Abstract:
Disclosed is a bi-directional optical link and method to facilitate bi-directional optical communications with a single optical fiber. Briefly described, the bi-directional optical link comprises a thin film detector having an upper surface facing a predetermined direction to receive incident light. Also, the link includes a thin film emitter stacked over the upper surface and oriented to direct a beam of light toward the predetermined direction. The thin film detector is relatively wide and flat, where the thin film emitter can be placed on the thin film detector while occluding only a portion of the thin film detector. Thus, the thin film detector can receive incident light from a single optical fiber facing the emitter/detector from the predetermined direction while at the same time emitting a beam of light into the same single optical fiber.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of application Ser. No. 09/384,112, filed Aug. 26, 1999, which claims the benefit of U.S. Provisional Application No. 60/098,049, filed Aug. 26, 1998, and U.S. Provisional Application No. 60/097,946, filed Aug. 26, 1998. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     The U.S. government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of EEC-9402723 awarded by the National Science Foundation of the U.S.  
       TECHNICAL FIELD  
       [0003]     The present invention relates generally to the field of communications, and more particularly, to a system and method for optical bi-directional communication.  
       BACKGROUND OF THE INVENTION  
       [0004]     Current communications systems and networks are becoming faster and more complex, using any one of number of mediums. Communication using these systems and networks is typically bi-directional in that signals are exchanged between two sources as they communicate for various purposes. More recently, fiber optic networks have been employed as the medium for communications. Typically, the optical fibers themselves may be very small and, consequently, coupling various devices to the optical fibers to create a communications link can be difficult and expensive.  
         [0005]     An additional problem with current optical fiber communications systems is that optical fibers are often used for unidirectional communications. This is due in part to the difficulty of physically transmitting and receiving an optical signal on a single optical fiber that may be, for example, a single micron thick. Unfortunately, this results in the need for two optical fiber links to establish bi-directional communications.  
       SUMMARY OF THE INVENTION  
       [0006]     Embodiments of the present invention include a bi-directional optical link and method to facilitate bi-directional optical communications with a single optical fiber. Briefly described, in one embodiment the bi-directional optical link comprises a thin film detector having an upper surface facing a predetermined direction to receive incident light. Also, the link includes a thin film emitter stacked over the upper surface and oriented to direct a beam of light toward the predetermined direction. The thin film detector is relatively wide and flat, where the thin film emitter can be placed on the thin film detector while occluding only a portion of the thin film detector. Thus, the thin film detector can receive incident light from a single optical fiber facing the emitter/detector from the predetermined direction while at the same time emitting a beam of light into the same single optical fiber.  
         [0007]     Embodiments of the present invention can also be viewed as including methods for establishing a bi-directional communications link. In this regard, in one embodiment the method can be broadly summarized by the steps of positioning a thin film detector having an upper surface so as to face a predetermined direction to receive incident light, stacking a thin film emitter over the upper surface, and, orienting the thin film emitter to direct a beam of light toward the predetermined direction.  
         [0008]     Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0009]     The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.  
         [0010]      FIG. 1A  is a drawing of a bi-directional communications link according to an embodiment of the present invention;  
         [0011]      FIG. 1B  is a top view of the bi-directional communications link of  FIG. 1A ;  
         [0012]      FIG. 2A  is a side view drawing of a detector used in the bi-directional communications link of  FIGS. 1A and 1B .  
         [0013]      FIG. 2B  is a bottom view drawing of a detector used in the bi-directional communications link of  FIGS. 1A and 1B ;  
         [0014]      FIG. 3  is a drawing of a bi-directional communications link according to another embodiment of the present invention;  
         [0015]      FIG. 4A  is a drawing of a bi-directional communications link using an intermediate host according to another embodiment of the present invention; and  
         [0016]      FIG. 4B  is a bottom view of the intermediate host assembly of  FIG. 4A . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     With reference to  FIG. 1A , shown is a bi-directional optical link  100  according to an embodiment of the present invention. The bi-directional optical link  100  includes a stacked arrangement of a thin film detector  103  and thin film emitter  106 . The detector  103  is located on a host substrate  109  as shown. The detector  103  is preferably flat in shape with a relatively small thickness. The detector  103  includes an upper surface  113  oriented to receive incident light  116  from a predetermined direction  123 , that is, for example, normal to the upper surface  113 . The incident light  116  propagates, for example, from an optical fiber  119  as shown where the optical fiber  119  has a core  126  and a cladding  129 . The emitter  106  is stacked over the detector  103 . Both the detector  103  and the emitter  106  include electrical contacts from which these devices are driven. Both the detector  103  and the emitter  106  are independently optimized and bonded, for example, to a transceiver circuit (not shown) located on the host substrate  109 .  
