INFLUENCE OF THIN SUPERSTRATE FILMS ON EVANESCENT WAVES IN SURFACE WAVE-GUIDES

Surface waveguide structures in glass substrates arranged as Mach-Zehnder interferometers allow the measurement of slightest influences on a superstrate coated on one arm of the integrated optical device. The refractive indices of the substrate's, the waveguide's, and the superstrate's material determine the characteristics of the modal field in the interferometer arms. For this reason, any influence on the effective refractive index in the superstrate can be monitored with high sensitivity. By use of the wave theory of light, the amplitudes and the phase correlation of the interfering waves can be calculated for any given combination of the materials mentioned above. To prove these considerations, one arm of the Mach-Zehnder interferometer is coated with polysiloxane as a thin superstrate film (0,5 - 2-mu-m). If organic hydrocarbons are passed over the superstrate, the molecules penetrate this interface and change its refractive index in dependence on the gas concentration. This experimental arrangement allows to compare measurements and calculations. Furthermore, the set-up can be used to monitor physical and chemical effects taking place in the thin superstrate layer. Therefore, besides a verification of the theory, the Mach-Zehnder chip can be used as a sensor. It can be shown that multi-wavelength measurements increase the informations obtained.