Amplifying optical fiber, and an optical fiber amplifier and resonator using the same

A even when the high-order mode is excited, the amplifying optical fiber which can output light of excellent beam quality, and to provide an optical fiber amplifier using the same. An amplifying optical fiber 50 includes a core 51, and a clad 52 coating the core 51, the core 51 is at least LP01 mode light of a predetermined wavelength, and, LP02 mode, and, LP03 mode in propagates in the core 51, in the case of normalizing a power of LP01 mode and the LP02 mode and the LP03 mode, LP01 strength mode, LP02 mode and LP03 least one of at least a portion of the strong area than the intensity of mode in the active element is added at a concentration higher than the center portion of the core. .Field