Acoustic nonlinearity parameter measurement using optical beam deflection by ultrasonic waves

A technique of optical beam deflection by ultrasonic waves was developed in order to measure acoustic nonlinearity parameters B/A in liquids. Finite-amplitude ultrasonic waves of 6.65 MHz propagating in a liquid sample deflect a laser beam of normal incidence. The deflection angle was detected with a photodiode having double elements, yielding a distorted waveform. Spectral analysis of the waveform yielded the ratio of the amplitude of the second-harmonic wave to that of the fundamental wave. The absolute value of the fundamental sound-pressure amplitude was obtained independently from the Raman-Nath parameter measured using the same experimental system under the condition that a laser beam waist is larger than sound wavelength. From these measurements, the nonlinearity parameters were evaluated to be 5.3 for water and 10.3 for ethyl alcohol. These results are in good agreement with established values.