MULTICLAD OPTICAL FIBER, OPTICAL FIBER MODULE, FIBER LASER, AND FIBER AMPLIFIER

摘要

The multi-cladding optical fiber of the present invention is provided with a core with an average refractive index n1 that guides a signal light and a cladding composed of an inner cladding with an average refractive index n2 that is formed on the periphery of the core, an intermediate cladding with an average refractive index n3 that is formed on the periphery of the inner cladding, and an outer cladding with an average refractive index n4 that is formed on the periphery of the intermediate cladding to confine an excitation light to the inner side of the intermediate cladding, characterized by the refractive indices n1 to n4 satisfying the relationship of n1 > n2 > n3 > n4; there being two or more axisymmetric modes in the core at a wavelength of the signal light; the two or more axisymmetric modes being composed of 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 an inner cladding mode, so that only the fundamental mode substantially propagates through the core.