embodiments, thin film resistor  109  deposited in region  605  can be rated for about 12 Ohm±2% at about 250 mW of power and can be formed from aluminum. Thatched areas  607  can be plated with a conductor which can act as a thermal conductor. Thatched areas  607 , then, are electrically isolated from region  605  and  604 . The plating in area  607  can, for example, be formed by about 0.1 micron of titanium, about 0.2 micron of lead, and about 1.5 micron of gold. The plating can be tested and should not be damaged (e.g., no peeling or discoloration) after, for example, a timed bake (e.g., about 3 minutes at about 400° C.).  
         [0053]    Region  603  provides a mount for laser  119 . Region  603  may be electrically coupled to the plating in region  607 , which would provide an electrical ground for laser  119 . Region  603  includes further metallization in order to mount and provide electrical and thermal contact with laser  119 . In some embodiments, the plating as shown in region  607  is further covered with about 0.5 micron of platinum and about 4.0 micron of gold-tin solder. In some embodiments, the gold-tin solder should be about 76%±2% gold by weight.  
         [0054]    Region  604  provides an area on which to mount spiral inductor  103 . Spiral inductor  103  can be mounted, for example, with a non-conductive epoxy material.  
         [0055]    [0055]FIG. 6C shows top portion  601  mounted to bottom portion  602  for a particular embodiment of laser submount assembly  402  according to the present invention. FIG. 6D shows dimensions of metallization features for a particular embodiment of top portion  601  of laser submount assembly  402 . In the particular embodiment of top portion  601  shown in FIG. 6D, the thickness of top portion  601  shown is prior to metallization. Further, the dimensions of resistor area  605  is a maximum area for the printed resistor.  
         [0056]    [0056]FIGS. 6E and 6F illustrate an embodiment of bottom portion  602  of laser submount assembly  402 . As shown in FIG. 6E, a portion of the top surface of bottom portion  602  is partially plated in region  610  with conducting material. In some embodiments, for example, about 0.1 microns of titanium is deposited on the top surface of bottom portion  602 , about 0.2 microns of lead is deposited over the titanium, and about 1.5 microns of gold is deposited over the lead. Other metallizations can, of course, be utilized as well. In some embodiments, other conductors such as copper, silver, or aluminum may be utilized to form the metallizations. As before, the metallization should withstand further processing and may be tested, for example, by a timed bake (e.g., about 3 minutes at about 400° C. to check for peeling, blistering or discoloration). As shown in FIG. 6F, the entire bottom surface of bottom portion  602  can also be metallized to allow coupling of laser submount  404  with heat sink  125  of feed-through assembly  101 . The bottom surface  611  of bottom portion  602  may, for example, be coated with a gold-tin solder. In some embodiments, the gold-tin solder can be about 76%±2% gold by weight.  
         [0057]    When top portion  601  is coupled with bottom portion  602 , vias  606  provide electrical and thermal connections between region  607  of top portion  601  and region  610  of bottom portion  602 . Therefore, heat from laser  603  is conducted through region  610  and bottom portion  602  to heat sink  125 . Further, heat from resistor region  605  can be conducted through the material of top portion  601  to conducting portion  610  and finally to heat sink  125 .  
         [0058]    Although particular dimensions are illustrated in FIGS. 6A through 6F, those dimensions are included to describe a particular example embodiment of laser submount assembly  402 . The present invention is not limited to these dimensions. The particular dimensions shown in FIGS. 6A through 6F are, unless otherwise stated, in units of millimeters.  
         [0059]    [0059]FIG. 4B shows a planar view of an embodiment of the assembled optical assembly  100  shown in FIG. 4A. FIG. 4C shows a view of the assembled optical assembly  100  shown in FIG. 4A along the A-A line as shown in FIG. 4B. FIG. 4D illustrates the view along the G-G line of the embodiment of the assembled optical assembly  100  shown in FIGS. 4B and 4C.  
