Ultrasonic wave field determination of laser-acoustic sources with arbitrary shapes

Laser systems for excitation and detection of ultrasonic waves incorporate the benefits of a completely contact free method into ultrasonic NDT systems. Techniques using this huge potential were investigated for a long time but in most cases with focus on symmetric sources for surface waves. To study the material bulk properties strong and surface damaging laser pulses were necessary. However, by shortening the pulse length down to some nanoseconds and below one can excite ultrasonic waves in the bulk of the material without any damage. Important for ultrasonic testing methods is the shape of the ultrasonic field which is widely studied for simple geometries like point or line like sources but not for arbitrary intensity distributions in a general manner. The developed method offers to handle realistic source shapes which enables the link between the optical and acoustical design of contact free ultrasonic systems. Statement of Contribution and Methods: This work delivers valid arguments on improving the prediction of laser acoustic wave fields in the bulk of the medium towards a better understanding of the governing mechanisms. A hybrid point source synthesis HPSS is proposed combining laser beam profile and surface displacement measurements with the common algorithm behind a point source synthesis. Based on the calculated wave fields discretized in space and time the impact of asymmetries in the laser beam on the excited wave field is investigated. Results: Comparing idealized Gaussian shaped excitation to realistic laser intensity distribution reveals the dislocation of the center of the ultrasonic field. For both, the longitudinal and transversal acoustic wave, the natural focal dislocation increases with rising asymmetries of the laser beam profile.