Abstract:
Various embodiments of the present invention provide optical multi-channel free space interconnects that provide optical channel isolation, thereby reducing crosstalk.

Description:
[0001]     This invention was made with Government support under contract no. DAAH01-98-C-R150 awarded by DARPA and administered by the U.S. Army Aviation and Missile Command. The Government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the invention  
         [0003]     The present invention relates to optical interconnects.  
         [0004]     2. Discussion of the Background  
         [0005]     The coupling of optical signals between components (e.g., circuit boards or other components) is an area of growing interest. Metal interconnections (e.g., metal backplanes) appear to have reached their speed limits, and therefore, optical backplanes and optical interconnect schemes are being considered as the next generation board-to-board interconnect solution. Single channel optical connections have been used for years, but the limit of pulse coded modulation (PCM) through a single optical channel is also reaching a practical limit.  
         [0006]     To solve this problem, multiple parallel optical “paths” or “channels” are used. By paralleling paths, one can achieve higher data rates. The greater the number of paths the greater the overall throughput. In some applications, arrays of low cost lasers (e.g., a vertical cavity surface emitting laser (VCSEL)) are used for transmitting the optical signals and low cost photodiode arrays (e.g., GaAs photodiodes) are used for detecting the optical signals.  
         [0007]     Problems exist when one is trying to interface the transmitting and detecting arrays. For example, the lasers used for transmission may emit light of 0.85 micrometers wavelength into a twenty-degree cone. The large cone angle creates cross-talk problems when attempting to couple each individual laser to an individual detector. Further, the lasers are usually situated on a recessed horizontal surface, thus emitting light into cones with vertical axes. This arrangement creates logistic problems because the light must be moved up (from a typical board) about an inch and a half followed by about two inches horizontally to an adjacent board, then down an inch and a half to the upward looking detectors.  
         [0008]     What is desired is an optical multi-channel interconnect that provides maximum optical isolation of adjacent channels (i.e., minimum crosstalk) while also providing minimal optical signal power loss.  
       SUMMARY OF THE INVENTION  
       [0009]     Various embodiments of the present invention provide optical multi-channel free space interconnects that provide a significant degree of optical channel isolation, thereby reducing crosstalk.  
         [0010]     An optical multi-channel free space interconnect according to one particular embodiment of the present invention includes: a first transparent block of material positioned in front of a transmitter array and having a first side and a second side perpendicular with the first side, wherein light transmitted from each transmitter of the transmitter array enters through the first side and exits through the second side; a second transparent block of material positioned in front of the transmitter array and between the transmitter array and the first side of the first block so that the light transmitted from each transmitter of the array passes through the second block before entering the first block; a coupling lens positioned adjacent the second side of the first block such that the light exiting the second side passes through the coupling lens; and a collimator positioned adjacent the coupling lens, the coupling lens being positioned between the first block and the collimator, wherein the light passing through the coupling lens also pass though the collimator.  
         [0011]     The above and other features and advantages of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The accompanying drawings, which are incorporated herein and form part of the specification, help illustrate various embodiments of the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use embodiments of the invention.  
         [0013]      FIG. 1  is a schematic of an optical, multi-channel, free-space interconnect  100 , according to one particular embodiment.  
         [0014]      FIG. 2  illustrates a data processing system, according to one embodiment, that utilizes an optical multi-channel free space interconnect. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0015]     A free space interconnect is defined as a non-electrical and non-fiber-optic interconnect for coupling an optical transmitter (e.g., a laser or other optical transmitter) with an optical detector (e.g., a photodiode or other optical detector).  
         [0016]     Referring now to  FIG. 1 ,  FIG. 1  is a schematic of an optical, multi-channel, free-space interconnect  100 , according to one particular embodiment of the invention, for providing a transmission path between an array of transmitters  102  (e.g., 400 or more transmitters) and an array detectors  104  (e.g., 400 or more detectors). In some embodiments, each transmitter is a laser (e.g., VCSE lasers) and each detector is a photodiode.  
         [0017]     As shown in  FIG. 1 , interconnect  100  uses air and bulk optics to conduit optical signals from the transmitter array to the detector array. More specifically, in the embodiment shown in  FIG. 1 , interconnect  100  includes a transmitting half  191  and a receiving half  192 .  
         [0018]     In the embodiment shown, the transmitting half  191  of interconnect  100  includes a transparent block of material  111   a  and another transparent block of material  112   a  for folding the optical signal emitted from the transmitter array  102 . Blocks  111   a  and  112   a  are both positioned in front of the transmitter array  102  so that the light transmitted from each transmitter of the array  102  passes through block  111   a  and into block  112   a . In some embodiments, block of material  111   a  is a block of sapphire and block of material  112   a  is a block of glass (e.g., a prism or other block of glass capable of folding light). Preferably, as shown, block  111   a  is positioned between the transmitter array  102  and block  112   a.    
