Abstract:
A modular, hermetically sealed optical signal receiver subassembly for converting a modulated optical signal to a corresponding electrical signal including an optical demultiplexer coupled to a fiber optic connector receiving a multi-wavelength optical signal having a plurality of information-containing signals each with a different predetermined wavelength and functioning to demultiplex the optical signal into distinct first and second optical beams corresponding to the predetermined wavelength and substrate is provided that forms an optional reference path of the first and second beams respectively, the photodiodes functioning to convert the respective optical signals into an electrical signal.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is related to copending U.S. patent application Ser. No. ______ filed Jun. ______, 2004, entitled Modular Optical Transceiver, assigned to the common assignee. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to optical receivers, and in particular to hermetically sealed assemblies or modules that provide a communications interface to an optical fiber, such as used in fiber optic communications links, and methods for assembling and aligning an optical fiber with optoelectronic components in such module.  
         [0004]     2. Description of the Related Art  
         [0005]     A variety of optical transceivers are known in the art which include an optical transmit portion that converts an electrical signal into a modulated light beam that is coupled to an optical fiber, and a receive portion that receives an optical signal from an optical fiber and converts it into an electrical signal. Traditionally, optical receive sections include an optical assembly to focus or direct the light from the optical fiber onto a photodetector, which in turn, is connected to an amplifier/limiter circuit on a circuit board. The photodectector or photodiode is typically packaged in a hermetically sealed package in order to protect it from harsh environmental conditions. The photodiodes are semiconductors chips that are typically a few hundred microns to a couple of millimeters wide and 100-500 microns thick. The package in which they are mounted is typically 3-6 mm in diameter, 2-5 mm tall and has several electrical leads coming out of the package. These electrical leads are then soldered to the circuit board containing the amplifier/limiter.  
       SUMMARY OF THE INVENTION  
       [0006]     1. Objects of the Invention  
         [0007]     It is an object of the present to provide an improved optical receiver subassembly in a hermetically sealed enclosure.  
         [0008]     It is another object of the present invention to provide a hermetic package for use with multiple optoelectronic components mounted on a circuit board.  
         [0009]     It is also another object of the present invention to provide a modular optical receiver subassembly for use in an optical transmission system within an industry standard XENPAK housing.  
         [0010]     It is still another object of the present invention to provide a method for assembling components in an optical receiver module for use in an optical wavelength division multiplexed (WDM) transmission system.  
         [0011]     It is still another object of the present invention to provide an optical transceiver capable of field upgrades of both hardware and software modules.  
         [0012]     It is still another an object of the present to provide an improved optical receiver using an optical demultiplexer and multiple photodetectors in a single modular subassembly.  
         [0013]     It is another object of the present invention to provide an improved method for aligning an optical fiber with an optical demultiplexer and an array of optoelectronic components.  
         [0014]     It is also another object of the present invention to provide a hermetic seal between an optical fiber and a receiver subassembly.  
         [0015]     2. Features of the Invention  
         [0016]     Briefly, and in general terms, the present invention provides an optical transceiver for converting and coupling an information-containing electrical signal with an optical fiber including a housing including a fiber optic connector adapted for coupling with an external optical fiber; and a modular, hermetically sealed receiver subassembly in the housing for converting a modulated optical signal into a corresponding electrical signal.  
         [0017]     The present invention further provides a receiver subassembly including an optical demultiplexer coupled to a fiber optic connector for receiving a multi-wavelength optical signal having a plurality of information-containing signals each with a different predetermined wavelength and demultiplexing the optical signal into distinct optical beams corresponding to the predetermined wavelengths; and a substrate forming an optical reference plane and including first and second photodiodes disposed thereon in the path of the first and second beams respectively, the photodiodes functioning to convert the respective optical signals into an electrical signal.  
         [0018]     The present invention further provides a receiver subassembly including an optical block with a plurality of wavelength selecting elements and reflectors operative to direct the optical beams emitted from each respective wavelength selecting element to respective ones of a plurality of spatially separated image positions corresponding to the locations of individual photodetectors in a photodetector array.  
         [0019]     In another aspect of the invention, there is provided a frame consisting of a ceramic substrate, a metal-ceramic or metal ring, and a metallic lid that are utilized to hermetically package an optical demultiplexer, a photodiode array, and associated electronic components in a single, modular subassembly.  
         [0020]     Additional objects, advantages, and novel features of the present invention will become apparent to those skilled in the art from this disclosure, including the following detailed description as well as by practice of the invention. While the invention is described below with reference to preferred embodiments, it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional applications, modifications and embodiments in other fields, which are within the scope of the invention as disclosed and claimed herein and with respect to which the invention could be of utility. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     These and other features and advantages of this invention will be better understood and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:  
         [0022]      FIG. 1  is an exploded perspective view of an optical transceiver in an exemplary embodiment in accordance with aspects of the present invention;  
         [0023]      FIG. 2  is an exploded perspective view of a hermetically sealed receiver subassembly.  
