Polarization diversity techniques for the use of coherent optical fiber submarine cable systems

Recent research and development work on coherent transmission systems, with emphasis on its application to optical-fiber submarine cable systems, are described. The authors demonstrated the feasibility of a polarization diversity scheme in a laboratory environment as well as in the first sea trial of a coherent frequency-shift-keying (FSK) system. The authors propose a novel external cavity laser module and a polarization coupling/dividing module in order to improve the system performance. The polarization fluctuation speed in optical-fiber submarine cable reaches 50 Hz in the worst case, and the polarization diversity scheme is a simple and very powerful technique under such polarization fluctuation conditions