Abstract:
Present thermal solutions to conduct heat from pluggable optical modules into heat sinks use a metal heat sink attached with a spring clip. The interface between the pluggable module and the heat sink is simple metal-on-metal contact, which is inherently a poor thermal interface and limits heat dissipation from the optical module. Heat dissipation from pluggable optical modules is enhanced by the application of thermally conductive fibers, such as an advanced carbon nanotube velvet. The solution improves heat dissipation while preserving the removable nature of the optical modules.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority from U.S. Patent Application No. 61/817,382 filed Apr. 30, 2013, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an optical module, and in particular to a sliding heat sink for a pluggable optical module. 
     BACKGROUND OF THE INVENTION 
     Conventionally, optical transceivers with data rates up to 4 Gb/s are packaged in small form factor (SFF or SFP) packages, while optical transceivers with higher data rates, e.g. 10 Gb/s, are in larger packages, such as XFP, X2, and XENPAK. A conventional XFP arrangement is illustrated in  FIG. 1 , in which an XFP transceiver module  1  is plugged into a host cage assembly  2  mounted on a host circuit board  3 . The host cage assembly  2  includes a front bezel  4 , a cage receptacle  5 , and a host electrical connector  6 . The transceiver module  1  is inserted through an opening in the front bezel  4 , and through an open front of the cage receptacle  5 , until an electrical connector on the transceiver module  1  engages the host electrical connector  6 . The cage receptacle  5  has an opening  7  in the upper wall thereof through which a heat sink  8  extends into contact with the transceiver module  1  for dissipating heat therefrom. A clip  9  is provided for securing the heat sink  8  to the cage receptacle  5  and thereby into contact with the transceiver module  1 . With this arrangement, the heat sink  8  can be changed to suit the owner&#39;s individual needs without changing the basic transceiver module  1 . 
     Examples of conventional heat sinks are disclosed in U.S. Pat. No. 6,916,122 issued Jul. 12, 2005 in the name of Branch et al. 
     Pluggable optic module thermal dissipation requirements are increasing with the continued advancement of features and performance. 10 Gb/s modules with added features, e.g. EDC, tenability etc., have increased the power density of pluggable optics, and speed increases to 40 Gb/s and 100 Gb/s are pushing power densities even higher. A fundamental problem for all pluggable (removable) optical modules in telecom systems is that the need to make them removable limits the thermal conduction path. Improvements to the thermal conduction path will reduce the need for faster cooling air speeds or larger heat sinks, which are not always capable of keeping the modules within the operating temperature ranges specified. 
     The most common approach to connecting a heat sink to a pluggable optical module is the use of the MSA-suggested heat sink  8 , which clips to the cage  2  using the spring clip  9 . The spring clip  9  enables the heat sink  8  to move slightly, i.e. up and down, side to side, forwards and back, when the pluggable optic module  1  is inserted/extracted, while maintaining a tight interface between the surface of the module  1  and the heat sink  8 . However, the surfaces of the heat sink  8  and the pluggable optic module  1  are made of hard, non-conforming metal. This metal-to-metal contact is the weak link in the thermal path. Microscopic imperfections in the heat sink  8  and surfaces on the module  1  limit the flow of heat across the interface. Thermal contact resistance causes large temperature drops at the interfaces, which negatively affect the thermal performance of the system. Thermal management can be significantly better if there are no high resistance interfaces in the system. 
     In non-sliding applications a thermal interface material, e.g. gel, is often used to improve the thermal interfaces by filling the imperfections and improving heat flow. However, in a sliding application, e.g. pluggable optics modules (SFP, SFP+, GBIC, XFP, XENPAK, XPAK, X2) traditional thermal interface materials are undesirable because the thermal interface for pluggable optics is transient in nature. Modules will be extracted and inserted multiple times. Thermal interface materials leave residue on modules as they are removed, they dry out when no module is present (shipping) and are generally awkward to apply. 
