A. P. Brysev

Time-reversed sound beams of finite amplitude

Numerical simulations based on the nonlinear parabolic wave equation are used to investigate time reversal of sound beams radiated by unfocused and focused sources. Emphasis is placed on nonlinear propagation distortion in the time-reversed beam, and specifically its effect on field reconstruction. Distortion of this kind, due to amplification during time reversal, has been observed in recent experiments [A. P. Brysev et al., Acoust. Phys. 44, 641-650 (1998)]. Effects of diffraction introduced by time-reversal mirrors with finite apertures are also considered. It is shown that even in the presence of shock formation, the ability of time reversal to retarget most of the energy on the source or focal region of the incident beam is quite robust.

Control of time structure for acoustic pulses at parametric phase conjugation of acoustic beams

The feasibility of controlling the time profile of a periodic sequence of acoustic pulses is studied within the framework of a general phenomenological description. The control is implemented for the case of a parametric spatial phase conjugation of a beam under the conditions of pulsed modulation of sound velocity in a phase-conjugating system. General expressions for the amplitudes and phases of equidistant spectral components of a phase-conjugate sound wave are obtained depending on the Fourier spectrum of the pumping field in the phase-conjugating medium. The time dependence of the profile of the phase-conjugate sound wave on the form of pumping is studied on this basis under different conditions, including the mismatch of the carrier frequencies of pumping and acoustic field.

Phase conjugation of the second harmonic of a focused ultrasound beam as a method for improving C‐scan acoustical imaging in nonlinear inhomogeneous media

Acoustical imaging in complex media (e.g., biological tissue) can be affected by phase aberrations introduced in a wave during propagation. Wave phase conjugation (WPC) of ultrasound is known for its ability to compensate for phase distortions due to inhomogeneity of the propagation medium, and it can be used for improvement of acoustical imaging under these conditions. In a nonlinear medium harmonics are generated during propagation of an intense beam of ultrasound, and this principle is used in tissue harmonic imaging. The parametric method of WPC permits phase conjugation of a selected frequency component of the probe beam. In this way the peculiarities of WPC can be combined with advantages of harmonic imaging. Automated WPC‐focusing of the conjugated second‐harmonic component of a focused nonlinear probe beam is studied experimentally and theoretically for the case of a homogeneous medium, and experimentally for a medium with pseudo‐random inhomogeneities. The generated conjugate wave can also be suf...

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

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 8MPa and strongly distorted, asymmetric waveforms were measured at the focus of conjugate beams having a fundamental frequency of 5MHz. 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 i...

Parametric wave phase conjugation of nonlinear ultrasound waves

Real time acoustic wave phase conjugation (WPC), based on parametric self‐consistent physical mechanisms, was realized up to the present time only for the monochromatic waves [A. P. Brysev et al., Phys.‐Usp. 41, 793 (1998)]. Here the possibility of WPC of nonmonochromatic ultrasound waves is considered. For simultaneous WPC of the entire series of spectral components generated by nonlinear propagation of the incident wave we propose the use of phonon‐plasmon interaction in piezosemiconductors. WPC of nonlinear acoustic waves can be accomplished by modulation of the electron density provided by a sequence of short laser pulses pumping the sample. If the periodicity of the optical pulses is half the period of the fundamental component of the acoustic wave, such wide‐band, excitation leads to self‐synchronized parametric conjugation of each spectral component in the incident wave. The conjugation efficiency depends sharply on relations between acoustical frequency content, laser pulse duration, and interband...

Wave phase conjugation of the second harmonic in a focused ultrasonic beam

Wave phase conjugation of the second‐harmonic component generated nonlinearly in a focused beam of ultrasound is investigated experimentally and theoretically. The incident field in this case is radiated from an extended volume of the fluid between the acoustic source and the phase conjugation system. A tone burst of frequency f=3 MHz was radiated into water and focused at a point midway between the source and the conjugator. Phase conjugation of the second harmonic 2f was performed inside a magnetostrictive ceramic modulated by a magnetic pump field at frequency 4f. The conjugate beam at frequency 2f reproduces quite accurately the incident second‐harmonic beam everywhere between the focal plane and the conjugator. The agreement deteriorates somewhat between the focal plane and the acoustic source, because it is mainly in this region where second‐harmonic generation occurs. Experimental observations are supported by analytical and numerical results. Phase conjugation using the nonlinearly generated secon...

Nonlinear acoustics of phase conjugate ultrasound waves

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 nondisper...

Acoustical waveform reconstruction by acousto-optic data processing

Experimental results of the study of nonlinear propagation of phase conjugated ultrasonic beam in water are presented. Time waveforms were measured using spectral analysis of light diffraction by the sound field, which permits digital acquisition of signal pulses. Data processing and analysis were carried out in approximation of Raman-Nath conditions. For high intensity ultrasound the method based on direct connection between amplitude of Raman-Nath spectra and Fourier components of ultrasonic wave was developed.