OPTICAL TRANSMISSION SYSTEM

PROBLEM TO BE SOLVED: To suppress deterioration of transmission characteristics and control high quality optical transmission, changing flexibly the gain according to the change in light signal power. SOLUTION: An optical signal power discrimination measure 11 discriminates the optical signal power for each gain bandwidth against the transmitted light power. A tone signal setting measure 12 sets tone signals of different frequencies for each gain bandwidth and changes the components of the tone signals according to the optical signal power. An optical transmission measure 13 transmits the optical signal including the tone signal. Optical amplifier 21 is provided for each gain bandwidth and amplify the light at gains according to the control signals. A tone signal extraction measures 22 extract tone signal and create extracted signals. Gain control measures 23 compare the extracted signals and the normal signals and create control signals for variable control of gains so as to eliminate gain difference between each gain band.