Optical amplifier

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PROBLEM TO BE SOLVED : To compensate the temperature dependence of the gain spectrum by changing the optical attenuation of a variable optical attenuator inserted between amplifier stages according to the EDF temp., the input signal light intensity, the inter-stage part insertion loss and the output signal light intensity. SOLUTION : A signal light passed through an optical isolator 4b is attenuated through a variable optical attenuator (variable optical attenuation means) 9, the optical attenuation of which is variable controlled and is incident on an optical component 10 inserted between a rare earth-added optical fiber 8a constituting a former stage of amplifier and a rare earth-added optical fiber 8b constituting a latter stage of amplifier. An arithmetic control circuit 12 has an optical attenuation cable for varying the attenuation of the variable optical attenuator 9 and is capable of varying the optical attenuation of the variable optical attenuator 9 according to the EDF temp., the input signal light intensity, the insertion loss of an optical component and the output signal light intensity upon receipt of a signal from a temp. sensor or signal light monitors PD3a, 3b.