An ultra-sensitive photonic crystal fiber methane sensor utilizing two-loop parallel configuration based on Vernier effect

Abstract Methane is not only the primary cause of gas explosions in coal mines but also a potent greenhouse gas. Accurate monitoring of its concentration is of great significance for ensuring production safety and maintaining ecological stability. In this paper, based on the Vernier effect, an ultra-sensitive photonic crystal fiber (PCF) methane sensor utilizing the two-loop parallel configuration is proposed. Two Sagnac loops function as the sensing and reference arms, with each containing a different length of methane-sensitive film-coated PCF. A combined design of microholes and semi-elliptical grooves is employed in the PCF, which not only achieves a high birefringence effect but also overcomes the difficulty of depositing the methane-sensitive film in the air holes. A theoretical analysis was carried out on the sensing performance of the proposed PCF methane sensor using the finite element method. Simulation results showed that the sensor achieved a maximum sensitivity of up to 952 nm/% within the methane concentration range of 0.00%–1.50%, corresponding to an 11.33-fold enhancement over the single-loop configuration. In addition, the PCF had a fabrication tolerance of ±1° for the semi-elliptical grooves. As far as we know, the proposed PCF methane sensor exhibits the highest sensitivity compared with other existing methane sensors. Owing to its merits of ultra-high sensitivity, simple structure, and high fabrication tolerance, the proposed methane sensor exhibits considerable potential for application in coal mine gas monitoring.