Abstract

Fibre-coating techniques with hydrogels (hydrophilic polymers) in conjunction with optical time domain reflectometry, offer the possibility of producing a distributed chemical sensor for detecting the presence of water (Michie et al., 1995). In tins paper, we extend the range of possible applications of such hydrogel based sensor, to measure water content of soils. The sensor is based on measurements of the changes in the Rayleigh backscattering signal induced by quasi-sinusoidal distortions of fixed periodicity on a multimode graded index optical fibre. In the presence of water molecules, the swelling action of a poly (ethylene oxide) based hydrogel coating, which is deposited on a GFRP support cable, causes the fibre to deform locally by squeezing it against a kevlar thread, helically wound along the length of die sensor cable (Fig. 1). The resonant period for die microbend is equal to the pitch length of the kevlar thread and is given from Δ_m=2πr/(2δ)^–1/2 where r is die core radius and δ is the maximum refractive index difference between die core and cladding, forcing die system to operate at a resonant mode.

© 1996 IEEE