Multi-cladding optical fiber, long-period optical fiber grating written therein and writing method thereof

Abstract

A multi-cladding optical fiber, a long-period optical fiber grating written in the optical fiber, and a writing method thereof are provided. The multi-cladding optical fiber includes a core made of germanium-doped silica (GeO,—SiO,), for guiding light, an inner cladding made of fluorine-doped silica (F—SiO,), the inner cladding having a refractive index smaller than that of the core and surrounding the core, and an outer cladding made of silica, the outer cladding having a refractive index smaller than that of the core and larger than that of the inner cladding and surrounding the inner cladding. Therefore, an optimized optical fiber can be designed. That is to say, an optical fiber having desired characteristics can be fabricated by adjusting at least one of the parameters including the amount of F doped into the inner cladding, the thickness of the inner cladding, the amount of GeO,doped into the core, the composition of SiO,in the outer cladding and the drawing tension of the optical fiber. Also, the refractive index of the core is periodically changed by periodically annealing the multi-cladding optical fiber, so that a stress-released long-period optical fiber grating can be obtained.