Abstract:
In an optical transceiver and/or receiver for converting and coupling an information-containing electrical signal with an optical fiber, an opto-electronic subassembly including an opto-electronic device for converting between an information-containing electrical signal and modulated optical signal corresponding to the electrical signal; and a Faraday shield for minimizing electromagnetic interference entering or leaving the opto-electronic device, extending adjacent to and around the portion of the connector adapted to couple to the periphery of the optical fiber.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to transmitter and receiver subassemblies for use in optical transceivers, and in particular to electromagnetic interference shields in such subassemblies.  
         [0003]     2. Description of the Related Art  
         [0004]     Optical transceivers used in optical fiber communications systems use a transmitter optical sub-assembly (TOSA) to convert the electrical signal to an optical signal for data transmission, and a receiver optical sub-assembly (ROSA) to convert received optical signal back into an electrical signal. The TOSA or ROSA is typically manufactured by optically aligning an OSA barrel (or “OSA support or housing”) with an opto-electronic (OE) device package such as a TO (transistor outline) can to form the sub-assembly.  
         [0005]     The TOSA or ROSA typically also includes an optical lens formed either inside the OSA barrel or on top of a device package such as a lens TO cap. Mounting an optical lens on the device package requires a high degree of precision in device package design in order to attain desired performance, which results in manufacturing difficulty and higher cost. For that reason, the optical lens is typically mounted on OSA barrels.  
         [0006]     Two types of materials, namely, plastic and metal, are currently used to fabricate OSA barrels. The plastic part is made using a precision injection molding technique that enables tight tolerances and complex lens designs (e.g., aspherical lenses) to be integrated in the barrel for efficient fiber coupling. Epoxies are used to assemble the barrel with the device package to form a TOSA or ROSA. Metal-based barrel is constructed by machining using hard metal material such as stainless steel. It is then either epoxied or laser-welded to the device package to form a TOSA or ROSA. Optical lens is either installed on a TO can, or is snapped on manually to the metal barrels so that a TO can with a flat window can be used. In both cases, the optical lenses are typically simple ball lenses due to manufacturing limitations. The uncorrected image aberrations from the simple ball lenses will cause poor optical coupling and degrade TOSA and ROSA efficiency.  
         [0007]     As any other device operating in the RF frequency range, transceivers based on opto-electronic devices generate significant electromagnetic interference (EMI), and also are very vulnerable to EMI from external sources. Typically, EMI generated from an RF source goes to all directions. The EMI between receiving and transmitting circuits in the same transceiver, which is referred herein as side EMI, causes significant device cross-talks and therefore degrades device performance. In addition, the EMI emitted through the front of a transceiver, which is referred herein as front EMI, affects the surrounding electronic environments. Transceiver designers typically try to reduce the EMI by shielding the RF sources, for example, by enclosing the transceiver in a grounded metal housing.  
         [0008]     There are a number of references that describe implementation of the metal housing for transceivers, including U.S. Pat. No. 4,840,451 entitled “Shielded Fiber Optic Connector Assembly” and U.S. Pat. No. 6,483,719 entitled “Conforming Shielded Form for Electronic Component Assemblies.” However, they disclose using a large opening at the front end of the transceiver in order to allow external fibers to access the TOSA and ROSA. These large openings may not be major EMI concerns for low speed transceivers, but it is of an important concern for higher speed transceivers such as those operating at 10 Gigabits per second (Gbps) and beyond.  
         [0009]     The non-conductive nature of the plastic OSA barrel poses a significant difficulty for transceiver designers to reduce EMI, especially for front EMI emanating in the direction where the optical fiber is coupled to the assembly. This is because transceivers typically rely on OSA barrels to connect to optical fibers for data transmission. Metal-based OSA barrels can solve this problem, but typically are comparatively more expensive. Further, it is generally more difficult to incorporate a complex optical lens (e.g., custom design optics) into the metal barrel as discussed above. A ceramic ferrule is typically also needed for the metal barrel due to the high precision requirement for fiber insert (within +/−2 micron) and it is very expensive for metal barrels to be machined with that precision.  
         [0010]     Therefore, it is desirable to provide an apparatus and method for reducing front EMI of an OSA, especially when a plastic barrel is used, other than the grounded metal housing having a large opening as described above. It is also desirable to provide an apparatus and method to reduce side EMI of the OSA to reduce cross-talks, which tend to degrade device performance.  
