Single-ended long period fiber grating refractive index sensor based on metal-coated surface plasma resonance

Abstract

A novel single-ended reflecting long period fiber grating (LPFG) sensor with thin metal film overlay and the sensing system is described. An all-fiber reflection based surface plasma resonance (SPR) LPFG sensor with three-layer structure (core, cladding and metal) is established experimentally and fabricated with a pulsed CO₂ laser writing system and vacuum evaporation coating system. Different nm-thick thin metal films are deposited on the reflected LPFG sensor for the excitation of surface plasma waves (SPWs) and the characteristics of the reflection resonance spectra of the LPFG sensor for measuring refractive index of fluids are studied. It is found that different thicknesses of different metal films show different measuring sensitivity. Through the comparison of the resonance wavelength obtained in air, water, alcohol, glycerin and their mixture solution, different sensitivities of the reflected SPR-LPFG sensor have been achieved. The proposed SPR-LPFG sensing scheme offers an efficient platform for achieving high performance fiber sensors for the measurement of ambient refractive index.