Optical amplification device and optical transmission line monitoring device

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PROBLEM TO BE SOLVED : To minimize loss of signal light, before the signal light is made incident on a rare earth-doped fiber as an amplifying medium since its increase portion is directly reflected in the noise characteristic (noise figure) deterioration of a light amplifier, because the conventional light amplifying method makes the signal light to be amplified pass through a wavelength-dependent optical coupler, whose loss is relatively large until the signal light reaches the rare earth doped fiber. SOLUTION : Exciting light outputted from an LD for excitation is made incident from the back of a 1st rare earth doped fiber 3 via an optical circulator 1 to reach an optical filter 4 for excitation light reflection. The major part of the excitation light is reflected from the filter 4 to be made incident on the fiber 3 again from the front. Light amplification of low noise can be realized, because the excitation light is made to flow in both directions to increase optical electric field strength in the incident part of the fiber 3, so that a population inversion state can be formed efficiently.