PURPOSE: To obtain a large light output and to attain an ultra-long distance large capacity wavelength multiplexing PSK optical fiber communication system by controlling constantly the intensity of a phase or frequency modulated light.

CONSTITUTION: To transmitters T1∼T10, separate signals S1∼S10 are inputted, and the output light is multiplexed in the same propagation direction with a multiplexing device 4. For example, in the transmitter T1, a semiconductor laser 12 is continuously oscillated, the output is given through a light phase modulator 13 to a laser diode saturable light amplifier 14 and the output intensity is constantly corrected. A signal light shaped to a constant intensity regardless of a phase modulation with the amplifier 14 is multiplexed with other channel by the multiplexing device 4 and made incident on an optical fiber 5.