4. Conclusion

Optical methods for measuring mechanical surface displacements have a resolution of some ${10}^{-6}\text{ nm/}\sqrt{\text{Hz}}$ . Although less sensitive than the piezoelectric detector, they have several advantages, the first of which is that they do not require mechanical contact, and the second is that they offer a much wider bandwidth. They lend themselves to local examination without disturbing elastic wave propagation. Heterodyne techniques are attractive for their low sensitivity to ambient noise. Probes are easy to calibrate. Their efficiency is enhanced when they are implemented as compact devices, with reduced optical paths. It should be noted, however, that these heterodyne probes...