The influence of the acousto-optic effect on LDV measurements of underwater transducer vibration and resultant field predictions

A scanning Laser Doppler Vibrometer (LDV) provides a potential method of measuring the surface velocity of an immersed underwater transducer. Scanning the laser beam across a transducer in this way has the potential to be a fast, non-invasive method for source characterisation and, in turn, field prediction. Such measurements are, however, significantly complicated by the acousto-optic interaction – that is, the effect on the measurements of the acoustic field through which the laser beam passes. A detailed simulation of the LDV measurement of a circular, plane piston transducer emitting a toneburst has been created. This use of a transient pressure field is important both for simulation and experiment, such that measurements can be made over a time window which ends before any acoustic signal reaches the water tank boundaries. The simulation results show a significant acousto-optic artefact contribution to the surface velocity data, but also that for some applications useful field predictions may be made in spite of this. To complement the simulations, experimental measurements have been made using a commercial LDV (Polytec PSV-400) on 500 kHz circular transducers