Ultra-high sensitivity methane gas sensor based on vernier effect in double D-shaped and cryptophane-A film-coated photonic crystal fiber: Design and FEM simulation

Abstract

Utilization of the vernier effect in fiber optic interferometers represents the latest approach to enhance the sensitivity and resolution in fiber optic sensors. In this paper, the Vernier effect in Sagnac interference (SI) with a double D-shaped photonic crystal fiber (PCF) was proposed for the methane gas sensing. The sensing arm in the SI was composed of a double D-shaped PCF which was coated with cryptophane-A film, while the polarization maintaining photonic crystal fiber (PM-PCF) is used as the reference arm. The sensing arm and the reference arm were cascaded in parallel to generate the Vernier effect. The influence of the structural parameters in the PCF on the sensing performance was analyzed using the finite element method. The sensing fiber uses a double-D structure, so methane gas is more easily in contact with the sensitive film. According to the simulation results, the wavelength sensitivity of the parallel Sagnac loops within the methane gas concentration range of 0–3.5% is as high as 170.58 nm/%, which is 3.14 times that of a single Sagnac loop. The proposed sensor exhibits a significant enhancement in sensitivity compared to the other fiber optic-based methane gas sensors, thereby providing a reference sample for the development of high-sensitivity methane gas sensors.