Development of a coated long period grating sensor for monitoring contaminants in ground water

The work presented here is part of a larger project to develop inexpensive, portable fiber-optic sensor arrays for on-site monitoring of organic and inorganic water contaminants. The chemical sensors are based on the change of optical properties of a long-period grating as a function of the refractive index of their immediate environment.; Previously long period gratings (LPG) have proved to be sensitive to environmental change, but the magnitude of the response and their chemical selectivity was insufficient for environmental applications. By coating the LPG with a layer of polymer film we were able to improve on the sensitivity and selectivity for our target analytes---mainly organic solvents diluted in water. The absorbed analytes cause a change in the refractive index of the coating, consequently altering the transmission properties of the grating. As the response of one single LPG sensor element is non-specific, an array of different sensor elements will need to be established in the future. Each sensor will then have a different coating which provides a unique absorption property to different analytes. Then a multivariate analysis tool would be used to analyze the different response from each sensor element and thus get information about individual analytes.; In order to characterize possible films, two different refractometers were set up for the measurement of materials' refractive indices. One of them, an Abbe-type refractometer, was used to investigate the response of polymer films to different analytes.; Among the many combinations of modified polymer film that were prepared and tested, one polymer/silica composite material was found to be stable in aqueous solution test and showed selectivity to different contaminants. The material was poly (ethylene vinyl acetate) (EVA) containing an ordered mesoporous silica material (SBA15) from which the template block co-polymer (Pluronic 123) was not removed. A group of related EVA films were synthesized as microextraction matrices for a prototype gas sensor.; The behaviour of an LPG coated with EVA+SBA15 (P123 in) was studied. It showed chemical selectivity towards saturated organic vapours including n-hexane, cyclohexane, benzene, TCE and xylene, as well as saturated contaminant solutions including cyclohexane, benzene and xylene. Its sensitivity was found to be superior to that of an uncoated LPG, albeit still far from optimal.; A calculation of cladding modes transition based on coupled-mode theory was carried out to explain the behavior of the coated LPG and how to improve the sensitivity of the method. Using thin coatings with high refractive indices may prove to be a new approach to produce the high sensitivity to the uptake of analytes.