An open-cavity Fabry-Perot interferometer with PVA coating for simultaneous measurement of relative humidity and temperature

A fiber-optic open-cavity Fabry-Perot (FP) interferometer (FPI) with polyvinyl alcohol (PVA) coating is proposed and experimentally demonstrated for simultaneous measurement of relative humidity (RH) and temperature. The FPI consists of an etched side-hole fiber (SHF) sandwiched in between two leading single-mode-fibers. The PVA film is plated on the fiber surface functioning as a humidity-to-refractive index (RI) transducer. Theoretical analysis details the RH and temperature influence on the spectral fringes of the FPI. Experimental results confirms and show that, the reflection loss of the FPI nonlinearly decreases more than 15 dB when the RH increases from 30%RH to 90%RH with a highest sensitivity of −1.2 dB/%RH, while the dip wavelength of the spectral fringe exhibits linear RH and temperature sensitivities of −23.1 pm/%RH and −6.14 pm/°C, respectively. Thus, by monitoring the reflection and wavelength of the interference dip, the proposed sensor enables accurate simultaneous measurement of RH and temperature. A fast and reversible time response has also been demonstrated. Such excellent sensing performance with compactness, ease of fabrication makes our sensing structure a highly promising candidate for RH and temperature monitoring applications.