       SUMMARY OF THE INVENTION  
       [0011]     Briefly, and in general terms, the present invention provides an optical transceiver and/or receiver for converting and coupling an information-containing electrical signal with an optical fiber, including an opto-electronic subassembly having a housing including an optical fiber connector adapted for coupling with an external optical fiber for transmitting and/or receiving an optical communications signal; and an opto-electronic device in the housing for converting between an information-containing electrical signal and modulated optical signal corresponding to the electrical signal. More particularly, the invention provides a Faraday shield for minimizing electromagnetic interference entering or leaving the opto-electronic device, extending adjacent to and around the portion of the connector adapted to couple to the periphery of the optical fiber.  
         [0012]     In an exemplary embodiment of the present invention, an optical subassembly (OSA) includes an opto-electronic (OE) device, and a barrel for optically aligning the OE device with an external device. A metal shield is disposed between the OE device and the barrel so as to reduce the electromagnetic interference (EMI) of the OSA between the opto-electronic device and other circuitry in the optical transceiver. The metal shield has an aperture for allowing an optical beam to pass through. The aperture is positioned proximately to the focal point of the lens associated with the OE device, so that the size of the aperture can be minimized, thereby increasing the amount of shielding. Such reduction to the aperture size is highly desired for higher speed transceivers such as those operating at high speeds such as 10 Gbit/s. The OE device may be a photodetector, for example.  
         [0013]     In another exemplary embodiment of the present invention, an optical transceiver includes a metal housing and an OSA mounted on the metal housing. The OSA includes an OE device, and a barrel for optically aligning the OE device with an external device. A metal shield is disposed between the OE device and the barrel so as to reduce the front EMI of the OSA. The metal shield has an aperture for allowing an optical beam to pass through. The aperture is positioned proximately to a focal point of the associated lens, so that the size of the aperture can be minimized. The OE device may be a photodetector, for example. The optical transceiver may also include a second OSA, which includes an OE package having a second OE device and a metal case containing the second OE device. The optical transceiver may further include a printed circuit board (PCB) having a PCB ground. The metal case is electrically coupled to a metal housing of the optical transceiver, so as to reduce side EMI within the metal housing. The second OE device may be a laser, for example.  
         [0014]     In yet another exemplary embodiment of the present invention, an optical transceiver includes an OSA including an OE package having an OE device and a metal case containing the OE device. The optical transceiver also includes a printed circuit board (PCB) having a PCB ground, and a metal housing for holding the OSA and the PCB. The metal case is electrically coupled to the metal housing, so as to reduce side EMI within the metal housing. The OE device may be a laser, for example. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a perspective view of an optical transceiver in an exemplary embodiment in accordance with aspects of the present invention;  
         [0016]      FIG. 2A  is a perspective view of a hybrid optical sub-assembly (OSA) in another exemplary embodiment of the present invention;  
         [0017]      FIG. 2B  is a partially unassembled view of the hybrid OSA of  FIG. 2A .  
         [0018]      FIG. 2C  is an OSA of the optical transceiver of  FIG. 1 ;  
         [0019]      FIG. 3A  is a front view of the OSA of  FIGS. 1 and 2 .  
         [0020]      FIG. 3B  is a cross-sectional view taken along the line A-A of the OSA of  FIG. 3A ;  
         [0021]      FIG. 4A  is an OSA in an alternate exemplary embodiment in accordance with aspects of the present invention;  
         [0022]      FIG. 4B  is a metal plate in the OSA of  FIG. 4A ;  
         [0023]      FIG. 5A  is a front view of the OSA of  FIG. 4A ;  
         [0024]      FIG. 5B  is a cross-sectional view taken along the line A-A of the OSA of  FIG. 5A ;  
         [0025]      FIG. 6  is a rear view of a conventional transmitter optical sub-assembly (TOSA);  
         [0026]      FIG. 7  is a cross-sectional view taken along the line C-C of the conventional TOSA of  FIG. 6 ;  
         [0027]      FIG. 8  is a rear view of a conventional TO-46 package; and  
         [0028]      FIG. 9  is a rear view of a TO package in an exemplary embodiment in accordance with aspects of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0029]     The present invention addresses the high EMI problems of current plastic-based optical sub-assembly (OSA), such as transmitter OSA (TOSA) and receiver OSA (ROSA), that are particularly important for high date rate applications. One exemplary embodiment in accordance with the aspects of the present invention provides a method and apparatus to provide a ground shield for the TOSA and ROSA to reduce the EMI to the outside of an optical transceiver. In another exemplary embodiment according to the present invention, a modified opto-electronic (OE) device package (e.g., TO can) for reducing EMI inside the transceiver is provided. Such modified OE device package should reduce cross-talk within the transceiver. These embodiments may be used either individually or jointly to realize superior EMI performance for TOSA and/or ROSA products without significantly increasing cost.  
