Abstract:
A pluggable optical/electrical module is disclosed. One or more features operate to decrease electromagnetic interference are implemented, which features include deforming the portions that mate together to form the housing, placing elbow deformities on extending fingers to more properly seal the housing convex shape to housing to seal gaps between multiple sections, and placing an EMI insulating material within an opening that is formed for the latch that locks the module in place in a chassis.

Description:
TECHNICAL FIELD 
     The present invention relates to electromagnetic interference (EMI) shielding, and more particularly, to an improved method and apparatus for shielding electronic modules from EMI entering or exiting. The invention has particular applicability in small form factor pluggable (SFP and SFP+) optical transceivers, small pluggable modules that are typically installed in a shelf or chassis and used in optical communications systems. 
     BACKGROUND OF THE INVENTION 
     Small form factor pluggable optical transceivers (“SFPs”) are known in the art. Typically, such transceivers consist of an elongated module with at least optical two ports, one for receiving light pulses and another for transmitting light pulses to a remote location. Such devices also typically include an electrical interface. Examples of such devices are disclosed in U.S. Pat. Nos. 7,314,384, and 7,186,134. 
     These SFP modules typically plug into a shelf or chassis to be used in an optical switch or router. Such modules often include fingerstock that extend outwardly and upwardly from the device in a manner that leaves the end of the fingers not in contact with the SFP. The ends of the fingers resiliently press the rack or chassis and serve to connect the outside of the SFP module to the chassis. One such finger is shown in  FIG. 1 , which depicts the open end  101  of a finger for resiliently pressing against a chassis, and a length  102  of the finger that extends along the outside surface of the SFP module. The length  102  of the finger is shown extremely magnified. 
     One problem is that the point of contact between each finger and the SFP housing is somewhat undefined. Among numerous fingers for a particular SFP module, there may be different contact points. This is due largely to imperfections in the outside surface of the SFP module and the bottom surfaces of the fingers, as depicted in  FIG. 1 . The point of actual contact between each finger and the surface of the SFP module is thus less than exact. 
     The distance between the end of the finger that resiliently presses against the chassis, to the part of the finger that contacts the outer surface of the SFP module, represents a source of EMI leakage. Because of the variability of this distance among the plural fingers for a particular SFP module, in some cases, this distance may be longer than the wavelength of signal which represents the EMI (Electromagnetic Interference). This means the gap under the finger permits EMI interference to pass. This problem is particularly acute in relatively high frequency systems, wherein the wavelengths of interest are relatively short. 
     Another problem with prior art arrangements such as that shown in  FIG. 1  is that the modules are typically built from the upper and lower housing, shown as  112  and  114  in  FIG. 1 . Because the seal  113  is never exactly perfect, gaps are left which also provide for EMI leakage. 
     A still further problem relates to the latch used to maintain the SFP module in the chassis in which it is typically installed. More specifically, there is often a slidable latch or similar type mechanism that clips the SFP into the chassis. However, this movable part also presents a source of EMI leakage because the EMI signals may leak in around the slidable part. 
     In view of the foregoing, there exists a need in the art for a more effectively sealed module in order to prevent EMI leakage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts an exploded view of a prior art “finger” installed on the outer surface of an SFP module; 
         FIG. 1A  depicts an assembled prior art SFP module, 
         FIG. 1B  shows an exploded view of a portion of  FIG. 1A , showing leakage at the seam of the two connected portions; 
         FIG. 2  depicts a prospected view of an exemplary embodiment of an SFP module in accordance with the present invention; 
         FIG. 3  depicts another view of the SFP module of  FIG. 2 ; 
         FIG. 4  shows an exploded view of one of the fingers made in accordance with the present invention, when the SFP module is installed in a chassis; 
         FIG. 5  depicts a nearly assembled view of two portions of the SFP module in accordance with the present invention; 
         FIG. 6  depicts an assembled view of the SFP module of  FIG. 5  in accordance with the present invention; 
         FIG. 7  is a bottom view of an exemplary embodiment of the present invention; 
         FIG. 8  is an additional bottom view of the arrangement of  FIG. 7 , with the bail extended; 
         FIG. 9  is a top view of the module with the bail in an unlocked position with a latch assembly and a latch located in an opening in the chassis of the module; and 
         FIG. 10  is a top of view of the module with the bail and the latch of  FIG. 9  in locked positions. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 3  depicts a prospective view of an exemplary SFP module of the present invention. The arrangement of  FIG. 2  includes a first and second portions  204  and  205  which are placed together to form the module. Fingers  202  are shown extending outwardly from the surface of the module  201 . A bail  203  controls a latch as shown in  FIGS. 7 and 8 , in a manner such that lifting the bail slides the latch out of a opening and permits removal of the module from the chassis. Various such arrangements for using a bail to slide a latch are known. 
