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Title: Polarization compensator for optical communications
Claims: What is claimed is: 1. A method of transmitting digital data from a first site to a second site comprising: at the first site, polarization modulating an optical carrier from a polarizing energy source to a selected one of a pair of values in response to an input of a binary signal value from a binary signal producing means; transmitting the modulated carrier from the first site to the second site via an optical path that distorts the polarization state of the carrier in a random manner to introduce cross-talk between the polarizations for the binary signal values; receiving the carrier containing cross-talk at the second site; introducing a differential phase retardation on mutually orthogonal polarization components of the carrier when received at the second site in response to the amplitude of the polarization components of the received wave to remove the cross-talk from the carrier at the second site; and the step of introducing a differential phase retardation including the steps of; coupling the received carrier through an optical differential phase retardation cell, and controlling the cell by a feedback means so that optical waves of substantially only one linear polarization are coupled through it at a particular time instant when the optical carrier received at the second site has a first polarization corresponding to a first binary value. 2. The method of claim 1 wherein the cell is controlled by the feedback means by comparing amplitudes of two linear, orthogonal polarizations coupled through the cell in response to the received carrier, and varying the cell by the feedback means until the compared amplitudes differ from each other by a maximum amount. 3. A digital communication system comprising: an optical carrier source for transmitting polarized optical energy; means for polarization modulating said optical energy derived from said source to a selected one of a pair of values in response to an input of a binary signal value from a binary signal producing means; means for transmitting the modulated optical carrier to a remote site via an optical path; the optical path having a tendency to distort the transmitted polarizations in a random manner causing crosstalk between the polarizations for two binary values; an optical receiver at the remote site for receiving the modulated carrier; said receiver including means for detecting the polarization of the carrier received at the remote site; and said detecting means including; an optical differential phase retardation cell responsive to the received carrier for introducing a differential phase retardation on mutually orthogonal polarization components of the carrier when received at the second site in response to the amplitude of the polarization components of the received wave for substantially eliminating the cross-talk between the two polarization states, means responsive to optical energy derived from the received carrier coupled through the cell for deriving a control signal indicative of the relative amplitudes of the received carrier in the pair of polarizations, and a feedback means responsive to the control signal for controlling the cell by the feedback means so that optical waves derived from the received carrier of substantially only one polarization are coupled through the cell at a particular time instant to indicate a binary value. 4. The system of claim 3 wherein the control means includes means for maximizing differences between the amplitude of the pair of polarizations of the received carrier. 5. The system of claim 4 wherein the maximizing means includes means for applying a dither signal to the cell for superimposing the dither signal on the control signal, means for phase comparing a replica of the dither signal applied to the cell with the dither signal superimposed on the control signal to derive an error signal, and means responsive to the error signal for activating the cell to control the phase retardation of the optical energy passing through the cell. 6. The system of claim 4 wherein the feedback means includes an absolute value circuit responsive to the control signal. 7. A detector responsive to an optical carrier polarization modulated to a selected one of a pair of values in response to an input of a binary signal value from a binary signal producing means comprising: an optical receiver responsive to the optical carrier; the received optical carrier having a polarization state that is randomly distorted by an optical path terminating at the optical receiver relative to the polarization of the carrier as it was initially modulated in response to the binary signal valve causing cross-talk in the received carrier between the binary values; said receiver including means responsive to the optical carrier for substantially eliminating the cross-talk; and said responsive means including; an optical differential phase retardation cell responsive to the received carrier for introducing a differential phase retardation on mutually orthogonal components of the carrier when received at the second site in response to the amplitude of the polarization components of the received wave, means responsive to optical energy derived from the received carrier coupled through the cell for deriving a control signal indicative of the relative amplitude of the received carrier in the pair of polarizations, and a feedback means responsive to the control signal for controlling the cell by the feedback means so that optical waves derived from the received carrier of substantially only one polarization are coupled through the cell at a particular time instant to indicate a binary value. 8. The detector of claim 3 wherein the control means includes means responsive to the control signal for maintaining the cell responsively fixed in response to the control signal for indicating that the amplitude of the received carrier for one polarization is a maximum relative to the amplitude of the received carrier for the other polarization. 9. The detector of claim 7 wherein the control means includes means for maximizing differences between the pair of polarization of the received carrier. 10. The detector of claim 9 wherein the feedback means includes an absolute value circuit responsive to the control signal. 11. The detector of claim 9 wherein the maximizing means includes means for applying a dither signal to the cell for superimposing said dither signal on said control signal, means for phase comparing a replica of the dither signal applied to the cell with the dither signal superimposed on the control signal to derive an error signal, and means responsive to the error signal for activating the cell to control the phase retardation of the optical energy passing through the cell. 12. The detector of claim 11 wherein the means for activating includes means for integrating the error signal. 13. The detector of claim 11 wherein the feedback meand includes an absolute value circuit responsive to the control signal. Other info: Inventors: Fitzmaurice, Michael W. (Gambrills, MD, US) Abshire, James B. (Greeneville, TN, US) Application Number: 463925 Filing Date: 1974-04-24 Publication_date: 1976-07-27 Assignee: The United States of America as represented by the Administrator of the (Washington, DC) Primary Class(es): 398/213 398/184 Other Classes: US Patent Ref:
Other Refs: Primary Examiner: Griffin, Robert L. Assistant Examiner: Ng, Jin F. Attorney: Kempf; Robert F., Marchant; Robert D., Manning; John R. |
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