         [0030]      FIG. 1  is an optical transceiver in an exemplary embodiment in accordance with aspects of the present invention. The optical transceiver includes a printed circuit board (PCB)  103  and one or more integrated circuit (IC) chips  105  mounted thereon for processing the transmission and reception of optical communication signals.  
         [0031]     The optical transceiver has a metal housing  100 , which provides case ground (or chassis ground), for example. The metal housing  100  has mounted thereon optical sub-assemblies (OSAs)  102  and  110 . For example, the OSA  102  may be a TOSA and the OSA  110  may be a ROSA, or vice versa. It should be noted that even though the optical transceiver as shown in  FIG. 1  has a single TOSA and a single ROSA for a single-channel, serial transceiver, transceivers in other embodiments may include multi-channel, parallel optical transceivers with multiple ROSAs and/or multiple TOSAs.  
         [0032]     The OSA  102  includes a metal plate (or a metal insert)  104  disposed between a housing  106  and a barrel  108 . An OE device package is disposed within the housing  106 . Of course, the OE device package may be an optical transmitter or receiver depending on whether the OSA  102  is a TOSA or a ROSA.  
         [0033]     The metal plate  104  serves as a shield to reduce the front EMI emanating from the OE device package to outside of the metal housing  100 . In other embodiments, the metal shield may have any suitable shape and may not necessarily be a plate. By way of example, the metal shield for reducing the front EMI may be any suitable bent piece of metal, a metal collar or a metal mesh with small grid. In still other embodiments, the metal shield may be formed by metal plating a non-metal material having a suitable size, shape and properties.  
         [0034]     The barrel  108  has a flange  109  typically used to secure the OSA  102  on the metal housing  100 . The metal plate  104  is held in place between the barrel  108  and the OSA housing  106 . The barrel  108  is accessible from outside of the metal housing  100  in the exemplary embodiment. In the exemplary embodiment, the OSA housing  106  and the barrel  108  are formed from plastic. In other embodiments, the housing and barrel may be formed from other suitable materials.  
         [0035]     In other embodiments, as shown in  FIGS. 2A and 2B , a hybrid OSA barrel  180  has a metal portion  184  which is near the metal housing  100  and a plastic portion  182  affixed to the metal portion  184 , such that the metal portion of the barrel and the plastic portion of the barrel are affixed to one another (e.g., through gluing or any other suitable method) to form a single barrel. The metal portion  184  in and of itself may be referred to herein as a barrel, and the plastic portion  182  may be referred to as a body or a housing. The plastic portion  182  may also be made of any other suitable non-metal material.  
         [0036]     When the metal portion  184  of the hybrid barrel  180  is connected electrically to the transceiver metal housing  100 , the metal portion  184  shields an external device from the front EMI generated by the OE device package. Therefore, an additional metal shield, such as the metal plate  104 , may not be needed.  
         [0037]     In more detail, the metal portion  184  has a generally cylindrical member (“barrel”)  186  having an opening  187  for aligning the OE device inside the barrel  180  to an external device. The metal portion also has a flange  188  and a number (e.g., four) of feet  190  for engaging the metal housing  100 . By way of example, the wall surrounding the opening (for mounting the hybrid barrel  180 ) of the metal housing  100  may be fitted between the flange  188  and the feet  190 . The aperture (not shown) in the metal portion  184  for passage of the optical signal should be close to the focal point of the optical signal passing through the barrel such that the aperture can be made as small as possible (i.e., the size of the aperture can be minimized). By way of example, the aperture may be 1 mm or less (e.g., 0.2 mm) in diameter.  
