Abstract:
An electro-optic interconnect for use with optical signals and a probe beam is disclosed. A plurality of first optoelectronic detectors responds to an optical signal and develops a plurality of first electrical signals. A probe laser generates the probe beam. Means are responsive to the plurality of first electrical signals and changes a characteristic of the optical probe beam. A second optoelectronic detector responds to the changed characteristic and develops an output electrical signal representative of the optical signals. Alternatively, the second optoelectronic detector can be eliminated and direct optical signal processing can be implemented. In another aspect, a time compensation network serves to synchronize the probe and the signal beams. The electro-optic interconnect can be configured to add and subtract the optical signals.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application is related to co-pending U.S. patent application Ser. No. 09/587,125, filed Jun. 2, 2000, entitled: “Optical Interconnect Capable Of Performing Addition/Subtraction” by Stephen Palese, now U.S. Pat. No. 6,516,503, assigned to the same assignee as this present invention, and its contents are incorporated herein by reference. 
     Also related to U.S. patent application Ser. No. 09/587,126, filed Jun. 2, 2000, entitled: “Time Compensation Architectures For Controlling Timing Of Optical Signals” by Stephen Palese, now U.S. Pat. No. 6,580,540, assigned to the same assignee as this present invention, and its contents are incorporated herein by reference. 
     Also related to U.S. patent application Ser. No. 09/587,124, filed Jun. 2, 2000, entitled; “Optoelectronic Communication System In Turbulent Medium Having Array of Photodetectors and Time Compensation” by Stephen Palese, now U.S. Pat. No. 6,585,432, assigned to the same assignee as this present invention, and its contents are incorporated herein by reference. 
     Also related to U.S. patent application Ser. No. 09/586,513, filed Jun. 2, 2000, entitled: “Optical Communication System Using Optical Transcription Material” by Stephen Palese, assigned to the same assignee as this present invention, and its contents are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an optoelectronic device, and more particularly to an electro-optic interconnect that can be configured for adding and subtracting optical signals. 
     2. Description of the Prior Art 
     The telecommunications industry is switching rapidly from an electronic system to a hybrid platform which utilizes both electronics and photonics to increase the operating bandwidth of the communication system. Today&#39;s communication systems consist of optical fiber networks, fiber amplifiers, optical diode transmitters, and high speed semiconductor receivers. However, there are problems in that the industry does not have a satisfactory optoelectronic interconnect, especially one that can be useful for directly adding or subtracting optical signals when processing received optical signals. 
     What is needed, therefore, is an electro-optic interconnect device that is capable of adding and/or subtracting optical signals and converting the result into a sum signal. 
     SUMMARY OF THE INVENTION 
     The preceding and other shortcomings of the prior art are addressed and overcome by the present invention that provides generally an electro-optic interconnect. It can be configured with a probe beam for selectively adding or subtracting optical signals. The electro-optic device comprises a plurality of optoelectronic detectors, each being responsive to an optical signal and operative to develop an electrical signal. A probe laser generates an optical probe beam. An electro-optic crystal senses electrical signals and changes the polarization rotation or phase of the optical probe. A second detector is responsive to the changed characteristic of the probe beam and develops an output or resultant signal representative of the sum of the optical signals. The modulator can be adjusted to cause its associated probe beam at different interconnect sites to be subtracted, rather than summed. 
     Alternatively, the second detector can be eliminated and direct optical processing of the probe beam can be implemented. 
     In other aspects, the bias voltage on the photodetectors is reversed in polarity or the electro-optic crystal orientation can be changed to cause subtraction of its associated optical signal. The output signal may be electrical or optical. 
     The foregoing and additional features and advantages of this invention will become apparent from the detailed description and accompanying drawing figures below. In the figures and the written description, numerals indicate the various elements of the invention, like numerals referring to like elements throughout both the drawing figures and the written description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of an electro-optic interconnect used in accordance with the present invention. 
