Hygroscopic polymer microcavity fiber Fizeau interferometer incorporating a fiber Bragg grating for simultaneously sensing humidity and temperature

This work proposes a hygroscopic polymer microcavity fiber Fizeau interferometer (PMFFI) incorporating a fiber Bragg grating (FBG) for the simultaneous measurement of relative humidity (RH) and temperature (T). The PMFFI was fabricated by attaching the hygroscopic polymer to a single-mode fiber endface to form a low-finesse Fabry-Perot resonant microcavity. This work is the first to investigate polymer of the Norland Optical Adhesive (NOA) series with particular porous structures that responds well to the moisture and can thus be used in the fiber-optic sensing of relative humidity. Additionally, the NOA materials have a high thermal expansion coefficient and so can be used in the highly sensitive measurement of temperature. The adsorption/desorption of water molecules and variations in temperature both change the optical path of the microcavity, shifting the fringes in the interference spectra. Incorporating a general FBG that is nonreactive to RH but sensitive to T judges the variation in T. The combined sensor supports the simultaneous measurement of both parameters RH and T by gauging of the individual spectral responses of PMFFI and FBG, respectively. Experimental results demonstrate that the RH and T can be simultaneously measured with high sensitivity and accuracy using the proposed sensing configuration. (C) 2015 Published by Elsevier B.V.