Recent progress in heterodyne/coherent optical fiber communications

The prospects for heterodyna/coherent systems of optical fiber communication replacing the presently used intensity-modulation/direct detection (IM/DD) scheme are assessed. The three main advantages of heterodyne/coherent systems over IM/DD systems is their sensitivity improvement, the possibility of frequency-division multiplexing with very fine carrier separation, and the possibility of direct light amplification in the receiver. The history of research on heterodyne/coherent systems is summarized, as are the technical problems which must be overcome before a practical heterodyne system is realized. Some results of the bit-error rate measurement of a heterodyne-type optical communication system are presented.