Abstract:
A fiber optic connector system for connecting at least one optical fiber cable mounted near the edge of a planar substrate to a backplane, each optical fiber cable including a plurality of optical fibers and a terminating ferrule, the longitudinal orientation of the optical fibers within the terminating ferrule defining a longitudinal axis and a forward direction, the ferrule having a first longitudinal range of motion x 1  and a ferrule spring element having a longitudinal ferrule spring force f n ,. The optical connector system includes a substrate housing assembly and a backplane housing assembly. The substrate housing assembly is designed to be mounted on the planar substrate and includes at least one ferrule receiving cavity for receiving the optical fiber ferrule, and a substrate housing assembly spring. The substrate housing assembly has a longitudinal freedom of motion with respect to the substrate, the housing assembly spring controlling movement of the substrate housing assembly along the longitudinal axis and having a longitudinal spring force h, wherein  
       h   &gt;       ∑   1   n          f   n                             
 
     The backplane housing assembly defines at least one longitudinal receiving cavity, the receiving cavity having a frontal opening along the first surface of the backplane member and a rear opening along the second surface of the backplane member. A frontal door covers the frontal opening and a rear door covers the rear opening.

Description:
RELATED APPLICATIONS  
       [0001]    The present application is a Divisional of commonly-owned U.S. patent application Ser. No. 09/643,333, filed Aug. 22, 2000, which is a continuation-in-part of U.S. patent application Ser. No. 09/443,713, filed Dec. 1, 1999, and which are hereby incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to an optical fiber connector system. More particularly, the present invention relates to a connector assembly for optically coupling a circuit card to a backplane.  
           [0003]    The use of optical fibers for high-volume high-speed communication is well established. As the volume of transmitted information grows, the use of optical fiber cables including multiple optical fibers, and of systems using multiple optical fiber cables, has increased.  
           [0004]    It has long been desirable to increase the number of fibers that can be removably connected within a given space. Until recently fiber optic interconnects were limited to single or duplex formats utilizing industry standard connectors, such as the SC, ST, LC, and the like. These solutions are analogous to single end electrical cable terminations prevalent prior to the invention of electrical ribbon cable and mass-terminable IDC connectors.  
           [0005]    Fiber optic terminations currently are evolving from single terminations to mass terminations. Within the past few years, ribbonized multi fiber cables have been developed. In conjunction with these cable development efforts, multi-fiber mounting ferrules also have been developed.  
           [0006]    The design of traditional electronic cabinets is now being utilized to accommodate optical and opto-electronic devices. In traditional cabinet designs, the cabinet comprises a box having a plurality of internal slots or racks, generally parallel to each other.  
           [0007]    Components are mounted on planar substrates, called as circuit boards or daughter cards, which are designed to slide into the slots or racks within the cabinet.  
           [0008]    As with electrical cables, the need exists to provide a means to allow the fiber signals to be passed through the backplane of electronic cabinets. A backplane derives its name from the back (distal) plane in a parallelepipedal cabinet and generally is orthogonal to the board cards. The term backplane in the present invention refers to an interconnection plane where a multiplicity of interconnections may be made, such as with a common bus or other external devices. For explanation purposes, a backplane is described as having a front or interior face and a back or exterior face.  
           [0009]    An example of a backplane connectivity application is the interconnection of telephone switching equipment. In this application, cards having optical and electronic telecommunication components are slid into cabinets. The need exists to have a removable fiber termination from both the front side and the back-side of the backplane.  
           [0010]    Furthermore, as a function of inserting and removing an optical driver card from a rack coupled to the backplane, coupling and uncoupling of the optical connections in the card is to be completed in a blind mating manner.  
           [0011]    In order to maintain appropriate transmission of light signals, optical fiber ends are to be carefully aligned along all three movement (x, y, and z) axes, as well as angularly. Alignment challenges increase and tolerances decrease geometrically as the number of optical fibers to be aligned increases. Blind mating of a card-mounted component to a backplane connector has been found to create special challenges with regards to alignment and mating force issues along the axis of interconnection.  
           [0012]    For the purposes of the present description, the axis of interconnection is called the longitudinal or x-axis and is defined by the longitudinal alignment of the optical fibers at the point of connection. Generally, in backplane applications, the longitudinal axis is collinear with the axis of movement of the cards and the axis of connection of the optical fibers in and out of the cabinets. The lateral or y-axis is defined by the perpendicular to the x-axis and the planar surface of the card. Finally, the transverse or z-axis is defined by the orthogonal to the x-axis and the backplane surface. The angular alignment is defined as the angular orientation of the card with respect to the x-axis.  
