Patent ID: 8705168

Claim:
An apparatus comprising: an optical amplifier for amplifying an input light by excitation power; a control unit controlling feeding of the excitation power to the optical amplifier; and a signal light monitoring unit monitoring an input state of a signal light input into the optical amplifier, wherein the signal light monitoring unit comprises: an optical branching unit branching a branched light of light input into the optical amplifier in four directions; a polarization component extraction unit extracting four polarization components having mutually different polarization parameters from lights branched in four directions by the optical branching unit; and a determination unit determining input/non-input of the signal light based on the four polarization components extracted by the polarization component extraction unit before outputting a result of the determination to the control unit, wherein the control unit turns on feeding of the excitation power to the optical amplifier when the signal light is being input based on the result of the determination from the determination unit and turns off feeding of the excitation power to the optical amplifier when the signal light is not being input.