MULTIMODE FIBER, OPTICAL AMPLIFIER, AND FIBER LASER

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The purpose of the present invention is to increase the efficiency of amplification by a rare-earth ion added to a multimode fiber while maintaining the beam quality of outgoing light. A multimode fiber (11) in which a rare-earth ion is added to a core and which has a normalized frequency of not less than 2.40 is provided with a filter portion (111) configured by bending some or all of sections of the multimode fiber (11). The filter portion (111) has a minimum diameter (diameter R1) which is set so that : (1) only the modes of LP01, LP11, LP21, and LP02 are propagated, or only the modes of LP01 and LP11 are propagated; and (2) the loss of a highest-order mode among the propagating modes is not more than 0.1 dB/m.