Nonlinear distortion of finite‐amplitude ultrasonic waves investigated by optical heterodyne interferometry

The study of asymmetrical patterns of the light diffracted by ultrasonic waves has been utilized in the last decades to investigate ultrasonic wave distortions caused by the nonlinearity of the medium. In contrast to the investigation of diffraction patterns in space, a novel optical method is proposed in this work to analyze, in the frequency domain, the phase modulation (related to the Raman–Nath parameter) of the probe beam. Using a heterodyne interferometric method, the effects of ultrasonic waves are described as sideband generations (Doppler shift) about a central frequency (optical heterodyning) in the frequency spectrum of the photocurrent. Asymmetry resulting from nonlinear effects was observed upon the spectrum envelope. This effect increases both with ultrasonic intensity and with propagation distance. Experimental results obtained in water confirmed qualitatively the theoretical analysis which includes the second and third harmonics in the formulation.