Ultimate performance of heterodyne/coherent optical fiber communications

Research and development of heterodyne/homodyne and coherent optical fiber communications have expanded rapidly because of the expectation for repeater separation elongation and superwide-band frequency-division multiplexing. In this paper, recent progress in heterodyne/coherent optical fiber communications is reviewed with emphasis on theoretical and experimental studies pursuing its ultimate performance. The topics discussed are: the recent and future trend in the system design, the transmitter power limit due to fiber nonlinearity and the resulting repeater separation limit, the laser noise problem, the fiber polarization control and polarization diversity, the possibility of narrow frequency-separation frequency-division multiplexing, and finally, a comparison of heterodyne/coherent and photon-counting communications.