Relation between phase differences and small layer thicknesses

On laboratory scale a new NDT technique `acousto-optics´ is elaborated for the quality control of layered structures. This technique measures the modulation of ultrasound reflected by a layered material optically in amplitude and phase using laser beams crossing the ultrasonic beams (incident and reflected). The most important characteristics of layered structures investigated are: the thickness and the thickness variations within a layer and the presence of artificial defects. Only when the ultrasound is incident in the Rayleigh angle of the substrate material, the phase changes obtain important information on the quality of the layered structure. The experimental results underline the results already obtained from the theoretical models elaborated for this technique