Abstract:
This disclosure is generally concerned with optical modules. In one example, an optical module provided that includes a pair of optical subassemblies, each of which includes a port housing within which a corresponding optical component is disposed. The optical module further includes a pair of electrically conductive elements configured to facilitate control of EMI. Each of the electrically conductive elements is disposed on a respective port housing such that a gap is present between the first and second electrically conductive elements and the first and second electrically conductive elements do not contact each other.

Description:
RELATED APPLICATIONS 
   This application is a continuation, and claims the benefit, of U.S. patent application Ser. No. 10/367,435, now U.S. Pat. No. 6,817,782, entitled OPTICAL MODULE WITH SIMPLEX PORT CAP EMI SHIELD, filed Feb. 13, 2003, which, in turn, claims the benefit of U.S. Provisional Patent Application Ser. No. 60/357,190, entitled OPTICAL MODULE WITH SIMPLEX PORT CAP EMI SHIELD, filed Feb. 15, 2002. All of the aforementioned patent applications are incorporated herein in their respective entireties by this reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to electromagnetic interference (EMI) shields for transceiver and transponder modules. 
   2. Description of the Related Art 
   It is desirable to provide electromagnetic interference (EMI) shielding in transceiver and transponder modules. One portion of a transceiver/transponder module for which EMI shielding is desirable is in a fiber connector end of the module. High-speed electronics, such as transmitter or receiver circuits operating at data rates greater than 1 Gb/s, may generate significant EMI if not properly shielded. Consequently, in many applications EMI shielding is required. 
   One application requiring EMI shielding of port housings is a small form factor transceiver, including both hot-pluggable and non-pluggable (i.e., hard soldered) varieties. An industry-wide Multi-Source Agreement (MSA) governs the size and pin arrangement of small form factor transceivers. Conventionally, the port housings of a transceiver/transponder module are fabricated from metal. However, there is increasing interest in plastic port housings. Plastic port housings provide several potential advantages, such as the ability to integrate a plastic lens into the housing. However, compared with metal port housings, plastic port housings have lower mechanical strength. Moreover, plastic port housings may suffer more from thermal stress. Additionally, plastic port housing are more difficult to shield from EMI. 
   Therefore what is desired is an EMI shield compatible with the requirements of plastic port housings and which has desirable EMI shielding and mechanical properties. 
   BRIEF SUMMARY OF AN EXEMPLARY EMBODIMENT OF THE INVENTION 
   In general, exemplary embodiments of the invention are concerned with the implementation of EMI control structures and systems in modules such as optical transceivers. In one embodiment, an optical module, such as an optical transceiver, is provided that includes a pair of optical subassemblies, each of which includes a port housing within which a corresponding optical component is disposed. The optical module further includes a pair of electrically conductive elements configured to facilitate control of EMI. Each of the electrically conductive elements is disposed on a respective port housing such that a gap is present between the first and second electrically conductive elements and the first and second electrically conductive elements do not contact each other. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which: 
       FIG. 1A  is a perspective view of a non-pluggable small form factor transceiver. 
       FIG. 1B  is a perspective view of a hot-pluggable small form factor transceiver. 
       FIG. 1C  is a connector end view of the small form factor transceiver of FIG.  1 A. 
       FIG. 2A  is a block diagram of one embodiment of a transceiver module utilizing port housing EMI shields in accord with the present invention. 
       FIG. 2B  is a plan view illustrating an individual optical sub-assembly. 
       FIG. 2C  is an exploded perspective view of a port housing, a portion of a yoke, and a portion of a restraining bar. 
       FIG. 3  is an exploded perspective view of one embodiment of a module of the present invention. 
       FIG. 4A  is a top view of the module of FIG. 3 . 
       FIG. 4B  is a side view of the module of FIG.  3 . 
       FIG. 5  is a cross-sectional view of the module of  FIG. 4A  along line A—A. 
       FIG. 6  is a cross-sectional view of the module of  FIG. 4A  along line B—B. 
       FIG. 7  is a cross-sectional view of the module of  FIG. 4A  along line C—C. 
   

   The figures depict a preferred embodiment of the present invention for purposes of illustration only. One of skill in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods disclosed herein may be employed without departing from the principles of the claimed invention. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A  is a top perspective view of an example of a non-pluggable small form factor transceiver, such as those manufactured by the Finisar Corporation of Sunnyvale, Calif. In this example, an internal optical module (e.g., see FIG.  2 A), for example a transceiver module or transponder module, is housed inside a shell  105  with appropriate pin connectors  115  for communicating with internal electronics. The transceiver is hard soldered or otherwise attached to a printed circuit board. It is non-pluggable in the sense that it is difficult to change the internal optical module without first detaching the device from the printed circuit board. 
