US Patent 11114811 Multimode fiber, optical amplifier, and fiber laser

An object is to improve the efficiency of amplification by rare earth ion while maintaining beam quality of output light in a multi-mode fiber doped with rare earth ion. A multi-mode fiber (11) that includes a rare-earth-ion-doped core and that has a normalized frequency of not less than 2.40 includes a filter portion (111) that is formed by bending a partial section of or entirety of the multi-mode fiber (11), the filter portion (111) having a smallest diameter (diameter R1) that is set so that (1) only LP01, LP11, LP21, and LP02 modes propagate or only LP01 and LP11 modes propagate and (2) a loss of a highest-order one of the modes that propagate is not more than 0.1 dB/m.