Patent ID: 7760423

Claim:
An optical amplifier for amplifying signal light inputted therein, said optical amplifier comprising: a pumping light source for outputting pumping light; a first rare-earth-element-doped optical fiber for amplifying the signal light propagating therethrough by supply of the pumping light outputted from said pumping light source; an input signal light monitor section for monitoring a power of the signal light inputted into said first rare-earth-element-doped optical fiber; an output signal light monitor section for monitoring a power of the signal light outputted from said first rare-earth-element-doped optical fiber; and a control section for inputting therein respective monitoring results of said input and output signal light monitor sections, and regulating said pumping light source such that a gain determined in accordance with the monitoring results becomes a target gain by power control of the pumping light to be supplied to said first rare-earth-element-doped optical fiber, wherein said first rare-earth-element-doped optical fiber has a cutoff wavelength longer than a pumping light wavelength but shorter than a signal light wavelength and mainly allows a pumping light component in a fundamental mode to propagate therethrough, and wherein the pumping light and signal light are made incident on said first rare-earth-element-doped optical fiber, said first rare-earth-element doped optical fiber having an optical characteristic such that only the pumping light component in the fundamental mode and a signal light component in the fundamental mode propagate through said first rare-earth-element-doped optical fiber in a state where said first rare-earth-element-doped optical fiber extends straight.