         [0060]    [0060]FIG. 4E is a schematic diagram along the A-A line shown in FIG. 4B of an embodiment of optical assembly  100  shown in FIG. 4A. Fully assembled, optical assembly  100  includes feed-through assembly  101 , photodiode assembly  105  mounted on feed-through assembly  101 , laser assembly  104  mounted on heat sink  125  of feed-through assembly  101 , and cap  102  with ball lens  110  mounted on feed-through assembly  101 . Feed-through assembly  101 , in some embodiments, includes printed insulating ceramic layers  121 ,  122 ,  123 , and  124  coupled together. One skilled in the art will recognize a number of metallization geometries which allow electrical connections to photodiode assembly  105  and laser assembly  104  along with a laser driver  106  (see FIG. 1A), which can be mounted in access  129  of printed ceramic layer  121 . Feed-through assembly  101  also includes heat sink  125 . In the embodiment shown in FIG. 2E, laser assembly  104  is mounted directly on heat sink  125 . In other embodiments, other arrangements may be made to thermally couple laser assembly  109  with heat sink  125 . Further, laser driver  129  can be mounted on ceramic layer  122  and, through ceramic layers  122 ,  123 , and  124 , is also thermally coupled to heat sink  125 . Better thermal stabilization of optical assembly  101 , then, can be achieved by this arrangement. Feed-through assembly  101  also includes spacer  120  and supports  126  and  127 , which are mechanically coupled to form a hermetic seal around ceramic layers  121 ,  122 ,  123 , and  124  and heat sink  125 . An embodiment of photodiode assembly  105  is illustrated in FIGS. 5A through 5E and discussed above. An embodiment of laser assembly  104  is illustrated in FIGS. 6A through 6F and discussed above.  
         [0061]    As illustrated in FIG. 4E, laser  119 , which is part of laser assembly  104 , provides an output optical beam directed in the forward direction through ball lens  110 . The optical beam from laser assembly  104  can then be coupled to an optical fiber as illustrated in FIG. 1B. Laser  119  also generates optical radiation directed in the backward direction towards photodiode  402  of photodiode assembly  105 . Photodiode  402  can provide feedback to laser driver  106  mounted in access  129  in order to monitor the output power of laser  119 .  
         [0062]    [0062]FIGS. 4F and 4I illustrate layout and alignment of laser assembly  104  and photodiode assembly  105  on feed-through assembly  101 . FIG. 4F shows a top-down view of optical assembly  100  with photodiode assembly  105  and laser assembly  104  mounted on feed-through assembly  101 . Laser  119  of laser assembly  104  is optically aligned with ball lens  110  for coupling into an optical fiber. Further, photodiode  402  is optically aligned with laser  119  to provide feedback for laser driver  106  which can be mounted in access  129 . FIG. 4I is a blow-up of the area in circle D shown in FIG. 4F.  
         [0063]    As shown in FIG. 4I, photodiode  402  is mounted on photodiode subassembly  401 . Photodiode  402  can be, for example, an KPDE030C-15-13T InGaAs photodiode produced by Kyosemi (Kyosemi USA, 368 S. Abbott Ave, Milpitas, Calif. 95035). Typically, photodiode  402  includes two electrical connectors that provide power to photodiode  402  and from which a signal indicating the incident optical power on photodiode  402  can be determined. Photodiode  402 , therefore, is mounted on conducting area  502  (see FIG. 5D) to provide one electrical contact. A wire bond to conducting area  503  is provided to form the other electrical connection for photodiode  402 . Electrical connections to traces on insulating ceramic layers  121 ,  122 ,  123 , and  124  are provided by the metallization  501  of photodiode subassembly  401 , which is mounted on ceramic layer  121  such that the conducting traces on photodiode assembly  105  are in contact with electrical traces on ceramic layer  121 .  
         [0064]    Inductor  103  is connected by wire bonding through wire  411  to resistor area  605  and by wire  410  to traces on insulating ceramic layer  121 . Ground straps  413  provide electrical and thermal connections between area  607  and areas of insulating ceramic layer  121 . Laser  119  is electrically and thermally coupled through its base to area  607 . Further, laser  119  is electrically coupled through wire  412  to resistor area  605 .  
         [0065]    Photodiode  402  can be aligned by an outside reticle or other externally controlled marks which can be “dialed in” to external features of the header. Markings formed on laser submount  404  can also be utilized to position laser  119 .  
         [0066]    [0066]FIG. 4H shows a magnified view of laser  119  with ball bonded wire coupling to resistor area  605 . The area shown in FIG. 4H is that area designated as area T in FIG. 4F. Laser fiducials  415 , which can be utilized for optically aligning laser  119 , are also shown in FIG. 4H.  
