Low Temperature Cross-Sensitivity Humidity Sensor Based on a U-Shaped Microfiber Interferometer

A simple and sensitive optical fiber relative humidity (RH) sensor is proposed and demonstrated in this paper. The optical microfiber inter-mode interferometer is fabricated by tapering PM-elliptical core fiber down to micrometer scale in diameter, the coupling mainly occurs between the LP 01 and LP 11 modes. The microfiber interferometer (MFI) is fixed as U-shape, making it conveniently to measure. The RH changes are determined from the wavelength shift of the interference pattern induced by the evanescent-field interaction. Experimental results show that MFI with a thinner diameter of $4.7~\mu \text{m}$ offers the best RH sensitivity of 114.7 pm/RHU (per relative humidity unit) ranging from 30% to 90% and good linearity between wavelength and RH. The temperature response has also been experimentally studied, and the temperature sensitivity is 7.6 pm/°C from 30 °C to 90 °C, which indicates that this microfiber device possesses a low temperature cross sensitivity. The proposed RH sensor does not need any functional coating, with advantages of compact size and simple fabrication procedure, which make it attractive for several applications.