Optimization of low-power laser generated ultrasound

Laser generated ultrasounds are lately gaining importance in the research world, thanks to the possibility of investigating a mechanical component structural integrity through a non-contact and non-destructive testing procedure. The ultrasonic outputs are, however, very low in amplitude, making it useful to use a cross-correlation operation between the trigger signal (a TTL which drives the laser) and the output. The current work studies how several important parameters can influence and improve the ultrasonic output S/N ratio if a low-power laser is employed: the number of pulses N_pulses, the number of averages N_averages, the pulse duration δ and the randomness of the pulse sequence. A FEM model of the problem has been initially developed, in order to analytically study the longitudinal and surface ultrasound generation mechanism and the obtainable outputs. At a later time, experimental tests have shown that the randomness of the sequence is not particularly important, even though literature approches employ pseudo-noise PN sequences only; in addition to this, δ becomes the most influent variable on thermoelastic response of the material, varying also the ultrasonic output frequency band. This feature, in particular, can be crucial based on the type of receiving probe used (broad or narrow band probes).