Patent ID: 7365902

Claim:
An optical amplifier that multiplexes pump lights having different wavelengths output from a plurality of pump light sources on a wavelength-division multiplexed signal light propagating through a transmission path to provide Raman amplification, comprising: an intensity modulator configured to modulate intensity of a pump light to be output from any one of the pump light sources, at a predetermined frequency; a photoelectric converter configured to convert a part of the wavelength-division multiplexed signal light Raman-amplified by the pump light to electrical signals; a gain modulation signal detector configured to extract a gain modulation signal having a component of the frequency from the electrical signals; and a controller configured to control only the intensity of pump lights that are output from the pump light source, of which intensity is modulated at the frequency to control Raman amplification gain so that an amplitude of the gain modulation signal becomes a predetermined value, wherein the intensities of pump lights output from individual pump light sources are sequentially controlled to obtain a predetermined value of the Raman amplification gain.