Distributed fibre optics polarimetric chemical sensor

Abstract

A new distributed fibre optic chemical sensor based on evanescent wave polarimetric interferometry is proposed with the underlying objective to apply the technology to gas chromatography. It allows real-time monitoring of the displacement of a chemical substance along a capillary. Theoretical analysis, modelling and fabrication of a special fibre containing an off-axis capillary is presented. Proof of the principle is experimentally demonstrated with liquid droplets.

Description of the response of a fiber optic velocity sensor applied to capillary gas chromatography

“…wave extends a few microns into the capillary [7]. By increasing the thickness of the stationary phase (for the present experiments it was 0.5 m), this evanescent wave will interact with more material, thus increasing the mode coupling strength.…”

“…The first model of the polarimetric sensor associated the vapor zone to a birefringent plate tilted at an angle ϕ [7] and calculations were done using Jones matrices [11]. The model gives the variation of the intensity of the light transmitted by the sensor (the signal) during the migration of a vapor zone at a velocity v. The resulting equation reads as:…”

“…For instance, numerical simulations have shown that the rotation is around 8 • for a refractive index change of 0.12 (the difference between water and silica) [7]. Moreover, the refractive index change caused by the concentration of the analyte absorbed in the stationary phase will be much smaller; it cannot be higher than few percents of the difference between the refractive index of the stationary phase itself and the one of the analyte.…”

“…All of these fiber optic sensors are not distributed sensors, and once decoded, the information provided by these detectors is not much different from those of other detectors used in GC: they measure the amount of analyte passing in the detector over time. The polarimetric sensor that we developed [1,[6][7][8][9][10] differs from the previous ones in that it is really a distributed sensor. Light is injected at the entrance of the capillary fiber, propagates along the entire length of the capillary, where it interacts with the analyte present in the column, and is then detected at the capillary output.…”

Fiber-Optic Chemical Sensors and Biosensors

“…Hence, hollow core microstructured polymer optical fibers can be used for sensing chiral species. A distributed fiber-optic polarimetric sensor was reported by Caron et al (179). The sensor is based on evanescent wave polarimetric interferometry and is intended for use in gas chromatography.…”