         [0067]    [0067]FIG. 4G shows a view along direction G-G as illustrated in FIG. 4C. As shown in FIGS. 4F, 4G and  4 I, the laser emission line and the optical axis of lens  110  need not be completely aligned. In accordance with an aspect of the present invention, cap  102  may be offset slightly so that the optical axis of ball lens  110  does not coincide completely with the laser emission. Offsetting cap  102  can allow better coupling of light out of optical assembly  101  and also allows for correction of misalignment of laser  119  during production.  
         [0068]    In some embodiments of the invention, the facet location of laser  119  can be located by machine vision and cap  102  can be offset such that ball lens  110  is placed within a small tolerance zone of the laser emission axis. Cap offsetting in response to the position of the facets of laser  119 , once mounted, can allow for much higher OSA yields than does the practice of simply bonding cap  102  to feed-through  101 .  
         [0069]    In some embodiments, offsetting of cap  102  does not need to be an active capping process, which usually involves a large amount of alignment time. Since typical tolerances of the optical design of optical assembly  100  allows for a small deviation of laser  119  relative to ball lens  110 , individual optical assemblies  100  can be classified as one of several “types” on production. These individual types represent various zones of the location of the facets of laser  119  with respect to ball lens  110  after final assembly. For example, a “type 1” header might have a laser facet which is offset by 0.050 mm, for example, in a certain direction, “type 2” might indicate 0.100 mm in the same or other direction. These different types represent the most typical placement areas observed for the laser facets and a composite picture of the overlapping areas of “coverage” each “type” allows would cover nearly the entire range of laser and ball lens placements which would occur from industry-standard TO56 header assembly processes. In this way, the offset capping can be reduced to 5 or 6 “types” or “settings” such that matching their (laser facet) offset with a corresponding, pre-determined cap offset becomes practical. The offset of the cap can be accomplished via cap mandrels which have offset pockets and which are indexed to the rotational orientation of the header. The measurement of the laser facet will be made via magnified machine vision and will be taken relative to features which are later also used to locate the position of the cap prior to attachment.  
         [0070]    Therefore, when cap  102  is attached to sealing ring  128 , first the facet of laser  119  is located and optical device  100  is typed. Cap  102  is positioned and attached in accordance to the typing of optical device  100 . The facet of laser  119  can be located automatically in a machine vision position tool.  
         [0071]    [0071]FIG. 7A shows an assembly process  700  for producing optical assembly I  00  according to the present invention. In step  710 , feed-through assembly  101  is assembled. Feed-through assembly  101  can be produced, for example, by Kyocera (Kyocera International, Inc., 8611 Balboa Ave, San Diego, Calif. 92123-1580), NTK (NTK Technologies, 3255-2 Scott Boulevard, Suite  101 , Santa Clara, Calif. 95054), or Sumitomo (Sumitomo Corporation of America, 600 Third Ave., New York, N.Y. 10016-2001). In step  720 , photodiode assembly  105  is produced. In step  730 , laser assembly  104  is produced. In step  743 , photodiode assembly  105  is attached to feed-through assembly  101 . In step  750 , laser assembly  104  is attached to feed-through assembly  101 . In step  760 , electrical connections between traces on feed-through assembly  101 , laser assembly  104 , and photodiode assembly  105  are made. In step  770 , the partially assembled optical assembly  100  is typed based on the emission axis of laser  119  of laser assembly  104 . In step  780 , cap  102  with ball lens  110  is attached to feed-through assembly  101  based on the type determined in step  770 . In step  790 , optical assembly  100  can be tested. In some embodiments, certain of these steps may be rearranged. For example, steps  720  and  730  may occur in a different order. Similarly with steps  730  and  740 , for example. One skilled in the art will recognize several variations of the flow chart shown in FIG. 7A.  