     FIG. 2 is a diagram illustrating another alternative electro-optic interconnect used in accordance with the present invention. 
     FIG. 3 is a diagram illustrating another alternative embodiment of the electro-optic interconnect in accordance with the present invention. 
     FIG. 4 is a diagram of an alternative embodiment of an electro-optic interconnect for use in a velocity matched configuration in accordance with the present invention. 
     FIG. 5 is a diagram illustrating a configuration for providing time compensation for use with a serial reading technique in accordance with the present invention. 
     FIG. 6 is a diagram of another embodiment for providing two dimensional time compensation in accordance with the present invention. 
     FIG. 7 is a diagram of an electro-optic interconnect arranged for adding and subtracting signal beams in accordance with the present invention. 
     FIG. 8 is a diagram of an electro-optic interconnect arranged in an alternative embodiment for adding and subtracting signal beams in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in the block diagram of FIG. 1, the present invention provides an electro-optic interconnect or device, generally designated by the numeral  10 . 
     Referring now to FIG. 1 the electro-optic interconnect  10  comprises a small area high speed photodetector  12  that converts an optical signal into an electrical signal having a magnitude depending upon the amount of light incident on it, and has an intrinsic response time of 5 ps. A bias voltage +V is applied across its electrodes  13  and  15  that are separated by a guardrail  17 . 
     Alternatively, the detector may be a photocathode microchannel, a superconducting detector or any other photoactivated compound device. The device  10  includes two optoelectronic detectors  12 . FIG. 2 shows one of the optoelectronic detectors  12  in detail. An optical fiber  14  is attached to the detector and propagates an optical beam or signal  16  onto the detector  12 . 
     An electro-optic crystal  20  or waveguide extends longitudinally in an orientation orthogonal to the detector  12 . The crystal  20  is formed from an electro-optic material (for example LiTaO 3 , LiNiO 3 , GaAs, or birefringent polymers), and as will be described with the birefringent axis properly oriented with respect to the electric field and an optical probe beam, shown diagrammatically by the arrow  22 . A probe laser  24  generates the probe beam, or reading beam, longitudinally through a single mode optical fiber  26  and the crystal  20 . The probe beam is preferably a continuous wave beam to provide real time signal processing. A microwave stripline  30  formed on a nonconducting substrate  32  extends perpendicularly to and supports the crystal  20  and propagates the electrical signal developed by the photodetector  12  to interact with the optical probe beam  22 . The stripline  30  is terminated in a 50 ohm load  34  to prevent reflections from propagating back down the stripline which would degrade the temporal resolution. The substrate  32  is connected to the ground to prevent noise or extraneous signals from interfering with the electric signal on stripline  30 . 
     More particularly, the electrical signal carried by the stripline  30  generates an electric field shown diagrammatically by the curved arrows  38  that penetrates into the underside of the electro-optic crystal  20  inducing a time dependent polarization rotation or phase change on the probe beam  22 . This change varies with the electric field strength and therefore with the intensity of the light incident on the photodetector. The rotation depends on the crystal axis orientation relative to the electric field  38  created by the stripline signal. 
     With respect to FIG. 2, an array of optoelectronic detectors is shown. The optoelectronic detectors in the array are identical to the photodetector shown in FIG.  1 . As illustrated by the numerals 1-m there are m detectors in the array, each having an end attached to one fiber  14  of the array of optical fibers  52 . Each fiber  14  propagates an optical beam onto one detector  12 . 
     Each signal from the two photodetectors  12  contributes to the total polarization rotation of the probe beam  22 . These additive or subtractive changes are converted to an amplitude modulated signal on the fiber  22  with a λ/2 waveplate  42  and a polarizer  44 . The λ/2 waveplate  42  can be adjusted to either homodyne or heterodyne the response. In heterodyne operation, the waveplate  42  is adjusted to allow some of the unmodulated probe beam  22  to pass through the waveplate. This sets up a local field that mixes with the signal and is used for linear operation. 