         [0038]     The OE device is disposed inside the plastic portion  182 . The plastic portion  182  has a generally box-like housing  192  and a substantially cylindrical barrel  194  protruding therefrom. The cylindrical barrel  194  has an opening  195  formed thereon for allowing an optical signal to pass through. The surface of the feet  190  facing away from the flange  188  is mounted on a surface  196  of the box-like housing  196 , and is attached (e.g., via gluing) thereto.  
         [0039]     In other embodiments, the barrel may have a thin metal layer plated/coated over. The metal coating provides an EMI shield when it is connected electrically to the transceiver metal housing  100 . For the same reason, an additional metal shield, such as the metal plate  104 , may not be needed.  
         [0040]     The OSA  110  is substantially the same structurally as the OSA  102  except that the OE chip package installed within the OSA  110  includes an optical receiver (e.g., a photodetector) as opposed to the optical transmitter (e.g., a laser) installed within the OSA  102 . Similar to the OSA  102 , the OSA  110  includes a housing  114 , a metal plate  112  and a barrel  116 , where the OSA housing  114  and the barrel  116  may be formed of plastic or any other suitable material. The barrel  116  has a flange  117  typically used to secure the OSA  110  on the metal housing  100 . The barrel  116  is accessible from outside of the metal housing  100  in this exemplary embodiment.  
         [0041]      FIG. 2C  is an enlarged view of the OSA  102  of  FIG. 1 . Since the OSA  110  is substantially the same structurally as the OSA  102 , only OSA  102  will be discussed hereinafter with the understanding that the description applies equally as well to the OSA  110 , except that OSA  102  includes an optical transmitter (e.g., laser) in the OE device package while the OSA  110  includes an optical receiver (e.g., photodetector).  
         [0042]     In the described exemplary embodiment, the metal plate  104  has a substantially rectangular shape, and has formed thereon a protruding member (i.e., a step) about the middle of each side (i.e., each edge) of the rectangle. Each of the protruding members  120 ,  122 ,  124  and  126  is substantially rectangular in shape, length of its longer side is approximately one third the length of the respective side from which it protrudes, and is substantially parallel to the respective side. One or more of the protruding members may allow easy integration of the OSA  102  with the metal housing (transceiver chassis) for ground connection by engaging (i.e., interlocking with) the metal housing  100 . In other embodiments, the metal plate  104  and/or the protruding members may have other shapes and/or dimensions.  
         [0043]      FIG. 3A  is a front view of the OSA  102 , and  FIG. 3B  is a cross-sectional view of the OSA  102  taken along the line A-A of  FIG. 3A . It can be seen in  FIG. 3B  that the metal plate  104  has formed thereon pins  132  and  134  on its surface facing an OE device package  107 . The metal plate  104  also has formed thereon pins  136  and  138  on its surface away from the OE device package  107 . The pins  132  and  136  may be considered a single pin that traverses through the metal plate  104 . In addition, the pins  134  and  138  may be considered a single pin that traverses through the metal plate  104 . The metal plates in other embodiments may also have additional pins that traverse therethrough.  
         [0044]     The pins  132  and  134  penetrate into the housing  106 , and the pins  136  and  138  penetrate into the barrel  108  so that the housing  106 , the metal plate  104  and the barrel  108  are fixedly coupled to one another. Hence, in a sense the metal plate  104  is embedded within the plastic barrel (package or housing) formed by the housing  106  and the barrel  108 . In practice, the plastic barrel (including the housing  106  and the barrel  108 ) may be formed by over-molding plastic material on the metal plate  104  during injection molding.  
         [0045]     The metal plate  104  has formed about its center a substantially circular opening (or an aperture)  142  that is aligned with the optical signal path between the housing  106  and the barrel  108 . The barrel  108  has through its length a generally cylindrical cavity  140 , through which an optical fiber (in a fiber ferrule), for example, is inserted to interface with the OE device package  107  within the housing  106 . The opening  142  in the exemplary embodiment may have a diameter that is different than or equal to the diameter of a cavity  144  formed within the housing  106 .  
         [0046]     The metal plate  104  is positioned close to the focal point of the optical signal (e.g., laser beam) in the OSA barrel formed by the housing  106  and the barrel  108 . The focal plane is often at the “stop” location for the fiber ferrule and is often referred to as the optical plane of the OSA barrel. When the metal plate  104  is close to the optical plane, the opening  142  can be made as small as possible (i.e., the size of the opening can be minimized) to provide better shielding for front EMI while still allowing the optical signal to pass substantially unobstructed.  
