Abstract:
An evanescent optically coupled electronic device including: a backplane wave guide or mother board including a set of parallel carriers that define a first plurality of parallel channels and include a first array of optical fibers having exposed cores in the first plurality of parallel channels; at least one electronic card or daughter board including a high speed optical waveguide bus; a flexible fiber ribbon or film including waveguides made up of individual optical fibers of locally increased refractive index joined by a web of suitable material forming the high speed optical waveguide bus and optically connecting the backplane waveguide and the at least one electronic card with no 90° angle turns; and a mechanism for retaining the first array of optical fibers having exposed cores in abutting and facing evanescent optical contact with the individual optical fibers in the flexible fiber or ribbon.

Description:
This invention was made with Government support under contract DASG60-02-P-0206 awarded by U.S. Army Space and Missile Defense Command. The Government has certain rights in the invention. 

   FIELD OF THE INVENTION 
   The present invention relates to optical interconnects for computing and processing systems and more particularly to an array waveguide evanescent coupler (AWEC) for daed-to-backplane optical connection. 
   BACKGROUND OF THE INVENTION 
   Ongoing research and development efforts in very large scale integrated (VLSI) circuits have led to a dramatic decrease in component size and an increase in overall chip size. The increase in complexity and density of the IC&#39;s is expected to increase the speed and reliability of the systems in which they are used and, at the same time, reduce the amount of power consumed. One major limitation of packaged chips which use electrical interconnects is the relatively long distances required to interconnect devices and circuits on a common substrate or to connect chip packages on different circuit boards. Often, the interconnections use aluminum or polysilicon lines. Ohmic power losses, long delay times escessive wafer space, and complex patterning techniques, are some of the other limitations associated with electrical interconnects. Operating speed, for example, can be limited by external electromagnetic interference (EMI) from connecting lines, which can keep speeds to less than a few hundred MHz for compact systems. This greatly limits its ability for fast computing and image data processing for both military and commercial applications. 
   Personal computers are currently making dramatic advancements. The CPU speed has increased significantly in a short time period. Flexible USB memory key is gradually replacing floppy drives. Compact disk (CD) is advancing to CD-R, CD-RW, DVD, and DVD-RW. However, there is no significant improvement on the computer motherboard because the existing interconnect still relies on electrical means that limit its improvement potential as mentioned above. 
   In comparison to electrical interconnects, optical interconnects offer several advantages, including the ability to achieve high data rate signal transmission, large fanout densities, and the ability to reduce capacitive and inductive loading effects. Because of their high speed and wide bandwidth capabilities, with interconnect parameters independent of interconnection distance, optical interconnects are excellent candidates to replace electrical interconnects in a variety of applications, including those which require high data rate operation (&gt;1 Gb/s), long distance signal propagation (intra-board chip-to-chip, board-to-board (or card-to-card), and system-to-system), low power consumption, and immunity to radiation and EMI. A comparison between optical and electrical interconnects, based on power and speed considerations, has shown that, at data speeds &gt;3 GHz, the switching energy for electrical interconnects increases abruptly, making them unsuitable for practical systems. Crosstalk and EMI effects further degrade their performance, thereby making optical interconnects very attractive for chip-to-chip and card-to-card applications. 
   For card-to-card (sometimes also called board-to-board) optical interconnects, free space interconnect architecture has been extensively investigated. The use of vertical cavity surface emitting laser arrays (VCSELs) and photodetector arrays has been considered a promising approach. Although holographic based interconnection schemes and substrate backplane interconnect schemes have been examined, the preferred schemes use microlens arrays to collimate and deliver the array interconnect beams from a VCSEL array to photodetector arrays located on different circuit cards. This type of card-to-card optical interconnect has its limitations, namely it is unsuitable for interconnecting cards with optical blocking by other cards placed in between. It further suffers from alignment sensitivity since each card, after plug-in to the backplane, could be tilted in its orientation. The adjustment to achieve optical alignment is complicated by the array VCSELs and photodetector arrangement (four-axis alignment). The aligned system suffers further from vibration sensitivity. All these are the major drawbacks of current card-to-card free-space optical interconnect architectures. 
