Optical amplifier

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PROBLEM TO BE SOLVED : To improve pumping efficiency by providing an optical fiber grating for combining a waveguide mode contributing to pumping rare earth metals with a waveguide mode not contributing to pumping rare earth metals out of a plurality of waveguide modes of pumping light entering an optical amplifier. SOLUTION : Periodic perturbations are formed in an optical fiber constituting an optical amplifier 20, whereby the optical fiber is provided with a grating function. The period Λ of perturbations formed in the amplifier 20 is set to a value such that (B0-Bn)=2π/A1 is satisfied if the sound propagation constant of the 0th-order waveguide mode contributing to pumping rare earth metals is denoted as B0, and the sound propagation constant of a predetermined higher- order waveguide mode other than the 0th-order mode is denoted as Bn out of a plurality of waveguide modes present in pumping light entering the amplifier 20. As a result, the predetermined higher-order mode can be combined with the 0th-order mode, whereby a portion of the optical power of the pumping light, which is used for pumping dopants can be increased, and thus the pumping efficiency of the amplifier 20 is improved.