     An object of the present invention is to overcome the shortcomings of the prior art by providing heat-sinking pluggable optical modules which addresses the need to be able to insert and remove MSA standard or other optical modules. The solution provides greatly improved thermal conductivity between the optical module and the heat sink within the system. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention relates to a cage assembly mountable on a printed circuit board for receiving an optical module comprising: 
     a cage for slidably receiving the optical module; 
     an electrical connector mountable on the printed circuit board for electrically connecting the optical module to the printed circuit board; and 
     a heat sink assembly mounted on the cage for dissipating heat from the optical module, the heat sink assembly comprising: 
     a thermally conductive heat sink separated from the optical module by a gap; and 
     a first thermal interface mounted on an underside of the heat sink, including thermally conductive fibers extending across the gap into the cage for contacting the optical module. 
     Another aspect of the present invention relates to an optical module for sliding into a cage assembly, which includes a cage, a first electrical connector with an opening in an upper wall, and a heat sink assembly mounted on the cage over the opening, comprising: 
     a housing defining a gap with the heat sink assembly when inserted in the cage; 
     optical and electrical components disposed in the housing for converting optical signals into electrical signals and electrical signals into optical signals; 
     a second electrical connector extending from the housing for connection to the first electrical connector; 
     an optical connector extending from the housing; and 
     a second thermal interface mounted on the housing including thermally conductive fibers for extending through the opening and across the gap into contact with the heat sink assembly for dissipating heat from the housing. 
     Another feature of the present invention provides an optical system including: 
     the aforementioned cage assembly; and 
     the aforementioned optical module; 
     wherein the thermally conductive fibers from each of the first and second thermal interfaces have a length between 0.6× and 1.0× a width of the gap between the optical module and the heat sink for engaging each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
         FIG. 1  is an exploded view of a conventional optical module cage system; 
         FIG. 2  is an exploded view of an optical module cage system in accordance with the present invention; 
         FIG. 3  is a cross-section view of the optical module cage system of  FIG. 2 ; 
         FIG. 4  is an isometric view of optical module; 
         FIG. 5  is a perspective view of a heat dissipating velvet of the optical module cage system of  FIG. 2 ; 
         FIG. 6  is a isometric view of a second embodiment of the present invention in which a single heat sink is utilized for a plurality of optical module cage systems; 
         FIG. 7  is an exploded view of the optical module cage system of  FIG. 6 ; 
         FIG. 8  is an isometric view of a third embodiment of the present invention in which the velvet is mounted on the optical module; and 
         FIG. 9  is an isometric view of a fourth embodiment of the present invention in which velvets are mounted on both the optical module and the heat sink. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIGS. 2 and 3 , the present invention relates to a cage assembly  12  for receiving a pluggable optical module  11 . The cage assembly  12  includes a rectangular, metal cage  13 , as is known in the prior art, mounted on a printed circuit board  15 , as in  FIG. 1 . The cage  13  includes a first opening  14  in a front wall for receiving the pluggable optical module  11 , and a second opening  16  in an upper wall for receiving a heat sink assembly  17 . The second opening  16  is at least half of the area of the upper wall, and preferably at least ¾ of the area of the upper wall, e.g. up to 90% of the area of the upper wall. An electrical connector  18  is mounted in the cage  13  on the printed circuit board for receiving a mating electrical connector on the pluggable optical module. The printed circuit board  15  includes trace electrical connectors for electrically connecting the connector  18  to a host computer system, within which the printed circuit board  15  is received. 
     The optical module, e.g. SFP, SFP+, GBIC, XFP, XENPAK, XPAK, X2, CFP, CFP2, CFP4, or QSFP transceiver, generally indicated at  11  in  FIG. 4 , typically includes a ROSA  21  mounted in a housing  22  alongside a TOSA  23 . A PCB  24  includes TOSA and ROSA control and monitoring circuitry, e.g. chip  25 . An electrical connector  27  extends from a rear end of the housing  22  for mating with a host mounted electrical connector  6 . For a pluggable transceiver the electrical connector  27  includes a card edge connector formed in the end of the PCB  24 . Bores  33  and  34  form an optical connector on a front end of the housing  22  for receiving an duplex optical connector. Other types of electro/optical modules are possible. 