   The guided-wave approach for card-to-card optical interconnection offers excellent interconnect path stability and is suitable for multi-card interconnects. It is seen as the only feasible means to accommodate the large bisection bandwidth of future military and civilian processing systems. Fiber array is promising for longer distance interconnection while local channel waveguide array is suitable for shorter distance multi-card optical interconnections. The major obstacle for card-to-backplane optical interconnects using a backplane waveguide is the requirement for a 90° out of plane turn by the optical waveguide (using a 45° etched waveguide endface (mirror). This turn greatly increases the cost of manufacture and degrades backplane reliability. The out of plane waveguide coupling is not energy efficient in general and thus the systems suffer significant power loss. When some cards are not plugged in (as in the case of a computer), these 90° out of plane turns still consume optical power. This situation is different from that in an electronic backplane that does not consume electrical power when the cards are not plugged in. It is, therefore, desirable to develop an optical coupling technology with the following requirements: 
   Able to tap optical signal power from the backplane waveguide (or fiber) to card waveguide (or fiber) or vise versa when the card is plugged into the backplane. 
   The coupler is not a 90° out of plane coupler with an angle etched mirror. 
   When the card is not plugged in, there is no coupling power loss on the backplane waveguide. In other words, there are no terminating ends on the backplane waveguides near the card plug-in locations. 
   The coupler must be fabricated at low cost and the coupling performance must be reliable. 
   The coupler is easy to handle by any person without special training. Furthermore, it is preferred that the coupler can be operated by one hand since this would be useful for space based applications. 
   OBJECTS OF THE INVENTION 
   It is therefore an object of the present invention to provide an optical coupler that is not a 90° out of plane coupler. 
   It is another object of the present invention to provide an optical coupler system that does not result in coupling power loss on the backplane waveguide when a card is not plugged in. 
   It is yet another object of the present invention to provide a cost effective optical coupler whose production costs are relatively low and whose reliability is high. 
   SUMMARY OF THE INVENTION 
   According to the present invention, there is described a novel array waveguide evanescent coupler (AWEC) for card-to-backplane or other appropriate optical interconnections. Evanescent coupling is similar to the well-known waveguide directional coupling. The coupler comprises: (A) a coupler frame; and B) a pair of opposing, parallel carriers that are housed within the frame and define facing parallel and coaxial channels that receive: 1) backplane waveguide optical fibers or another first array of optical fibers having a exposed cores; and 2) a second array of optical fibers or AWEC ribbon fibers having a exposed cores and retain them in facing evanescent optical contact. A securing mechanism in the form of locking screw(s) or spring(s) in or inserted into the coupler frame provide for the retention of the optical fiber cores of a pair of fiber ribbons or backplane waveguide fiber ribbon cores and AWEC ribbon fiber cores in evanescent optical contact. Optionally, a layer of index matching fluid between facing optical fibers or fiber ribbons may be used. The method for achieving evanescent coupling is also described. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of a conventional prior art two-channel waveguide showing directional coupling between them. 
       FIG. 2  is a schematic, cross-sectional end view of one embodiment of an AWEC device in accordance with the present invention. 
       FIG. 3  is a schematic end view showing one method of fabricating the evanescent coupler of the present invention. 
       FIG. 4  is a schematic enlarged side view of the AWEC coupling of the present invention for card-to-backplane optical interconnection. 
       FIG. 5  shows an alternative embodiment of the optical coupler of the present invention. 
       FIG. 6  is a schematic representation showing the card-to-backplane optical interconnect using the array waveguide evanescent coupler of the present invention. 
       FIG. 7  is a perspective, cross-sectional view of a D-shaped optical fiber of the type useful in the coupler of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a conventional prior art waveguide directional coupler structure  10 . In this conventional configuration, a substrate  12  has mounted therein an input channel  16  and an adjacent output channel  18 . The directional coupling results in the spatial periodic modulation of light intensity as shown by sinusoidal curve  20 . Assume that the input light is in channel waveguide  16  at the point z=0 (see  FIG. 1 ), the light intensities of the propagating mode in the two channel waveguides can be evaluated by the coupled mode theory and are given as follows
   I   1 ( z )= I   0  COS 2 (κ z ),   I   2 ( z )= I   0  sin 2 (κ z ),  (1) 
where I 0 =E 0   2 . κ is the coupling coefficient between the modes in the two waveguides  16  and  18  which depends strongly on the shape of the mode tails in the channel waveguides and the channel separation. κ can be determined by
 
                   κ   =       2   ⁢     h   2     ⁢   p   ⁢           ⁢     ⅇ     -   ps             β   ⁡     (     w   +     2   p       )       ⁢     (       p   2     +     h   2       )           ,           (   2   )               
where
 
                                 p   2     =       ⁢       β   2     -       n   s   2     ⁢     k   0   2           ,                   h   2     =       ⁢         n   g   2     ⁢     k   0   2       -     β   2         ,                 tan   ⁡     (   hw   )       =       ⁢         2   ⁢   p   ⁢           ⁢   h         h   2     -     p   2         .                   (   3   )                 
Here, k 0 =ω/c. w is the channel width, s is the channel separation, β is the waveguide mode propagation constant in the z direction, h and p are respectively the propagation constant and the extinction coefficient in the y direction. n g  and n s  are the refractive indices of channel waveguides and the substrate, respectively.
