Optical Detection of Ultrasound

The readout of the ultrasound signal can be obtained by interferometric techniques using coherent  light. The most challenging tasks will be to provide sufficient sensitivity and uniformity over the cross section.

Principles

Figur 1: Fabry-Perot cavity, also called etalon. Output signal depends on surface motion ΔL.

• Coherent light incident to the cavity undergo multiple reflections
• Reflected light from reflector 1 and 2 will interfere depending on the phasedifference of the two waves
• Incoming acoustic waves will make reflector 1 oscillate, and thus change cavity length
• The output signal (interference signal) will change as a function of a change in the cavity length
• The reflectances, R, of the reflectors, will affect the output intensity

The following graphs represent the output intensity of the Fabry-Perot cavity, for different values of R:

Out of these three graphs we see that there is a trade-off between sensitivity (how small cavity changes we can detect) and the maximum deflection possible to detect. When choosing parameters, these factors need to be considered.