A fiber-optic sensor system for the distributed measurement of organic chemicals is presented in this paper. The system uses the technique of optical time-domain reflectometry (OTDR) and a polymerclad optical fiber that is sensitive to nonpolar substances. The location of the chemicals is attained by measuring the time delay between a short laser light pulse entering the fiber and a discrete change in the backscatter signal caused by the enrichment of the analyte in the fiber cladding. Chemical substances enriched in the cladding of the sensor fiber lead to changes in the OTDR response signal, because the light-guiding properties of the fiber are affected through the evanescent wave. The enrichment of an analyte with a higher refractive index than the fiber cladding, for example, will induce a light loss because of mode stripping. This light loss is followed by a step drop in the OTDR response signal. If the analyte penetrating into the fiber cladding absorbs the emitted laser light pulse, a step drop also occurs in the backscatter signal because of the light loss due to the absorption. A fluorescent substance in the fiber cladding leads to a characteristic peak in the OTDR response signal. The intensity of the different signals is correlated with the refractive index and the concentration of the analyte, the interaction length between analyte and sensing fiber, and the temperature, fiber diameter, and bend radius of the fiber.
Truly distributed sensing systems for nonpolar hydrocarbons are described that are built from a chemically sensitive polymer-clad silica fiber adapted to different optical time domain reflectometer (OTDR) setups. OTDR measurements allow to locate and detect chemicals by measuring the time delay between short light pulses entering the fiber and discrete changes in the backscatter signals that are 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. Such arrangements are developed to monitor hydrocarbon leakage from spatially extended technical installations or contaminated areas.Data are presented on the distributed sensing of fluorescent polynuclear aromatic hydrocarbons (PAH) that can be located by combining the fiber with an OTDR setup and a pulsed UV laser light source. This setup allows spatially resolved sensing of PMIs, e.g. fluoranthene, in the low micrograms-per-liter concentration range. However, due to the strong aUenuation ofthe UV excitation light in the fiber, the maximum fiber length is limited to about 100 m. Much longer sensing lengths are possible if OTDR measurements are performed in the near-infrared spectral range. First data on the distributed sensing of chlorinated hydrocarbons (CHCs) with a commercially available mini-OTDR adapted to a sensing fiber of nearly one kilometer length are described. Here, a laser diode emiUing at the 850-nm telecommunication wavelength was applied to locate the CHCs by analyzing the step drop (light loss) in the backscatter signal that is caused by refractive index changes in the silicone cladding induced by analyte enrichment.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.