Characteristics of coated long-period fiber grating sensor under refractive index modulation

A rigorous model of refractive index modulation is established with the method of transfer matrix. The non-uniform grating is divided into a number of uniform pieces. The closed-form solutions for each uniform piece are combined by multiplying matrixes associated with the pieces. Firstly, the vector components of the electric field for the HE cladding mode are plotted under sine modulation of the core index, and the coupling constant between the core mode and the cladding mode is analyzed. Further, the transmission spectrums of chirped fiber gratings and the sensitivity to the refractive index of LPFG film sensor were studied The film parameters (refractive index, film thickness) and chirp parameters (chirp coefficient, chirp factor) of the sensor structure have been investigated to optimize the sensor for practical application. It provides an important theoretical guide to the optimization of the design of LPFG sensor.