Thermotropic ultra-large mode field fiber

The invention discloses a thermotropic ultra-large mode field fiber, and aims to solve the problem of reduced and limited numerical value and aperture of a fiber core and big manufacturing difficulty. The thermotropic ultra-large mode field fiber is composed of the fiber core and a coating layer; the refractive index of the fiber core material is increased along with the temperature increase, and the fiber core includes an inner fiber core and an outer fiber core, and the inner fiber core is located at the center of the outer fiber core, the refractive index of the inner fiber core is less than or equal to the refractive index of the outer fiber core; a gain medium is mixed in the inner fiber core, and the outer fiber core excludes gain medium; the diameter of the inner fiber core is more than or equal to 20 microns, the ratio between the diameter of the inner fiber core and the diameter of the outer fiber core is less than or equal to 50%; the coating layer excludes the gain medium, and includes an inner coating layer and an outer coating layer; the outer coating layer coats the inner coating layer, and the inner coating layer coats the outer fiber core; the refractive index of the inner coating layer is less than that of the outer fiber core; the refractive index of the outer coating layer is less than the refractive index of the inner coating layer. The numerical value and aperture of the inner fiber core are further reduced, the manufacturing difficulty is lowered, and the light filtering pressure of the coating layer is released.