         [0024]     The novel features and characteristics of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to a detailed description of a specific embodiment, when read in conjunction with the accompanying drawings. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]     Details of the present invention will now be described, including exemplary aspects and embodiments thereof. Referring to the drawings and the following description, like reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of exemplary embodiments in a highly simplified diagrammatic manner. Moreover, the drawings are not intended to depict every feature of actual embodiments or the relative dimensions of the depicted elements, and are not drawn to scale.  
         [0026]     Referring more particularly to  FIG. 1 , there is provided an optical transceiver  100  for operating over both multimode (MM) and single mode (SM) fiber using multiple laser light sources, multiple photodetectors, and an optical multiplexing and demultiplexing system. This enables a single transceiver module to communicate over multiple protocols and at maximum distance goals. The transceiver  100  and its housing  102  are designed such that maximum operating efficiency is achieved cost effectively and at reduced electromagnetic interference (EMI) and thermal levels in an industry standard form factor or package design.  
         [0027]     Advantageously, the transceiver  100  is manufactured in a modular manner preferably using three separately mounted circuit boards mounted in the housing—a transmitter subassembly, a receiver subassembly, and a protocol processing board, with each board having dedicated functions and electrically connected to each other using either flex circuitry, mating multipin connectors, land grid arrays, or other electrical interconnect devices. This enables the basic transceiver module to be configured to different protocols and to support different optoelectronic devices using a simple subassembly configuration change, thus minimizing manufacturing costs and eliminating the need for manufacturing different transceivers for each different application. In addition, the use of flex circuitry or detachable connectors to interconnect the boards allows for a modular interchangeable board design (e.g., receiver, transmitter and PCS functionality each on separate boards). Although the preferred design uses three boards, any two of the functions may be combined on a single board for an even more compact design.  
         [0028]     The modularity of the board design also enables the placement of heat-sensitive components in the optimal location with respect to the heat-generating components (lasers and ICs) within the module housing  102 . It also makes it convenient and realistic to test and troubleshoot separate modular subassemblies independently before final assembly. In addition, the flex or other interconnects allow for manufacturing of the various boards (RX, TX, PCS) to proceed in parallel instead of in serial, hence reducing the manufacturing time for the entire unit.  
         [0029]     Referring now to  FIGS. 1 and 2 , an exemplary optical transceiver module  100  is shown according to a preferred embodiment of the present invention. In this particular embodiment, the module  100  is compliant with the IEEE 802.3ae-2002 10GBASE-LX4 Physical Media Dependent sub-layer (PMD) and the XENPAK™ form factor. It is to be noted, however, that the transceiver module  100  may be configured to operate under various other compliant protocols (such as Fibre Channel or SONET) and be manufactured in various alternate form factors such as X2. The module  100  is preferably a 10 Gigabit Coarse Wavelength Division Multiplexed (CWDM) transceiver having four 3.125 Gbps distributed feedback lasers and providing 300 meter transmission over legacy installed multimode fiber and from 10 to 40 km over standard single mode fiber.  
         [0030]     The transceiver module  100  includes a two-piece housing  102  with a base  104  and a cover  106 . In addition, contact strips  152  are provided to ground the module to chassis ground as well. The housing  102  is constructed of die-cast or milled metal, preferably die-cast zinc, although other materials also may be used, such as specialty plastics and the like. Preferably, the particular material used in the housing construction assists in reducing EMI. Further EMI reduction may be achieved by using castellations (not shown) formed along the edges of the housing  102 .  
         [0031]     The front end of the housing  102  includes a faceplate  153  for securing a pair of receptacles  124 ,  126 . The receptacles  124 ,  126  are configured to receive fiber optic connector plugs  128 ,  130 . In the preferred embodiment, the connector receptacles  128 ,  130  are configured to receive industry standard SC duplex connectors (not shown). As such, keying channels  132  and  134  are provided to ensure that the SC connectors are inserted in their correct orientation. Further, as shown in the exemplary embodiment and discussed further herein, the connector receptacle  130  receives an SC transmitting connector and the connector receptacle  128  receives an SC receiver connector.  
