
3
Coherent optical communication
systems
Kazuro Kikuchi
Department of Frontier Informatics, University of Tokyo, Kashiwa,
Chiba, Japan
3.1 INTRODUCTION
3.1.1 Coherent Optical Communi cations Twenty Years Ago
The research and development in optical fiber communication systems started around
the first half of the 1970s. Such systems used intensity modulation of semiconductor
lasers, and the transmitted optical signal intensity was detected by a photodiode,
which was regarded as a square-law detector. This combination of the transmitter and
receiver is called the intensity modulation/direct detection (IMDD) scheme, which
has been commonly employed in curre ...