Focused nonlinear phase-conjugate waves generated by a solid parametric amplifier

Abstract

An experimental and numerical study is reported on focused, phase-conjugated sound beams of finite amplitude produced in water by a solid-state parametric amplifier, referred to as the conjugator. The sound beams incident on the conjugator were radiated by a focused circular source. Field distributions were measured along the beam axis and in the focal plane of the source. Both linear and nonlinear propagation were investigated. Apertures of different diameters were placed in front of the conjugator to demonstrate diffraction effects associated with size of the active surface of the conjugator. Peak-to-peak acoustic pressures of 8 MPa and strongly distorted, asymmetric waveforms were measured at the focus of conjugate beams having a fundamental frequency of 5 MHz. Numerical simulations of harmonic generation in the conjugate beams are in agreement with the measurements. The study reveals that the conjugate beams accurately reproduce the focal region of the incident beams when a sufficiently large aperture is used, even when substantial waveform distortion due to nonlinear propagation effects are observed in the conjugate beams.