Optical measurement of transient ultrasonic shock waves

In order to measure acoustic shock waves generating pressures of some tens of MPa in water, an ideal sensor has to be calibrated, wideband and sufficiently robust to support acoustic cavitation. We measure high intensity ultrasonic displacements and shock waves in water, using the phase modulation of an optical beam reflected by a thin immersed membrane materializing moving particles. With either an analog or a digital demodulation process, a 1-μm ultrasonic transient displacement (corresponding to a 22-MPa acoustic pressure in water) has been measured with a standard heterodyne interferometer. Results are in good agreement with those given by a hydrophone designed to measure high pressure waves. This sensitive optical method provides absolute measurements with a 50- μm lateral resolution, in a large bandwidth (from 20 kHz to 50 MHz). If the low cost membrane used in the experimental set-up is damaged by the acoustic cavitation, it can be easily replaced. We compare three phase detection techniques that require no calibration. The maximum measurable displacement is limited by the frequency bandwidth of the instrument. Since the photodetector cuts frequencies higher than 120 MHz, the interferometer is able to measure a 22-MPa peak acoustic pressure in water. This performance can be improved by increasing the frequency shift of the probe beam, to obtain a wider bandwidth. For measuring acoustic pressures up to 45 MPa, we have built an interferometer operating at a 140-MHz carrier frequency.