The optical modelling and design of Fabry Perot Interferometer sensors for ultrasound detection

The work in this thesis documents the optical modelling and design of Fabry Perot Interferometer (FPI) sensors for the detection of ultrasonic waves. The ultrasonic waves modulate the optical phase of the beam through a change in the cavity spacing of the FPI sensor. The optical sensitivity is defined as the change in the reflected light per unit change in the cavity thickness. An optical model to simulate Interferometer Transfer Functions (ITFs) for a Gaussian beam propagating in a Fabry Perot Interferometer is implemented. An understanding of the Gaussian beam phase propagation in a Fabry Perot Interferometer is presented to help in explaining the shape of ITFs simulated. The model is experimentally validated. The model is applied as a design tool for the purpose of optimising FPI sensors. This is achieved by choosing the beam radii, mirror reflectivities and cavity spacing’s which lead to high optical sensitivity. A FPI sensor with high optical sensitivity and pressure linearity is achieved. A high pressure linearity can be achieved by creating a highly asymmetric ITF, by a combination of a highly diverging beam and aperturing the reflected beam. The understanding presented in this work helps in designing optimised FPI sensors for ultrasound detection, as well as in providing a general understanding of the effects of Gaussian beams or other types of divergent beams illuminating FPIs.