Design optimization of a long-period fiber grating with sol–gel coating for a gas sensor

The design of a novel scheme long-period fiber grating (LPFG) film sensor is presented. The sensor is composed of a long-period grating coated with sol–gel derived films, which are sensitive to the surrounding medium (gases). Based on the coupled-mode theory, a four-layered numerical model has been developed to study the characteristics of the transmissivity of the LPFG film sensor. By analyzing the relationship between the sensitivity S n and the thin film optical parameters (thickness h 3 and refractive index n 3 ) and the fiber grating parameters (the grating period Λ, the core index change σ and the grating length L ), the optimal design parameters of the LPFG film sensor can be obtained. Data simulation shows that the resolution of the refractive index of this LPFG film sensor is predicted to be 10 −8 . Experimentally, a long-period fiber grating film sensor for the detection of C 2 H 5 OH was fabricated, and a preliminary gas-sensing test was performed. The results indicate that a novel optical film sensor scheme with structure optimization has higher sensitivity.