THE LASER-GENERATED ULTRASONIC PHASED-ARRAY - ANALYSIS AND EXPERIMENTS

Focused ultrasonic waves have been generated in a solid by irradiating its surface with a multiple beam-pulsed YAG laser. A set of 16 rectilinear sources is used, equivalent to a phased array of ultrasonic transducers. Longitudinal waves are focused in the sample by introducing an appropriate time delay between each laser pulse. The elastic waves are detected either by a broadband optical heterodyne probe to analyze the wide ultrasonic signal spectrum (0-20 MHz), or by a narrow-band piezoelectric transducer to achieve the sectorial acoustic beam scanning of the sample. Neglecting heat diffusion in the solid and considering the source as a surface center of expansion, the impulse directivity patterns of laser-generated longitudinal acoustic waves have been computed. Experiments performed on duraluminum samples in the thermoelastic regime and steel samples in the ablation regime are presented and compared with this analysis. It is shown that a high focusing and a significant improvement of the signal sensitivity for longitudinal waves can be achieved with this technique.