Direct contact evanescent wave absorption enabled fiber optic refractive index sensor operating in the dynamic range of 20 °C to 60 °C

Abstract

The study of refractive index provides the complete picture of material bodies with reference to their major properties like physical, chemical, optical, etc., and hence decides the behavior of the material body at macro level. Measurement of refractive index in small volume samples using a U-shaped extrinsic fiber optic sensor based on the principle of direct contact evanescent wave absorption can be employed to detect such information. In order to obtain such measurements, a U-shaped glass probe connected between a light transmitter and an optical detector using two PCS multimode optical fibers of the same diameters (200/230 μm) was developed and investigated in the present study. Liquid mixtures prepared using Toluene and Acetic acid taking at different proportions making total volume equivalent to 20 ml employing a two burette system. When the liquid mixtures are applied on the U-shaped probe the light reaching the detector from the source was shown varying depending upon the refractive index of the mixture. The changes in the transmitted power spectra were measured as a function of absorption of higher order evanescent wave modes at various refractive index values at different temperature levels ranging from 20 °C to 60 °C of the mixture and by repeating the experiment with various operating wavelengths (630 nm, 660 nm, 820 nm & 850 nm). The calibrated curves were plotted, which can be used to determine the refractive index of liquids at various temperatures with high degree of accuracy, precision and sensitivity measurements, exploiting all the benefits offered by the optical fiber communication systems.