     The heat sink assembly  17  includes any conventional heat sink  41 , comprised of metal or other suitable thermally conductive material, preferably with a plurality of thermally conductive fins or fingers extending upwardly therefrom, enabling cooling air to pass over, around and between. The heat sink assembly  17  also includes a first sliding thermal interface  42   a  in the form of a velvet or brush comprised of a plurality of thermally conductive whiskers, filaments or fibers disposed between the housing of the optical module  11  and the heat sink  41 , whereby the whiskers, filaments or fibers extend through the second opening  16  and across gap  19  between the optical module  11  and the heat sink  41 . In an alternate embodiment a second sliding thermal interface  42   b  is mounted on the optical module  11 , in place of or in conjunction with the first sliding thermal interface  42   a , whereby the whiskers, filaments or fibers extend upwardly from the optical module  11  through the second opening  16  into contact with the heat sink assembly  17 , i.e. the first sliding interface  42   a  or all the way to the heat sink  41 , if the first sliding interface  42   a  is absent. 
     Ideally, the heat sink assembly  17  covers the entire area of the second opening  16 , and the first (or second) sliding thermal interfaces  42   a  and/or  42   b  covers at least 50% of the second opening  16 , preferably at least 75% and more preferably up to 90%. Typically, each fiber is between 3 and 12 um in diameter, with a packing density of from 0.1% to 24%, preferably 3% to 15%, and more preferably 4% to 6%. Typically, the velvet  42   a  and/or  42   b  has a thermal conductivity greater than 500 W/m 2 K, preferably between 1000 and 10,000 W/m 2 K, and more preferably about 2000 to 5000 W/m 2 K. Ideally, carbon nanotubes ( FIG. 5 ) are used, which provide excellent thermal conductivity while maintaining mechanical compliance. Examples of carbon nanotubes are found in U.S. Pat. No. 7,416,019 issued Aug. 26, 2008 in the name of Osiander et al, and U.S. Pat. No. 8,220,530 issued Jul. 17, 2012 in the name of Cola et al, which are incorporated herein by reference. 
     With reference to  FIG. 5 , ideally, the “velvet”  42   a  and/or  42   b  is comprised of carbon nanotubes in the form of a foil substrate  43  with an array of carbon nanotubes  44 . The preferred embodiment uses a specifically designed carbon nanotube velvet to connect the pluggable optical module  11  to a heat sink  41 . The many fibers in the velvet  42   a  and/or  42   b  can move independently to fill the voids in the surfaces of the pluggable optic module  11  to improve the heat flow therebetween. The independent and flexible nature of the fibers also enables the surfaces to slide while still maintaining thermal contact. The improved contact lowers the temperature of the pluggable module  11  more than the standard metal-on-metal contact of the MSA-specified heat sink design shown in  FIG. 1 . 
     In the primary embodiment of the invention, the velvet  42   a  is mounted on the heat sink  41  of the cage system  12  into which the pluggable optic module  11  is being inserted, In this particular application, the carbon nanotube array  44  can be a velvet called VEL-THERM® procured from ESLI (Energy Science Laboratories, Inc.) disclosed in U.S. Pat. No. 7,132,161 issued Nov. 7, 2006 to Knowles et al, which is incorporated herein by reference. The velvet  42  must be precut (die cut) to the precise size required to extend through the second opening  16  in the optical module cage  13 . The thickness of the velvet  42   a  or  42   b  is precisely controlled to provide optimal contact with the pluggable optic  11  for optimization of both thermal performance and the insertion and removal of the module  11 . Typically, the thickness of the velvet  42   a  or  42   b  is larger than the gap  19 , e.g. 1.2 mm, between the module  11  and the heat sink  41 . Preferably, the thickness of the velvet  42   a  or  42   b  is between 1.5× and 2.0× the width of the gap  19 , e.g. 1.8 mm to 2.4 mm, and ideally 1⅔× the width of the gap  19 , e.g. 2 mm. 
     Another important consideration is the control of stray carbon nanotubes. Every effort is made to ensure that the pre-cut velvet  42   a  and/or  42   b  have no loose carbon nanotube fibers, which could dislodge and interfere with the electrical operation of the circuit board  15  on which the optical module  11  is placed. An additional precaution is the application of an electrically insulating coating to the velvet  42   a  and/or  42   b , which reduces or eliminates any electrical conductivity of the velvet  42   a  and/or  42   b . A coating, such as a Parylene coating, improves fiber retention, but most importantly reduces the electrical conductivity of loose individual fibers, whereby detached fibers would not fall onto the printed circuit board  15  and short circuit any electrical circuitry. 
