Otdr Distributed Sensing of Liquid Hydrocarbons Using Polymer-clad Optical Fibers

A distributed sensing system for apolar hydrocarbons is presented which is built from a chemically sensitive polymer-clad silica fiber adapted to an optical time domain reflectometer (OTDR) setup. OTDR measurements allow locating and detecting chemicals by measuring the time delay between short light pulses entering the fiber and discrete changes in the backscatter signals that are caused by local extraction of hydrocarbons into the fiber cladding. The light guiding properties of the fiber are affected by interaction of the extracted chemicals with the evanescent wave light field extending into the fiber cladding. Distributed sensing of pure liquid hydrocarbons (HC) and aqueous HC solutions with a commercially available mini-OTDR adapted to sensing fibers of up to one kilometer length could be demonstrated. A pulsed laser diode emitting at the 850-nm telecommunication wavelength was applied in the mini-OTDR to locate the HCs by analyzing the step drop (light loss) in the backscatter signal, which is induced by refractive index changes in the silicone cladding due to analyte enrichment. The prototype instrument can be applied for monitoring hydrocarbon leakage in large technical installations such as tanks, chemical pipelines or chemical waste disposal containments.