Phase-conjugated ultrasonic beams of finite amplitude

Abstract

Generation of intense, phase-conjugated ultrasonic beams by a single active magnetic element in a supercritical parametric mode has been demonstrated experimentally and theoretically. Specifically, phase conjugation with amplification greater than 80 dB, producing intensities of several hundred W/cm2 at MHz frequencies, is accomplished via parametric electromagnetic pumping of magnetoelastic solids. Reversal of the sound field incident on the conjugator has been confirmed by observation of the self-targeting of beams on scattering sites in water [Usp. Fiz. Nauk 168, 877–890 (1998) (in Russian)]. The large amplification provided by the conjugator leads to nonlinear distortion and shock formation in the conjugate field. These nonlinear effects are studied experimentally with measurements based on light diffraction for conjugated beams in water having intensities of 2 W/cm2 and frequencies near 5 MHz. Numerical simulations corresponding to experimental conditions take into account absorption, diffraction, nonlinearity, and the finite size of the conjugator. The simulations reveal the presence of shocks, yet accurate reversal of the incident field despite the strong waveform distortion [Phys. Acoust. (in press (1998)]. Potential applications of this technololgy are discussed. [Work supported by RFBR (Grants 96-02-17301, 98-02-16761), CRDF (Project RE1-270), and ONR.]

<title>Acoustical waveform reconstruction by acousto-optic data processing</title>

“…A correct reconstruction of the waveform by (2) requires an information about the whole diffraction spectrum. A correct reconstruction of the waveform by (2) requires an information about the whole diffraction spectrum.…”

“…Vm exp(jmx) = Vm m exp(jmx) (5) After multiplying and equalization of coefficients with the same exponent we get a system of linear equations which uniquely connect Fourier components of ultrasonic wave ak with amplitudes of Raman-Nath diffraction orders Vm. k .…”

“…In the last years a great progress was achieved in investigation of ultrasonic wave phase conjugation (WPC) [1,2]. Application of parametric WPC principles to the magnetostrictive ceramics with parametric pumping by alternating magnetic field made it possible in practice an ultrasound WPC with a gain more than 80 dB and high conjugate wave intensity (of about hundreds W/cm"2) [3].…”

“…In this respect, no-lens systems using the phase conjugation effect for the selffocusing of ultrasound beams [4,5] seem to be more promising. The studies on quality ofphase conjugation ofultrasound in a nonlinear medium are important for practical applications of high-intensity conjugate ultrasonic beams.…”

“…For experimental study ofconjugate sound beams propagation the methods based on light diffraction by ultrasound are the most adequate [1,2]. The main advantage of optical methods is a complete absence of distortions introduced to an acoustic field.…”