Optical fiber amplifier

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PROBLEM TO BE SOLVED : To stably output even a low-level light signal by connecting a light attentuator to the output terminal of an excitation light source. SOLUTION : An optical fiber amplifier 1 is provided with a light signal input optical fiber 2 and a light signal output optical fiber 8, first and second isolators 3 and 9, a rare earth element doped optical fiber 4, a branching filter 6, and an excitation light source 5. Also, it is provided with an optical attenuator 7 that is connected to the output terminal of the light source 5. The optical attenuator 7 in a configuration where, for example, excitation light is attenuated by an axis deviation fusion and an attenuation film with improved heat resistance is embedded is used. Then, by supplying excitation light that is outputted from the light source 5 by the optical attenuator 7 as needed and then supplying it to the branching filter 6, the level of a light signal that is emitted to the optical fiber 8 is changed, thus stably outputting a light signal with a small level that can normally be obtained by starting the light source 5 near a threshold.