Studies of laser-generated ultrasonic waveforms at different orientations

The ultrasonic waveforms generated by a pulsed laser incident at two positions on a steel plate are studied as a function of laser energy. At the 45° position there is a steady increase in compression wave amplitude with energy when passing from thermoelastic to ablation regimes, in contrast to the epicenter where there is a large increase in amplitude and change of pulse shape. It is concluded that the thermoelastic and ablation sources are comparable in generation efficiency per unit laser energy, except close to the epicenter, and that the chief effect of raising the power density is to vary the angular distribution of ultrasonic energy.