 
   From Equation (1), it can be seen that the fundamental guided beam intensity does indeed transfer back and forth between the two parallel waveguides as a function of propagation length. The period Λ of the light intensity within one of the waveguides is given by Λ=π/κ. λ is the light wavelength. 
   As used herein the following terms are meant to have the indicated meanings and to refer to the defined structures: “fiber ribbon(s)” is meant to describe and refer to any array of optical fibers, glass or plastic or polymeric, in the form of a ribbon or film including polymer waveguides comprising individual optical fibers or linear regions of locally increased refractive index joined by a web of a suitable material. Thus, while in the description, the terms AWEC waveguide and fiber ribbon are used to describe different, but similar structures they may both be considered “ribbons” for purposes of their use in this application. Additionally, although portions of the description herein refers to “D-shaped optical fibers” it will be readily understood by the skilled artisan that such language is being used only because the most common form of optical fiber currently supplied is in the form of a round structure that most approximates the final shape that can be juxtaposed in evanescently coupled relationship with another similar optical fiber having an exposed core as described below. Thus, it will be readily understood by the skilled artisan that square, rectangular or even ovate fibers can be similarly used in the device of the present invention. Also, as alluded to above, it is clearly intended to include polymer waveguides as well known to the skilled artisan within the term “fiber ribbon(s)”, since such structures inherently and effectively include optically conductive fiber cores of the type necessary for the successful practice of the present invention. 
   As described below and shown in  FIGS. 2 and 4  in our geometry, the two channel waveguides or fiber ribbons are located on two separate carriers  22  and  24 . One is on the AWEC ribbon  48  (see  FIG. 4 ) formed by the combination of ribbon fiber cores  26  in ribbon carrier or substrate  22  while the other is on the backplane waveguide  52  formed by the combination of ribbon fiber cores  28  and ribbon substrate or carrier  24 . Nevertheless, the coupling concept is the same as for the prior art device just described. The input waveguide is the backplane waveguide  52  as shown in  FIGS. 2 and 4  and described in detail below. Z is the effective coupling length, i.e., the useful coupling region length of the AWEC ribbon  48  or the length over which fibers  26  and  28  abut in coupler  40 . The coupling constant κ must be controlled so that I 2  from Equation (1) achieves a reasonable value. The control of coupling constant is done by adjusting the pressure through the locking screws  30  and  31  shown in  FIG. 2  or the spring shown in  FIG. 5 . As described below, a layer of index matching fluid  46  may be added to improve coupling performance but is not necessary. 
   Referring now to  FIGS. 2 and 4  that show, in  FIG. 2  a schematic end view of the coupler of the present invention and in  FIG. 4  a schematic side view of the optical coupler of the present invention, coupler  40  comprises an opposing pair of substrates or carriers  22  and  24  that, when abutted as shown in  FIG. 2 , bring ribbon fiber cores  26  and  28  into intimate and evanescent optical contact. According to a preferred embodiment of the present invention these two ribbon fiber cores are the optical fiber cores of a backplane waveguide and those of a general fiber ribbon for coupling with the backplane waveguide. A frame  42  is provided for mounting locking screws  30  and  31  that apply pressure to a pressure plate  44  to assure uniform application of pressure across carrier  22  and consequently ribbon fiber cores  26  and  28  bringing them into intimate and evanescent contact. An optional thin layer of index matching fluid  46  is shown in  FIG. 2  and described more fully below. As will be apparent to the skilled artisan, locking screws  30  and  31  are preferably threaded into and through frame  42 , but any other suitable structure that allows locking screws  30  and  31  to apply suitable pressure to coupler  40  as shown in  FIG. 2  would be appropriate. 
