PMMA-coated fiber Bragg grating sensor for measurement of Ethanol in liquid solution: manufacturing and metrological evaluation

The sensitivity of Fiber Bragg Gratings (FBGs) to the variation of a specific parameter can be improved by choosing a suitable coating material. This paper explores the possibility of measuring the concentration of Ethanol in aqueous solutions by using poly (methyl methacrylate) (PMMA) as coating material of a single-mode FBG. The basic measurement principle relies in the absorption of liquid Ethanol which produces controlled swallowing of the coating, straining the underneath Bragg grating. The PMMA coating was deposited on the FBG by means of a specifically designed dip coating test bench, able to accurately control the thickness of the applied layer. Metrological performances of the developed PMMA-coated FBG Ethanol sensor have been assessed for Ethanol concentrations in the range of 0-38 % in distilled water at 25 °C constant temperature. The prototype shows good repeatability and better sensitivity when compared to an uncoated FBG, especially at low Ethanol concentrations. The dynamic behavior of the sensor (which depends on the kinetics of the absorption mechanism) has been assessed as well, showing a response time of about 290 s, while the recovery time (660 s) was almost twice.