Abstract: In a conventional photo signal amplification transmission system, since control is performed to make constant an output power, total output powers of all wavelengths are made constant when a multi-wavelength transmission is made. An output power of one wavelength is decreased and deviated from an optimum power. To solve the problem, a transmission photo amplifier 3, a relay photo amplifier 4 and a receiving photo amplifier 5 are provided with wavelength number information management sections 11, 12 and 13, respectively. An amplifying means is controlled in accordance with the number of wavelengths of the multi-wavelength signal which is obtained by the wavelength number information management sections. The wavelength number information management sections 11 to 13 multiplex a wavelength number information signal onto an optical fiber cable 6 and renews the signal.

Abstract: As an optical signal to be transmitted, a transmitting station 100 multiplexes a monitoring/control signal SC at a different optical wavelength to the main signal SM, and the optical signal is transmitted via at least two parallel optical transmission lines 10 and 20. A receiving station 200 can process the monitoring/control signal SC by separating it from the main signal MS, which thus need not be converted to an optical signal, and the circuit scale is not increased. In addition, a trouble occurring in either system may be coped with by adopting the optical system which is transmitted via the other system.

Abstract: In an optical transmission system including a plurality of stations, each of the stations includes a monitor/control circuit and a light signal output circuit. When the monitor/control circuit detects no arrival of a first light signal from the upstream-side station due to cutting of an optical cable or the like, a second light signal multiplexed with an alarm light signal of a predetermined wavelength is sent to the upstream-side station. The upstream-side station separates in wavelength the received multiplexed light signal so as to extract the alarm light signal therefrom and interrupts an optical output of the light signal output circuit toward the downstream-side station, i.e. a sender of the alarm light signal.