Laser generation of ultrasonic Lamb waves using low power optical sources

Abstract

Laser generated ultrasound is a convenient noncontact broad bandwidth source of test radiation for ultrasonic nondestructive evaluation (NDE). However, conventional laser ultrasound exploits high pulse energies in short duration pulses typically involving peak powers in the tens to hundreds of megawatts range. Consequently, surface damage is frequently problematic. The prospects are examined for low peak power sources based on laser diode excitation of mechanical samples. With peak powers of significantly less than 1 W and a 2% duty cycle, the authors have demonstrated broadband Lamb wave excitation in a thin metal sample, including observation of the dispersion curves. The same principle can be extended to full continuous wave (CW) operation. This enhances the received SNR while involving minimal probabilities of sample damage.