High-temperature refractive index sensor with high sensitivity and wide measurement range based on the fiber open...

High-temperature-resistant refractive index (RI) sensors with high sensitivity and a wide measurement range are urgently required in many industrial fields. We propose an open fiber Fabry-Pérot interferometer (FPI) gas RI sensor based on hollow-core fibers (HCFs) offset fusion splicing. The gas molecules diffuse into the FP cavity through a microscopic channel of the HCF to alter the RI, which leads to a change in optical path difference (OPD) of the FPI. The RI is measured by analyzing the OPD variation through a fast Fourier transform filter and the least mean square error algorithm. The experiments demonstrate that the RI sensitivity of the sensor is proportional to the cavity length, achieving a gas RI sensitivity of 1972.24 µm/RIU within the RI range of 1.00027-1.03782. For a cavity length of 778 µm, the sensor presents an ultra-low RI-temperature cross-sensitivity of 3.78 × 10^-7 RIU/°C at a temperature range of 20-800 °C. A fiber Bragg grating is integrated with the FPI structure to enable simultaneous detection of temperature and RI, while compensating for thermal expansion-induced error. The sensor features high-temperature resistance, compact size, high sensitivity, and wide measurement range, demonstrating significant application potential in harsh environments.