           [0013]    In preferred embodiments, the motion of sliding the card into a receiving slot simultaneously achieves optical interconnection. The “optical gap” distance along the longitudinal axis between the optical fiber ends and interconnected optical components is an important consideration. A large gap will prevent effective connection, thereby causing the loss of the optical signals. On the other hand, excessive pressure on the mating faces, such as that caused by “jamming in” a card, may result in damage to the fragile optical fiber ends and mating components. Traditional optical gap tolerances are in the order of less than one micron.  
           [0014]    Current connector assemblies include forward biased spring mounted ferrules. The purpose of the said bias springs is twofold, one, to absorb a limited amount of over travel of the ferrules during mating and two, to provide a predetermined spring biasing force thus urging the ferrules intimately together when the ferrules are in their mated position.  
           [0015]    An additional subject of concern is card gap, especially when dealing with backplane connector systems. Card gap is defined as the space remaining between the rear edge of a circuit card and the interior or front face of the backplane. In general, designers and users of backplane connection systems find it exceedingly difficult to control the position of a circuit card to a backplane within the precision range required for optical interconnects. Card gap, otherwise defined as card insertion distance, is subject to a multiplicity of variables. Among these variables are card length, component position on the surface of the card, card latch tolerances, and component position on the backplane.  
           [0016]    Over insertion of a circuit card relative to the interior surface of a backplane presents a separate set of conditions wherein the backplane connector&#39;s components are subjected to excessive compressive stress when fixed in a mated condition. In certain instances the said compressive stress may be sufficient to cause physical damage to the connector&#39;s components and the optical fibers contained therein.  
           [0017]    The need remains for a connector system that prevents component damage due to excessive operator force, compensates for longitudinal card misalignment, yet provides accurate control of optical gap distance and mating force.  
           [0018]    Another consideration is radial misalignment of the card. When an operator inserts a card on a slot, it is often difficult to maintain the card edge perfectly aligned in parallel with the lateral axis of the backplane. FIG. 1 illustrates an angularity misaligned card  10  having a connector  12  mating to a backplane connector  14 . The card is otherwise correctly aligned along the y and z-axes. At the point of contact between connectors  12  and  14 , the angular misalignment prevents correct gap spacing between optical fibers  16  and causes undue pressure on one end of the connector and the respective optical fiber end faces.  
           [0019]    Other considerations exist in backplane interconnection systems other than correct alignment. With the advent of laser optical signals and other high-intensity light sources, eye safety is a major concern associated with backplane connector users today. The safety issues are further escalated by the fact that ribbonized fiber arrays present a greater danger than the single fiber predecessors because the amount of light is multiplied by the number of fibers.  
           [0020]    Previous systems, such as that discussed in U.S. Pat. No. 5,080,461, discuss the use of complex door systems mounted on terminating fiber connectors, but mainly for the purpose of preventing damage or contamination of fiber ends. As the lighttransmitting core of a single mode fiber measures only ˜8 microns in diameter, even a minute accumulation of dust particles may render the fiber inoperable. However, prior systems require complex terminations at each fiber end and only may be mated to another corresponding male-female connector pair, not to standard connectors, making their use cumbersome.  
           [0021]    EMI (electromagnetic interference) control also has arisen as an issue in backplane connector design. As connection of optoelectronic devices through a backplane often necessitates forming of a physical opening through the backplane of an electronic cabinet, the potential exists for EMI leakage through the said backplane. Electrical interconnection has attempted to address this problem through the use of several elaborate EMI shielding techniques. However, current optical fiber connectors have failed to satisfy this concern.  
           [0022]    Finally, another concern regarding backplane optical connector applications is bend radius control. Horizontal cabinets connections are often subject to bend stresses due to gravity, operator misuse, or physical constraints, such as when a cabinet is pressed against a wall. Optical fibers are made of glass and rely on total internal reflection to transmit light signals. When an optical fiber is bent beyond a certain critical angle, fractures may appear in the glass, causing the fiber to break or become damaged. Also, at certain bend angles, even if the glass fiber does not break, the optical signal may be lost or may deteriorate, as the complete light signal is no longer contained inside the fiber.  
           [0023]    Several methods and apparatus for controlling the bend radius of an optical cable have been attempted. Among those are preformed boots that are slid over the cable, external devices such as clips or clamps, and elaborate injection molded components that are shaped such that when attached to a cable, the cable assumes the shape of the molded structure.  
