Nonlinear acoustics of phase conjugate ultrasound waves

Abstract

Nonlinear propagation of phase conjugate waves (PCW) in liquid is studied experimentally and simulated numerically. A supercritical parametric technique is used for ultrasonic (5–10 MHz) wave phase conjugation (WPC) with enormous amplification (more than 80 dB). The technique provides sufficient PCW intensity for vivid manifestation of acoustic nonlinearity. Quantitative studies of refocused PCW beams have been performed. The space–time structure of nonlinear PCW was measured and results were compared with calculations. Finite aperture size and conjugator gain were taken into account in a numerical model based on the nonlinear parabolic wave equation. Accurate refocusing of strongly nonlinear PCW beams is demonstrated for sufficiently large apertures. The second and higher harmonics generated by PCW are localized within the focal region of the incident wave even after PCW refraction in a randomly inhomogeneous medium. The phenomenon is explained theoretically by phase locking of PCW harmonics in nondispersive acoustic media. The results have been generalized to include narrow-band WPC of selected harmonics of the incident wave when the latter is nonlinear. Specifically, WPC can both remove phase distortion and amplify the second harmonic sufficiently that imaging can be performed with the resulting fourth harmonic. This novel imaging technique is demonstrated in an example of C-scan acoustic microscopy.