Distributed measurement of chemicals using fiber optic evanescent wave sensing

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Abstract
A truly distributed sensing system for nonpolar organic chemicals has been built up by adapting a chemically sensitive polymer-clad silica fiber to an optical time domain reflectometry (OTDR) set-up. This arrangement allows to measure the time delay between a short light pulse entering the fiber and the discrete signals of backscattered light caused by chemical effects in the fiber cladding. The backscatter signals originate from changes in the light guiding properties of the fiber, which are affected by the enrichment of chemicals in the cladding through the evanescent wave. The shape and magnitude of signals caused by penetrating chemicals either due to changes in refractive index, or absorption and fluorescence properties of the fiber cladding, have been examined. Changes in the optical properties of the cladding were produced either by contacting the fiber with solvents (e.g. tetrachloroethane) or organic dyes such as methylene blue and rhodamine 800. Typical parameters, that influence the intensity of the OTDR response signal are the refractive index, concentration and molar absorptivity of the analyte, as well as the power of the light source.

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