         [0047]     When the metal shield has a shape other than that of a plate, the whole metal shield may not be aligned with the optical plane of the OSA barrel. In those cases, the opening for the optical signal should be positioned close to the focal point of the optical signal, so as to make the opening as small as possible (i.e., the size of the opening can be minimized)  
         [0048]     The metal plate  104  may be used in either a TOSA or ROSA, and not necessarily both, of an optical transceiver. Further, the TOSA and ROSA barrels, multiple TOSA barrels, and/or multiple TOSA barrels may be coupled (e.g., electrically) together through respective metal plates formed therein. In an alternative embodiment, the OSA barrel (e.g., a plastic barrel) may be coated with metal, either completely or partially for EMI shielding.  
         [0049]      FIG. 4A  is a perspective view of an OSA  150  (ROSA or TOSA) in an alternate exemplary embodiment in accordance with aspects of the present invention. When used, one or more of TOSAs and ROSAs of this type may be installed on a metal housing of the transceiver.  
         [0050]     The OSA  150  includes a metal plate (or a metal insert)  152  disposed between a housing  154  and a barrel  156 . An OE device package is disposed within the housing  154 . Of course, the OE device package may be an optical transmitter or receiver depending on whether the OSA  150  is a TOSA or a ROSA. The barrel  156  has a flange  158  used to secure the OSA  150  on a metal housing, such as the metal housing  100  of  FIG. 1 . The metal plate  152  is held in place between the barrel  156  and the OSA housing  154 . The barrel  156  is accessible from outside of a metal housing when mounted on the same. In the exemplary embodiment, the OSA housing  154  and the barrel  156  are formed from plastic. In other embodiments, the housing and barrel may be formed from other suitable materials. In still other embodiments, the OSA barrel may be made partly of metal and partly of plastic, wherein the metal portion of the OSA barrel serves as an EMI shield. Here, the metal portion of the OSA barrel may be referred to as a barrel, and the non-metal (e.g., plastic) part of the OSA barrel may be referred to as a housing.  
         [0051]      FIG. 4B  is a perspective view of the metal plate  152  of  FIG. 4A . In the alternate exemplary embodiment, the metal plate  152  has a substantially rectangular shape, and has formed thereon a protruding member (i.e., a step) about the middle of each side (i.e., each edge) of the rectangle. Each of the protruding members  162 ,  164 ,  166  and  168  is substantially rectangular in shape, length of its longer side is approximately one third the length of the respective side from which it protrudes, and is substantially parallel to the respective side. One or more of the protruding members may allow easy integration of the OSA  152  with the metal housing (transceiver chassis) for ground connection by engaging (i.e., interlocking with) the metal housing. In other embodiments, the metal plate  152  and/or the protruding members may have other shapes and/or dimensions.  
         [0052]     The metal plate  152  also has formed thereon a cylindrical member  169  about the center of the metal plate  152 . The cylindrical member  169  has a generally circular surface  171  at the end away from the metal plate  152 . The cylindrical member  169  has formed thereon a circular opening (or an aperture)  174  at the center of the generally circular surface  171 . The cylindrical member  169 , for example, may be formed by stamping a metal plate. When the cylindrical member has been formed using metal stamping, the cylindrical member is of a single integrated piece with the metal plate  152 , and has a hollow interior with an opening to the hollow interior formed at the end close to the metal plate  152 .  
         [0053]      FIG. 5A  is a front view of the OSA  150 , and  FIG. 5B  is a cross-sectional view of the OSA  150  taken along the line A-A of  FIG. 5A . It can be seen in  FIG. 5B  that the cylindrical member  169  of the metal plate  152  connects the OSA housing  154  with the barrel  156  formed by injection molding plastic.  
         [0054]     The barrel  156  has through much of its length a generally cylindrical cavity  172 , through which an optical fiber (in a fiber ferrule), for example, is inserted to interface with an OE device package  157  within the housing  154 . The cylindrical member  169  of the metal plate  152  fits inside the barrel  156 . This way, the OSA housing  154 , metal plate  152  and the barrel  156  are fittably joined together. In practice, the plastic barrel (including the housing  154  and the barrel  156 ) may be formed by over-molding plastic material on the metal plate  152  during injection molding.  