     Another limitation of the MSA-specified heat sink  8  is that one heat sink can only be applied to one pluggable module  1 , i.e. one heat sink  8  cannot be used to cool multiple pluggable modules  1 . This is due to the floating nature of the MSA-specified design. When attached to a single pluggable optic module  1 , the heat sink spring clip  9  can account for any tolerance mismatch and maintain contact between the heat sink  8  and the pluggable module  1 . But when additional pluggable modules  1  are added, it is impossible to contact all of the surfaces due to standard tolerance variation. 
     With reference to  FIGS. 6 and 7 , the use of brushes or velvets  42   a  and or  42   b , e.g. carbon fiber nanotubes, eliminates the need for the heat sink  8  to move because the individual fibers accommodate the variations in the surfaces of the heat sink and the optical modules  11 . Therefore, a plurality of pluggable optic cages  13  can be mounted on a single printed circuit board  56 , with a combined electrical connector  57  for connection to a host device (not shown). Accordingly, only a single stationary heat sink  58  can be used to dissipate heat from each and every one of a plurality of optical modules  11  received within the cages  13 . One or both of the velvets  42   a  and/or  42   b  is provided for each module  11 , either mounted on the heat sink  58  or on each module  11  or both. The heat sink  58  can cover just the area above the cages  13  or it can cover, and provide protection and heat dissipation, for the entire printed circuit board  56 . 
     In the illustrated multi-unit embodiment of  FIGS. 6 and 7 , the heat sink  58  includes a front wall  61  including a plurality of apertures  62  providing access to the openings  14 , and a rear wall  63  including an access port  64  through which the combined electrical connector  57  extends. Side walls  66  and  67 , preferably include an array of openings, enabling air to circulate through the side walls and over the electrical elements on the printed circuit board  56 . The upper wall  68  of the heat sink  58  includes a series of fins or fingers  69  in the area over top of the cages  13 , i.e. velvets  42   a , for increased heat dissipation. Additional vent openings and/or heat dissipating fins or fingers can also be provided over top of the other sections of the printed circuit board  56 , as required by their thermal dissipation needs, such as required for any processors, FPGA&#39;s and memory chips provided in the multi-unit module. 
     Some pluggable optic modules are not designed for heat sinks. In these cases, the pluggable optic module is inserted into a cage on the PCBA. There is a gap between the pluggable module and the cage that inhibits the flow of heat. Placing carbon fiber nanotube velvet between the pluggable optic module and the cage will create thermal contact between the parts and promote heat flow. This can be accomplished by attachment of the velvet to both or either of the optical module and the cage. 
     Accordingly, in another embodiment of the invention, illustrated in  FIGS. 8 and 9 , an optical module  81 , e.g. SFP, is insertable into a cage  83 , which is mounted on a printed circuit board  85  including an electrical connector  86 . A velvet  82  is mounted directly on the upper surface of a pluggable optical module  81  or on the inside surface of the upper wall of the cage  83 , so that the velvet  82  extends between the optical module  81  and the cage  83 , i.e. across the gap therebetween. A heat sink  84  is mounted on the outer surface of the upper wall of the cage  83 , whereby heat is conducted from the optical module  71  through the velvet  82 , through the upper wall of the cage  83  to the heat sink  84 . A second velvet or a conventional thermally conductive material  88 , e.g. gel or pad, can be added between the cage outer surface of the upper wall of the cage  83  and the heat sink  84  to enhance thermal conductivity. Accordingly, the heat sink assembly includes The materials and dimensions of the velvet  82  are the same as those of the velvet  42 , relative to the gap between the optical module  71  and the cage  83 , e.g. preferably 0.1.2× to 2.0× the gap, more preferably 1.5× to 2.0× the gap, and most preferably 1.66× the gap. Ideally, the velvet  82  covers over 25%, preferably greater than 50%, and more preferably greater than 75% of the upper surface of the optical module  81  or the inside surface of the upper wall of the cage  83 .