Recent progress in heterodyne/coherent optical-fiber communications

Research and development of heterodyne/coherent optical-fiber communications are being accelerated because of the possibility of 20-30-dB receiver sensitivity improvement. In this paper, after brief descriptions of the history of the R & D and the classification of various techniques, recent achievements in the bit-error rate (BER) measurement of heterodyne/coherent systems are reviewed and compared with theory. The experimental systems described range from simple on-off-keying (PCM-PSK) heterodyne to sophisticated phase-shift keying (PCM-PSK) heterodyne or homodyne systems.