         [0055]     The circular opening  174  at the center of the cylindrical member  169  is aligned with the optical signal path between the OSA housing  154  and the barrel  156  during injection molding process. The opening  174  in the exemplary embodiment may have a diameter that is different than or equal to the diameter of a cavity  176  formed within the housing  154 .  
         [0056]     The circular opening  174  of the metal plate  152  is positioned close to the focal point of the optical signal (e.g., laser beam) in the OSA barrel formed by the housing  154  and the barrel  156 . The focal plane often is at the “stop” location for the fiber ferrule and is often referred to as the optical plane of the OSA barrel. When the circular opening  174  is close to the optical plane, it can be made as small as possible (i.e., minimized) to provide better shielding for front EMI while still allowing the optical signal to pass substantially unobstructed.  
         [0057]     The metal plate  152  may be used in either a TOSA or ROSA, and not necessarily both, of an optical transceiver. Further, the TOSA and ROSA barrels, multiple TOSA barrels, and/or multiple TOSA barrels may be coupled (e.g., electrically) together through respective metal plates formed therein. In an alternative embodiment, the OSA barrel may be coated with metal, either completely or partially for EMI shielding.  
         [0058]      FIG. 6  is a rear view of a conventional TOSA  200 , which has a standard transistor outline (TO) package  202  mounted thereon. The TO package  202  has formed thereon glass-sealed feed-throughs  204 ,  206  and  208  for carrying signals to and/or from the TO package  202 .  FIG. 7  is a cross-sectional view of the conventional TOSA  200  of  FIG. 6  taken along the line C-C. The TO package  202  includes a TO header  210  welded with a TO cap  212 . The TO header/cap is used as a signal ground. The TO can (including the TO header  210  and the TO cap  212 ) is not coupled to the chassis ground (e.g., the metal housing  100  of  FIG. 1 ). Therefore, the TO can does not provide sufficient shielding protection against side EMI.  
         [0059]     This is also the case for a standard TO-46 package  220 , rear view of which is illustrated on  FIG. 8 . The TO package  220  has formed thereon glass-sealed feed-throughs  222 ,  224 ,  226  and  228  for carrying signals to and/or from the TO package  220 . The signal ground in the TO package  220  is provided by a TO header  221  welded to a TO cap.  
         [0060]     As seen on  FIG. 8 , the number of feed-throughs is usually limited to a maximum of four. This typically is not enough for today&#39;s multi-GHz transceivers, especially on the receiver side. To conserve connections, the signal grounds are usually connected to the header  221 . This makes the packages susceptible to side EMI coupled through the signal ground within the transceiver.  
         [0061]     In an exemplary embodiment in accordance with the aspects of the present invention as illustrated in  FIG. 9 , a TO package  230  includes glass filled feed-throughs  232  and  234 . The TO package  230  acts as a mechanical, optical and electrical platform for the active and passive components within the optical subassemblies (OSAs). The windowed “can” provides a hermetic environment for the components, and the electrical connections are made through the glass-sealed feed-throughs on the TO header  231 .  
         [0062]     The TO package  230  also includes an elongated (e.g., oblong shape) glass filled feed-through that has embedded therein four in-line pins (or lead frames)  236 ,  238 ,  240  and  242 . Hence, instead of using the TO header  231  and/or the TO cap as the signal ground, one or more of the in-line pins  236 ,  238 ,  240  and  242  can be used to make contact with the PCB circuit ground as the signal ground. The number of in-line pins in other embodiments may vary (for example, 3 or 5).  
         [0063]     None of the in-line pins is directly connected to the TO case, and form a coplanar GSG (ground-signal-ground, e.g., a three in-line pin configuration) or GSSG (ground-signal-signal-ground, e.g., a four in-line pin configuration) transmission line through the header  231 . The TO case (including header and cap) is electrically connected to the transceiver housing (e.g., a chassis ground). In this way, the internal components are shielded from the side EMI.  
         [0064]     In an alternate embodiment, a similar scheme is used with a ceramic header by providing the suitable metalization on the header. By way of example, the metalization of the header may be provided by metal plating an external circumferential periphery of the header. Here glass-sealed feed-throughs are replaced by electrical vias through the ceramic. Together with the metal plate for shielding front EMI described above, the shielding provides effective EMI suppression when using low-cost plastic barrels for optical subassemblies.  
         [0065]     It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character hereof. The present description is therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.