Optical amplifier

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PROBLEM TO BE SOLVED : To easily adjust the gain and thereby prevent change of dependence on wavelength of the gain due to the change of gain by isolating and combining the feedback beam and signal beam of the wavelength band in the course of the rare earth doped optical fiber and then attenuating the isolated light beam with an optical attenuator. SOLUTION : One rare earth doped optical fiber is divided into a couple of rare earth doped optical fibers 1a, 1b which are excited respectively by a pumping light sources 2a, 2b. The amplified signal beam and oscillated beam are in different wavelengths in the band and the signal beam and oscillated beam are isolated to pass different optical paths using optical branching units 9a, 9b between the two amplifying stages. An optical attenuator 8 and a variable optical attenuator 10 are provided respectively in the optical path of oscillated beam and signal beam. In the range where the internal oscillation is maintained, reflection coefficient of optical fiber grating filters 5a, 5b and oscillated beam fix the gain of amplifying medium with amount of attenuation of optical attenutator 8. Meanwhile, the signal beam can adjust the gain without changing depending on wavelength by changing attenuation amount of the variable optical attenuator 10.