A gas sensor based on an integrated optical Mach-Zehnder interferometer

In this paper we demonstrate a sensor system for measuring gaseous compounds using an integrated optical Mach-Zehnder interferometer (IO-MZ chip). One of the surface waveguide arms of the IO device is covered with a polysiloxane layer, sensitive to organic solvents. The refractive index of the polymer layer changes continuously when exposed to the vapours of such solvents. The influence on the evanescent field of the guided mode causes a change in the optical pathlength in the covered arm. Incoupled light of a semiconductor laser diode results in an interference pattern at the output of the IO-MZ chip. The phase shift of the signal is dependent on the gas concentration and the solvent type. We have determined the values for some hydrocarbons, chlorohydrocarbons and aromatic compounds. The extremely short response time of the system allows it to be used like a gas chromatographic detector. The applicationn of Maxwell's equations for the propagation of light in waveguides offers a simulation of the interference signals. By this means, the experimental signal obtained can be correlated to changes in refractive index of the polymer layer. The dependence of the interference patterns on wavelength is explained. White light interference is obtained by use of a xenon lamp. © 1992.