Abstract:
A novel optical detection apparatus is disclosed comprising a plurality of photodetectors and a plurality of transimpedance amplifiers wherein the photodetectors and the amplifiers are electrically connected to each other and are located in close proximity to each other, thus allowing the detecting of high frequency optical signals over a large detection area. Further, logical circuitry is disclosed for processing the signals generated from the photodetectors and for determining the strength of incoming light signals on various portions of the detection area.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/714,084, filed on Sep. 2, 2005. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of optical detection apparatuses used in optical communication systems. 
     BACKGROUND OF THE INVENTION 
     In free space optics an optical transceiver sends and receives optical signals from a second transceiver located some distance away. Alignment of the two transceivers is crucial for error free transmission of the signals exchanged between the two transceivers. 
     A photodetector is used in each of the transceivers to convert the optical signals to electrical signals to be processed by the electronic components of the transceiver. However, even if the transceivers are initially aligned properly, optical aberrations due to weather conditions or aging of the optical or mechanical components of the transceivers and misalignment of the transceivers due to mechanical forces from wind or other factors can lead to improper focusing of the incoming beam onto the photodetector, thus leading to errors in the transmission. Also, to detect high-speed optical signals it is necessary to use small low capacitance photodiodes, which further reduces the detection area and makes optical alignment of the receiver and transmitter difficult. 
     In this patent, a novel detection apparatus is introduced that uses a combination of photodetectors and transimpedance amplifiers in close proximity to reduce capacitance and distributed over a wide detection area, thus eliminating the issues related to prior art photodetectors. 
     SUMMARY OF THE INVENTION 
     A novel detection apparatus is disclosed that includes a plurality of photodetectors and a plurality of transimpedance amplifiers. Each photodetector is electrically connected to one transimpedance amplifier, which is located in close proximity to the photodetector. Each photodetector and amplifier combination forms a low capacitance optical detection cell capable of detecting high-speed optical signals. The presence of multiple such cells in the apparatus of this invention provides a wide detection area for incoming optical signals. Further, the apparatus of this invention includes logical circuitry to process the signals generated from the photodetectors and to determine the strength of incoming light signals on various portions of the detection area. 
    
    
     
       LIST OF FIGURES 
         FIG. 1   a  shows one embodiment of the receive optics of the transceiver of this invention. 
         FIG. 1   a  shows one embodiment of the transmit optics of the transceiver of this invention. 
         FIG. 2  shows one embodiment of the photodetector of this invention. 
         FIG. 3  shows one embodiment of the electronic components of the transceiver of this invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1   a  shows one embodiment of the receive optics of the transceiver of this invention. It includes a lens  22 , such as a plano-convex lens from Thorlabs, attached to the front end of a tube  6 . A single planoconvex lens or a system of lenses  23  (plano-convex) and  24  (ball lens), along with lens  22  (plano-convex) focus an incoming laser beam  21  onto a photodetector  25 , as described later in this invention. Several lens vendors offer lenses that can be used in this invention, such as those from Thorlabs or Edmund Optics. The photodetector  25  is attached to a motorized X-Y-Z stage  26 , such as a set of three MX80L Parker Daedal stages in an X-Y-Z configuration with an associated driver such as a ViX driver. A band-pass optical filter  29 , such as an Edmund Optics DWDM dielectric bandpass filter, is also used in the optical path to allow only predetermined optical wavelengths to pass through. 
       FIG. 1   b  shows one embodiment of the transmit optics of the transceiver of this invention. It includes an optical fiber  35  entering one end of tube  7 . A system of lenses  34 ,  33  (both plano-convex lenses) and  32  (ball lens), expand the laser beam carried by the fiber and emit it into free space  31 . 
       FIG. 2  shows one embodiment of the photodetector  25  of  FIG. 1   a  of this invention. It includes a plurality of single chip photodiodes,  101 ,  102 ,  103 ,  104 , such as Fermionics FD100 photodiodes, attached to a substrate  109 , such as an alumina ceramic substrate. The photodiodes are connected to a plurality of transimpedance amplifiers (TIA)  105 ,  106 ,  107 , and  108 , such as Maxim MAX 3657 TIAs, by means of wirebonds,  110 , and  111  for photodiode  102 , and transimpedance amplifier  106 , and similarly for the remaining photodiodes and TIAs. Each one of the TIAs has two differential outputs, one plus and one minus, for example  112  for plus and  113  for minus for TIA  106  and one output for the photodiode average current,  114  for TIA  106 . All the plus outputs of all TIAs are connected together by, for example, connecting all the plus wires such as  112  together. Similarly all the minus outputs of all TIAs are connected together by, for example, connecting all the minus wires such as  113  together. Also, connections can be done by wirebonding the TIA outputs to pads on the alumina substrate and then using copper traces in the alumina substrate to connect the outputs together. 
       FIG. 3  shows one embodiment of the electronic components of the transceiver of this invention. The differential outputs  112  and  113  of the photodetector  25  of  FIG. 1   a  is input into a limiting amplifier  207 , and the photodiode average current output  114  that is converted to voltage by means of a resistor  205  is input into an analog to digital converter (ADC)  204 , such a National Semiconductor ADC0801. The output of the ADC  204  is input into a field programmable gate array (FPGA)  209 , such as a Xilinx Virtex FPGA. 
     The FPGA  209  receives the output of the ADC  204 , and generates control signals  27  that drive the servomotors of the X-Y-Z stage  26 , to maximize the photodiode current  114 . The FPGA can either implement a Verilog code, or have an embedded CPU that executes instructions to generate the appropriate motor control signals. There are several Prior Art techniques for controlling a set of servomotors to maximize a feedback variable. 
     Further in  FIG. 3 , the limiting amplifier  207  is connected to a transceiver IC (SERDES)  208 , such as an Intel LXT971A. The SERDES  208  is connected to the FPGA  209 . The FPGA  209  is connected to an Ethernet transceiver IC  210 , such as an Intel LXT971A (note that the same type of IC is used as SERDES and Ethernet transceiver). The Ethernet transceiver IC  210  is connected to an isolation transformer  211 , such as four Pulse PE68515, which in turn is connected to a RJ-45 connector  212 . The SERDES  208  is also connected to a laser driver  213 , such as a Maxim MAX 3668, that drives a single mode laser module  214 , such as a Sumitomo SLT4460. The fiber  35  is connected at one end to the output of the laser module  214  and coupled at the other end to the transmit optics  7 . 
     Numerous other embodiments of the present invention are also possible. For example, the receive and transmit optics can be replaced by a different lens or system of lenses or mirrors (singlets, doublets, parabolic reflectors) that accomplish the function of focusing the incoming laser beam for the receiver, and expanding and collimating the outgoing laser beam. Also, several different types of motors and controllers can be used in place of the servomotors, such as stepper motors and controllers, linear stages, piezoelectric motors etc. Also, manual control of the X-Y-Z stage  26  can also be implemented, by for example attaching micrometers, as is well known to the prior art. 
     Also, in  FIG. 2 , the photodiode array could be in a separate substrate from the TIAs, provided that the connections between the photodiodes and the TIAs do not add significant capacitance to the system, which could be detrimental to its performance. A single chip solution for both the photodiode array and the TIAs could also be implemented. Further, although four photodiode/TIA combinations are shown in  FIG. 2 , any number of photodiodes/TIA combinations could be implemented. Further, the TIAs need not be connected to each other thus providing information on the strength of the incoming signal at the various portions of the photodetector array. That information can be used by the transceiver to adjust the position of the photodetector array, by means of the motors, to maximize the strength of the incoming signal.