Modelling of confocal Fabry-Perot interferometers for the measurement of ultrasound

A mathematical and physical description of a confocal Fabry-Perot interferometer used for the measurement of ultrasound is presented. It analyses the interferometer performance in terms of ultrasonic transfer functions derived for rays of types 1-4. Instrument performance is modelled for two different schemes of operation, namely transmission and reflection. The frequency response is derived from a combination of ray types. In the case of the reflection scheme, a new beam separation method is suggested, which makes full use of the instrument etendue. Additionally, this latter scheme offers a significant increase in sensitivity over a wide frequency range extending towards the free spectral range of the interferometer, typically exceeding 100 MHz.