Polymer Microtip-Based Fabry–Perot Interferometer for Water Content Determination in the Gas and Liquid Phase

In this work, we present a Fabry-Perot interferometer (FPI) based on a polymer microtip for water content determination in both the gas and liquid phase. The polymer tip of pentaerythritol triacrylate (PETA) is fabricated at the end of an optical fiber by self-guiding photopolymerization, forming at the same time a low-fineness Fabry-Perot interferometer and a sensing layer for water thanks to hydroxyl groups present in PETA. The PETA tip shows a clear interferometric signal, which is highly sensitive to the change of the water content in the environment. The FPI signal shifts with a constant sensitivity of 90 pm/%RH, which corresponds to a relative sensitivity of 104 ppm/% RH, in the range of relative humidity from 30 to 80%. In liquid, the FPI sensor shows a nonlinear sensitivity, up to 158 pm/wt % as the water content is below 40 wt % in water/glycerol mixtures. The cross effect of the temperature on the PETA tip is demonstrated to be negligible as the FPI signal is insensitive to temperature changes from 23 to 70 °C. More importantly, the interaction between the tip and the environment affecting the FPI signal is demonstrated experimentally. The proposed FPI sensor is therefore promising for the direct, sensitive, and reliable determination of the water content of products.