Rare-earth-doped fiber and an optical fiber laser using the same

A rare-earth-doped fiber includes a center core, made of silica glass into which a rare-earth element is doped; a plurality of outer cores; an inner cladding, disposed around the center core and the outer cores; and an outer cladding, disposed around the inner cladding. The refractive index of the inner cladding is less than that of the inner and outer cores, and the refractive index of the outer cladding is less than that of the inner cladding. At least one of the center core and the outer cores is disposed in an area such that the distance between it and the center of the rare-earth-doped fiber should be greater than 0.71r, where r indicates an outer radius of the inner cladding.