Without significant changes in the noise characteristics Relates to a long wavelength band erbium excited by the excitation light source of the 1530nm wavelength band that improves the gain characteristic-doped fiber amplifier, the present invention for this is here unit for exciting the excitation light source of the input stage, 1530nm wavelength band of the optical signal of a long wavelength band type, the excitation comprises including erbium-doped fiber for amplifying the input optical signal in the inverted state with an energy level that by the wavelength division multiplexing coupler, and the excitation light source to combine the light source and the input light signal, the excitation unit said 1530nm where the light source is equal to the input signal light having a wavelength band forward direction here, characterized by using a direction opposite the reverse direction here, any one of two-way or a two-stage amplifier where the structure using the forward and reverse here here together.
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