         [0032]     In particular, the housing  102  holds three circuit boards, including a transmit board  108 , a receive board  110  and a physical coding sublayer (PCS)/physical medium attachment (PMA) board  112 , which is used to provide an electrical interface to external electrical systems (not shown). An optical multiplexer (MUX)  114  interfaces to the transmit board  108  via an assembly of four distributed feedback (DFB) lasers  116  in TO-cans. The lasers  116  are secured in place at the bottom of the housing  102  using a laser brace  118 . The laser brace  118  also functions as a heat sink for cooling the lasers  116 . In addition, the transmit board  108  and receive board  110  are connected to the PCS/PMA board  112  by respective flex interconnect  120 , or other board-to-board connectors. Thermally conductive gap pads  160  and  161  are provided to transmit the heat generated by the lasers or other components to the base  104  or cover  106  of the housing, which acts as a heat sink. The receiver subassembly  110  is directly mounted on the housing base  104  using a thermally conductive adhesive to achieve heat dissipation. Different subassemblies therefore dissipate heat to different portions of the housing for more uniform heat dissipation. The output of the four lasers  116  is then input into the optical MUX  114 . The MUX  114  is mounted on a flexible substrate  140 . The substrate  140  may be an optical flexible planar material, such as FlexPlane™ available from Molex, Inc. of Lisle, Ill., although other flexible substrate may be used as well. The optical fibers originating from the laser assembly  116  and being input into the MUX  114  are mounted to the substrate  140 . The output of the MUX  114 , which is routed to the transmit connector plug  130 , is also attached to the substrate  140 . The fibers are routed and attached in such a manner as to minimize sharp bends in the optical fibers to avoid optical loss and mechanical failure.  
         [0033]     The substrate  140  includes an opening  142  or hole in a portion of the material that is located directly above the retimer IC or other heat generating components mounted on the PCS/PMA board  112 . The opening  142 , which is substantially an area the size of the unused portion of the substrate  140 , enables the heat sink on the cover to contact a heat transmission gap pad  160 , so as to provide access to the mounted components on the board. This area normally would be inaccessible if not for the opening  142 . For example, a heat sink may be installed without interfering with the routing of the optical fibers on the substrate  140  and without removing the mounted substrate  140  to allow access to the PCS/PMA board  112 .  
         [0034]     Several additional advantages are realized in using the flexible substrate  140 . In particular, attaching the fibers to the substrate  140 , rather than allowing the fibers to move about freely within the transceiver module housing  102 , neatly maintains the routing of the optical fibers to prevent unwanted tangling and breakage during assembly of the transceiver. Furthermore, attaching the optical fibers to the substrate  140  greatly reduces the stress on the fibers, thereby reducing the incidence of microcracks forming in the fiber coatings.  
         [0035]     In the case of WWDM receive sections there needs to be a detector for each wavelength. It is evident that the use of multiple photodetectors in separate hermetic cans would result in a large receive section for such multi-wavelength receivers. Instead, a single multi-element photodiode array is mounted directly to the circuit board containing the amplifier/limiter circuit. A miniature optical demultiplexer is aligned to the photodiode array.  
         [0036]     As shown in  FIG. 2 , in order to provide a hermetic environment to the receiver components, the receiver components are enclosed in a hermetic package. The bottom of the package is the circuit board itself. Since standard fiberglass and glass-epoxy materials are not impervious to water, the circuit board is made from ceramic materials such as LTCC (low temperature co-fired ceramic). A moisture-impervious enclosure is then attached to the LTCC board to surround the photodiode array, miniature optical demultiplexer and, amplifier/limiter IC.  
         [0037]     The enclosure can be composed of a frame  300  and a metal lid  302 . The frame is sealed to the circuit board  222  using standard hermetic sealing methods. One such method is to place a solder preform  224  on the LTCC board  222 . This preform  224  is sandwiched between a metal trace  236  on the LTCC board  222  and the frame  300  walls. Alternatively, solder paste can be deposited on the metal traces and the metal frame can be placed on the paste. This allows for standard SMT assembly techniques to be used. The frame  300  can be placed during the same SMT operation as other components, and the solder is reflowed to attach the frame  300  to the LTCC board  222 . The optical demultiplexer  226  is then aligned to the photodiode array  220  and fixed to the circuit board  222 . The optical fiber  250  is then aligned and fixed to the demultiplexer  226 . The lid  302  is attached to the frame  300  using standard hermetic sealing techniques such as soldering, welding, or seam sealing.  
         [0038]     The frame  300  contains a hole  301  or cutout that serves as a feedthrough for the optical fiber  250 . The optical fiber  250  needs to be metalized in the region where it penetrates the cutout  301 . This allows the feedthrough to be hermetically plugged with solder. The approach described hereinabove results in a package that is fully hermetic.  
