Abstract:
The invention relates to a resilient, non-conductive, RF absorptive, strain-relief bushing mounted on a electro-optical module for limiting the amount of electromagnetic interference emanating from the housing of the electro-optical module. The electro-optical module includes an optical sub-assembly for converting electrical signals into optical signals or vice versa, and an input/output port for transmitting the optical signal to the optical sub-assembly via an optical fiber. The bushing is in the form of a collar, which surrounds the input/output port of the electro-optical module, or a boot, which extends from one end of the input/output port down a portion of the length of the optical fiber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present invention claims priority from U.S. Patent Application No. 60/571,841 filed May 17, 2004, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to a bushing for an electro-optical module, and in particular to a resilient, non-conductive, radio frequency (RF) absorbing bushing for reducing electromagnetic interference (EMI) emissions from within the module, while providing strain relief for an optical fiber and/or a feed-through tube extending from the module.  
       BACKGROUND OF THE INVENTION  
       [0003]     Conventional EMI shields found on electro-optical modules, e.g. transmitter optical sub-assemblies (TOSA) and receiver optical sub-assemblies (ROSA), consist of a piece of sheet metal cut or bent into shape and placed in the front or rear of the electro-optical module. Examples of conventional EMI shielding for transceivers are disclosed in U.S. Pat. No. 6,200,041 issued Mar. 13, 2001 in the name of Gaio et al; U.S. Pat. No. 6,335,869 issued Jan. 1, 2002 to Branch et al; U.S. Pat. No. 6,659,655 issued Dec. 9, 2003 to Dair et al; and U.S. Pat. No. 6,817,782 issued Nov. 16, 2004 to Togami et al. All of the aforementioned EMI shields consist of a solid conductive material for electrically interconnecting the electro-optic component, e.g. laser or photo-detector, to the module housing, which is then grounded to a host device. Accordingly, existing EMI shields require small, accurately made and assembled structures, which add to the base and assembly cost of the module.  
         [0004]     An object of the present invention is to overcome the shortcomings of the prior art by providing a resilient, non-conductive and RF absorbing shield that isolates the optical component from the module housing, while reducing EMI emissions and providing mechanical support and strain relief for portions of the components extending from the housing.  
       SUMMARY OF THE INVENTION  
       [0005]     Accordingly, the present invention relates to an electro-optical device comprising: 
        an electro-optical component for converting between electrical and optical signals;     a housing for supporting the electro-optic component having an input/output port for supporting an optical fiber, which transmits optical signals to or from the electro-optical component; and     a resilient, non-conductive, RF absorbing collar mounted in close proximity to the input/output port, thereby reducing EMI emissions from the housing, and thereby providing mechanical support to the input/output port.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:  
         [0010]      FIG. 1  is a cross-sectional view of an embodiment of the present invention mounted on an electro-optical module housing;  
         [0011]      FIG. 2  is an isometric view of an embodiment of  FIG. 1 ;  
         [0012]      FIG. 3  is an isometric view of the invention of  FIGS. 1 and 2 ;  
         [0013]      FIG. 4  is an isometric view of an alternative embodiment of the present invention mounted on an electro-optical module housing;  
         [0014]      FIG. 5  is an isometric view of the embodiment of  FIG. 4  out of position on the electro-optical module housing; and  
         [0015]      FIG. 6  is an isometric view of an alternative embodiment of the present invention mounted on an electro-optical module housing. 
     
    
     DETAILED DESCRIPTION  
       [0016]     With reference to  FIGS. 1 and 2 , an electro-optical module, generally indicated at  1 , includes a housing  2  enclosing an electro-optical component  3 , e.g. a laser or a photo-detector, along with any electrical circuitry  4  for controlling and monitoring the electro-optical component  3 , typically provided on a module printed circuit board (PCB). An electrical connector  6  extend from the side of the housing  1  for electrically connecting the control and monitoring circuitry to a host device, within which the electro-optical module  1  is mounted. The illustrated electrical connector  6  comprise pins  7 , which are soldered directly to a host printed circuit board (PCB)  8  in the host device  9 . Alternatively, the electrical connector  6  can be a card edge connector formed in an end of the module PCB, or any other pluggable electrical connector, for plugging into a corresponding electrical connector on the host PCB.  
         [0017]     An input/output (I/O) port  10  extends through a wall of the housing  2  enabling light to travel between the electro-optical component  3  and an optical fiber  12 , which can be provided with a conventional strain relief boot  13 . The I/O port  10  can take many forms depending on the structure of the electro-optical component  3  and the housing  1 . The I/O port  10  could be a feed-through tube  11  enabling the optical fiber  12  to extend therethough into the housing  1  into close proximity to the electro-optical component  3  (as in  FIGS. 1 and 2 ) or the I/O port  10  could be an optical coupler for receiving an end of an optical fiber encased in an optical fiber ferrule.  
