Patent Publication Number: US-2005129407-A1

Title: Imaging lens for multi-channel free-space optical interconnects

Description:
FIELD OF INVENTION  
      This invention relates to multi-channel free-space optic interconnects.  
     DESCRIPTION OF RELATED ART  
      Free-space optical interconnects are intended for systems in which data must be transferred across short distances and there exists an unobstructed line of sight between the transmitter and the receiver. In these systems, an optical fiber is not used as a transport medium to carry the light from one end of the link to the other. Instead, the light is allowed to propagate freely in air as it travels from one device to the next. In links that require large amounts of data to be moved, parallel arrays of lasers and detectors are employed to push more data through the system at the same time. Conventionally, parallel arrays are composed of multiple copies of a single channel solution, with each channel using its own individual coupling optics. This kind of architecture demands that the lasers and detectors be built on a spacing that is large compared to their diameters and economically becomes a poor use of the semiconductor material.  
       FIG. 1  illustrates such a conventional free-space parallel optical interconnect  10 , which uses two identical but independent lens systems. Both channels are constructed as independent links with individual optics assembled in an array. Optical interconnect  10  includes a transmitter  12  having a die  14  with multiple (e.g., two) lasers  16 . Each laser has its own lens  18  to collimate light emitted by the laser into a beam toward a receiver  20 . Receiver  20  includes a die  22  with multiple (e.g., two) detectors  24 . Each detector has its own lens  26  to focus the light beam onto the detector. Lasers  16  and detectors  24  must be manufactured on a large pitch P 1 , which is dictated by the required aperture size of the lenses  18  and  26 . In other words, the pitch of lasers  16  and detectors  24  is forced to have the same pitch as the aperture of the lenses  18  and  26 . Two 10 micron (um) laser apertures are then separated by 250 um, resulting in a large area of expensive and wasted semiconductor real estate. Thus, what is needed is a free-space parallel optical interconnect that addresses the space inefficiencies of the conventional optical interconnect  10 .  
     SUMMARY  
      In one embodiment of the invention, a free-space parallel optical interconnect includes a first module and a second module. The first module includes (1) a first die having an array of light sources each emitting light and (2) a first common lens for directing the light from each light source to the second module. The second module includes (1) a second die having an array of detectors and (2) a second common lens for directing the light from each light source to a corresponding detector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a conventional free-space parallel optical interconnect.  
       FIGS. 2, 3 ,  4 ,  5 ,  6 , and  7  illustrate a free-space parallel optical interconnect and its modules in various embodiments of the invention. 
    
    
      Note that the light rays shown in various figures are for illustrative purposes only and may not be accurate.  
     DETAILED DESCRIPTION  
      In one embodiment of the invention, a free-space parallel optical interconnect uses a single lens to simultaneously couple all laser channels so that the laser channels can be spaced closer together. Instead of spacing the channels by 250 um, as is common for parallel arrays with individual optics, a single, common optic would drive the laser spacing to a separation of, for example, only 50 um. This is a greater than a five (5) time reduction in the semiconductor area for an equal number of lasers. The laser cost, which is typically the dominant cost of a module, is roughly linearly related to the area it occupies on a wafer. Using a single coupling optic for all channels could dramatically reduce the cost of both laser and photodetector components in a module.  
       FIG. 2  illustrates a free-space parallel optical interconnect  100  in one embodiment of the invention. Optical interconnect  100  includes a module  112  (e.g., a transmitter) having a die  114  with an array of light sources  116  (e.g., two lasers  116  as shown in  FIG. 3 ). Both laser channels in the array share a common coupling optic  118 , which encourages the laser spacing to be as small as possible.  
       FIG. 3  illustrates the details of transmitter  112  in one embodiment. Optics  118  is a collimating lens that collimates light from lasers  116  into overlapping beams directed toward a module  120  (e.g., a receiver). Although the communication channels are shown as overlapping in the space between transmitter  112  and receiver  120 , due to the nature of optical imaging, the data channels will eventually be separated and isolated at receiver  120  with negligible amounts of crosstalk. The pitch between lasers  116 , measured from their centers, is selected to be compatible with the pitch of corresponding detectors in receiver  120 . In one embodiment, two 10 um laser apertures are spaced apart by a pitch P 2  of 50 um on die  114 , which provides a real estate savings of over five (5) times compared to conventional die  14 . Lasers  116  can be vertical cavity surface emitting lasers (VCSELs), edge-emitting lasers, or light emitting diodes (LEDs).  
      Referring back to  FIG. 2 , receiver  120  includes a die  122  with an array of detectors  124  (e.g., two detectors  124  as shown in  FIG. 4 ). Both detector channels in the array share a common coupling optic  126 , which again encourages the detector spacing to be as small as possible.  
       FIG. 4  illustrates the details of receiver  120  in one embodiment. Optics  126  is a converging lens that separates the overlapping light beams and focuses each on a corresponding photodetector with negligible amounts of crosstalk. Note that pitch P 2  can vary depending on the size of detectors  124 , which can range from 30 to 80 um. In one embodiment, two detectors  124  are spaced apart by a pitch P 2  of 50 um on die  122 , which provides a real estate savings of over five (5) times compared to conventional die  22 . Detectors  124  can be positive-intrinsic-negative (PIN) photodiodes.  
      Thus, free-space parallel optical interconnect  100  uses a common lens system to provide the simultaneous coupling for both parallel channels. The advantage of this design is that the semiconductor devices can be produced at much higher density, leading to dramatically lower cost components and modules.  
       FIG. 5  illustrates the details of a transceiver  112 A in one embodiment. Transceiver  112 A is similar to transmitter  112  except that die  114  further includes an array of detectors  166 . For clarity, only one laser  116  and one detector  166  are shown. Here, common lens  118  is used to direct light from lasers  116  to another module (e.g., another transceiver) and to direct light from the other module onto corresponding detectors  166 .  
       FIG. 6  illustrates the details of a transceiver  112 B in one embodiment. Transceiver  112 B is similar to transmitter  112  but further includes a die  166 . Die  166  includes an array of detectors  166 . For clarity, only one laser  116  and one detector  166  are shown. Again, common lens  118  is used to direct light from lasers  116  to another module (e.g., another transceiver) and to direct light from the other module onto corresponding detectors  166 .  
       FIG. 7  illustrates the details of a transceiver  112 C in one embodiment. Transceiver  112 C is similar to transmitter  112 B ( FIG. 6 ) but uses a separate common lens  226  for directing light from the other module (e.g., another transceiver) onto corresponding detectors  166 .  
      Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. Although only two channels are illustrated, optical interconnect  100  can include additional channels. Although only certain components of transceiver  112  and receiver  120  are shown in the figures, one in the art understands these modules can contain additional integrated circuits that assist in the operation of optical interconnect  100 , such as serializer/deserializer circuits, driver circuits, error processing circuits, and signal processing circuits. Numerous embodiments are encompassed by the following claims.