Patent ID: 8660396

Claim:
A multi-cladding optical fiber comprising: a core with an average refractive index n 1 that guides a signal light; and a cladding comprising an inner cladding with an average refractive index n 2 that is formed on the periphery of the core, an intermediate cladding with an average refractive index n 3 that is formed on the periphery of the inner cladding, and outer cladding with an average refractive index n 4 that is formed on the periphery of the intermediate cladding to confine a pumping light to the intermediate cladding or the inner cladding: wherein the refractive indices n 1 to n 4 satisfy the relationship of N 1 >n 2 >n 3 >n 4 ; two or more axisymmetric modes exist in the core at a wavelength of the signal light; the two or more axisymmetric modes include a fundamental mode and at least a high-order mode that is an axisymmetric mode of a higher order than the fundamental mode and when bent at a predetermined bending diameter the high-order mode in the core disperses within the inner cladding due to coupling with the inner cladding mode, so that only the fundamental mode substantially propagates in the core through the fiber wherein the outer diameter of the inner cladding is two to five times the outer diameter of the core, and 130 μm or less, wherein the outer diameter core is 25-50 μm; and the relative refractive index difference between the core and the inner cladding is .05 to 0.13%.