AMPLIFYING OPTICAL FIBER, AND OPTICAL FIBER AMPLIFIER AND OSCILLATOR USING SAME

The invention provides an amplification optical fiber, which can output light with a good beam quality even when a higher-order mode is excited, and an optical fiber amplifier using the amplification optical fiber. An amplification optical fiber 50 has a core 51 and a clad 52 covering the core 51. The core 51 propagates light with a predetermined wavelength in at least an LP01 mode, and an LP02 mode, and an LP03 mode. When the LP01 mode, the LP02 mode, and the LP03 mode are standardized by power, in at least a part of a region where the intensity of the LP01 mode is larger than at least one of the intensities of the LP02 mode and the LP03 mode, the active element is added to the core 51 at a higher concentration than the central portion of the core.