OPTICAL FIBER AMPLIFIER

PROBLEM TO BE SOLVED: To reduce the length of an optical amplification optical fiber by a method wherein an optical fiber amplifier is provided with a first multiplexer, which is made to couple with pumping light having a prescribed wavelength through an optical path, and a second multiplexer, which separates the pumping light to output the separated pumping lights, and the outputs of the separated pumping lights are made to feedback-input in the optical fiber. SOLUTION: Pumping light remains in emitted light, which is outputted from an optical amplification optical fiber 28, but the remained pumping light is made wavelength division by a third multiplexer 30, is outputted through a seventh optical path 32 and is coupled with a second multiplexer 26 through feedback loops, which are respectively constituted of the path 32 and an eight optical path 34. The pumping light, which is fed back through the feedback loops 32 and 34, is refed to the optical fiber 28 by making the pumping light couple with a fourth ray path 27. That is, the pumping light, which is outputted from a laser diode 22, passes through the fiber 28 through the loops 32 and 34 extending over at least two times. Accordingly, the length of the fiber 28 can be significantly reduced.