Gas sensor based on an integrated optical interferometer

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Abstract
An integrated optical sensor for the detection of gases like CO2 and SO2 is described. The working principle is as follows: The gas to be measured is absorbed by a sensitive film, which changes its refractive index with gas absorption. The sensitive film is deposited onto an integrated optical interferometer. As the intensity distribution of waveguide modes is not totally confined to the waveguide, interferometer phase changes with the refractive index of the film. Tests have been made with interferometers of the Fabry-Perot-type. Interferometer phase is measured continuously using the serrodyne detection scheme. Organically modified silicates are used as sensitive films. These glassy films are prepared by the solgel technique, which allows the incorporation of functional organic groups within a network of siloxane bonds. The ability of selective gas absorption of these materials is determined by chosing appropriate functional groups. Films sensitive toward CO2 and SO2 have been fabricated and tested under different gas atmospheres. Temperature dependence of the sensor signal and cross sensitivities toward other gases and humidity have been investigated.

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