Interaction of target molecules with the evanescent wave of light guided in optical fibers is among the most promising sensing schemes for building up smart chemical sensor technologies. If the technique of optical time domain reflectometry (OTDR) is combined with silicone-clad quartz glass fibers distributed chemical sensing is possible. Hydrocarbon (HC) detection and location is done by automated identification of the position of the corresponding step drop (light loss) in the backscatter signal induced by local refractive index increase in the silicone cladding due to a penetrating HC compound. A commercially available mini-OTDR was adapted to sensing fibers of up to nearly 2-kilometer length and location of typical HC fuels could be demonstrated. The instrument is applicable for fuel leakage monitoring in large technical installations such as tanks or pipelines with spatial resolution down to 1 m. A similar technique using measurements in the Vis spectral range is being developed for health monitoring of large structures, e.g., for early detection of corrosion caused by water ingress and pH changes in reinforced concrete. Here, a pH indicator dye and a phase transfer reagent are immobilized in the originally hydrophobic fiber cladding, leading to a pH induced absorption increase and a step drop signal in the backscatter curve. The configuration of the distributed sensing cables, the instrumental setups, and examples for HC and pH sensing are presented.
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 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 PAHs, e.g. fluoranthene, in the low micrograms-per-liter concentration range. However, due to the strong attenuation of the 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 emitting 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.
In situ measurements with the prototype of a portable fiber- optic sensor system for the monitoring of nonpolar hydrocarbons (HC) in ground water or industrial waste water are presented. This sensor system can be used for quantitative in situ analysis of pollutants such as aromatic solvents, fuels, mineral oils or chlorinated HCs in a broad concentration range from around 200 (mu) g(DOT) L-1 up to a few 100 mg(DOT) L-1. The sensing principle is based on solid phase extraction of analyte molecules into a hydrophobic silicone cladding of a quartz glass optical fiber and the direct absorptiometric measurement of the extracted species in the polymer through the evanescent wave. The sensor can be connected via all-silica fibers with a length of up to 100 m to a filter photometer developed at the IFIA, thus allowing even remote analysis in monitoring wells. This instrument provides a sum concentration signal of the extracted organic compounds by measuring the integral absorption at the C-H overtone bands in the near-infrared spectral range. In situ measurements with the sensor system were performed in a ground water circulation well at the VEGAS research facility (Universitat Stuttgart). Here, the sensor proved to trace the HC sum concentration of xylene isomers in process water pumped from the well to a stripper column. In further experiments the sensor was combined with an oil sampling device and was tested with simulated waste waters of a commercial vehicle plant contaminated with different types of mineral oil. In this case the sensor system was able to detect the presence of mineral oil films floating on water or oil-in-water emulsions with concentrations greater than 20 ppm (v/v) within a few minutes.
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.
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.
Investigations into the optimization of a long-path integrated optical evanescent field absorbance sensor for the detection of nonpolar organic substances in water are described. The sensor is based on a multimode strip waveguide produced by Na+Ag+ ion- exchange in a borosilicate glass substrate and a hydrophobic silicone sensing layer deposited on the IO structure, that reversibly enriches organic contaminants from water or air. Light from a tungsten-halogen lamp in launched into the planar structure and evanescent wave absorption measurements of the organic species in the silicone superstrate are performed with a near-infrared diode array spectrograph. Polymethyl(phenyl)siloxanes with varying refractive index were prepared and tested as sensitive coating for the IO structure. The light transmission through the sensor may decrease up to 90% if the coated sensors come in contact with water. These losses caused by light scattering effects due to the formation of H2O micro- emulsions in the silicone superstrate can be minimized by using polysiloxanes with a higher degree of cross-linkage. Measurements of aqueous trichloroethene samples were successfully performed in the region of the C-H first overtone vibration band. The sensitivity of the measurement can be raised distinctively by using polymethylphenylsiloxanes, which have a higher refractive index than polydimethylsiloxane. Kinetic experiments with aqueous trichloroethene samples showed a reversible sensor response with t90 values in the range from 7-20 minutes.
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