           [0024]    Since backplane connection frequently involves connecting an increasing number of optical fibers in a small space, the need exists for an apparatus for controlling the bend radius of the optical fibers.  
         SUMMARY OF THE INVENTION  
         [0025]    The present invention relates to an optical fiber interconnect system that provides longitudinal and angular alignment control, contamination control, visual safety and bend radius control. In certain embodiments, the optical interconnect system of the present invention provides for interconnecting arrays of optical fiber cables in a individual or collective fashion.  
           [0026]    The fiber optic connector system of the present invention is designed for connecting at least one optical fiber cable mounted near the edge of a planar substrate, a card, through a backplane. Each optical fiber cable includes a plurality of optical fibers and a terminating ferrule, the longitudinal orientation of the optical fibers within the terminating ferrule defining a longitudinal axis and a forward direction towards the backplane. Each optical fiber cable is terminated by a ferrule having a first longitudinal range of motion x 1  with respect to a retaining member and a ferrule spring element having a longitudinal ferrule spring force f n .  
           [0027]    The optical connector system comprises a card housing assembly and a backplane housing assembly. The card housing assembly is mounted on the planar substrate or card and includes at least one ferrule-receiving cavity for receiving the optical fiber ferrule. The card housing assembly includes a card housing spring. The card housing assembly has a longitudinal range of motion x 2  with respect to the card, the card housing assembly spring controlling movement of the card housing assembly along the longitudinal range of motion. The card spring has a longitudinally directed spring force h, wherein  
         h   &gt;       ∑   1   n          f   n         ,                         
 
           [0028]    that is, the spring force of the card spring can counteract the opposite spring force of all the ferrule springs. It should be understood that the ferrule spring may comprise one or more individual spring elements. In one embodiment of the present invention, the card spring includes two or more springs laterally spaced from in each other, to create an independent card suspension that compensates for angular misalignment along the x-y plane.  
           [0029]    The backplane member has a first surface and a second surface. The backplane housing include at least one longitudinal receiving cavity, matching a respective cavity in the card housing assembly. The receiving cavity has a frontal opening along the first surface of the backplane member and a rear opening along the second surface of the backplane member. A frontal door covers the frontal opening and a rear door covers the rear opening. In a particular embodiment, the doors are spring elements made of a flexible, conductive material and biased towards a closed position. To provide EMI protection, the doors may be electrically connected to ground. In another particular embodiment, the backplane housing comprises two members, one coupling to the first side of the backplane and the second coupling to the second side of the backplane. To provide EMI protection, one of the members may include an electrically conductive material electrically connected to ground.  
           [0030]    The interconnect system also may include one or more optical cables including a bend radius control member for controlling the bend radius of an optical fiber cable. The bend radius control member comprises a deformation resistant heat-shrinked outer jacket wrapped around the optical fiber cable, wherein the heat-shrunk outer jacket has a desired bend radius curvature. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    [0031]FIG. 1 is a side elevation view of an angularly misaligned card and a backplane connector.  
         [0032]    [0032]FIG. 2 is an isometric cut-away view of a first embodiment of a connector system in accordance with the present invention in a coupled card position.  
         [0033]    [0033]FIG. 3 is an isometric view of the connector system illustrated in FIG. 2 in an uncoupled card position.  
         [0034]    [0034]FIG. 4 is an exploded isometric view of the connector system illustrated in FIG. 2.  
         [0035]    [0035]FIG. 5 is an isometric cut-away view of the backplane housing assembly of the connector system illustrated in FIG. 2.  
         [0036]    [0036]FIG. 6 is an isometric view of the card housing assembly of the connector system illustrated in FIG. 2.  
         [0037]    [0037]FIG. 7 is an isometric view of the card-facing face of the housing assembly of the connector system illustrated in FIG. 2.  
         [0038]    [0038]FIG. 8 is a side elevation view of a backplane connection system wherein the connector components are aligned along the axis of the interconnection even though the circuit card is angular with respect to the said axis of interconnection.  
         [0039]    [0039]FIG. 9 is an isometric view of the plug portion of the connection system illustrated in FIG. 4.  
         [0040]    [0040]FIG. 10 is an isometric exploded view of plug illustrated in FIG. 4 showing the plug fully assembled except for the installation of the cover.  
         [0041]    [0041]FIG. 11 is an isometric view of the plug illustrated in FIG. 4 with its cover being installed.  