         [0018]     The detector  103  and the emitter  106  may be bonded to the transceiver circuit using a variety of materials, including metals, conductive polymers, and conductive epoxies, etc. In addition, either thick or thin bonds may be used to adhere and electrically connect the detector  103  and the emitter  106  to a transceiver or other circuit on the host substrate  109 . These thick or thin bonds may include a circuit, glass, plastic, laminate, polymer, etc.  
         [0019]     During operation of the bi-directional optical link  100 , the emitter  106  generates a beam of light  133  that propagates in the predetermined direction  123  into the core  126  of the optical fiber  119 . Generally, the optical fiber  119  is positioned so as to receive the beam of light  133  and to ensure that the incident light  116  propagates from the optical fiber  119  onto the upper surface  113  of the detector  103 . In other words, the end of the optical fiber  119  is positioned so as to face the detector  103  from the predetermined direction  123 . Both the incident light  116  and the beam of light  133  are modulated accordingly. In this manner, the bi-directional optical link  100  advantageously allows a single optical fiber  119  to be used for bi-directional communications.  
         [0020]     The emitter  106  may be, for example, a vertical cavity surface emitting laser or a light emitting diode (LED), or other suitable light source. The actual component chosen as the emitter  106  depends in part on the capabilities and attributes of the component and the particular application. For example, vertical cavity surface emitting lasers allow higher speed operation than LED&#39;s, but also include higher power consumption. The emitter  106  includes electrical contacts that are bonded to the host substrate  109  using suitable conductive bonding agents such as, for example, metals, conductive polymers, conductive epoxies, or other suitable conductive bonding agents. In particular, the electrical contacts are preferably bonded to counterpart electrical contacts on the host substrate  109 , where the circuit that drives the emitter  106  is also located on the host substrate  109 .  
         [0021]     The leads that run between the emitter  106  are located over the detector  103  in order to reach the emitter  106 . These leads are relatively small in width resulting in minimal occlusion of the detector  103  and may be placed over a transparent insulation layer placed over the detector  103  as will be discussed. Likewise, the detector  103  includes electrical contacts that are bonded to the counterpart contacts on the substrate  109  in a similar manner to the emitter  106 .  
         [0022]     The host substrate  109  may comprise, for example, an actual circuit, glass, plastic, laminate, polymer, or other material, etc. The optical fiber  119  is held into place using suitable means.  
         [0023]     Turning to  FIG. 1B , shown is a top view of the bi-directional optical link  100 . The detector  103  is spherical in shape with a diameter d, for example, that is larger than the diameter c of the core  126 , and, given its relatively small thickness, is in the shape of a disk. The emitter  106  is generally located over the detector  103  at a position approximately near the center of the detector  103 . Interposed on the detector  103  are outlines of the core  126   a  and  126   b  of the optical fiber  119  ( FIG. 1A ) in first and second positions over the detector  103 .  
         [0024]     The first and second positions of the cores  126   a  and  126   b  illustrate a positioning tolerance of the bidirectional optical link  100  that is a significant advantage of the present invention. The core  126   a  is shown substantially centered in the detector  103 , whereas the core  126   b  is off to the side. However, the emitter  106  is still positioned within the periphery of both cores  126   a  and  126   b , thus resulting in the propagation of the beam of light  133  ( FIG. 1A ) into the core  126  of the optical fiber  119 . It is a significant advantage that the optical fiber  119  need only be positioned relative to the detector  103  and the emitter  106  so as to capture the beam of light  133  generated by the emitter  106 , while at the same time, illuminating enough of the detector  103  with the incident light  116  ( FIG. 1A ) so as to be detectable. This is due, in part, to the relatively large diameter d of the detector  103  and the relatively small size of the emitter  106 . In terms of actual measurements, for example, given that the diameter c of the core  126  is approximately 1 micron, and the diameter of emitter  106  is much smaller, the core  126  may be positioned over the detector  103  off center by the positioning tolerance X. Note, however, that the emitter  106  may be any size or shape.  
