Full Matrix Capture and the Total Focusing Imaging Algorithm Using Laser Induced Ultrasonic Phased Arrays

Laser ultrasonics is a technique where lasers are used for the generation and detection of ultrasound instead of conventional piezoelectric transducers. The technique is broadband, non-contact, and couplant free, suitable for large stand-off distances, inspection of components of complex geometries and hazardous environments. In this paper, array imaging is presented by obtaining the full matrix of all possible laser generation, laser detection combinations in the array (Full Matrix Capture), at the nondestructive, thermoelastic regime. An advanced imaging technique developed for conventional ultrasonic transducers, the Total Focusing Method (TFM) [2], is adapted for laser ultrasonics and then applied to the captured data, focusing at each point of the reconstruction area. In this way, the beamforming and steering of the ultrasound is done during the post processing. A 1- D laser induced ultrasonic phased array is synthesized with significantly improved spatial resolution and defect detectability, without the need of complicated optical setups, optical fibres or use of multiple laser beams. Laser ultrasound generates longitudinal, shear and surface acoustic waves, simultaneously. In this study, shear waves are used for the imaging, since they are more efficiently produced than longitudinal waves in the nondestructive, thermoelastic regime. A model for laser generation and detection of shear waves is presented that has been used to simulate results in the samples used. Experimental results are presented from nondestructive, laser ultrasonic inspection of aluminum samples with side drilled holes and slots at depths varying between 5 and 20mm from the surface and are compared with the model ones. The results using the TFM imaging algorithm are also compared with those using the Synthetic Aperture Focusing Technique (SAFT) which previous authors [2] have used with laser ultrasonics