Patent ID: 7929201

Claim:
An optical amplifier, comprising: an input port inputting signal light; an output port outputting the signal light; a plurality of amplifying parts including an optical amplification medium amplifying the signal light and a pumping light source generating pump light for pumping the optical amplification medium, respectively, the amplifying parts serially connected to each other between the input port and the output port, the plurality of amplifying parts including a first amplifying part which is arranged by the input port and another amplifying parts, the plurality of amplifying parts having a ratio of the pump light power of the first amplifying part to the pump light power of the another amplifying parts; an optical attenuator optically connected between the amplifying parts, the optical attenuator variably attenuating the signal light from upstream of the optical amplifier; a gain controller controlling pump light power of each pump source of the amplifying parts in order to maintain a sum of generating gains of the amplifying parts; and an optical attenuator controller controlling attenuation of the optical attenuator in order to maintain the sum of generating gains of the amplifying parts at a gain set value instructed externally in accordance with a state of the signal light inputted into the input port; wherein the gain controller determines an upper level of a multi pass interference (MPI) between the input port and the output port and a lower level of a noise figure (NF) of deterioration of each of the amplifying parts by amplifying characteristics of the optical amplifier, and controls the pump light power of the pump sources, respectively, in such a way that the ratio decreases in accordance with the gain set value increasing and the ratio has a value between the upper level and the lower level.