Multi-cladding optical fiber, optical fiber module, fiber laser, and fiber amplifier

Abstract

Provided is a multi-cladding optical fiber which includes: a core with an average refractive index nand a cladding including an inner cladding with an average refractive index nformed on the periphery of the core, an intermediate cladding with an average refractive index nformed on the periphery of the inner cladding, and an outer cladding with an average refractive index nformed on the periphery of the intermediate cladding where n>n>n>nTwo or more axisymmetric modes exist in the core at a wavelength of the signal light; the two or more axisymmetric modes including a fundamental mode and at least a high-order mode. When the fiber is bent at a predetermined bending diameter, the high-order mode in the core disperses within the inner cladding due to coupling with an inner cladding mode, so that only the fundamental mode substantially propagates through the core.