         [0025]     The size of the emitter  106  is optimized, keeping a couple of competing parameters in mind. On one hand, one wishes to maximize the size of the emitter  106  to ensure that a beam of light  133  of maximum size is generated and coupled into the optical fiber  119 . On the other hand, one wishes to minimize the size of the emitter  106  to reduce the occlusion of the detector  103  by the emitter  106 . Thus, the actual sizes chosen for the emitter  106  should be specified with these competing interests in mind. Specifically, one should determine the size of the emitter  106  in light of the desired strength of the beam of light  133  that is to be transmitted through the optical fiber  119  as well as a desired signal strength from the detector  103  in a worst case position of the optical fiber  119  over the detector  103 .  
         [0026]     With reference to  FIGS. 2A and 2B , shown are side ( FIG. 2A ) and bottom ( FIG. 2B ) views of an inverted metal-semiconductor-metal (MSM) photodetector  103   a  that is preferably employed as the detector  103  ( FIGS. 1A and 1B ). The MSM photodetector  103   a  is described in detail in Jokerst, N. M. et al.,  Thin Film Inverted MSM Photodetectors , IEEE Photonics Technology Letters, Vol. 8, No. 2, (February 1996), that is incorporated herein by reference in its entirety. It is understood, however, that any detector that provides a suitable physical shape and electrical properties may be employed as the detector  103 .  
         [0027]     To provide an overview, the MSM photodetector  103   a  includes a first portion  139  of semiconductor material that generates photogenerated excess carriers when exposed to the incident light  119 . The photodetector  103   a  also includes electrodes  143  and  146  with inter-digitated fingers  149 . When a bias voltage is applied to the inter-digitated fingers, an electric field is formed between the adjacent fingers  149  that causes the photogenerated excess carriers to be swept from the semiconductor material into the fingers, creating a photocurrent that is proportional to the power of the incident light  119 .  
         [0028]     With respect to  FIG. 3 , shown is a bi-directional optical link  100   a  according to another embodiment of the present invention. The bi-directional optical link  100   a  includes a detector  103   a  with electrodes  143 / 146 . The electrodes  143 / 146  are electrically coupled to contacts of a transceiver circuit, for example, located on the host substrate  109  via conductive “bump bonds”  153 , the bump bonds being known to those skilled in the art. In fact, the bump bonds  153  are the mechanism by which the detector  103   a  is originally held to the host substrate  109 . Thereafter, a first insulation layer  156  is placed over the detector  103   a . The materials used to create the first insulation layer  156  are transparent with respect to the incident light  116  so as to allow the incident light  116  to reach the detector  103   a . Such materials are generally known to those skilled in the art.  
         [0029]     Placed on the first insulation layer  156  is a first electrical lead  159  to connect a first contact of the emitter  106  to a corresponding contact on the host substrate  109 . A second insulation layer  163  is located over the first electrical lead  159  and surrounds the emitter  106 . The second insulation layer  163  may be comprised of the same materials as the first insulation layer  156 . A second electrical lead  166  is placed on top of the second insulation layer  163  that electrically connects a second electrical contact of the emitter  106  to a counterpart contact on the host substrate  109 . Thus, the emitter  106  is driven by the transceiver circuit located on the host substrate  109  via the first and second electrical leads  159  and  166 . Likewise, the photocurrent generated by the detector  103  is provided to the transceiver circuit via the electrical connections established by the bump bonds  153 .  
         [0030]     With reference to  FIG. 4A , shown is a bi-directional optical link  100   b  according to another embodiment of the present invention. The bi-directional optical link  100   b  includes a detector  103   a  and an emitter  106  bonded to an intermediate host  169  as shown. The detector  103   a  includes first and second detector leads  173  and  176  that electrically couple the electrodes  143 / 146  of the detector  103   a  to the bump bonds  183  as shown. The emitter  106  includes contacts that are coupled to bump bonds  186  via first and second emitter leads  189  and  193 . Insulation material  196  is located at various points as shown to hold the various components in position. The intermediate host  169  with the detector  103  and the emitter  106  is bonded to the substrate host  109  via the bump bonds  183  and  186  that adhere to appropriate contacts  199  on the substrate host  109 . The intermediate host  169  is comprised of a transparent material to allow both the beam of light  133  and the incident light  116  to travel therethrough.  FIG. 4B  shows a bottom view of the bi-directional optical link  100   b  of  FIG. 4A .  
         [0031]     Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the present invention.