In the past, two laser beams were used for the accousto-optic technique. One of the laser beams was diffracted by the incident ultrasound while the other was diffracted by the reflected ultrasound. Comparing the optical near field signals the phase difference between the incident and the reflected ultrasound can be measured. Experimental measurements revealed that the geometrical shape of a plate, the thickness of a thin layer as well as the presence of defects inside a layered material influence the phase measurements. this combined influence made the experimental results rather hard to interpret. Therefore the accost-optic technique based on the use of two laser beams is only used to detect the geometrical shape of a plate. The Luder's bands present on a tensile specimen of an aluminium alloy could in this way be visualised as well as the surface roughness of an arbitrary sample.

To measure the thickness of a thin layer exactly or to detect the presence of "small" defects in a layered material a new experimental setup was developed in which only one laser beam was used, travelling through the overlapping region of the incident and the reflected ultrasound. Phase differences are measured between the optical near field signal and a reference signal. Layer thicknesses less than 1µm could be detected as well as very small defects inside a material. REFERENCE

1. M. Wevers, S. Devolder, O. Leroy and P. De Meester, "Accost-optic technique: a new non-destructive technique to evaluate thin layered structures", Appl. Phys. Lett., Vol. 66, pp. 1466-68, 1995.