         [0019]     In one embodiment, as mentioned above, block  112   a  functions to fold the light transmitted from the transmitter array. Accordingly, in some embodiments, as shown, block  112   a  is implemented with a prism. Prism  112   a  includes a first side  151  and a second side  152  that is perpendicular to first side  151 . Side  151  faces towards block  111   a  and is positioned adjacent thereto so that light passing through block  111   a  also passes through side  151 , thereby entering prism  112   a . Preferably, prism  112   a  is constructed such that, on entering the prism  112   a , each light ray converges slightly less and proceeds to a third side  153  of prism  112   a  where the light rays reflect (total internal reflection) off of the third side and then proceed to exit prism through the second side  152 . This is illustrated in  FIG. 1 .  
         [0020]     As also illustrated in  FIG. 1 , a coupling lens  161   a  may be positioned adjacent side  152  of prism  112   a  such that the light rays exiting side  152  pass through lens  161   a . In some embodiments, lens  161   a  is a plano-convex lens with the flat side of the lens  161   a  positioned adjacent side  152  and directly facing side  152 . A collimator  162   a  may be positioned adjacent the convex side of lens  161   a . Collimator  162   a  functions to collimate the light passing through lens- 161   a . Collimator  162   a  may include one or more lenses.  
         [0021]     After the light rays pass through collimator  162   a , the light rays travel through free space (e.g., air) until they reach the detector half  192  of interconnect  100 .  
         [0022]     As shown in  FIG. 1 , detector half  192  includes the same components as transmission half  191 . That is detector half  192  includes a collimator  162   b , a coupling lens  161   b , a transparent block  111   b , and another transparent block  112   b.    
         [0023]     The components of detector half  192  are configured such that the light rays leaving transmission half  191  first pass through collimator  162   b , then pass through lens  161   b , then through block  112   b , then though block  111   b . After passing through block  111   b , the light reaches the detector array  104 , which converts the optical signal into an electrical signal.  
         [0024]     Like lens  161   a , lens  161   b  may be a plano-convex lens, wherein the planar side of the lens faces and is adjacent to block  112   b . Similarly, like block  112   a , block  112   b  functions to fold the transmitted light rays and may be a prism. And, like block  111   a , block  111   b  may be an optical flat made out of sapphire.  
         [0025]     As illustrated in  FIG. 1 , a housing  144  may be employed to house elements  161   a ,  162   a ,  161   b  and  162   b . Additionally, retainers or fasteners  145  may be employed to fasten blocks  112   a  and  112   b  to housing  144 , respectively.  
         [0026]     In one embodiment, each transmitter of transmitter array  102  is a VCSEL and the rays from the VCSELs pass through several thin layers or “windows” (e.g., block  111   a ) in front of the VCSEL. All these windows have plane surfaces so their optical effect is to shift, very slightly upwardly, the apparent location of the VCSELs. The windows are optical flats which shifts an image by an amount equal to t*(n−1)/n, where t is the thickness of the optical flat and n is its refractive index. In some embodiments, the thickness of the optical flats should not exceed about 0.010 inches.  
         [0027]     Embodiments of the present invention account for the fact that the VCSELs are not classical Lambertian light sources sending light over 180 degree angle (a hemisphere), but rather are regularly spaced light sources emitting into 20 degree cones. The design discussed above exploits the fact that ray divergence decreases upon entering a higher refractive index medium. The (relatively) high refractive index of blocks  112   a  reduces the beam spread from the VCSELs by a factor equal to the refractive index of the block material. A refractive index equal to 1.6 reduces the beam spread by one third. The lens  161   a  acts as a field lens, that together with the high index of the block  112   a  material contains the total ray bundle spread, coming from all the VCSELS, to within a circle diameter of slightly more than two and a quarter millimeters at the output block  112   b  face. The VCSEL locations and beam angles, the optical location of the VCSELs relative to the lens, the prism length and refractive index all control the creation of spots on the detector array  104 .  
         [0028]     Referring now to  FIG. 2 ,  FIG. 2  illustrates a data processing system  200 , according to one embodiment, that utilizes an optical multi-channel free space interconnect  202 . Interconnect  202  may be implemented as shown in  FIG. 1  and described above. Data processing system  200  includes transmitter array  102  connected to a first circuit board  211  and detector array  104  connected to a second circuit board  212 . Interconnect  202  functions to couple the transmitter array  102  with the detector array  104  such that the light rays transmitted by array  102  are detected by array  104 .  
         [0029]     While various embodiments/variations of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.