Optical detection of toluene in water using an IGI optical fiber with a short sensing region

Optical detection of toluene in aqueous solutions by means of a sensor employing a 4 cm-long sensing region produced by dip-coating a thin dimethylpolysiloxane layer on the core of a fiber with an inverted parabolic-index profile was investigated. The toluene in solutions was detected through changes of the output signal from the fiber at a wavelength of 850 nm caused by induced changes of the refractive index of the fiber cladding. In experiments, two types of exposures were used—a static exposure, in which the fiber, which was initially in air, was for some time exposed to a solution in rest; and a dynamic exposure, in which the fiber which was initially in flowing water, was for some time exposed to a flowing solution. In 5-min static exposures and 2.5–3-min dynamic exposures detection limits in the low mg/l range were obtained. A possibility of improving the sensitivity by reducing the rate of sampling of the photodetected signal was shown. Effects associated with the observed cross-sensitivity of the fiber to water and an effect of a change of the solution temperature on the output signal were also examined.