<title>Optical fiber sensors for the distributed measurement of hydrocarbons</title>

Abstract

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.