         [0042]    [0042]FIG. 12 is an isometric view of the plug illustrated in FIG. 4 fully assembled.  
         [0043]    [0043]FIG. 13 is an isometric view of the plug assembly illustrated in FIG. 11 wrapped about a forming fixture.  
         [0044]    [0044]FIG. 14 is an exploded isometric view of a backplane housing assembly. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0045]    [0045]FIGS. 2 and 3 illustrate an embodiment of an optical interconnect system  100  in accordance with the present invention. The optical interconnect system  100  couples a circuit card or daughter card  102  with and through a backplane  104 . The card  102  is a planar substrate, such as a circuit card or daughterboard, which may include optical, optoelectronic, and electronic components. The card  102  may be slideably inserted in a slot defined by card guides  106 . The backplane  104  includes a through opening  108 , a first interior surface  110  and a second, exterior surface  112 .  
         [0046]    The optical interconnect system  100  includes a backplane housing  120  disposed within opening  108 . The backplane housing  120  includes, in the present embodiment, a first portion  122  and a second portion  124 . The first portion  122  includes male locating features  126  that engage with corresponding female features (not shown) on a rear face of the second portion  124 . Locating features three help ensure accurate alignment between the backplane housing portions  122  and  124  during assembly. It should be understood that in alternative embodiments housing portions  122  and  124  do not need to be separate and could be molded as one piece. Splitting off the housing portions  122  and  124 , however, may allow for more freedom in mold core design.  
         [0047]    In the present embodiment, fasteners  128  secure the backplane housing assembly  120  to the backplane  104 . Fasteners  128  include threaded metal inserts inserted through matching bores  130  in the first and second portion  122  and  124  of the backplane housing  120 . Those skilled in the art will readily appreciate that mounting screws are used in conjunction with fasteners  128  and that a variety of fastening mechanisms, adhesives, interference fitting, and other devices known in the art may be used to align and secure the backplane housing assembly  120 .  
         [0048]    The backplane housing assembly  120  defines an array of four receiving cavities  132 . Alternative embodiments may include a single receiving cavity or any other necessary number of cavities to accommodate various optical fiber cable connections. Each one of the cavities  132  includes a front opening  134  and a rear opening  136 . For the purpose of the description of the present invention the terms rear, front, forward or backward are merely illustrative to help describe the depicted embodiments with respect to the figures. The folding front doors  138  are coupled to close the front opening  134  and rear doors  140  are coupled to close rear openings  136 . The front and rear doors  138  and  140  in the present embodiment include flat spring metal members hingedly coupled to the front and rear openings  134  and  136 . The doors  138  and  140  are designed to fold down flat when a plug is inserted into the opening of the receiving cavity  132 . In the present embodiment, the backplane housing assembly  120  comprises molded plastic pieces of a dielectric material that exhibit the structural strength and dimensional stability required to maintain control of the optical fiber&#39;s position. Such materials include, but are not limited to, thermoplastic injection moldable polymers that are filled or unfilled with reinforcement agents, and transfer moldable polymers such as epoxy. The doors  138  and  140  are made of a conductive metal material, such as tempered stainless steel, beryllium/copper alloys or other materials, and are coupled to provide a grounding electrical path. The doors  138  and  140  provide three functions:  
         [0049]    1) to provide a physical barrier to limit ambient contamination from entering the assembled connector housing,  
         [0050]    2.) to absorb and route to ground electric magnetic interference that may otherwise leak through the cavities  132  through the backplane  104 ; and  
         [0051]    3) to provide eye safety from emitted light signals from either end of the backplane.  
         [0052]    The backplane housing assembly  120  may include mating features corresponding to common plugs or ferrules. The dual door design allows for the sealing of the optical connection without the need to include special gated terminations at each connector. The double door arrangement also allows for at least one door to be closed any time a receiving cavity is not filled by both a rear and a front plug. Finally, the use of conductive metal doors retained in a conductive housing assembly  24  allows for the containment and grounding of EMI components, using a relatively simple and elegant design. In embodiments where the user is not concerned with any of the above issues, the use of doors may be optional without effecting the performance and function of the backplane housing assembly  120 .  
         [0053]    Another useful feature of the housing assembly  120  is the use of side latch receiving features  142 . While traditional plug retaining features, such as that in a conventional phone plug, are placed on top of a connector plug and receiving housing, it was found that such an arrangement unnecessarily interfered with the stacking of ribbon flat optical fiber cables. The present invention addresses this problem by placing the latch receiving features along the same plane defined by the optical fiber array in an optical fiber ribbon cable. This allows for vertical stacking of a number of flat ribbon cables in a reduced space.  
