CHARACTERIZATION OF A FIBEROPTIC SYSTEM FOR THE DISTRIBUTED MEASUREMENT OF LEAKAGES IN TANKS AND PIPELINES

A fiber-optic sensor system for the distributed measurement of organic chemicals is presented in this paper. The system uses the technique o f optical time-domain reflectometry (OTDR) and a polymer-clad optical fiber that is sensitive to nonpolar substances. The location of the ch emicals is attained by measuring the time delay between a short laser light pulse entering the fiber and a discrete change in the backscatte r signal caused by the enrichment of the analyte in the fiber cladding . Chemical substances enriched in the cladding of the sensor fiber lea d to changes in the OTDR response signal, because the light-guiding pr operties of the fiber are affected through the evanescent wave. The en richment of an analyte with a higher refractive index than the fiber c ladding, for example, will induce a light loss because of mode strippi ng. This light loss is followed by a step drop in the OTDR response si gnal. If the analyte penetrating into the fiber cladding absorbs the e mitted Laser light pulse, a step drop also occurs in the backscatter s ignal because of the light loss due to the absorption. A fluorescent s ubstance in the fiber cladding leads to a characteristic peak in the O TDR response signal. The intensity of the different signals is correla ted with the refractive index and the concentration of the analyte, th e interaction length between analyte and sensing fiber, and the temper ature, fiber diameter, and bend radius of the fiber.