         [0072]    [0072]FIG. 7B illustrates an example method for performing step  710  to assemble feed-through assembly  101 . In step  711 , each of ceramic layers  121 ,  122 ,  123 , and  124  are produced. Ceramic layers  121 ,  122 ,  123 , and  124  can be formed to the appropriate shapes from a ceramic material, for example AN271 alumina tape. Further, access areas and vias can be formed in ceramic layers  121 ,  122 ,  123 , and  124  to accommodate electrical and thermal conductivity throughout feed-through  101 . Metallization for creating electrical traces on ceramic layers  121 ,  122 ,  123 , and  124  can be formed by first depositing, for example, a layer of Kapton in certain portions (i.e., to the right of line B in FIG. 3I, for example). In the embodiment shown in FIG. 3I, between the lines labeled A and B of ceramic layer  121 , a first layer of SnPb solder about 0.013 mm thick and about 0.160 mm wide is deposited on the ceramic material. A second layer of copper trace about 0.025 mm thick and about 0.160 mm wide is deposited over the first layer of SnPb solder. Finally, a top layer of, for example, Kapton about 0.025 mm thick can be deposited over the copper trace. To the left of the line marked A on FIG. 3I, a top layer of copper plating about 0.025 mm thick is placed. Metallization is formed both to conduct heat and provide electrical connectivity through feed-through  101 . One skilled in the art will readily determine a number of ways to metallize ceramic layers  121 ,  122 ,  123 , and  124  to achieve this purpose. Ceramic layers  121 ,  122 ,  123 , and  124  can be attached, both electrically and mechanically, to one another during cofiring. In other words, in some embodiments traces are printed on green ceramic layers  121 ,  122 ,  123  and  124  by a thick film process and then the layers are cofired together to form a cohesive, rigid, multilayered unit.  
         [0073]    In step  712 , heat sink  125  is attached to the bottom of ceramic layer  124 . Heat sink  125  is attached to provide a position for the later mounting of laser assembly  104  and to provide a heat sink for a laser drive  106 , which can be placed in access  129  in ceramic layer  121 , in contact with ceramic layer  122 . An example of the placement of heat sink  125  is shown in FIG. 3E. Heat sink  125  can be formed from any thermally conducting material, for example a copper-tungsten material (10CU90W) that can be obtained from Kyocera.  
         [0074]    In step  713 , ceramic spacer  120  can be attached by epoxy to the top of ceramic layer  121 , as shown in FIG. 3B. FIG. 3B also shows the placement of ceramic spacer  120  for a particular embodiment of optical assembly  100 .  
         [0075]    In step  714 , supports  127  and  126  are positioned onto spacer  120  and heat sink  125  around ceramic layers  121 ,  122 ,  123 , and  124 , and attached so that a hermetic seal is produced around ceramic layers  121 ,  122 ,  123 , and  124 . In some embodiments, supports  126  and  127  can be formed from an iron-nickel alloy, Fe—Ni50 Kovar, which can be obtained from a number of suppliers. In some embodiments, supports  126  and  127  are sealed to ceramic spacer  120 , heat sink  125 , and ceramic layers  121 ,  122 ,  123 , and  124  by a silver copper brazing technique. In some embodiments, the He leak rate through feed-through  101  is less than about 1×10 −8  atm cc/sec. The metallization performed on ceramic layers  121 ,  122 ,  123 , and  124  should not be damaged by the bonding techniques used.  
         [0076]    In step  715 , sealant ring  128  is positioned and attached to supports  126  and  127 . In some embodiments, sealant ring  128  is formed of the same material as is supports  126  and  127  and is bonded to supports  126  and  127  concurrently with the bonding of supports  126  and  127  to spacer  120 , ceramic layers  121 ,  122 ,  123 , and  124 , and heat sink  125 .  
         [0077]    In step  716 , feed-through assembly  101  is inspected. Inspection of feed-through assembly  101  can include a visual inspection through a microscope to check for blistering or discoloration of the metallization. Inspection step  716  may also include electrical tests to check the electrical connectivity of various metallization traces through feed-through  101 .  
         [0078]    [0078]FIG. 7C illustrates assembly of photodiode assembly  104 . In step  721 , photodiode  402  can be inspected. Photodiode  402  can, for example, be an InGaAs PIN diode or any other device which provides an electrical signal in response to light incident on a surface of photodiode  402 . Inspection of photodiode  402  can include a visual inspection, for example with a 10-100× microscope.  
         [0079]    In step  722 , photodiode submount  401  can be inspected. Submount  401 , as illustrated in the embodiment shown in FIGS. 5A through 5E, can be formed from alumina or other insulating material, such as AlN or BeO, for example. Submount  401  is metalized to provide electrical connections to photodiode  402 . Inspection step  722  can include a visual inspection, for example, with a 10-100× microscope, of the metallization as well as the material of submount  401 . Submount  401 , with metallization as described here, can be obtained from Stellar Industries (Stellar Industries Corp., 225 Viscoloid Ave., Leominster, Mass. 01453-4388), ATP (Advanced Thermal Products, Inc., P.O. Box 249, 328 Ridgeway Rd., St. Marys, Pa. 15857), NTK, or Kyocera.  