         [0039]     Referring to  FIGS. 1 and 2  the receiver subassembly  10  with the circuit board  222  acts as an optical bench for the attachment and alignment of the demultiplexer  226  to the photodiode array  220 . In particular, there is shown a miniature optical demultiplexer  226  aligned to the photodiode array  220 , resulting in a compact receive section. The circuit board  222  not only serves as a substrate for the electrical circuitry, but also serves as an optical bench for the optical components. Particularly, the surface of the circuit board  222  acts as the optical reference plane  228  for the optical components. Optionally, the receiver board  222  is a printed circuit board (PCB) formed from PCB materials having higher glass content and providing less signal loss under high frequency (RF) operation compared to less expensive PCB materials. A suitable material is Rogers RO4003, available from Rogers Corp. of Chandler, Ariz., which is less expensive than either ceramic or silicon. The use of ceramic or silicon provides the ability to make the package hermetic.  
         [0040]     The surface of the circuit board  222  is the optical reference plane  228 . The top surface of the photodiode array  220  is set to a predetermined height by controlling its thickness to within 50 microns and the thickness of its attachment material such as glue or solder (not shown). The demultiplexer  226  is also attached to this surface. The demultiplexer output (not shown) is thus at a predetermined height of within 50 microns above the photodiode array  220 .  
         [0041]     More particularly, the photodiode array  220  has a variable thickness from lot to lot and is attached to the circuit board  222  with epoxy, solder or eutectic metal bonding of variable thickness. The thickness of the bond material is manufactured to a controlled thickness such that the active surface of the photodiodes is at a predetermined height above the circuit board surface so as to match the focus distance. The miniature optical demultiplexer  226  is then aligned relative to the active areas of the photodiode array  220  in a plane parallel to the photodiode array surface. The demultiplexer  226  has a precise thickness such that when it rests on the optical reference plane  228  defined by the circuit board surface, the optical exit surfaces of the demultiplexer  226  are at the correct height above the photodiode array  220 .  
         [0042]     The demultiplexer  226  utilized and implemented in the present invention is preferably that described in U.S. Pat. No. 6,542,306, hereby incorporated by reference, and includes an optical block with an upper surface and a lower portion. The optical block has at least one optical element and a plurality of wavelength selecting elements and reflectors. The optical block is specifically positioned on top of a beam-directing member. In the preferred embodiment of the present invention, both the optical block and beam-directing member are optically transparent.  
         [0043]     In particular, as described in the above noted U.S. patent, at least one optical element is disposed generally on the upper surface of the optical block. Its function is primarily to focus and direct a multi-wavelength optical signal along a prescribed optical signal path. Further, the wavelength selecting elements are disposed generally below the upper surface of the optical block. The wavelength selecting elements are designed and operative to receive the optical signal from the optical element. Moreover, a plurality of reflectors are disposed generally on the upper surface of the optical block and opposite from the wavelength selecting elements. Due to such strategic positioning and orientation, the reflectors are able to direct the optical signal from one wavelength selecting element to an adjacent wavelength selecting element. Thereafter, the beam-directing member, which is disposed about the lower portion of the optical block, operates to redirect and focus the optical signal from the wavelength selecting elements to the photodiode array  220 . Although the demultiplexer described above is preferred, other optical configurations for demultiplexing the signals may be used as well, and such alternative configurations are within the scope of the present invention.  
         [0044]     The present invention implements the transceiver  100  utilizing the four standard, commercially available fiber pigtailed lasers  116  which interfaces to a Fused Biconic Tapered (FBT) coupler  114  to collect and multiplex laser radiation into a single fiber. The fiber that is used in the fiber pigtailed lasers  116  and the FBT  114  is affixed to the flexible substrate material  140 . This prevents fiber tangling and breakage while remaining flexible and therefore easy to work with. The flexible substrate material  140  may be an optical flexible planar material, such as FlexPlane™ available from Molex, Inc, of Lisle, Ill., or Kapton™ available from E.I. Dupont de Nemours and Company of Wilmington Del. Other flexible substrates may be used as well. A conforming coating is used over the entire flex  140  is used to secure the fibers to the flex  140 .  
         [0045]     As previously noted above, several additional advantages are realized when using the flexible substrates  140  rather than allowing the fibers to move about freely within the transceiver module housing  102 , neatly maintains the routing of the optical fibers to prevent unwanted tangling. Furthermore, attaching the optical fibers to the substrate  140  greatly reduces the stress on the fibers, thereby reducing the incidence of microcracks forming in the fiber coatings. The fibers are routed and attached in such a manner as to minimize sharp bends in the optical fibers.  
         [0046]     It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types described above.  
         [0047]     While the invention has been illustrated and described as an optical receiver subassembly embodied in a transceiver for an optical communications network, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.  
         [0048]     Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.