     FIG. 1B  is a bottom perspective view of an example of a hot-pluggable small form factor transceiver, such as those manufactured by the Finisar Corporation of Sunnyvale, Calif. In this approach, a cage housing  175  is soldered or otherwise attached to a printed circuit board. It has an open end  110  to permit an optical module  200  to be inserted/removed. A bottom open portion  185  of cage  175  permits an electrical interface connector, such as pins, to be located within cage  175 . In this manner, electrical connection can be made with internal electronics. The design is hot-pluggable since the internal optical module  200  may be changed by removing (by sliding, in this case) the current module and replacing it with a different module, without detaching the cage housing  175  from the printed circuit board. 
     FIG. 1C  shows a view of an internal transceiver module from the fiber connector end  110 . In this example, the transceiver is non-pluggable but the following remarks also apply to the hot-pluggable variety. Two ferrules  140  and  150  include bores to receive and guide optical fibers into position. The fiber connector end  110  may be designed to accept any suitable dual fiber connector, such as an LC type, SC, or MT-RJ type connector. 
   The present invention generally comprises an optical module having an EMI shield integrated onto each port housing to reduce the aperture for EMI emission and to provide an additional ESD protection path. 
     FIG. 2A  depicts a top view of one embodiment of an optical module  200  having components disposed in a conductive shell  218  in accord with one embodiment of the present invention. The following example is a pluggable transceiver module, but the optical module may be either a transceiver or transponder module, including both non-pluggable and pluggable varieties. During normal use, optical module  200  is inserted into cage housing  175 . 
   Transceiver module  200  includes a transmitter optical subassembly (TOSA)  212 , which includes a light source capable of being modulated, such as a laser transmitter, Tx. A receiver optical subassembly (ROSA)  214  includes an optical detector. Each optical subassembly (OSA) is electrically coupled to a printed circuit board assembly (PCBA)  216  having respective transmitter and receiver electronic circuits. In one embodiment, each OSA is coupled to the PCBA  216  using its own flex electrical connector  211 . 
   The OSAs are disposed in an electrically conductive shell  218 . A neck portion  231  of a port housing  238  for the OSA is located in an electrically conductive yoke  233 , proximate a fiber connector end  220 . An electrically conductive restraining bar  232  (not shown in  FIG. 2A ) seats onto the portion of the necks  231  not supported by yoke  233 . Yoke  233  and restraining bar  232  are electrically coupled to the shell  218  and provide EMI shielding with respect to the fiber connector end  220 . 
     FIGS. 2B-2C  illustrate aspects of the port housing  238 , yoke  233 , and restraining bar  232 .  FIG. 2B  is a side view of the port housing  238 .  FIG. 2C  is an exploded perspective view of the port housing  238 , yoke  233 , and restraining bar  232 . An individual OSA has a port housing  238  comprising a body portion  240 , a neck portion  231 , and a ferrule head portion  230 . Body portion  240  has a receptacle end shaped to house a header  299 , such as a receiver or transmitter header  299 . The neck portion  231  is bounded by a first stop surface  287  disposed on body portion  240  and a second stop surface  281  disposed on an inner surface of collar  234 . Collar  234  defines an annulus about the ferrule head end  230 , which has a diameter less than that of the body portion. Ferrule head end  230  includes a bore  235  with a bore opening for receiving an optical fiber. 
   An electrically conductive cap  225  is disposed on an outer surface  224  of collar  234 . The conductive cap  225  may be formed on the port housing  238  or be fitted onto the port housing. Referring to  FIG. 2C  which show half of an electrically conductive yoke  233  and electrically conductive port restraining bar  232 , neck  231  is shaped to be held in a yoke  233  having a region  292  preferably shaped to form a slide-in or snap-in connection. A restraining bar  232  is shaped to fit about the portion of neck  231  not held in yoke  233 . The shape of the yoke  233  and port restraining bar (PRB)  232  may be selected in combination with the separation of stop surfaces  287  and  281  to permit a limited range of motion of neck  231  with respect to yoke  233 . 
   The port housing  238  of each OSA may be fabricated from a material that does not significantly block EMI, such as a composite plastic with optical grade plastic in regions that are used to couple light. The use of plastic port housings permits a plastic lens (not shown) to be incorporated into the port housing to couple light between the OSA and an optical fiber (not shown) in the bore  235  of a receiving ferrule  230  of the port housing. Electrically conductive cap  225  provides additional shielding on the front surface  224  of collar  234 . Conductive cap  225  is preferably shaped so that an electrical contact is made along at least one point of cap  225  to restraining bar  232  or yoke  233  to provide a path to ground via the shell  218 . Alternately, the cap  225  may make direct electrical contact with the shell  218 . Consequently, cap  225  provides an additional conductive surface that attenuates EMI and grounds ESD. 
   Cap  225  provides an aperture through which EMI passes, which increases the attenuation for EMI. Note that cap  225  may shield substantially the entire annular region of collar  234 , forming an aperture having a smaller diameter than body portion  240 . In some embodiments, cap  225  may form an aperture having a smaller diameter than neck  231 . 
   Referring again to  FIGS. 2B-2C , compliant movement of the port housings  238  is facilitated by coupling the OSAs to the PCBA  216  with a flexible connector  211 . In one embodiment each OSA is electrically coupled to its corresponding PCB electronics by its own flex connector, in order to facilitate each port housing moving independently of the other port housings in response to loads. Microwave frequency flex connectors may, for example, comprise microwave transmission lines formed or embedded within a flexible material. Port restraining bar  232  may be shaped to apply a sufficient pressure such that neck portion  231  may move in response to light loads. 
