Detection of toluene by using specially coated PCS fibers excited by an inclined collimated beam

This paper deals with the detection of toluene dissolved in water by means of straight polymer-clad silica (PCS) detection fibers with short detection regions. The approach is based on the detection of refractive index changes of the polymer cladding induced by toluene. The fibers were excited by an inclined collimated light beam with the aim of finding an alternative to the coiled axially excited fibers which would achieve similar detection sensitivity. Two types of siloxane polymers, thermally curable one with the refractive index of 1.407, and UV curable polymer with the refractive index of 1.423 were used for the modification of fiber properties in the detection area. The polymers were applied as thin layers (5-20 mum) on the silica core of the sensing fibers. Angular distributions and time-responses of the output optical power to changes of toluene concentration in solutions were measured. Sensitivity curves were drawn on the basis of these results. The best value of the detection sensitivity of about 0.006 dB/(mg/l) was achieved with the UV curable siloxane. This value corresponds to the detection limit of toluene in water of about 15 mg/l. Sensing fibers with beveled input ends were prepared. They exhibited the same sensitivity as fibers with the end perpendicular to the fiber axis when they were excited by the uninclined beam. (C) 2002 Elsevier Science B.V. All rights reserved.