High-sensitivity H₂S gas sensor based on asymmetric modes in long-period fiber grating

A long-period fiber grating (LPFG) sensor coated with zinc oxide (ZnO) film is proposed for the detection of hydrogen sulfide (H₂S), which is based on the combined high-response characteristics of asymmetric cladding modes and the sensitive film. The asymmetric cladding modes are excited by fabricating the grating in an asymmetric dual-core fiber (ADCF). By simulating the sensitivity of the ADCF-LPFG sensor with different thicknesses in the mode transition region, the optimal film thickness is determined to be 120 nm. Experimentally, the sensor achieves a maximum H₂S concentration sensitivity of 63.21 pm/ppm in the range of 4.93-41.6 ppm. It also exhibits excellent stability of 0.0011% and a detection limit of 2.432 ppm. Additionally, the sensor demonstrates a temperature sensitivity of -83.17 pm/°C over the range of 10-40°C. The designed sensor features high sensitivity and compact structure, demonstrating significant potential for simultaneous detection of H₂S concentration and temperature.