The bending and temperature characteristics of long period gratings written in elliptical core step-index fibre

Abstract

A fibre long period grating (LPG) is an axially periodic refractive index variation inscribed in the core of a photosensitive single-mode optical fibre by ultra-violet irradiation, which couples light from the core of the fibre into the fibre cladding modes at discrete wavelengths. The index modulation change within the core of a single mode optical fibre is approximately 10-4 and has a period typically between 100-600 μm. The index modulation produces a set of attenuation bands seen in the transmission spectrum of the optical fibre core. The study of the LPG attenuation bands has yielded many potential applications in the field of sensing through their sensitivities to strain (ε), temperature (T), the refractive index of the surrounding medium (ns) and bending. The LPG's sensitivity to these parameters can manifest itself in two ways. Firstly the central wavelength of the attenuation band can shift in the spectral domain, which will be referred to as spectral sensitivity and secondly a change in the spectral transmission profile of the attenuation band may occur. The spectral shift of the attenuation band arises from the phase matching condition of the LPG1.