         [0054]    The front end of the backplane housing assembly  120  mates with a board housing assembly  150  when the card  102  is slid into the guide slots  106 . The board housing assembly includes a housing member  152 , including hollow protrusions  154  shaped in size to correspond and fit into front openings  134  of the backplane housing assembly  120 . The board housing assembly  150  includes board attachment features  156  having a barbed end  158 . The board attachment features  156  are designed to be inserted through a receiving slot  160  in the planar substrate  102 . While the board attachment features  156  secures the board housing assembly to the board in the transverse and lateral direction, a range of freedom of movement along the longitudinal axis is allowed. The present embodiment, the length of the slot  160  exceeds the width of the alignment feature  156 . Those skilled in the art will be readily aware of additional methods for attaching the board housing assembly  150  to the planar substrate  102 , while allowing freedom of movement in the x direction. Alternative embodiments may include attachment means such as mechanical fasteners, spring clips or the like.  
         [0055]    The protrusions  154  in the present embodiment are hollow and rectangular shaped and are terminated in a truncated pyramid shaped lead  162 . The pyramid shaped lead  162  allow for compensation of certain mating misalignments by directing the board housing assembly protrusions  154  into the receiving cavities  132  of the backplane housing assembly. Furthermore, the protrusions  154  are shaped to provide alignment with respect to the inside walls of receiving cavities  132 . Protrusions  154  also provide an automatic pressure for opening front doors  138  during mating. The inner walls of protrusion  154  define a stepped cavity  164  that provides guidance to a fiber optic ferrule  170  to be seated inside of the stepped cavity  164 . The present embodiment, the stepped cavity  164 , is shaped to receive an industry standard ferrule, such as the MT-Style optical ferrules. Step cavity  164  is designed in such a manner that it comprises a front and a rear rectangular opening  166  and  168 , respectively. The front opening  166  is sized to allow insertion of the ferrule  170  up to an internal flange  172 . A typical MT-style connector includes a ferrule  170  mounted on a stalk of optical fibers  174 , slidably connected to a detente body portion  176 . The ferrule  170  has a limited range of motion x 1  along the longitudinal axis. The stalk of optical fibers  174  is allowed to move with respect to the detente body portion  176 . A spring element located between the ferrule and the detente body portion forward biases the ferrule towards a forward end of the range of motion.  
         [0056]    In the present embodiment, the board housing assembly  150  includes rear openings  168  designed to accept the MT connector, including the detente body portion  176 . The detente body portion  176  is retained against flange  173  while the ferrule  170  is allowed to extend inside of protrusion  154  up to and through the rear opening  168 . The detente member  176  is designed in such a manner that as the member  176  is inserted into the front of the stepped cavity  164 , the spring  178  is compressed between detente member  176  and the ferrule  170 . The ferrule  170  is prevented from travelling freely through the rear opening  168  by a flange  180  formed in the ferrule  170 . The flange  180  is formed to act as a travel stop for the ferrule  170  when flange  180  is engaged with internal flange  172 . The detente member  176  is provided with a latch feature that engages the rear opening  168  of the board housing assembly  150 . Preferably, latching features are provided on both side surfaces of the housing assembly  150  and the detente member  176 . It may be desirable in some instances to remove detente member  176  from the housing assembly, and for these situations, a release feature is provided in the side of the housing. This release feature is cantilevered and allowed to pivot and thereby allowing the release feature to be sprung outwards to release the corresponding latch feature.  
         [0057]    The length of travel of the card  102  along the card guides  106  is selected such that when in the coupled position the board housing assembly  150  exerts spring force on the backplane housing assembly  120 . In a preferred embodiment, the width of the card gap should be greater than 0, preferably greater than the combined travel of the spring biased ferrules (typically 1 to 2 mm) relative to their respective housings.  
         [0058]    The range of motion x 2  of the board housing assembly  150  with respect to the card  102  is sufficient to correct for tolerance errors in the range of movement of the card  102  along the card guides  106 , and to absorb any excessive force imparted by the user when sliding the card before the card is stopped by the backplane housing  120  or by the stop features if present in the card guides  106 . The present invention addresses issues or overcompression by allowing the circuit card&#39;s attached connector components to move relative to the said circuit card. Accordingly, in the coupled position, the board housing assembly  150  is held tightly against the back of the backplane housing assembly  120  and is subject to a constant spring bias provided by spring assembly  184 . The advantage of providing the constant spring bias is to ensure that intimate contact is maintained between the housing assemblies  150  and  120  even in the event that the card  102  is subject to movement during its operation.  