       FIG. 3  depicts a different perspective view of the exemplary of the embodiment of  FIG. 2 . As shown in  FIG. 3 , ports  305  and  306  exist for receiving and transmitting optical fibers respectfully. 
     Turning to  FIG. 4 , shown therein is an exploded view of a finger  401  representing one of the plurality of fingers  300  shown in  FIG. 3 . As shown in  FIG. 4 , a slight elbow  402  is placed along the length of  FIG. 401  at a point where it is desirable to contact an outside surface of the module  406 . Two preferably elbow shaped deformities  402  and  410  are formed in the finger  401 . The deformities are formed at prescribed locations along the finger  401  so that the distance indicated as D can be controlled. More specifically, comparing the arrangements of  FIGS. 1 and 4 , it can be appreciated that the contact points for the fingers are certain and predictable in  FIG. 4 , rather than varying in a somewhat unpredictable as in  FIG. 1 . Moreover, by adjusting the distance D appropriately, it can be made shorter than the shortest wavelength of interest, thereby substantially eliminating EMI at the wavelengths of interest. 
       FIGS. 5 and 6  depict the two portions of the SFP module  502  and  503  that may be brought together to form the completed module. As indicated pictorially, the surface of one or both portions  502  and/or  503  may be curved. Such slight curvature causes a force to be exerted at the seam  510  when the far ends of the two portions are squeezed together and held that way with screw  509  or similar means. As a result, there is a strong pressure forcing the seam closed, assisting to seal it against EMI leakage. 
     The force pushing the seam together may arise by curving either or both portions. Moreover, by orienting the tab  513  and lip  512  slightly downwardly, rather than completely horizontally as shown, a prying force can be obtained which results in similar pressure being placed at the seam. However, the curved embodiment is more preferred and believed to result in a tighter seal. 
       FIGS. 7 and 8  depict two views from underneath the module, showing the module in the locked and unlocked position, respectively. As is well known in the art, when the SFPs are plugged into a chassis, the movement of bail  203  from the position of  FIG. 8  to that of  FIG. 7  locks the SFP in, typically by moving a latch or similar protrusion into an opening or the like. Many variations on this basic idea exist in the market. 
     The area  704  represents an opening in which a latch or suitable structure typically slides or otherwise moves. Because the latch must be able to engage some portion of the chassis into which the SFP module is installed, there is an opening through which EMI leakage may occur. 
     To minimize leakage here, an EMI gasket materials used to create a seal around the latch. The EMI seal is realized as a compressive/compliant conductive foam gasket or as a metallic spring finger. The positive electrical contact between the sliding latch mechanism and the optical transceiver provided by the conductive foam or metallic spring finger results in an effective EMI seal. The EMI material is preferably placed underneath any slidable, moving mechanism, such as a latch, and assists in further sealing the opening to EMI leakage. 
       FIG. 8  shows EMI gasket  706  according to one embodiment of the invention. Latch assembly  710  is exposed in an opening  704  ( FIGS. 9-10 ) of the chassis of the module  201 . Gasket  706  is shown at the bottom of opening  704  (when viewing module  201  of  FIG. 8  such that bail  203  is at the top). 
       FIG. 9  shows latch assembly  710  which includes latch  708  (which in turn includes latch portions  708 - a  and  708 - b ) exposed through opening  704  in the rightmost surface of the chassis and movable in response to rotation of bail  203  about pivot axis  902 .  FIG. 9  shows latch assembly  710  and latch  708  in an unlocked position. Gasket  706  is located within opening  704  and underneath latch  708  (gasket  706  is “underneath” latch  708  when viewing module  201  such that bail  203  is on top) and aids in maintaining an EMI seal.  FIG. 9  shows latch  708  proximate to chassis block  712 , but not yet engaged therewith. 
       FIG. 10  shows the apparatus of  FIG. 9  with latch  708  locked onto chassis block  712 . Bail  203  has been rotated clockwise (“clockwise” when viewing bail  203  from below in the view of  FIG. 9 ) about pivot axis  902  from the position of bail  203  shown in  FIG. 9 , thereby advancing latch assembly  710  and latch  708  leftward in the view of  FIGS. 9 and 10 , thereby enabling latch portions  708 - a  and  708 - b  of latch  708  to engage respective grooves in the chassis block  712  within opening  704  in the chassis. 
     The combination of the fingers with the deformities, one or more curved sections of the module, and additional of the EMI gasket results in the sealing of the SFP module to EMI to a greater degree than was previously thought possible, particularly at higher data rates (e.g.; above 10 GB/s). Any one of more of the foregoing may be used alone or in combination to assist in the diminishing EMI from interfering with the operation of the device. While the foregoing describes the preferred embodiment of the invention, various modifications and/or additions will be apparent to those of skill in the art.