OPTICAL AMPLIFIER

PROBLEM TO BE SOLVED : To provide an optical amplifier which uses an amplifying medium to which a broadly general rare earth ion is added, and which can keep a gain profile constant. SOLUTION : The optical amplifier has a rare-earth doped optical fiber or a rare-earth doped optical waveguide which has a rare earth ion as an amplifying medium doped to its core and/or to its clad; an exciting means to excite the amplifying medium; and an optical resonator which generates laser oscillation at a plurality of oscillation wavelength of amplified spontaneously emitted light which is generated in the amplifying medium and amplified. Also a control method is provided for controlling a gain profile of the optical amplifier constant.