Optimal Rayleigh waves generation by continuous wave modulated laser

Laser-ultrasound technology is typically employed in case of non-destructive, noncontact inspection of mechanical components. In particular, low power laser sources (diodes) allow to contain implementation costs; on the other hand, identification of the ultrasonic peak is complex due to the low Signal-to-Noise Ratio (SNR), requiring the use of specific signal processing techniques. Features of the ultrasounds generated by the laser excitation, both in terms of frequencies and SNR, cannot be foreseen in advance, depending from the type of material and its thermo-elastic characteristics: it is thus fundamental to dispose of criteria to set in an optimal way the signal acquisition parameters to effectively apply a correct processing procedure and retrieve the useful information. In the work, surface R (Rayleigh) waves generated by a Continuous Wave (CW) low power laser are characterized, using a particular processing technique in the time domain. To identify the most influential input parameters on SNR, a Design Of Experiments (DOE) and a specific analysis are introduced: overall, the distance between source and receiver and the number of ensemble average applied before acquisition strongly affect SNR; the pulse duration results on the other hand influential at the same time on SNR and on the generated ultrasound frequency. Finally, analogies with longitudinal (L) waves generated by the same source are highlighted, allowing also for information on how to set up the investigation based on the type of wave and acquisition instruments employed.