Integrated optic sensor for the detection of H2 concentrations

An integrated optic Hydrogen sensor has been implemented and tested. Its operation is based on the changes in the optical properties of a Pd membrane produced by hydrogen absorption and desorption (1). The sensiitive structure consists of an optical waveguide covered by a thin Pd film which couples to the evanescent optical field of the guide. Any changes in the properties of the Pd film affects the optical propagation, giving rise to changes in both absorption and refractive index of the waveguide. this last can be monitored with a high accuracy by interferometric tecniques. a schematic of the device is shown in Fig. 1. it consists of a Mach-Zehnder interferometer with one of the arms covered by the Pd film. the use of the Mach-Zehendre structure is very attractive since, because of the presence of the sensing and reference arms very close on the same substrate, it may provide a high rejection of common mode error sources such as temperature. the optical device was produced by diffusing titanium strips into y-cut LiNbO3 crystals with an appropiate choice of diffusion parameters in order to produce a single mode waveguide. Aftre diffusion the crystal end faces were cut and optically polished so to allow end fire coupling to single mode fibers. the system was feeded by a laser source and the ligth level at the output, upon hydrogen absorption and desorption, was monitored by a photodetector. the complete device results very compact in size and suitable for safe operation in hazardous surroundings.