SIMULTANEOUS SENSING OF PRESSURE AND TEMPERATURE USING A SELF-TEMPERATURE-COMPENSATED FABRY–PÉROT MEMS MECHANISM

Abstract

This thesis presents the design and development of a self-temperature-compensated sensor for measuring temperature and pressure in harsh environments using a combination of Fabry–Pérot interferometry and microelectromechanical systems (MEMS). A silicon-on-insulator (SOI) wafer is etched to form a dual mechanism consisting of a membrane and a solid block that is then coupled with two optical fibers contained in a unique and simple protective stainless-steel housing. The solid block uses the thermo-optical properties of silicon for temperature measurements, while the deflection of the membrane is used for pressure sensing. An empirically based model combines solid mechanics and optical theory and is in good agreement with experimental measurements. As part of this work, the thermo-optic coefficient (TOC) of the silicon was also investigated theoretically and experimentally. The results show a good agreement between the TOC extracted from the experimental data and such a coefficient in published literature. Furthermore, a novel optical model for the demodulation of the intensity-based pressure-sensing mechanism was developed. This model relates the whole sensor-response profile to the measured parameters and eliminates linear range limitations. By using this model, one can also obtain the initial cavity lengths of an FFPI sensor, which can be very challenging at the microscale. A series of experiments conducted to test the performance of this multi-functional sensor showed that it can easily withstand pressures up to 1,000 psi and temperatures of up to 120°C, where the range of the temperature measurements are restricted only by the fiber optic materials. The developed self-temperature-compensated multi-functional sensor therefore serves as a promising tool in the precise characterization of pressure and temperature in harsh and/or complex environments.