Laser induced ultrasonic phased array using Full Matrix Capture data acquisition and Total Focusing Method

Laser based ultrasound is a technique where a short pulsed laser is used to generate ultrasound and optical interferometry is used in order to detect the signal. Since both generation and detection of ultrasound is based on optical means, the technique is broadband, non-contact, and couplant free, suitable for large stand-off distances, inspection of components of complex geometries and hazardous environments. A data collection method (Full Matrix Capture) developed for ultrasonic arrays, is used for data collected using laser based ultrasound. In this method, a signal is captured from every possible transmitter-receiver array element combination. After the data capture, the imaging is done in post processing using the Total Focusing Method, in which the beam is focused, in both transmission and reception, on every pixel in the image to achieve improved defect detectability and very high spatial resolution. For the first time in laser ultrasonics, the beamforming and steering of the ultrasound is done during the post processing. In this way, a synthesised 1-D laser induced ultrasonic array operates, in post processing, as a phased array without the need of complicated optical setups, optical fibres or use of multiple laser beams. This work presents results from a non destructive laser ultrasonic inspection of aluminium samples with side drilled holes and slots at depths varying between 5 and 20mm from the surface.