Multimode interference structures of variable geometry for optical sensor application

The paper presents the analysis results of a waveguide sensor based on multimode interference structures. The multimode section was covered with sensor material. The change of optical parameters of the cover entails the change of propagation conditions of light in the structure. By measuring the light intensity at the output of the structure, we can define external physicochemical parameters to which the sensor layer is sensitive. The paper presents the method to adjust the sensitivity of the device through a proper selection of the thickness of the particular layers of multimode waveguides. We present, among others, the analysis results of optical systems whose parameters of the sensor layer corresponded with the parameters of wolfram oxide (WO3), being the material frequently applied as a gas-sensitive layer in gas sensors. We have proposed a configuration of a sensor based on the Mach-Zehnder interferometer whereof one arm is a multimode section and the other one a single mode waveguide. The detection method was narrowed down here to the measurement of phase difference between the waves propagating in the respective arms.