     The polarizer  44  converts the polarization rotation induced on the probe beam  22  to a time dependent amplitude modulation. For homodyne operation the waveplate  42  is adjusted and the polarizer  44  is crossed so no light leaks through when there is no voltage developed on a photodetector  12 . For both heterodyne and homodyne operation, the polarizer  44  analyzes changes in the phase and polarization of the probe beam  22  produced by the electro-optic effect. A probe high speed photodetector  51  converts the amplitude modulated probe beam into a resultant electrical signal on conductor  52  to an output  54  for application to a signal processor  56 . This represents the sum of the in-phase optical signals  16  projected through the fibers  14 . Each of the signal photodetectors  12  contribute to the total polarization rotation of the probe beam  22 . 
     In operation, optical signals on the optical fiber  14  propagate the optical signal to its optoelectronic detector  12 . The detector  12  converts the optical signal into an electrical signal. The electrical signal is propagated down the stripline  30  to the 50 ohm load  34 . The propagating electrical signal creates an electric field  38  outside the stripline  30 . The electric field penetrates into the surface of the electro-optic crystal  20  and induces a time dependent polarization rotation or phase change on the probe beam  22 . Each detector  12  induces a rotation or phase change corresponding to the received optical signal carried to it by its optical fiber, which are either additive or subtractive. The λ/2 waveplate  42  and polarizer  44  are adjusted to convert the total rotation or phase change to a resultant probe beam into a time dependent modulated optical signal. The probe high speed detector  50  converts this into an electrical signal representative of the optical signals. This is applied to the signal processor  56  which demodulates and further processes the electrical signal as desired. 
     Another technique for selecting whether to configure a photodetector in the summing mode or the subtractive mode is by changing the polarity of the bias voltage on the photodetector. With reference to FIG. 1 a bias voltage +V is applied to the electrodes  13  and  15  of the photodetector. A nonconducting guardrail  17  separates the electrodes. To sum the optical signals the two photodetectors are biased identically. To change photodetector to a subtraction mode, the polarity of the bias voltage is reversed. This may also require that the photodetector be changed from a pnp type to an npn type device. 
     An alternative embodiment of the electro-optic interconnect is illustrated in FIG.  3 . Many of the elements of the interconnect  70  are identical to like parts in the interconnect illustrated in FIG. 2 described above, and accordingly, there have been applied to each part of the interconnect in FIG. 3 a reference numeral corresponding to the reference numeral that was applied to the like part of the interconnect described above and shown in FIG.  2 . 
     The fundamental difference between the interconnect shown in FIG.  3  and the interconnect shown in FIG. 2 is that it comprises a Mach-Zehnder interferometer  72  and thus employs phase modulation. Accordingly, it does not incorporate a λ/2 waveplate or polarizer. Each signal photodetector contributes to the total phase change of the probe beam  22 . The fiber based Mach-Zehnder interferometer  72  comprises a reference leg  74  and an adjustable leg  76  which comprises the path of the probe beam through the electro-optic crystal  20 . As was described, passage of the probe beam through the crystal varied depending on the electric signals produced by the optoelectronic detectors and their striplines. 
     The system and method of this invention can utilize many other types of optical interferometers. The optical interferometer splits light into two separate optical paths and then recombines this light interferometrically to create optical outputs that can present constructive and destructive interference. The wavelength of light and the relative optical path lengths of the two legs in the interferometer determine the particular state of interference that takes place when the light is combined. This state of interference determines if the output, or destructive interference, in which case there is an absence of optical intensity output. When one of the two optical path lengths within the interferometer is made to be adjustable, the state of interference can be continuously varied between the constructive and destructive interference states. The interferometer  72  serves to translate this phase change into a time dependent amplitude modulation which is detected by the probe high speed photodetector  51 . 
