20-MHz acoustic waves from pulsed thermoelastic expansions of constrained surfaces

Repetitive pulses from lasers with pulse widths 5-10 nsec or a current generator with 10-25-nsec widths have been used to launch acoustic waves by thermoelastic expansions. For the laser case, when transparent media such as quartz plates are used to acoustically constrain the energy absorbing surface, an increase of up to 46 dB at 20 MHz was observed over that generated from a free surface. An experiment using a scannable laser to generate elastic waves for flaw detection in a metallic sample is described.