Abstract: The AM/FM converter of the present invention comprises an optical frequency modulation section which receives multi-channel AM video signals and outputs an optical-frequency-modulated optical signal beam; and an optical heterodyne detection section comprising an optical frequency oscillator composed of a semiconductor local laser which outputs a local oscillator optical beam having an optical frequency separated by a designated intermediate frequency, an optical coupler, and a photoelectric converter composed of a photodiode, which receives the optical signal beam and outputs an electrical batched FM modulation signal which is frequency-modulated and frequency-converted to the intermediate frequency. By means of this configuration, the invention offers an AM/FM converter wherein the frequency band is made considerably wider.

Abstract: In a transmitter side in an optical transmission system, an input binary signal is converted into a duobinary signal, and the duobinary signal is applied to an optical modulation device which provides an optical intensity modulation signal, wherein the optical intensity for a center value of the duobinary signal is a minimum, the optical intensity for the other two values of the duobinary signal is a maximum, and an optical phase for those two values is opposite to each other. In a receiver side, simple direct detection is carried out for receiving optical signal through an optical transmission line to provide a demodulated binary signal. Thus, an original binary signal is recovered without a duobinary decoder and receiver sensitivity degradation.