Random Sequence for Optimal Low-Power Laser Generated Ultrasound

Abstract

Abstract. Low-power 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 (NDT) procedure. The ultrasounds are, however, very low in amplitude, making it necessary to use pre-processing and post-processing operations on the signals to detect them. The cross-correlation technique is used in this work, meaning that a random signal must be used as laser input. For this purpose, a highly random and simple-to-create code called T sequence, capable of enhancing the ultrasound detectability, is introduced (not previously available at the state of the art). Several important parameters which characterize the T sequence can influence the process: the number of pulses , the pulse duration δ and the distance between pulses . A Finite Element FE model of a 3 mm steel disk has been initially developed to analytically study the longitudinal ultrasound generation mechanism and the obtainable outputs. Later, experimental tests have shown that the T sequence is highly flexible for ultrasound detection purposes, making it optimal to use high and δ but low . In the end, apart from describing all phenomena that arise in the low-power laser generation process, the results of this study are also important for setting up an effective NDT procedure using this technology.

Signal Enhancement in Surface Crack Detection with Gas-Coupled Laser Acoustic Detection

“…If not, the finite beam size reverses the behavior: a higher response for high acoustic wavelengths is observed, the beam diameter being the same [41]. As a non-contact detection device, GCLAD can sense variations in the refractive index associated with ultrasound induced by different types of contact or non-contact sources, like continuous wave lasers [42][43][44][45][46] or contact piezoelectric probes [37].…”

Crack detection with gas-coupled laser acoustic detection technique

“…For these reasons, the possibility of using cheaper laser sources like continuous modulable lasers is currently being investigated. These sources are characterized by a lower irradiance, resulting in a reduction in the risk of ablation but also in the amplitude of the generated ultrasonic signals; for further information, refer to previous works by the authors [28,[30][31][32].…”