         [0059]    [0059]FIG. 5 illustrates a detailed cutaway view of backplane housing assembly  120  having front and rear doors  138  and  140 . The doors  138  are designed such that when the protrusions  154  of board housing assembly  150  are inserted into the front opening  134 , the pyramid shaped lead  162  of the protrusions  154  forces the front door  138  to fold down. Similarly, when a plug  190  is inserted into rear opening  136 , the insertion of the plug  190  causes rear door  140  to fold down. Doors  138  and  140  are preferably formed of a spring-like material that withstands numerous cycles of being folded to an open position and then returning to a closed position when the plug  190  or protrusion  154  is removed. In instances where EMI protection is a concern, the rear doors  140  and the first portion  124  of the backplane housing may be constructed of a conductive material such as metal. When made of a conductive material, the rear door  140  and the first portion  124  will absorb the majority of any EMI radiation that would otherwise escape through the cavities  132 . The first portion  124  is then electrically coupled to a ground end feature. In alternative embodiment, either the doors  140  or the first portion of the backplane housing  122  may be constructed of a dielectric material, leaving only one conductive element. The remaining conductive portion would then be coupled to ground.  
         [0060]    By providing both a front door  138  and a rear door  140  covering both the front opening  134  and the rear opening  136 , the removal of either plug  190  or the card housing assembly  150  results in the closing of one of the doors, thus alleviating any possible visual safety risk. It should be understood that each door is allowed to function independently of the other. Accordingly, that means that if only one plug  190  is inserted into the rear opening  136 , the rear doors  140  of the remaining receiving cavities  132  will remain closed. To further assure the tight fit of the doors  138  and  140  within the openings  134  and  136 , frame features  144  may be formed on the side walls of the receiving cavities  132  that match the side profile and overlap the side edges of doors  138  and  140 . This further creates a tighter seal to prevent contamination, contain EMI, and prevent light leakage.  
         [0061]    [0061]FIGS. 6 and 7 illustrate the positioning of springs  184  inserted into spring receiving openings  186  and housing assembly  150 . Springs  184  are wire springs having a wire diameter sized such that the wire springs  184  provide a slight pressed fit between the spring, board attachment features  156  and the receiving boards slots  160 . With springs  184  inserted into the spring receiving openings  186 , the board attachment features  156  are prevented from flexing, thereby locking the housing assembly  150  to the card  102 . Referring in particular to FIG. 6, one may appreciate how slots  160  provide passage through card  102  for the board attachment features  156 . The barbed end  158  of the board attachment features  156  is designed as to grip the back side of card  102  thereby securing the housing assembly  150  along the transversed axis to the daughtercard  102 . The slots  160  are sized such that the board housing assembly  150  has a range of movement x 2  along the longitudinal axis on the surface of the card  102 . The combination of the forward bias of the spring assembly  182  and the freedom of movement x 2  of the housing assembly  150  allows to compensate for incorrect tolerances in the alignment of the card  102  with respect to the backplane  104 . The combined force of the springs  184  of spring assembly  182  is selected to be greater than the summation of all opposing spring forces such as those of the independent springs  178  of the individual ferrule assemblies. Otherwise, the combined force of the springs  178  of the ferrule assemblies would push the housing assembly backwards thus preventing the desired coupling between the board housing assembly  150  and the backplane housing assembly  120 . However, as the forward movement of the board housing assembly  150  will be limited by flange  151 , the independent ferrules still retain their range of movement, thus assuring a tight fit on each individual optical cable connection.  
         [0062]    As illustrated in FIGS. 6 and 7 the longitudinal movement of the board housing assembly  150  is controlled by a spring assembly  182 . The term spring refers to a resilient or elastic member, such as a coiled spring, a biasing clip, an elastic band, a compression foam, or other similar devices known in the art. In the present embodiment, the spring assembly  182  includes two spring clips  184  laterally spaced with respect to each other and located generally at the lateral ends of the board housing assembly  150 . The spring assembly  182  serves three functions (a) to exert a forward force along the longitudinal axis on the board housing asse1mbly  150 , thus creating a spring bias between board housing assembly  150  and the board  102  that the board housing assembly  150  is mounted on; and (b) to lock the board latching features  156 , thus preventing the board housing assembly  150  from inadvertently being removed from the board; and (c) to provide compensation for angular misalignment of the card.  
