Multimode interference self-imaging optical fiber sensor based on sol-gel silica for methane detection

Abstract

We reported a multimode interference (MMI) sensor based on single mode fiber-no core fiber-single mode fiber (SMF-NCF-SMF) structure incorporated with silica sol-gel nanostructure for detection of methane. Due to the core mismatch between SMF and NCF, a number of higher order modes was excited at the NCF region and recoupled back to the fundamental mode of SMF lead-out which resulted in multimode interference self-imaging. Deposition of silica sol-gel nanostructure formed the hybrid waveguide whose optical property changes according to the surrounding perturbation. The effect of silica coating upon its thickness was clearly demonstrated which can enhance the sensitivity of the sensor. As the concentration of methane varies, the effective refractive index of the waveguide also changes, hence introducing the resonant dip shifts in the transmission spectrum. The sensitivity achieved was 7.92 nm/% for 8-layers of coating, 5.47 nm/% 4-layers of coating, and 0.5 nm/% for uncoated sensor. In addition, the proposed sensor also exhibits good linear response within 0 - 0.175 % of methane concentration while the test of reproducibility confirmed the sensing stability of the sensor.