         [0018]     RF energy radiates within the housing  2  and is received by the body of the electro-optical component  3 , i.e. much like an antenna. The RF energy is then conducted through the wall of the housing  2  via the I/O port  10 . Moreover, the intersection of two elements, e.g. the feed-through tube  11  and the housing  2  or the feed-through tube  11  and the optical fiber  12 , forms gaps, which create a transfer point for any mechanical forces applied to one of the elements. In order to prevent the RF energy from being re-radiated outside of the housing  2 , a bushing collar  21  is mounted on the housing  2  using a suitable adhesive or other suitable means in close proximity to the gaps in the I/O port  10 , e.g. surrounding (or at least partially surrounding) the feed-through tube  11  covering the gaps with the optical fiber  12  and/or the housing  2 . With reference to  FIG. 3 , preferably the collar  21  is rectangular in shape, matching the shape of the housing  2 , with a cylindrical hole  22  extending therethrough having a diameter slightly less than that of the feed-through tube  11  providing a tight fit therebetween. To facilitate assembly, a slit  23  is provided in one side of the collar  21  enabling the collar  21  to be temporarily bent to fit around the feed-through tube  11 . The collar  21  is made of a sufficient mass of a resilient, RF absorptive, non-conductive material, e.g. nitrites, silicones, and polyeurethanes as bases, loaded with various magnetically-loaded products such as ferrous materials, carbons, and high-performance dialectrics. Preferably, the collar  21  is made of a magnetically loaded silicone rubber, which is RF absorptive over the frequency range of 800 MHz to 18 Ghz, such as a material sold under the trade name Eccosorb™. In addition to minimizing any EMI emissions from the housing  1 , the collar  21  provides mechanical support and strain relief for the feed-through tube  11 . Moreover, the resiliency of the collar  21  ensures a consistent seal between the housing  1  and the feed-through tube  11  during any shock, vibration or thermal expansion. However, the collar  21  does not require the precise control over position and compression required by traditional EMI gaskets.  
         [0019]     With reference to  FIGS. 4 and 5 , an alternative embodiment of an optical module  31  according the present invention includes a housing  32  with an electrical connecting  36  comprising pins  37  extending therefrom. An I/O port  40  includes a feed-through tube  41 , which receives an optical fiber  42  extending therethrough. The I/O port  40  includes a cylindrical snout  44 , which tapers to a small cylindrical opening  45  for receiving the feed-through tube  41 . An alternative bushing collar, according to the present invention, in the form of a strain relief boot  47  is positioned around the optical fiber  42  over top of an end of the feed-through tube  41  and the cylindrical opening  45 . Like the collar  21 , the strain relief boot  47  is made of a sufficient mass of a resilient, RF absorptive, non-conductive material, e.g. nitrites, silicones, and polyeurethanes as bases, loaded with various magnetically-loaded products such as ferrous materials, carbons, and high-performance dialectrics. Preferably, the strain relief boot  47  is a magnetically loaded silicone rubber, which is RF absorptive over the frequency range of 800 MHz to 18 Ghz, e.g. a material sold under the trade name Eccosorb™. In addition to minimizing any EMI emissions from the housing  31  and isolating the feed-through tube  41  from the housing  32 , the strain relief boot  47  provides mechanical support and strain relief for the feed-through tube  41  and the optical fiber  42 . Moreover, the resiliency of the strain relief boot  47  ensures a consistent seal between the housing  31  and the feed-through tube  41  and between the feed-through tube  41  and the optical fiber  42  during any shock, vibration or thermal expansion. Ideally the strain relief boot  47  is solid and slid over the optical fiber  47  during manufacture, but alternatively can initially be formed with a slit for enabling the strain relief boot  47  to be wrapped around the optical fiber  47  after the optical module  31  is assembled.  
         [0020]     With reference to  FIG. 6 , a one-piece collar/boot  51  can be sized to fit over the entire feed-through tube (not shown) into contact with the housing  52  of an optical module  53 . The one-piece collar/boot  51  includes a strain relief boot portion  54 , which covers the gap between the feed-through tube and an optical fiber  55 , thereby providing mechanical support and strain relief for optical fiber  55 , and a feed-through tube collar portion  56 , which covers the gap between the feed-through tube and the housing  52 , thereby providing mechanical support and strain relief for the feed-through tube and electrically isolating the feed-through tube from the housing  52 . As above, the one-piece collar/boot  51  is made of a sufficient mass of a resilient, RF absorptive, non-conductive material, e.g. nitrites, silicones, and polyeurethanes as bases, loaded with various magnetically-loaded products such as ferrous materials, carbons, and high-performance dialectrics. Preferably, the collar/boot  51  is made from a magnetically loaded silicone rubber, which is RF absorptive over the frequency range of 800 MHz to 18 Ghz, e.g. a material sold under the trade name Eccosorb™. As above, the optical module  53  has an electrical connector  57  in the form of pins  58  for electrically connecting an electro-optical component to a host device.