     In operation the light on the optoelectronic detector  12  propagated by the optical fibers  14  in turn causes a phase delay in the probe beam. When there is no light on the detector  12  no phase delay occurs. This consequently unbalances the interferometer  72  and provides a signal to the photodetector  51 . 
     An alternative embodiment of the fundamental optoelectronic interconnect  10  is illustrated in FIG.  4  and designated by the numeral  100 . This is known as an electro-optic velocity matched configuration. Many of its elements are identical to like parts in the interconnect  10  in FIGS. 1 and 2 described above, and accordingly, there have been applied to each part of the interconnect in FIG. 4 a reference numeral corresponding to the reference numeral that was applied to the like part of the interconnect described above and shown in FIG.  1 . 
     The fundamental difference between the device shown in FIG.  4  and the structure shown in FIG. 1 is that the electrical signal  102  and the probe beam  22  propagate in the same direction, comprises a velocity matched photodetector  104  that is preferably one that is available from the University of Rochester, and is back illuminated by the optical signal. Note that the signal is transmitted through the optical fiber  14  and the substrate  32  to the detector  104 . This facilitates connection of the optical fiber to the detector. Generally it is desired to have the fiber as close as possible to the detector. It has been found that for ultraviolet wavelengths of the signal beam, back illumination works best, otherwise front illumination is used. 
     In this embodiment, the signal beam is transmitted through the optical fiber  14  and the substrate  32  material to the photodetector  104 . This produces an electrical pulse which propagates down the microwave stripline  30  to the terminator  34 . The propagating electric field  38  penetrates into the electro-optic crystal  20  inducing a time dependent polarization rotation or phase change on the probe beam. In the velocity matched configuration, the electrical signal  102  and the optical probe beam  22  propagate in the same direction leading to smaller transit time broadening (high frequency response) and/or longer effective crystal lengths (increased modulation depth of the probe beam). 
     Two time compensation architectures are illustrated in FIGS. 5 and 6 and will be subsequently described. The time compensated methodology provides a means for correcting optical delays which occur in a serial reading beam geometry due to the time required for the reading optical beam to propagate. 
     This propagation time t=nd/c, where n is the material index of refraction, d is the distance, and c is the speed of light. For a large array this propagation delay can seriously degrade the receiver temporal response. For example, a thousand element array of 15 μm diameter detectors would take 90 ps to read. This is twenty times slower than a typical photodetector temporal response. With a time compensated architecture the arrival of the signal beam at the photodetectors is made to be synchronous (either on a row by row as shown in FIG. 5, or element by element basis as shown in FIG. 6) with the arrival of the reading (probe) beam so that propagation delays do not accumulate. This avoids degrading the bandwidth. This is accomplished by inserting equivalent (compensating) optical delays in a prescribed manner into the signal beams. 
     FIG. 5 shows a one dimensional time compensation system for a serial optoelectronic reading device, generally illustrated by the numeral  200 . Many of the parts of the system  200  are identical in construction to like parts in the interconnect illustrated in FIG. 2 described above, and accordingly, there have been applied to each part of the system in FIG. 5 a like reference numeral. The optical signal arrives at a time compensation element (TCE)  202 . The TCE  202  comprises an optical wedge of the kind that is commercially available from several optical device manufacturers. The optical wedge is configured as an m×i matrix. As the TCE  202  lies in front of the optical fiber array  52 . The optical signal at time t s =t os , transmits through the TCE  202  and propagates to the optoelectronic detector (not shown) at the end of the fiber at time 
     
       
         
           t 
           s 
           =t 
           os 
           +t 
           cm 
           +n 
           f 
           d 
           fmi 
           /c 
         
       
     
     where t cm  is the time delay in row m of the TCE  202 , n f  is the index of refraction of the optical fiber  14 , d fmi  is the length of the optical fiber in row m and column i, and c is the speed of light. 
