Abstract

Most current optical-fiber systems use intensity modulation and direct detection demodulation. Although impressive performance has been realized with systems employing this modulation/demodulation scheme^1,2 and experiments with a bit-rate x distance product of 35 GHz km have been re- ported,³ significant improvements are potentially possible by using heterodyne detection and modulation schemes such as FSK and PSK.⁴ Such techniques have been common in coaxial cable, satellite, and microwave radio transmission and were investigated for nonguided optical communication systems in the late 1960s.⁵ Recently a renewed interest has developed in heterodyning techniques mainly because of improvements in spectral purity and stability of semiconductor lasers. Not only could heterodyning provide significant improvement in receiver sensitivity, but It would also allow the frequency division multiplexing of several optical carriers with narrow spacing and thus allow a more efficient use of optical fiber transmission capacity.

© 1983 Optical Society of America