Abstract

There is a strong interest in combining the advantages of optical amplifiers with coherent optical communications. While optical amplifiers can increase the transmission span between regenerative repeaters, the coherent detection process provides the advantage of better optical filtering by limiting the background noise produced by the spontaneous emission from semiconductor laser amplifiers. This characteristic is especially attractive for densely packed frequency multiplexed systems.¹ Earlier, an amplitude shift keyed (ASK) experiment² with two in-line optical amplifiers showed a 21-dB net gain. Recently, a system demonstration was performed in a record 372-km transmission distance 400-Mbit/s coherent frequency shift keying (FSK) experiment.³ Also, a 100-Mbit/s phase shift keyed (PSK) experiment⁴ with one in-line amplifier achieved an 8-dB net gain. However, in this last experiment, use of an He–Ne laser transmitter imposed limits on wavelength tuning and IF choice.

© 1988 Optical Society of America