   As shown most clearly in  FIG. 4 , AWEC ribbon  48  incorporating optical fiber ribbon cores  26  forms the transition and connection with card  50 . In those areas of backplane  52  where optical ribbon fiber cores  28  do not contact or lie in evanescent optical contact with optical ribbon fiber cores  26 , i.e. outside the boundaries of optical coupler  40 , in area  51 , light travelling through ribbon fiber cores  28  is in a non-coupled mode while in those areas where evanescent contact between ribbon fiber cores  26  and  28  occurs, i.e. within the boundaries of optical coupler  40 , light is in a coupled mode. Thus, no optical energy is lost in the system when no coupler  40  is present as with more conventional electronic systems. 
   A key feature of the evanescent coupler  40  according to a preferred embodiment of the device of the present invention is the lateral alignment between the optical ribbon fiber cores  26  and the optical ribbon fiber cores  28 . In experimental uses visual alignment under a microscope can be used. However, in commercial applications, the coupling alignment can be done by pressing both ribbon fiber structures to the coupler  40  side wall  43  and bottom wall  45  (see  FIG. 2 ) and thus the alignment between the optical ribbon fiber cores  26  and the optical ribbon fiber cores  28  is rendered automatic. Of course, coupler  40  could be forced into any corner of frame  42 . 
     FIG. 6  is a schematic depiction of the novel evanescent coupler  40  in its use for multi-card backplane optical interconnects. Both the backplane waveguide  52  and the AWEC ribbon  48  are as described herein. Electronic cards  60  having memory chips  62 , interface chips  64 , dedicated processor chips  68  and other chips  66  as well as a reduced instruction set computer  70  and a slow speed electrical data bus  72  that connects to a slow speed electronic control line connector  74 , for example, are located on a backplane  76  that also incorporates high speed optical waveguide bus  53  formed by using the waveguide ribbon fibers. It should be noted that there are no 90° angle turns on backplane waveguide  52  and thus no terminal loss when a card  60  is not plugged in. 
     FIG. 5  shows an alternative embodiment of the optical coupler  40  of the present invention. According to this embodiment, coupler  40  utilizes a calibrated spring  32  to achieve the required coupling pressure described above. Calibrated spring  32  is located in this embodiment within a frame  78  that incorporates a latching mechanism  80 , a hinge point  82  and a retainer spring  83  that keeps coupler ribbon  48  and backplane waveguide ribbon  52  in proper abutting orientation and location within housing  78 . When latching arrangement  80  is “closed”, i.e. both portions  79  and  81  of latching mechanism  80  are engaged in the conventional fashion coupling pressure is applied as described above in connection with the use of locking screw  30 . The retainer spring  83  presses both coupler ribbons toward the common side wall of the housing  78  to achieve auto alignment of the ribbon fiber cores  26  and  28 . 
   The success of the directional coupling between backplane waveguide ribbon fibers  28  and AWEC ribbon fibers  26  prefers that optical ribbon fiber cores  26  and  28  and in particular the optically coupling portions of such ribbon fiber cores be fabricated from the same material and by the same process to ensure identical guiding effective index. Index matching fluid  46  preferably having a refractive index slightly smaller than that of optical ribbon fiber cores  26  and  28  can be added in the coupling contact region. Such index matching fluid can greatly enhance the backplane waveguide  52  to AWEC ribbon  48  coupling efficiency in a preferred embodiment. The choice of a slightly smaller refractive index for the index matching fluid is to preserve the waveguide mode propagation without causing local waveguide termination. As described below, an index matching fluid having an index of refraction the same as or slightly higher than that of the optical ribbon cores can also be used, but with some loss of coupling energy. Locking screw  30  or spring  32  can control the physical gap between backplane waveguide fiber cores  28  and the AWEC optical ribbon fiber cores  26  and thus control the coupling efficiency. We note here that the coupling efficiency depends on both the coupling region length and the optical ribbon fiber core separation. The control of the locking screw  30  or spring  32  pressure can be very effective in coupling efficiency control. Obviously, evanescent coupler  40  will serve as an effective means for tapping out optical signal from backplane waveguide data bus line  52  and can also insert an optical signal from the plug-in card to the backplane waveguide data bus lines  52 . The connection between AWEC ribbon  48  and interconnect waveguide  53  on card(s)  60  can be done by butt coupling using suitable packaging epoxy. Direct connection of ribbon fibers to the destination on the card is also possible. In this way the butt coupling is avoided. 
