Novel metal coated long period fiber grating liquid sensor based on dual-peak resonance

Abstract

A novel metal coated long period fiber grating (LPFG) liquid sensor is presented, in which the dual-peak resonance (DPR) combined with the surface plasma resonance is utilized to improve the sensitivity to liquid concentration. Based on the coupled-mode theory, the double-clad structural model is used to analyze the dual-peak resonant characteristics and refractive index sensing properties of this LPFG. The results show that dual resonant peaks are far away from each other, and shift in opposite directions with an increase of surrounding refractive index. In addition, the influence of metal film thickness and grating structural parameters (grating period Λ, average index change σ) on the sensitivity to refractive index is discussed for this LPFG sensor, which provides a foundation for designing high sensitivity liquid LPFG sensors. Experimentally, the silver films were coated on a LPFG by direct current sputtering, and the dual resonant peaks were observed in this LPFG. Then the salt solution concentration monitoring test was performed. The results indicate the metal coated LPFG based on DPR has higher refractive index sensitivity as compared with dual-peak based non-coated LPFG and general single peak based LPFG, the resolution of liquid refractive index is available to 10−5 for the proposed LPFG with suitable film and grating parameters.