在局域响应的光折变晶体中的相干光放大

理论和实验表明，在由光生伏打电荷迁移为主的局域非线性响应的薄光折变晶体（LiNbO₃∶Fe）中实现弱信号光放大是可能的．光能流从抽运光向信号光不可逆转移的起因是两束入射光与背面反射光之间的四波作用形成互倾斜、彼此有一定空间相移的两组相位栅．当入射抽运光在非布喇格条件下读出由反射光和其衍射光所写入的新光栅时，弱信号便获得相干放大．对理论计算与实验结果进行了比较． 关键词：     

超声检测用压电换能器瞬态特性可控可调

超声检测用压电换能器瞬态特性主要包括瞬态空间响应和瞬态时间响应。本文通过调整控制换能器和激励源,实现瞬态空间响应和瞬态时间响应的某些特性的可控可调。其中包括在空间响应方面消除边缘波以获得平行声束和消除平面波以获得聚焦绳声束;在时间响应方面,调整换能器的背衬阻抗以获得可调首次波幅比和调整换能器结构和激励电信号以获得任意检测信号等。     

Dynamic optical transfer function of an acoustooptic modulator with a damped sound wave

Based on a spatially dependent dynamic optical transfer function of an acoustooptic modulator, a dynamic model of light modulation by a sound signal under conditions of acoustic damping is developed. A system of equations describing the dynamics of the acoustooptic interaction is given. Solutions of this equation for arbitrary power level and spatial-time structure of a sound signal are found. It is shown that acoustooptic damping has the strongest effect in a nonlinear modulation regime. Here, the dissipation of an acoustic signal suppresses the higher harmonic of the dynamic optical transfer function. An analytical model of the dynamic optical transfer function for a low level of acoustooptic coupling is given. It is shown that when the pump beam aperture is much greater than the spatial size of an acoustic signal, the time response of the acoustooptic modulator response is identical within a phase factor to the amplitude profile of a pump beam apodized by the exponential dependence of the amplitude of a damped sound wave. Otherwise, the sound damping produces almost no distortions of a plane top of the acoustooptic modulator response to a pulsed signal and shows itself only under phase mismatch conditions. Here, the asymmetry of overshoots at the edges of the acoustooptic modulator response is observed. Calculated plots are presented, which illustrate the aforementioned specific features of the transient process under conditions of acoustic damping in the acoustooptic interaction in paratellurite.     

A prototype for a tomography system using third-order acoustic nonlinear effects

A prototype of a tomography system for reconstructing the distributions of acoustic nonlinear parameters is developed and manufactured on the basis of the effect of nonlinear noncollinear interaction of three primary waves. Application of coded primary signals with further correlation processing of a detected combination signal makes it possible to reconstruct the complete image of an object as a result of a single experiment using a small number of transducers, i.e., three radiators and one receiver. A mirror system is proposed, consisting of two coaxial conical acoustic mirrors that make it possible to transform the front of a wave from a cylindrical transducer into a homogeneous quasi-plane beam with a large width close to the real medical diagnostics requirements. Results of physical experiments are given.     

Increase in the efficiency of the shear wave generation in gelatin due to the nonlinear absorption of a focused ultrasonic beam

Experimental results and theoretical estimates are presented to demonstrate the prospects of using the acoustic nonlinearity of a gel-like medium for increasing the efficiency of the shear wave generation in it by a pulsed ultrasonic beam. The experiment is based on the propagation of a focused beam of longitudinal acoustic waves at a frequency of 1.1 MHz in a gelatin sample and on the detection of shear waves by the optical method [1]. It is demonstrated that the amplitude of the shear wave excited by a nonlinear acoustic pulse can be increased by an order of magnitude owing to the formation of shock fronts in the profile of this pulse.     

Acoustic scattering of a high-order Bessel beam by an elastic sphere

The exact analytical solution for the scattering of a generalized (or “hollow”) acoustic Bessel beam in water by an elastic sphere centered on the beam is presented. The far-field acoustic scattering field is expressed as a partial wave series involving the scattering angle relative to the beam axis and the half-conical angle of the wave vector components of the generalized Bessel beam. The sphere is assumed to have isotropic elastic material properties so that the nth partial wave amplitude for plane wave scattering is proportional to a known partial-wave coefficient. The transverse acoustic scattering field is investigated versus the dimensionless parameter ka(k is the wave vector, a radius of the sphere) as well as the polar angle θ for a specific dimensionless frequency and half-cone angle β. For higher-order generalized beams, the acoustic scattering vanishes in the backward (θ = π) and forward (θ = 0) directions along the beam axis. Moreover it is possible to suppress the excitation of certain resonances of an elastic sphere by appropriate selection of the generalized Bessel beam parameters.     