   As stated hereinabove, the optical ribbon fiber cores  26  and  28  form an important aspect of the present invention. The ribbon fibers can be assembled using D-shape fibers (see  FIG. 7 ). D-Shape fiber by its name has a D-shape cross-section. Such fiber can be purchased from KVH Industries, 50 Enterprise Center, Middleton, R.I. 02842. Such D-shape fiber&#39;s core may not be, and generally is not, exposed at its flat side. Suitable etching in a diluted hydrofluoric acid solution, for example, can be used to expose the fiber cores for the desired evanescent coupling applications described herein. 
   The large distance from the fiber core to the flat surface, prevents the evanescent coupling between D-shape fibers pressed together on the flat side as shown in the attached Figures. To facilitate the required evanescent coupling, the fiber cladding  75  (see  FIG. 7 ) must be partially removed to fully expose the fiber core  77 . 
   In order to obtain D-shaped fibers of a configuration to achieve the results described herein, fiber structure  26  in  FIG. 7  must be reduced or “flattened” to a point along the line  90 — 90  shown in  FIG. 7 . Such reduction or flattening is achieved using a diluted hydrofluoric acid with 5% concentration, and carefully etching D-shape fiber structure  26 , including cladding  75  to obtain the required D-shaped optical fiber. 
   After the D-shaped fiber etching to expose the fiber core  77 , it is ready for assembly to form the D-shaped fiber based AWEC ribbon  48 . The packaging of D-fiber based AWEC ribbon  48  can be done by laying the flat portion of the D-shaped fibers facing down on a smooth glass surface and applying a silicon V-groove to put D-shaped fibers into array format as shown in  FIG. 3 . Epoxy can be used to fix the D-shaped fibers in position. A piece of hard plastic or ceramic  44  is preferably attached to the back of silicon carrier  22  for stronger support. This can be done on both ends of D-shape fiber ribbon  48 . The D-shape fiber array can also be laid on backplane  52  in a similar manner with its flat face facing upward. At the designated coupling region, the optical ribbon fiber cores  26  and  28  can also be supported by silicon. In this way the fiber-to-fiber separations on the AWEC ribbon  48  and on the backplane waveguide  52  are identical being controlled by carriers  22  and  24  (shown in  FIG. 2 ). 
   Using a D-shaped fiber array prepared as just described, in evanescent coupler  40 , evanescent coupling between two such fiber arrays is achieved. A coupling length is on the order of just a few millimeters can yield significant coupling efficiency. 
   Another method for producing a suitable optical fiber array is by physically polishing or abrading a commercially available fiber ribbon array. Such commercially available fiber ribbon arrays come complete with ferrule ends. By polishing or abrading the fibers contained in such an assembly, a substantially fiber array of the type described herein can be obtained and a pair of such “polished fiber ferruled arrays” placed in facing relationship to obtain coupler  40  of the present invention. 
   The invention mentioned above uses the terms of “evanescent coupling” and “evanescent coupler” to describe the working principle of optical coupling from the pair of coupling fibers or waveguides. The illustrated coupling concept can be further extended to cover any optical coupling phenomenon involving the uses of additional gratings in the coupling region and or other non-evanescent coupling that can transmit optical power from one exposed core to the other. 
   While the invention has been described in the context of a frame that retains a plurality of optical fibers in evanescent contact, it will be readily understood by the skilled artisan that the term “frame” can be interpreted to include any of a vast number of structures capable of retaining a plurality or even a single pair of optical fibers in evanescent or optically coupled contact. Thus, any mechanism that serves to retain at least two optical fibers in contact such that wave energy in the electromagnetic spectrum such as visible, UV and or IR emissions can be transmitted from one fiber to another is contemplated as within the meaning of the term “frame” and such a structure is referred to in the appended claims as a “mechanism” for retaining one or more fibers in contact such that radiation may be transmitted from one to another. 
   Also, while the index matching fluid introduced between abutting optical fibers is preferably of a slightly lower index of refraction than that of the optical fibers for purposes of efficiency, it will be readily understood by the skilled artisan that the use of index matching fluids of the same or greater index of refraction than that of the abutting fibers may be used with some loss of coupling energy and efficiency. Such a loss of coupling energy and efficiency may be tolerable in certain other than ideal circumstances and thus, use of such a lower efficiency is contemplated as within the scope of the appended claims. 
   Finally, although the above description has been provided in the context of the coupling of “a plurality or opposed pairs” of optical fibers, (the most common and desirable context and use) it will be readily apparent to those skilled in the art that the apparatus and methods described herein can be equally effectively used to optically couple a single pair of optical fibers. 
   As the invention has been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the appended claims.