Method for improving mode instability inhibition capability of doped gain fiber and doped gain fiber

The doped gain fiber comprises a fiber core, and a first cladding, a second cladding,..., and an nth cladding which are coated outside the fiber core from inside to outside, n is greater than or equal to 3, and under the condition that the refractive index of the fiber core and the distribution condition of a gain dopant in the fiber core are fixed, the refractive index of the fiber core is greater than or equal to 3. The refractive index distribution of the first cladding, the second cladding,..., and the nth cladding is designed as follows: the refractive index of the first cladding is lower than the refractive index of the fiber core, and the refractive index of the second cladding is lower than the refractive index of the first cladding; and the refractive index of the nth cladding is lower than that of the (n-1) th cladding, so that the loss of a high-order mode in the doped gain fiber can be effectively increased, and the mode instability suppression capability of the doped gain fiber is improved. Under