A high-sensitivity optical fiber relative humidity sensor based on microsphere WGM resonator

In this paper, we proposed and experimentally demonstrated a high-sensitivity optical fiber relative humidity (RH) sensor, which consists of a tapered fiber and a silica microsphere resonator, for excitation of Whispering Gallery Modes (WGMs). Whispering gallery modes in the microsphere (200 µm) resonator are excited by evanescent coupling using the tapered fiber with ∼2.1 µm waist diameter. The variation of surrounding RH will induce the effective refractive index and coupling efficiency of the microsphere change, and result in transmission spectra change finally. Studies of the RH response sensitivity, the repeatability, long-term stability and temperature dependence of the proposed sensor are carried out. The sensitivity achieved in our experiments was 0.11 dB/%RH in the RH range from 20% to 70%, and 0.21 dB/% RH in the RH range from 70% to 90%. The proposed sensor with the advantages of compact size, high sensitivity, low hysteresis, good repeatability, and low cross sensitivity to temperature has good application prospects.