A performance study of a laser Doppler vibrometer for measuring waveforms from piezoelectric transducers.

The stresses at transducer contacts were estimated from accurate particle velocity measurements by using a laser Doppler vibrometer (LDV). We then evaluated the performance of LDV for ultrasonic waveform measurements in physical model experiments that are employed for modeling seismic observations. For such experiments, the characteristics of the source and detector should be exactly known. Disc-shaped compression and shear-mode piezoelectric transducers were attached on a flat surface of a steel semicircular column, and ultrasonic waves were excited by single-shot sine waves with 0.25, 0.5, and 1 MHz frequencies. Radial and transverse components were measured by LDV at a distance of 150 mm from the source. The maximum amplitudes of waves with respect to radiation angle give a radiation pattern of a transducer. Each observed radiation pattern was fitted to the calculated radiation pattern by assuming a harmonically oscillating stress distributed uniformly on a flat circular area. The observed radiation patterns show fairly good agreement with the calculated radiation patterns for both radial and transverse components when the source frequencies are 0.25 and 0.5 MHz. Because the best-fit stress values were independently estimated from the radial and the transverse radiation patterns, the 2 stress values should be equal for each source and frequency. The discrepancy between the estimated radial and transverse stress values becomes larger as the source frequency increases. Provided that coincidence of the 2 stress values indicates the validity of waveform measurements, the results suggest that LDV is applicable for measuring the 3-D particle-velocity at frequencies up to 0.5 MHz.