         [0080]    In step  723 , photodiode  402  is positioned on photodiode submount  401  and fixed to photodiode submount  401 . As shown in FIGS. 4I and 5D, photodiode  401  is also electrically coupled to region  502  of metallization on photodiode submount  401 . In some embodiments, photodiode  402  is attached to region  502  of submount assembly  401  with a silver epoxy or glass+paste. After visual inspection, for example with a 10-100× microscope, the silver paste epoxy, for example Epotek H20E (Epoxy Technology, 14 Fortune Dr., Billerica, Mass. 01821), can be cured. Epotek H20E, for example, is cured by heating for about 30 minutes at a temperature less than about 150° C. In some embodiments, photodiode  402  can be attached to submount  401  attached with Ag-filled glass preforms or paste. Such paste is also manufactured by Ablestik (Ablestik Laboratories, 20021 Susana Rd., Rancho Dominquez, Calif. 90221) (e.g., 2105S) and DieMat (Diemat, Inc., 19 Central St., Byfield, Mass. 01922) (e.g., DM2700PF).  
         [0081]    In step  724 , photodiode  402  is further electrically connected to metallization deposited on submount  401 . In some embodiments, a gold ball bonding technique can be utilized to attach a wire between a connection on the front of photodiode  402  to metallization region  503  on submount  401 . Once photodiode  402  is mounted and electrically connected to the metallization traces formed on photodiode subassembly  401 , photodiode assembly  105  is formed.  
         [0082]    In step  725 , electrical testing of photodiode assembly  105  can be performed. In some embodiments, a burn-in with a voltage of about 2V at a temperature of about 100° C. for about 96 hours can be performed. Other embodiments of the invention may utilize different procedures for testing photodiode assembly  105  electrically.  
         [0083]    [0083]FIG. 7D illustrates step  730  of process  700 , forming laser assembly  104 . In step  731 , upper portion  601  is formed and metallization is added. In embodiments as shown in FIGS. 6A through 6F, a resistor region  605  is formed with a thin film resistor material deposited in resistor region  605  to form resistor  109  as shown in FIG. 1B. Further, vias  606  are formed to provide electrical and thermal contact to bottom portion  602 . In some embodiments, upper portion  601  can be formed from aluminum nitride (AN271) and metallization can be formed from a first layer of Ti about 0.1 micron thick covered by a layer of lead about 0.2 micron thick and further covered by a layer of gold about 1.5 micron thick. Other metallizations can also be utilized. Further, in some embodiments region  603  can also include a layer of platinum 0.5 micron thick deposited over the gold layer and a layer of gold-tin solder about 4.0 micron thick.  
         [0084]    In step  732 , bottom region  602  can be formed. Bottom region  602  can be formed of an insulating material which is metallized on the top surface to provide electrical and thermal contact through vias  606  of top portion  601 . Further, the bottom surface of bottom region  602  can be metallized to provide thermal and electrical contact when mounted on heat sink  125 . In some embodiments, bottom region  602  can be formed of aluminum nitride (AN271) metallized with the same materials as were used for upper region  601 .  
         [0085]    In step  733 , laser subassembly is formed by positioning upper portion  601  on bottom portion  602  and bonding them together. Upper portion  601  can then be cofired with bottom region  602  to form laser subassembly  404 . Cofiring is the process by which “unfired” or “green” ceramic layers are sandwiched together and are brought to elevated temperature under pressure. This procedure fuses the ceramic layers together to form an integral, multi-layered part. The conductive traces which were applied to the ceramic prior to the cofiring process maintain their conductivity and function as a printed wire board.  
         [0086]    In step  734 , laser  119  is attached to region  603  of upper portion  601 . Laser  119  can be attached, for example, by bonding to the gold-tin solder deposited in region  603 . Bonding can be achieved by heating the assembly for a period of time, for example to about 300° C. for 30 minutes.  
         [0087]    In step  735 , inductor  103  is epoxied to region  604  of upper portion  601 . Finally, in step  736  electrical connections between laser  119  and the thin film resistor deposited in resistor region  605  and connections between inductor  103  and the thin film resistor deposited in resistor region  605  can be accomplished by a gold ball bonding method.  
