2. Optical detection of mechanical displacements

The aim of this section is to describe optical devices for measuring mechanical displacements of a surface. These devices are less sensitive than piezoelectric detectors, but they offer the advantage of local, non-contact examination with a very wide bandwidth.

The interaction of a light beam, diameter d, with an acoustic wave, wavelength λ, modifies :

• its direction if d < λ: the light beam is deflected by the corrugation of the surface ;

• its intensity if $d\gg \lambda$ : the light beam covering several wavelengths is diffracted by the phase grating associated with the elastic wave;

• its phase by the variation of the optical path...