Numerical and Experimental Studies towards Improvements in Laser-Acoustic Microscopy by Optical-Based Sound Beam Shaping

Incorporating Laser-Ultrasonic systems for excitation and acquisition of ultrasonic waves into NDT&E systems establishes the benefits of contact free methods to high-frequency ultrasonic measurements. Techniques using this huge potential were investigated for a long time but in most cases with focus to surface waves. To study the material bulk properties strong surface damaging laser pulses were mostly used. However, by shortening the pulse length down to some nano-seconds at moderate intensities one can excite ultrasonic waves in the bulk of the material without any damage. The work presented deals with the prediction of the acoustic fields shape based on arbitrary energy distribution across the excitation laser spot and its use in ultrasonic inspection. An algorithm is discussed which uses the laser beam profile, measured surface normal displacements and bulk wave speeds as input. Comparing simulated an measured data reveals basic agreements but also still some discrepancies caused by additional thermal diffusion in the excitation region. Using this algorithm already offers to analyse the presumed location and shape of longitudinal and transversal wave foci. Further, its capability for optimizing laser-acoustic systems is demonstrated.