A non-intrusive measuring technique, applied to sensing and measuring acoustic waves at ultrasonic
frequencies is considered. The method is optically based and so does not interfere with the
ultrasound field. The measurement procedure relies on the acousto-optic effect, that is the change in
refractive index which occurs with changing pressure in the ultrasound field. This change in
refractive index is detected through the change in the path length of a laser beam propagating
through the region of interest. Typically these changes are small corresponding to a physical change
of the order of 10−6 m. Fourier analysis is used to separate the component of the signal
corresponding to the pressure variation from background noise and vibrations which can be
dominant. Application of the technique is illustrated for an underwater ultrasound transducer.
Measurements are made using the optical technique and compared to measurements taken with a
hydrophone. The effectiveness of the optical measuring technique is discussed. It is shown that the
laser vibrometer produces a good estimation of the mean beam pressure provided an estimation of
the beam width is available, a restriction which is often satisfied; and the acoustic field can be
assumed to be approximately constant across the beam.