Communication Through Phase-Conjugated Optical Variants

Abstract

An optical transport system configured to transmit at least two phase-conjugated optical variants carrying the same modulated symbols, with the phase-conjugated optical variants in being different from one another in one or more of polarization of light, the time of transmission, spatial localization, optical carrier wavelength, and subcarrier frequency during transmission. The two phase-conjugated optical variants can be generated by a single polarization-diversity transmitter to be orthogonally polarized, and propagate through an optical transmission link with the same wavelength and spatial path. The optical variants are detected and processed at the receiver in a manner that enables coherent summation of the corresponding electrical signals prior to constellation de-mapping. The coherent summation tends to cancel out the deleterious effects of nonlinear distortions imparted on the individual phase-conjugated optical variants in an optical fiber transmission link because said nonlinear distortions tend to be opposite to each other.

What is claimed is: 
     
         1 . An apparatus comprising an optical receiver, the optical receiver comprising:
 a front-end circuit configured to convert at least two phase-conjugated optical variants carrying a same modulated payload symbol into a corresponding plurality of digital electrical signals; and   a processor configured to:
 process the plurality of digital electrical signals to generate a set of complex values representing the same modulated payload symbol; 
 sum the complex values of the set to generate a summed complex value; 
 map the summed complex value onto a constellation; and 
 determine based on the mapped summed complex value a bit-word represented by the same modulated payload symbol. 
   
     
     
         2 . The apparatus of  claim 1  wherein the at least two phase-conjugated optical variants differ from one another in one or more of polarization, time of arrival at the optical receiver, spatial localization, optical carrier wavelength, and subcarrier frequency. 
     
     
         3 . The apparatus of  claim 1  wherein the at least two phase-conjugated optical variants are complex conjugates in the time domain. 
     
     
         4 . The apparatus of  claim 1  wherein the at least two phase-conjugated optical variants are complex conjugates in the frequency domain. 
     
     
         5 . The apparatus of  claim 1  wherein one of the at least two phase-conjugated optical variants includes an optical version of a symbol for transmission. 
     
     
         6 . The apparatus of  claim 1  wherein another of the at least two phase-conjugated optical variants includes a complex conjugate version of the optical version of the symbol for transmission with a constant phase rotation. 
     
     
         7 . The apparatus of  claim 1  wherein the processor is configured to
 undo phase conjugation of the at least two phase-conjugated optical variants, and 
 generate at least two complex values representing the symbol intended for transmission. 
 
     
     
         8 . The apparatus of  claim 1  wherein the at least two phase-conjugated optical variants are orthogonally polarized. 
     
     
         9 . The apparatus of  claim 1  further comprising
 a polarization-diversity transmitter for generating at least two orthogonally-polarized phase-conjugated optical variants. 
 
     
     
         10 . The apparatus of  claim 1  wherein the front-end circuit comprises at least one polarization-diversity optical hybrid and at least one optical local oscillator. 
     
     
         11 . The apparatus of  claim 1  wherein the front-end circuit comprises at least four analog-to-digital convertors (ADCs). 
     
     
         12 . The apparatus of  claim 1  wherein:
 the front-end circuit comprises a wavelength de-multiplexer configured to de-multiplex the at least two phase-conjugated optical variants. 
 
     
     
         13 . The apparatus of  claim 1  wherein:
 the front-end circuit comprises an optical coupler configured to spatially de-multiplex the at least two phase-conjugated optical variants. 
 
     
     
         14 . The apparatus of  claim 1  further comprising
 a medium for conveying the at least two phase-conjugated optical variants, wherein the medium is one or more of single-mode fiber, multi-core-fiber, fiber bundle, and multi-mode fiber. 
 
     
     
         15 . The apparatus of  claim 1  wherein the processor configured to determine the bit-word represented by the same modulated payload symbol is configured to
 determine a FEC-based error correction based on a sequence of mapped constellations for a sequence of same modulated payload symbols. 
 
     
     
         16 . The apparatus of  claim 1 , wherein the processor configured to process the plurality of digital electrical signals to generate the set of complex values representing the same modulated payload symbol is configured to
 perform one or more of time synchronization, channel estimation, channel compensation, frequency estimation, frequency compensation, phase estimation, and phase compensation.   
     
     
         17 . The apparatus of  claim 16 , wherein processing the plurality of digital electrical signals includes use of pilot symbols. 
     
     
         18 . The apparatus of  claim 1 , further comprising:
 an optical transmitter configured to generate a second set of at least two phase-conjugated optical variants in response to a symbol of an input payload data stream, the at least two phase-conjugated optical variants of the second set differing from one another in one or more of polarization, time of transmission, spatial localization, optical carrier wavelength, and subcarrier frequency.   
     
     
         19 . A method of optical communication comprising:
 converting, at an optical receiver, at least two phase-conjugated optical variants carrying a same modulated payload symbol into a corresponding plurality of digital electrical signals;   processing the plurality of digital electrical signals to generate a set of complex values representing the same modulated payload symbol;   summing the complex values of the set to generate a summed complex value;   mapping the summed complex value onto a constellation; and   determining based on the mapped summed complex value a bit-word represented by the same modulated payload symbol.   
     
     
         20 . An apparatus comprising an optical transmitter configured to generate at least two phase-conjugated optical variants in response to a symbol of an input payload data stream, the at least two phase-conjugated optical variants differing from one another in one or more of polarization, time of transmission, spatial localization, optical carrier wavelength, and subcarrier frequency.