Focusing and steering of ultrasonic waves generated by a sixteen-laser source array

A set of sixteen rectilinear thermoelastic sources, equivalent to a phased array of ultrasonic transducers has been implemented on the surface of an isotropic solid from a multiple beam YAG laser. Introducing a time delay between each laser pulse, the elastic waves were focused in the sample. The ultrasonic beam is detected either by classical piezoelectric transducers or by a compact optical heterodyne probe which is sensitive to the normal displacement of the sample surface. Neglecting heat diffusion in the solid and considering the thermoelastic source as a surface center of expansion, the directivity patterns of laser generated longitudinal acoustic waves have been computed. Experiments performed on duraluminum samples in the thermoelastic regime are presented and compared with this analysis. It is shown that a high focusing and a very good sensitivity for longitudinal waves were achieved with this technique.