OPTICAL AMPLIFIER

PROBLEM TO BE SOLVED : To provide an optical amplifier which uses an amplification medium, doped with common rare-earth element ions and of which gain profile can be held fixed. SOLUTION : The present invention relates to an optical amplifier which uses an amplification medium, doped with a rare-earth element and of which gain profile can be controlled to be fixed. The optical amplifier includes a rare-earth-doped fiber or a rare-earth-doped optical waveguide as an amplification medium in which rare-earth-element ions are doped in its core and/or clad, an excitation means for exciting the amplification medium, an optical resonator which causes laser oscillation at a specified wavelength of an amplified spontaneously emitted light generated in the amplification medium, and a control unit which monitors light signal in the light amplifier, to control the excitation means. The present invention also relates to a method for controlling the gain profile of the optical amplifier to be fixed.