     If the fibers  14  are the same length and no time compensation element is included, then the probe beam  22  and the signal beam  16  accumulate a temporal error equal to n 1 d 11mi /c which degrades the array&#39;s temporal resolution. The time compensation optical element  202  with t cm =n 1 d m1mi  serves to synchronize the probe beam  22  and the optical signal beam  16  at a point in each row of the array so that the temporal degradation is reduced. 
     Thus, each row is compensated. More particularly, the center element in each row is synchronized with the probe beam  22 . Note the other elements in each row are not synchronized. Thus, this technique has some temporal resolution error associated with it, which would be analogous to time jitter. For a square fiber bundle  52 , the one dimensional time compensation scheme allows the number of fibers to be squared (i.e., 10 fibers becomes 100 fibers) with the same temporal resolution (frequency bandwidth). 
     FIG. 6 shows an element by element time compensation architecture. The optical signal beam arrives at the fiber array  52  at time t s =t os  and propagates to the end of the fiber at time 
     
       
         
           t 
           s 
           =t 
           os 
           +n 
           f 
           d 
           fm 
           /c 
         
       
     
     where n f  is the index of refraction of the fiber  14 , d fm  is the length of fiber m, and c is the speed of light. 
     If the fibers  14  are the same length and no time compensation element is included, then the probe beam  22  and signal beam  16  accumulate a temporal error equal to n 1 d 1m /c which degrades the array&#39;s temporal resolution. As, however, in accordance with this invention the fiber lengths are tailored such that n f (d fm −d f1 )=n 1 d 1m  the probe and signal beam are synchronized at each point in the array. Thus, this embodiment does not degrade the optical reading of the probe beam. 
     With reference to FIG. 7, an optoelectronic interconnect  300  is illustrated. As shown, there are separate optoelectronic arrays, one for addition  302  and one for subtraction  304 . 
     A modulator  18  is included in one arm and provides phase or polarization to the optical signal. Accordingly, the modulation can be adjusted to make the signals in-phase or out-of-phase by 180°. In-phase signals are additive and out-of-phase signals are subtractive. 
     The difference in the function of the arrays  302  and  304  results either from a change in the electro-optic orientation of the crystal  20  relative to the photodetector propagating electric field  38 , a change in the semiconductor structure of the photodetector  12  to allow for an inversion of the bias voltage (from positive voltage to negative voltage), or the insertion of an electro-optic modulator  18  (phase or polarization) before and/or after one of the photodetector arrays  302 . The signal on each individual fiber  14  in the array  52  is either routed to the positive or negative photodetector array with an electro-optic modulator  306  and polarizer  308 . Passive time compensation architectures need to be incorporated into the signal fiber array in order to synchronize the probe  22  and photodetector electric field  38  at each point in the interconnect  300 . Active time or nonsynchronous passive compensation architectures can be implemented to provide additional functional capability (for example, dynamically controlled probe sequences) or environmental corrections. The interconnect may be operable with amplitude or phase/polarization rotation embodiments. 
     FIG. 8 shows an electro-optic arithmetic interconnect  350 . In this embodiment an electro-optic modulator  18  (phase or polarization) is inserted in between each individual photodetector element. These additional modulators can electrodes  352  and  354  along the appropriate crystal axis and applying a voltage across the crystal  20 . These additional modulators  18  can either be resonant or traveling wave devices. The application of voltage across these electrodes  352  and  354  will in effect alter the polarization (phase) of the probe beam  22  so that addition or subtraction can occur at each separate photodetector  12 . Passive time compensation architectures as shown in FIGS. 5 and 6 need to be incorporated into the signal fiber array in order to synchronize the probe and photodetector electric field at each point in the interconnect. Active or nonsynchronous passive time compensation architectures can be implemented to provide additional functional capability (for example, dynamically controlled probe sequences) or environmental corrections. The interconnect may be operable amplitude or phase/polarization rotation embodiments. 
     Accordingly, an electro-optic interconnect that can be configured for adding and subtracting optical signals is provided. 
     Obviously, many modification and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.