Design of Palladium-Coated Long-Period Fiber Grating for Hydrogen Sensing

Abstract

We present a detailed numerical analysis that describes the influence of palladium (Pd) layer thickness on the spectral characteristics of long-period fiber gratings (LPFGs) and their response to the uptake of hydrogen. The investigation is carried out with a view of determining an optimal layer thickness to design high-sensitivity LPFG-based hydrogen sensors. Coupled differential equations for a four-layer waveguide structure have been solved using a matrix method considering a layer of Pd with finite thickness on the cladding. Response of higher order cladding modes of the Pd-coated LPFG at turn-around-point and also at mode transition could be computed. It has been shown that if properly designed, the resonant wavelength of a desired mode may shift by about 20 nm for 1% uptake of hydrogen. There is good match between simulations and experiments for LPFGs with coupling to higher order cladding modes.