Laser generation of narrowband and directed ultrasound for noncontact ultrasonic testing

The authors describe investigations directed at the use of spatial arrays and temporal beam modulation to generate ultrasonic signals which are both narrowband and may be directed (`steered´) over some angle within the test piece. By directing a portion of the ultrasonic energy, displacement amplitudes at the receiving site may be increased. Furthermore, the narrowband nature of the signal permits a corresponding reduction in the signal bandwidth of the laser receiver with a commensurate reduction in noise. Both factors, therefore, increase the ratio of the signal-to-noise and, as a result, improve the detection sensitivity of the laser-ultrasonic system. Experimental results on this technique are presented