     FIG. 3  is a perspective exploded view of one embodiment of a small form factor optical transceiver module  300 . In an electronics section of the shell  318 , PCBA  316  is used to mount receiver and transmitter electronics (not shown in FIG.  3 ). The PCBA  316  may be attached to a shell  318  using a suitable fastener  306 , such as a bolt or screw. Shell  318  is preferably an electrically conductive shell, such as shell comprised of a metal or having a metal foil. Two flex circuits  311 A and  311 B are attached to the PCBA to provide separate flexible electrical connections to the pins of the device headers of each OSA. A laser transmitter OSA (TOSA)  312  is disposed within a first port housing  338 . A photodiode receiver OSA (ROSA)  314  is disposed within a second port housing  338 . 
   Each port housing  338  includes a collar  334 . Each port housing includes a first stop surface  387  of body end  340  and a second stop surface  381  of collar  334 . The bore  335  of the port housing is shaped to receive an optical fiber. It has a smaller outer diameter than the body end of the port housing that houses the OSA receiver or transmitter electronics. An electrically conductive cap  325  is shaped to fit onto the each annular portion of collar  334 , extending over the front and side surfaces of the lip to form a conductive sleeve around the rim of the collar  334 . In one embodiment, the connector receptacles  309  have front openings shaped to receive a dual fiber connector, such as an LC type connector in the fiber connector end  320  of the module. A portion of shell  318  has a region  370  shaped to receive a restraining bar (not shown in FIG.  3 ). 
     FIG. 4A  is a top view of an assembled module  300  and  FIG. 4B  is a side view of the same module. Note that the separate flex circuits may each have a different length, permitting the laser port housing and OSA to have a different length than the optical detector port housing and OSA. The restraining bar  332  is illustrated in  FIGS. 4A and 4B . 
     FIG. 5  is a cross-sectional view along line A—A of  FIG. 4A  showing a view of a port restraining bar (PRB)  332  and yoke  333  holding an OSA  314 . In one embodiment, PRB  332  is shaped to contact the conductive cap  325  with a sufficient friction that the neck is movably coupled to yoke  333  with conductive cap  325  disposed in the front portion of connector end  320 . 
     FIG. 6  shows a cross sectional view along line B—B of FIG.  4 A through the photodiode port housing  338 . The conductive cap  325  subtends the annulus of collar  334 . Port restraining bar  332  and yoke  333  provide some EMI shielding. The aperture of conductive cap  325  further attenuates EMI. As can be seen in  FIG. 6 , cap  325  may have an inner diameter, d 1 , that is less than that an inner diameter d 2 , of the yoke  333 . Consequently, cap  325  increases the attenuation of EMI and also increases ESD protection. In this example, a ball lens  397  is used for optical coupling.  FIG. 7  shows a corresponding cross-sectional view along line C—C through the transmitter laser port housing  338 . An integrated plastic lens  398  is also shown. 
   The present invention provides several benefits. One benefit is that the effective aperture for EMI radiation is reduced to the inner diameter of the conductive cap. Substantially all of the area aside from the ferrule portions of the port housing may be chocked off with a conductive shield. The conductive shell, the conductive port caps, yoke, and other metal elements (e.g., the PRB) are electrically coupled together and may be suitably grounded to other elements, such as to a conductive enclosure. Moreover, in one embodiment, the shield elements are electrically coupled to an electrically conductive shell, which further facilitates protecting the PCBA electronics from ESD. 
   Another benefit of the present invention is that it permits each port housing a limited range of motion in response to loads. A pressure fit may be selected that engages the conductive port cap with sufficient pressure to form an electrical connection to the conductive cap. However, the port housing may still move in response to thermal expansion, mechanical loads, or vibration. This provides several advantages, since it facilitates maintaining the fiber in proper optical alignment within the ferrule of the port housing. Additionally, mechanical reliability of plastic components may also benefit. 
   Yet another benefit of the present invention is that the receiver port housing and the transmitter port housing may move independently of each other, facilitating each port housing maintaining reliable optical alignment to its fiber. 
   Still yet another benefit of the present invention is that it highly manufacturable, since it requires only a comparatively low cost manufacturing step to add a conductive port cap to a plastic port housing. 
   While the present invention has been describe in detail with regards to a transceiver module having separate receiver and transmitter OSAs, it will be understood that the present invention may be applied to any module having one or more plastic port housings, such as receiver, transmitter, and transceiver modules. Moreover, it will be understood that the present invention is not limited to modules configured to receiver LC-type fiber connectors but may be adapted to receive a variety of connector types, such as SC, MT-RJ type or other types of connectors. Additionally, while the present invention has been described in detail in regards to plastic port housings, it will be understood that it applies more generally to any type of electrically non-conductive or poorly conductive port housing. 
   While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention as defined in the appended claims.