US Patent 9431787 Amplification optical fiber and fiber laser device using the same

The refractive index of the first core portion 11a is higher than that of a clad 12, and the refractive index of the second core portion 11b is higher than that of the first core portion 11a. When light of the LP01 mode and light of the LP11 mode are standardized by power, in the core 11, an active element that stimulates to emit light of the predetermined wavelength is doped at a higher concentration in at least a part of an area where power of light of the LP01 mode is larger than that of light of the LP11 mode than at least a part of an area where the power of light of the LP11 mode is larger than that of light of the LP01 mode.