Hysteresis in response of nonlinear bistable interface to continuously varying acoustic loading

In many experimental situations it is an equation of the forced relaxator and not of the forced oscillator that describes a variation in the acoustic field of the interface width (i.e. of a characteristic distance between the surfaces composing the interface). The developed theory predicts that some types of the nonlinear relaxators (depending on the structure of the nonlinear interaction force between the surfaces) exhibit hysteresis in their response to continuous acoustic loading of first increasing and then decreasing amplitude. Nonlinear (unharmonic) variation of the interface width starts at threshold amplitude of the incident sinusoidal acoustic wave, which is higher than threshold amplitude for returning to sinusoidal motion. This dynamic hysteresis (and accompanying it bistability) are possible, in particular, if the dependence of the effective interaction force on the interface width admits two quasi-equilibrium positions of the interface (bistable interface) or if the force itself is hysteretic (hysteretic interface). These theoretical predictions are relevant to some recent experimental observations on the interaction of powerful ultrasonic fields with cracks.     

Experimental study on subharmonic and ultraharmonic acoustic waves in water-saturated sandy sediment

Experimental observations of the subharmonic and ultraharmonic acoustic waves in water-saturated sandy sediment are reported in this paper. Acoustic pressures of both nonlinear acoustic waves strongly depend on the driving acoustic pressure at a transducer. The first ultraharmonic wave reaches a saturation value as the driving acoustic pressure increases. The acoustic pressure levels of both nonlinear acoustic waves exhibit some fluctuations in comparison with that of the primary acoustic wave as the receiving distance of hydrophone increases in sediment. The subharmonic and the ultraharmonic phenomena in this study show close resemblance to those produced in bubbly water.     

Fluctuations of acoustic field in a granular medium

Results of an experimental study of sound propagation in a granular medium are presented. It is found that, in the case of excitation of a harmonic signal with a constant amplitude, the acoustic response of a single grain strongly varies in time. The dependence of the harmonic component amplitudes in the response spectrum on the level of signal excitation proves to be nonmonotonic and also strongly varies in time. The most intense fluctuations are observed in the subharmonic component of the propagating signal. The intensity fluctuation spectra of the harmonic components of the response are obtained for the frequency range of 10⁻⁴−10⁻¹ Hz. A possible mechanism that may be responsible for the slow fluctuations of an acoustic field in a granular medium is discussed.

     

基于布拉格声光衍射场的光强非均匀空间分布研究

为了保证合成孔径激光雷达测试中信号光的光束质量,从波动方程出发,推导出传统的衍射光源布拉格声光作用的耦合波方程。根据测试对光束质量的不同要求,借助有限差分方法分析衍射场光强的空间三维分布,发现声场振幅的不均匀分布会影响声光衍射场的光强分布,光强分布不均造成实验中信息光源的探测难度。通过光强空间分布研究增大有效通光孔径到3 mm,解决了声光调制器光束衍射质量问题,改进后的光束质量达到实验要求。该研究方法适用于改进声光器件工作参数。     

Competition between subharmonic and signal beams for photorefractive gain in BSO with two pump beams

It is shown that two pump beams incident upon a bismuth silicon oxide crystal give rise to a subharmonic beam bisecting the pump beam. Simultaneously a signal beam injected between the pump beams may be strongly amplified at the expense of the pump beams. It is shown that the amplified signal beam grows with a faster time constant than the subharmonic beam and that there is competition for gain in the steady state regime.     

Signal enhancement in Rayleigh wave interactions using a laser-ultrasound/EMAT imaging system

Enhancement of signal amplitudes from Rayleigh wave interaction at solid surface features has been investigated when signals were detected by an in-plane electromagnetic acoustic transducer (EMAT). A laser-ultrasound system was used to inspect surface-breaking slots, serving as artificial defects. Nd:YAG laser pulses were delivered onto a metal surface via an optical fiber and focused to a line source by a cylindrical lens. An in-plane EMAT receiver detected transient surface acoustic waves. A-scan signals and B-scan images from surface defects revealed increased signal amplitude up to 2.8+/-0.3 depending on the distance of the transducer from a slot. An explanation is based on the interaction of the EMAT sensor with the Rayleigh wave. A supporting computer model was derived to show that experimental signal enhancements were consistent with numerical predictions.     

