Interferometric detection of finite-amplitude ultrasonic waves

The investigation of asymmetrical patterns of the light diffracted by ultrasonic waves has been used in the past to study ultrasonic waveform distortions caused by the nonlinearity of medium. Rather than the spatial diffraction pattern in the optical far-field, we propose in this work to detect the phase modulation of a narrow probe beam in the optical nearfield. Using a heterodyne interferometer, the effects of ultrasounds are analyzed in frequency domain as sideband generations about a central frequency (Doppler shift). Asymmetry is observed in the frequency spectrum due to nonlinear effects, which increases both with ultrasound amplitude and with propagation distance. Experimental results obtained in water confirm qualitatively the theoretical calculation