Probing Focused Sound Fields Using Optical-Beam Deflection Method

Abstract

Optical-beam deflection was used to probe a focused sound field in water. A laser beam was deflected by ultrasonic waves radiating from a concave transducer at 2.1 MHz under conditions in which that the sound wavelength is larger than the laser beam width. The deflected signal was detected with a position-sensitive photodiode, yielding a waveform that was the time derivative of the sound pressure. Scanning the laser beam in the propagation and radial directions, we obtained a map of the focused sound field. The results were compared with those from hydrophone measurements and theoretical calculations. The comparison showed qualitative agreement, and the observed difference was explained by the optical integrated effect over a range of interaction between the laser light and ultrasound. The amplitude of the maximum pressure estimated from the deflection angle was roughly in agreement with that obtained from the hydrophone method.