Contactless Optical Fiber Interferometric Sensor to Monitor Water Content in Ethanol

A simple contactless optical fiber interferometric sensor for real-time measurement of low-level of water content in ethanol is proposed and demonstrated. The sensing head consists of a multi-cavity Fabry-Pérot interferometer formed with a fiber tip in close contact with a quartz spectrophotometer cell. Ethanol-water solutions introduced into the cell cavity produce a change in the optical path length of one cavity of the interferometer. The increment of water content induces refractive index changes, but more important the absorption of the light increases. Both effects produce a significant change in the interference spectra that can be easily monitored and quantified in the Fourier domain. We demonstrate that our technique can detect water content (0%-60%) in ethanol with a detection limit as low as 0.5%.