Patent ID: 8427739

Claim:
An optical amplifier comprising: an input port to which a plurality of optical signals with various wavelengths are input; an output port from which a plurality of optical signals with various wavelengths are output; an amplification medium which is allocated on an optical path between the input port and the output port and with which an rare-earth element for optical amplification is doped; a light source which supplies the amplification medium with an excitation light; a first monitor which monitors a total power of an optical signal of each wavelength to input into the amplification medium according to a monitor period which is longer than a transient response time of the amplification medium; and a controller which controls the light source so that a power of the excitation light is constant when a monitor value of the first monitor is equal to or smaller than a predetermined threshold value and which controls the light source so that an optical gain in the amplification medium is constant when the monitor value is larger than the predetermined threshold value.