A comparison of Q-switched and intensity modulated laser pulses for ultrasonic NDT

The purpose of this work was to investigate the potential of intensity modulated laser pulses for ultrasonic signal enhancement in NDT using a numerical model. Q-switched lasers produce broadband ultrasound which is susceptible to measurement noise. Intensity modulation can alleviate this problem by focussing the ultrasonic energy into a band centered around the modulation frequency. Ultrasonic waveforms and their DFTs are considered here in both aluminum and steel for a wide range of observation angles, for Q-switched laser pulses and for a 1 MHz intensity modulated laser pulse. These waveforms are useful to NDT researchers because they indicate what kinds of signals can be expected from unflawed materials. It is found that P waves are not as easily enhanced by modulation as are SV or Rayleigh surface waves. The time window which should be used to measure ultrasonic signals produced by intensity modulated laser pulses is determined. © 1998 Plenum Publishing Corporation.