A fully distributed fiber optic sensor for simultaneous relative humidity and temperature measurement with polyimide-coated polarization maintaining fiber

Abstract

A distributed fiber-optic sensor for simultaneous relative humidity (RH) and temperature measurement is proposed and experimentally demonstrated by means of optical frequency domain reflectometry (OFDR) with single sensing fiber configuration. The sensing element is a high-birefringence polarization-maintaining (HB-PM) fiber with polyimide coating that transfers strains on the fiber after being expanded by water uptake with the ambient RH changes. Either humidity-variation-induced strain applied on the fiber or temperature changes contribute to distinct spectral responses in the cross-correlation and auto-correlation results of Rayleigh backscatter signals from the polyimide-coated HB-PM sensing fiber when interrogated by the OFDR technique. These spectral responses can be successfully extracted for every segment of the sensing fiber with a centimeter-order spatial resolution, enabling distributed temperature and RH discriminative measurement along the sensing fiber. Experimental results show that the optimal sensitivities for RH and temperature measurement can reach up to 1.8 pm/%RH and 11.1 pm/°C, respectively. Measurement errors for simultaneous RH and temperature sensing are found to be 0.54 °C and 2.75%RH, respectively. The proposed sensing technique will open a brand-new door for developing reliable and practical distributed RH and temperature discrimination sensing systems with high sensitivities and low errors.