         [0088]    [0088]FIG. 7E illustrates an example of step  740  shown in FIG. 7A. In step  741 , photodiode assembly  105  is positioned on ceramic layer  121 . In step  742 , photodiode assembly  105  can be attached to and electrically coupled to traces on ceramic layer  121  of feed-through  101  with a Ag paste epoxy (e.g., Epotek H20E). Curing of the epoxy may be accomplished, for example, by heating for about 30 minutes at low temperature (e.g., less than about 150° C.). In step  743 , photodiode assembly  105  is tested on feed-through assembly  101 . For example, a visual inspection through a 10-100× microscope can be performed. Additionally, a dark current check can be performed. In some embodiments, the dark current from photodiode  402  should be less than about 100 nA.  
         [0089]    [0089]FIG. 7F shows an example of step  750  shown in FIG. 7A. In step  750 , laser assembly  104  is attached to heat sink  125  of feed-through assembly  101 . In some embodiments, laser assembly  104  is attached through a Ag epoxy or with a Au—Sn solder. In step  752 , a plasma cleaning process can also be performed in preparation for performing ball-bonding.  
         [0090]    [0090]FIG. 7G illustrates step  760  shown in FIG. 7A. In step  761 , ribbon bands  413 , wires  410 , and possibly wires  411  and  412  are attached, for example by ball bonding. As shown in FIG. 7F, a plasma cleaning process may be performed prior to the ball bonding step. In step  762 , a wire pull test may be performed. In a wire pull test, each of ribbon bands  413  and wires  410 ,  411 ,  412  and others are pulled slightly to ensure that they are firmly bonded.  
         [0091]    [0091]FIG. 7H shows an example of step  770  of FIG. 7A. In step  771 , laser facet location and orientation of laser  119  on laser assembly  104  is determined. Facet location and orientation can be determined, for example, by reflecting an optical beam from the laser surface and measuring the reflected beam. Other machine vision techniques for determining the facet location and orientation of laser  119  can also be used. In some embodiments, laser fiducial marks (see marks  415  shown in FIG. 4H) can be utilized to determine facet location and orientation. In step  772 , the laser emission axis offset is determined from the location and orientation of the laser facet. Once the laser emission axis is determined, in step  773  optical assembly  101  can be classified as belonging to a particular type. For example, “type 1” may indicate that the laser facet is offset by a certain distance in a first direction while “type 2” may indicate that the laser facet is offset by a certain distance in the opposite direction. There may be any number of types, and there should be a sufficient number of different types in the classification to allow cap  102  to be positioned within the allowable tolerances required of optical assembly  101 .  
         [0092]    [0092]FIG. 7I illustrates an example of step  780  of FIG. 7A. In step  780 , cap  102  is positioned according to type in step  781  and attached to sealant ring  128  in step  782 . In step  781 , cap  102  is positioned in order to focus the light emission from laser  119  to couple with an optical fiber. From the type of optical assembly  100 , the position of cap  102  can be determined. Therefore, based on type, cap  102  is positioned relative to laser  119 . In step  782 , cap  102  is attached, for example by brazing, to sealant ring  128 .  
         [0093]    In step  790  shown in FIG. 7A, optical assembly  100  is tested. For example, a leakage test to ensure that the area between cap  102  and sealant ring  128  is sealed can be performed. In some embodiments, a He leak of less than about 5×10 −8  atm cc/sec should be attained. Further, electronic testing can be performed. Further, a burn-in step can be performed. For example, a current of 50 mA to laser  119  at a temperature of about 100° C. for about 96 hours can be performed. A second set of electronic tests, for example LIV testing, can be performed. LIV testing is a standard DC test of laser performance involving testing the light output (L) of the laser at various currents (I) and voltages (V) and is typically performed over a range of temperatures. Finally, a high speed data test can be performed by modulating laser  119  at a particular rate and monitoring the emissions through lens  110 .  
         [0094]    One skilled in the art will recognize numerous variants based on the embodiments of the invention disclosed here. These variants are considered within the scope and spirit of this disclosure. Further, several of the figures include numerical dimensions that are intended to illustrate a particular embodiment of the invention and are not intended to be generally limiting. As such, the invention is limited only by the following claims.  
       We claim: