Broadband measurements of ultrasonic waves using optical beam deflection

Abstract

A technique of optical beam deflection by ultrasonic waves was developed to measure acoustic nonlinearity parameters B/A and ultrasonic absorption in liquids. The laser beam is deflected by ultrasonic waves under the condition that the sound wavelength is larger than the laser beam waist. The deflection angle was detected with a high-speed photodiode having double elements, yielding a time derivative of pressure amplitude of the ultrasonic waves. Two kinds of experiments were performed using this technique. The distorted waveforms of finite-amplitude ultrasonic waves of 6.65 MHz were observed. Nonlinearity parameters were obtained from the fundamental and second-harmonic wave amplitudes in water and ethanol. Second, a broadband ultrasonic pulse was observed as function of propagation distance. Spectral analysis of the pulsed waveform enabled absorption measurements in the 5–35 MHz range in gelatin sols and gels.