Distributed sensing of hydrocarbons using evanescent wave interactions in a silicone-clad optical fiber

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Abstract
A truly distributed sensing system for nonpolar organic chemicals is described which is built from a chemically sensitive polymer-clad silica fiber adapted to an optical time domain reflectometer (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 light guiding properties of the fiber are affected by the enrichment of chemicals in the cladding through the evanescent wave. Changes in the refractive index (RI) of the cladding were produced by contacting the fiber with different solvents (e.g. dichloromethane, 1,1,1-trichloroethane or tetrachloroethene). Hydrocarbon compounds with a higher RI than the fiber cladding penetrating into the polysiloxane layer will increase the refractive index of the cladding and lead to a distinct step decrease in the OTDR response signal of the fiber at the position of enrichment. The size of the step decrease can be quantitatively correlated to the concentration of the hydrocarbon compound. Furthermore, the intensity of the OTDR response signal is dependent on the power of the light source and on the RI of the compound. By using a 5-W laser diode backscatter signals from tetrachloroethene in aqueous solution could be measured even at concentrations in the ppm range. The width of the step drop is linearly dependent on the interaction length between chemical and sensing fiber.

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