Effect of a constant magnetic field on echo signals in high-temperature superconductor powders

The generation of acoustic and vortex oscillations in high-temperature superconductor (HTSC) powders excited by radiofrequency (rf) pulses was analyzed in detail in our earlier publications. The rf magnetic field stimulates oscillations of magnetic vortices on the surface of an HTSC grain, which are transformed into lattice vibrations via the pinning centers at the surface, thus inducing a propagating acoustic wave. The allowance for second-order nonlinearity in the gradient of deviation of the crystal lattice from its equilibrium position in the equation for the acoustic wave leads to a dependence of the natural frequency of crystal lattice vibrations on the amplitude and duration of pulses exciting these vibrations. Such a dependence is responsible for echo signals that can be detected experimentally. The proposed model makes it possible to interpret most experimental results for BiPbSrCaCuO superconducting samples. We consider the effect of a constant magnetic field on the amplitude and the echo signal decay time. We observed a clearly manifested peak that was not described by other authors. The model proposed here provides an obvious explanation for this peak.     

Spatial dependence of the dynamic optical transfer function of a transparent medium perturbed by an acoustic signal

Based on a spatially dependent (SD) dynamic optical transfer function (DOTF) of a medium perturbed by an acoustic signal, a dynamic theory has been developed of the acousto-optical interaction (AOI) of light beams in the field of an arbitrary acoustic signal under conditions of the diffraction distortion of its amplitude profile (DDAP). A system of integro-differential equations is derived for angular and frequency DOTF spectra of coupled waves in the AOI region. An analytical DOTF model has been constructed for a weak acousto-optical coupling. The characteristics of transient processes (TPs) in the diffracted-beam field have been investigated for the spatially dependent DOTF. It is demonstrated that in most practical situations this dependence is manifested rather strongly; however, it does not always affect the TP. Thus, when the light and acoustic beam apertures exceed some characteristic values d^* and L^*, related to the acoustic frequency and speed and the AOI geometry, the effect of the SD DOTF on the transient process can be neglected. Otherwise, the DDAP leads to the asymmetry of the dynamic dependences of the field amplitude of the off-axis angular-spectrum components of the diffracted beam. The parameters d^* and L^* have been calculated for the AOI of several types in lithium niobate and paratellurite crystals. The TP characteristics are illustrated by plots calculated for the DDAP in the case of the AOI in paratellurite. Tomsk State University of Control Systems and Electronics, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 40–48, May, 1999.     

Ultra-fast reconfigurable spatial switching between a quadratic solitary wave and a weak signal

Experimental and numerical investigations of an ultra-fast reconfigurable spatial switch based on the nonlinear interaction between a weak wave (the signal) and a solitary wave (the control) at 1548 nm are reported. The non-collinear interaction in a quadratic nonlinear film waveguide gives birth to a third switched optical beam (the idler). This beam could be steered according to the transverse spatial position of the control beam. PACS 42.65.Ky; 42.65.Re; 42.65.Wi     

Acoustooptic interaction of beams close to forbidden directions

The theory of acoustooptic interaction of beams taking into account the drift and spreading of beams in an anisotropic medium is presented. The problem is solved analytically for a weak interaction of Gaussian beams close to forbidden directions. The possibility of existence of collinear diffraction of wave beams along the direction forbidden for interaction of plane waves is shown theoretically. The interaction appearing close to such a direction was found to result in a certain distortion of the beam shape. The calculation performed for a paratellurite crystal shows that the efficiency of the diffraction along the forbidden direction can reach 10% at the acoustic power usually used.     

Amplitude measurement of weak sinusoidal water surface acoustic wave using laser interferometer

To determine the amplitude of weak sinusoidal water surface acoustic wave(WSAW), a method based on the spectrum analysis of the phase-modulated interference signal is developed. Calculated from the amplitude spectrum of the detection signal, a characteristic ratio indicates that the phase-modulation depth of a WSAW is suggested by determining the amplitude of a WSAW according to their functional relationship. Experimental investigations for a 4 k Hz WSAW evaluate the measurement's precision with an amplitude measurement standard deviation of 0.12 nm. The measurement accuracy also is demonstrated by the experimental investigations.The theory of this method is briefly described, and the experimental setup is presented.     

On the equivalence principle in quantum theory

The role of the equivalence principle in the context of non-relativistic quantum mechanics and matter wave interferometry, especially atom beam interferometry, will be discussed. A generalised form of the weak equivalence principle which is capable of covering quantum phenomena too, will be proposed. It is shown that this generalised equivalence principle is valid for matter wave interferometry and for the dynamics of expectation values. In addition, the use of this equivalence principle makes it possible to determine the structure of the interaction of quantum systems with gravitational and inertial fields. It is also shown that the path of the mean value of the position operator in the case of gravitational interaction does fulfill this generalised equivalence principle.     

Plasmon beams interaction at interface between metal and dielectric with saturable Kerr nonlinearity

We present a novel theory of surface plasmon polariton interaction on the surface of dielectric with saturable Kerr nonlinearity. The effect of the total internal reflection of a weak signal plasmon beam from a high-power reference beam is discussed. Both ray and wave theories are used to describe signal propagation. The effect of the signal tunneling through the narrow inhomogeneity induced by the reference beam is considered.