         [0063]    The spring assembly  182  preferably biases the board housing assembly  150  towards the front or mating edge of the daughter card, such that the board housing assembly  150  is forced to move against the resistance of springs  184  when the board housing assembly  150  is moved by an action opposite to that of the normal force of the springs  184 .  
         [0064]    Furthermore, as illustrated in FIG. 8, the placement of the two springs  184  at laterally spaced locations allows for the correction of angular misalignments, thus reducing the pressure and possible damage on the leading edge of the backplane housing assembly  150  and compensating for angular misalignment of the port.  
         [0065]    FIGS.  9 - 11  illustrate the plug assembly  190 . The plug assembly  190  is designed to receive a conventional MT-style connector ferrule and provide connectorization features to match the backplane housing assembly  120 . Those skilled in the art will readily appreciate that the plug assembly may be molded to receive different types of connectors. In alternative embodiments of the present invention, the backplane housing assembly may be shaped to receive directly traditional connector assemblies.  
         [0066]    The plug assembly  190  is comprised of a lower housing member  192  and housing cover  194 . As explained above, a MT style connector assembly includes a ferrule  170 , and a ferrule spring  178 . The MT style connector is used to terminate a multi-fiber ribbon cable  196  that is surrounded by a protective jacket  198 .  
         [0067]    The lower housing component  192  includes a front opening  200  defined by flange surfaces  202 , a receiving well  204 , and a spring-retaining lip  206 . The ferrule  170  has a front portion  171  and a flange  172 . The front portion  171  passes through opening  200 . However, opening  200  is sized such that the flange  172  is too large to pass through opening  200  and the flange  172  rests against the flange surfaces  202 . The end  179  of ferrule spring  178  when positioned properly within lower housing  192 , as seen in FIG. 10, rests within receiving well  204  and is compressed between flange  172  and the spring-retaining lip  206 . The compression of ferrule spring  178  results in a force being exerted against flange  172  and lip  206 , therein spring biasing ferrule  170  forward through opening  200 .  
         [0068]    [0068]FIG. 11 illustrates housing cover  194  positioned for attachment to lower housing  192 . This attachment is facilitated by placing engaging features  208  of housing cover  194  into engaging cavity  210  present in the sidewalls of the lower housing component  192 . As housing cover  194  is rotated in a downward direction, engagement features  208  are trapped within engagement cavity  210 . As the rotation progresses male snap latches  212  are engaged with the respective female latch receiving features  214 , locking lower housing component  192  and housing cover  194  together.  
         [0069]    An opening  216  is provided in lower housing component  192  to provide a path for strength members  218  to pass through. The strength members  218  are generally present in fiber optic cables and are typically attached to the housings of fiber optic connectors to relieve axial stress on the cable&#39;s optical fibers.  
         [0070]    The lower housing component  192  also includes cavities  220  into which posts  222  of the housing cover  194  are inserted during the assembly procedure to provide lateral locking and alignment of the housing cover  194  to the lower housing component  192 .  
         [0071]    [0071]FIG. 12 illustrates plug assembly  190  assembled onto the optical fiber cable  196  with a bend radius control member  230  installed. The bend radius control member  230  for purposes of this illustration is comprised of a shrinkable tubing that has been applied over a rear housing section  232  of plug assembly  190 , the cable&#39;s protective jacket  198 , and the cable&#39;s strength members  218 . The bend radius control member  230  is heated and shrunk into position therein securing cable  196  to the plug  190 .  
         [0072]    [0072]FIG. 13 shows a cable forming device  250  comprising a vertical support  255  fastened to a base plate  254  and one or more forming mandrels  256  that are attached to vertical support  252 . The radius of the mandrels  256  exceeds the critical bend radius for the optical fiber cable  196 . The angles of the mandrels  256  with respect to each other correspond to the expected or desired path for the optical fibber cable  196 .  
         [0073]    To apply the bend radius control member  230 , a shrinkable tubing or jacket  262  is first slid or wrapped over the plug assembly  190  and the optical fiber cable  196 . The term heat-shrinkable jacket or tubing is intended to include tubing, jackets, tapes, wraps or coatings comprising heat-shrinkable materials that may be wrapped around the desired portion of the optical fiber cable. The term heat-shrinkable jacket refers to a material that, when heated, collapses and compresses around the optical fiber cable, and remains in this collapsed shape upon returning to ambient temperature, such as heat-shrinkable plastics.  
