Liquid Sensors Based on Enhanced Fabry–Perot Etalons

Abstract

An optically enhanced Fabry–Perot etalon was demonstrated to accurately measure the refractive index of sugar solutions. The etalon consisted of two silver/SiO2-coated glass substrates separated by a spacer. The semi-transparent silver films of ~15-nm thickness greatly enhanced the interference of light. The SiO2 layer coated on the silver created a hydrophilic surface in addition to protecting the silver from oxidation. The hydrophilic behavior of the SiO2 films together with a capillary action allowed the tested liquids to easily flow into and wet the cavity between the two pieces of glass. Optical spectrophotometer was used to measure the transmission spectra of the etalon with and without a sugar solution. The refractive indices of sugar solutions with different brix concentrations were subsequently determined from the interference peak positions. The results showed a linear response of the refractive index to the brix concentration with a ratio of $\sim 1.7\times 10^{-3}$ refractive index per brix%, making the etalons promising for analyzing specific chemicals in liquids.