COHERENT OPTICAL COMMUNICATION SYSTEM WITH FSK HETERODYNE OR HOMODYNE DETECTION AND LITTLE INFLUENCE BY DISTORTION OF A MODULATED OPTICAL SIGNAL

摘要

A laser (21) is frequency modulated by an electrical modulating signal of a bit rate into a modulated signal of a first and a second modulated frequency. A combination of an optical mixer (27) and an optical detector (31) carries out optical heterodyne or homodyne detection on the modulated optical signal to produce a detected signal having a first and a second frequency component. A component separator (32) is for selecting only one of the components from the detected signal. A detector (33) detects the selected component to produce a demodulated electrical signal representative of the modulating signal. The laser is preferably a semiconductor laser. More preferably, the semiconductor laser is frequency modulated after the modulating signal is converted to a current of an mBnB code sequence. When the bit rate is below and above about 10 Mbit/second, either the modulating signal or the mBnB code sequence current is preferably used in frequency modulating the semiconductor laser after caused to pass through a high-pass and a low-pass filter, respectively.