         [0074]    The cable  196  and the shrinkable tubing  262  are wrapped about mandrels  256 . The illustrated device  250  produces a dual bend wherein the cable  196  is formed down and left thus creating a compound bend. The shrinkable tubing is then heated to a temperature sufficient to cause the tubing to shrink. In the present embodiment the heat exposure required to collapse the heat-shrinkable material is selected to avoid any detrimental effects to the optical fiber cable, yet to be higher than the normal operating range for the optical fiber cable. Heat sources may include hot air guns, irradiating heat elements, heated mandrels or other suitable heat sources. The heating may be done before placing the optical cable  196  on the mandrels  256  or afterwards. The shrinkable tubing  262  and the cable  196  remain wrapped about mandrels  256  while the tubing is allowed to cool. Once cooled, the cable  196  will assume the desired shape and bend radius. The stiffness of the formed cable may be controlled by the thickness and the durometer of the material from which the shrinkable tubing is formed.  
         [0075]    In certain instances it may be desirable to coat the inner surface of the shrinkable tubing with a heat activated adhesive that forms a bond with the protective jacket of the optical cable  196  and with the rear housing section  232 . The bend radius control member may be applied to any portion of the cable where a bend is expected or desired. Field applications may be performed using a wrapable shrink material and a portable heat source, such as a heat air gun or lamp.  
         [0076]    [0076]FIG. 14 shows a backplane housing assembly  120  according to the present invention including an alternative embodiment of the folding front doors  238  and folding back doors  240 . In this case, the structure of the folding front doors  238  and the folding back doors  240  includes a pair of substantially equally sized biasing members  242 , 244  connected by an elongate hinge plate  246  located between the biasing members  242 , 244  and integrally formed therewith. The general appearance of each of the folding doors  238 , 240  is that of a V-folded planar element including a substantially centrally located hinge plate  246  having biasing members  242 , 244  joined at opposing longitudinal edges of the hinge plate  246  and extending outwardly of the same side of the hinge plate  246 .  
         [0077]    After installation in the housing assembly  120 , the biasing members  242 , 244  of each of the folding doors  238 , 240  provide closure at either the front openings  134  or the rear openings  136  of a pair of adjacent receiving cavities  132 . In the embodiment shown in FIG. 14, installation of the folding doors  238 , 240  requires the placing of a first latch  248  and a second latch  250  adjacent to each of the longitudinal edges of the hinge plate  246 . The latches  248 , 250  engage an upper latch seat  252  and a lower latch seat  254  formed as recesses in the upper and lower faces of an intervening wall  256  between adjacent receiving cavities  132 . With the biasing members  242 , 244  positioned over e.g. openings  134  of an adjacent pair of receiving cavities  132 , the hinge plate  246  being aligned with the intervening wall  256  and latches  248 , 250  positioned to engage the latch seats  252 , 254 , application of pressure to the hinge plate  246  attaches the folding door  240  to the housing assembly  120 . This provides connection of the folding door  240  to the intervening wall  256  by interference-fit between the latches  248 , 250  and the latch seats  252 , 254 . Secure attachment of the hinge plate  246  adjacent to the intervening wall restricts movement of the hinge plate  246  but allows deflection of each biasing member  242 , 244 , independent of the other, during insertion of a plug  190  into a receiving cavity  132  or withdrawal therefrom. Fabrication of biasing members  242 , 244  requires the use of durable material that retains its shape for repeated cycling between a retracted condition, to allow access to a receiving cavity  132  and a closed condition in which a biasing member  242 , 244  fills an opening  134 , 136  and presents a barrier to contaminants such as dirt, dust moisture and the like. Preferably the durable material is a flexible metal, such as a stainless steel alloy, a beryllium/copper alloy or similar springy materials that return substantially to their original shape even after numerous applications of shape altering forces.  
         [0078]    It should be noted that this invention is not limited to the use of shrinkable tubing to provide strain relief and bend radius control; however the use of shrinkable tubing offers an inexpensive solution to an otherwise costly problem.  
         [0079]    Those skilled in the art will appreciate that the present invention may be used when coupling a variety of optical devices and even non-optical devices that require precise alignment. While the present invention has been described with a reference to exemplary preferred embodiments, the invention may be embodied in other specific forms without departing from the spirit of the invention. Accordingly, it should be understood that the embodiments described and illustrated herein are only exemplary and should not be considered as limiting the scope of the present invention. Other variations and modifications may be made in accordance with the spirit and scope of the present invention.