An optical sensor based on MMI structures has been studied. We have used the optical parameters of a new hybrid
material deposited by sol-gel method leading to waveguide structures after UV-photoinscription, and of ZnO, sensitive
material characterised in previous studies. The variation of optical properties such as the refractive index of the covering
sensitive material leads to a modification of waveguiding conditions in the MMI. A gas sensor can be developed by
measuring the variation of light intensity at the output of the MMI structure under gas exposure. Simulations have been
performed in order to optimise the output light intensity variations to increase sensitivity. We have shown that these
structures are more sensitive than Mach-Zehnder interferometers, and that the relation between dimensions and
sensitivity of the MMI is not trivial. Computations have to be performed to optimise the structure for given parameters.
Materials involved in gas sensing applications have been deposited by various methods in thin film form. Variation of the optical properties of the thin films were investigated under butane and ozone exposure using the m-lines technique. Effective index variations down to 10<sup>-4</sup> were observed. Concentrations of 100 ppm of butane diluted in the air or in nitrogen were detected.