Influence of relaxation on the acoustic self-induced transparency

Using a singular perturbation theory for the method of the inverse scattering transform, the influence of transverse relaxation on the acoustic self-induced transparency is considered. It is shown that account of the relaxation improves the quantitative agreement of the theory with experiment.     

Effect of sound speed profile on the structure of long-range acoustic pulse

The time arrival structure of acoustic pulse signals propagating in ocean waveguides is of great significance for underwater acoustic communication and navigation. Using the deep-sea sound propagation data from the experiments respectively conducted in the East Indian Ocean(EIO) and the South China Sea(SCS) with explosion sources near the sound channel axis(SCA), long-range transmission loss(TL) and time arrival structure of acoustic pulses for different sound speed profiles(SSPs) are compared. ...     

Effects of viscosity and chemical relaxation on acoustic pulse propagation in seawater

This paper considers acoustic pulse distortion in seawater due to effects of viscosity and chemical relaxation. The acoustic medium is modelled as a linear, time-invariant, space-invariant filter. The deformation of initially Gaussian-shaped pulses is estimated. The combined effects of the dissipative processes are that as a pulse propagates through the medium, the maximum amplitude is decreased, the pulse is stretched in time and a delayed tail is developed. Input pulses with high-frequency energy contents can undergo a considerable deformation due to these phenomena.     

Short optical pulse profile characterization using a nonlinear optical loop mirror

We propose in this work a technique for short pulse profile retrieval based on the Kerr effect in a fiber nonlinear optical loop mirror (NOLM). Under some assumptions, the profile can be determined from the energy transfer characteristic of the pulses through the NOLM, which can be measured with a low-frequency detection setup. Two numerical approaches are considered, both relying on the resolution of a system of nonlinear algebraic equations, and which differ in the way that the profiles are discretized. We show numerically that both approaches allow proper profile retrieval for a wide variety of pulse shapes. The two techniques are compared, and their advantages and drawbacks are discussed. The effect of the amplitude noise of the pulses is assessed, as well as the impact of an inaccurate knowledge of the NOLM transfer function, or of the energy transfer characteristic. The technique is demonstrated in the frame of the characterization of both ns and ps pulses.     

Influence of resonator shape on nonlinear acoustic field in a thermoacoustic engine

The influence of the resonator shape on nonlinear acoustic field in a thermoacoustic engine is studied.The resonator of themoacoustic engine is boundary driving by a piston at one end,and the other end of it is rigid closed.A one-dimensional wave equation that accounts for gas dynamic nonlinearities and viscous dissipation in the resonator is established based on the governing equations of viscous hydromechanics.The nonlinear wave equation is solved using approximate Galerkin method.The nonlinear acoustic field in four different types of shaped resonators including hyperbolical,exponential,conical and sinusoidal are obtained and compared with that of a cylindrical resonator.It is found that the amplitude and waveform of the pressure are strongly affected by the resonator shape,the driving amplitude and the oscillation frequency of the piston.Waveform distortion,resonance frequency shift and hysteresis are observed,when the piston oscillation amplitude is large enough.The advantages of shaped resonator for thermoacoustic engine lie in inhibition of higher order harmonics and improvement of pressure ratio,etc.     

谐振管形状对热声发动机内非线性声场的影响

研究了谐振管一端受活塞声源激励,另一端刚性封闭条件下,管道形状对热声发动机谐振管内部非线性声场的影响。基于流体力学基本方程建立了渐变截面谐振管内一维非线性声场的模型,考虑了黏性耗散及非线性效应的影响。利用伽辽金法数值求解了该模型的速度势方程,分析了谐振管形状、活塞振动速度及激励频率对管内声场的影响。将双曲形、指数形、锥形、正弦形等四种变截面谐振管内的非线性声场与圆柱形直管的情况进行了比较。结果反映了谐振管内声场的压力波动受活塞振动速度及谐振管形状的影响;显示了当活塞振动幅度较大时,谐振管内出现的波形畸变、频率曲线偏移、共振频率滞后等非线性现象;揭示了变截面谐振管在抑制管内的高阶谐波及提高压比等方面的优越性。     

Influence of the electromechanical coupling coefficient of a piezoelectric plate on the acoustic pulse duration in the presence of a compensating circuit

The dependence of the emitted pulse duration on the electromechanical coupling coefficient of a piezoelectric material is investigated by computation. The optimal value of the coupling coefficient is obtained.     

Ultrashort pulse measurements using a nonlinear mirror

The use of nonlinear optical phenomena at the boundary of a nonlinear medium for measuring the duration and the pulse shape of ultrashort light pulses are considered. Two different schemes are discussed.     

Excitation of coherent acoustic phonons by a femtosecond pulse

Physics of the Solid State - Excitation of coherent acoustic phonons in superlattices or layered materials by a femtosecond pulse is considered. In superlattices, coherent acoustic phonons with a...     

Free-vibration acoustic resonance of a nonlinear elastic bar

Free-vibration acoustic resonance of a one-dimensional nonlinear elastic bar was investigated by direct analysis in the calculus of variations. The Lagrangian density of the bar includes a cubic term of the deformation gradient, which is responsible for both geometric and constitutive nonlinearities. By expanding the deformation function into a complex Fourier series, we derived the action integral in an analytic form and evaluated its stationary conditions numerically with the Ritz method for the first three resonant vibration modes. This revealed that the bar shows the following prominent nonlinear features: (i) amplitude dependence of the resonance frequency; (ii) symmetry breaking in the vibration pattern; and (iii) excitation of the high-frequency mode around nodal-like points. Stability of the resonant vibrations was also addressed in terms of a convex condition on the strain energy density.     

Transmission of a light pulse by a nonlinear medium

Resonance interaction is considered for a pulse of monochromatic radiation in a medium of phthalocyanin type, on the basis of a two-level model. The change in pulse width and height are deduced, as is the degree of bleaching.     

Model experiments on acoustic tomography of the nonlinear parameter

It is shown experimentally that the scattering of sound by sound can be observed outside the region of interaction between primary waves and that it carries information on the spatial distribution of the scatterer’s nonlinear characteristics. Based on this effect, a simple method for reconstructing the distribution of the nonlinear parameter by using complex-modulated broadband primary signals and a small number of transmit and receive transducers is implemented. The possibility of reconstructing a two-dimensional distribution of a nonlinear parameter inhomogeneity with the use of only three transducers (two transmitters and one receiver) without additional scanning in the aspect angle is demonstrated. This simple scheme loses information on the low-frequency components of the scatterer’s spatial spectrum and reconstructs the distribution of only the spatial variations of the nonlinear parameter.     

Estimating the molecular weight distribution of a polymer using acoustic relaxation spectra

An approach to estimating the molecular weight distribution function of a polymer in a solution using acoustic spectroscopy data is presented. The estimation function is based on the relationship between relaxation spectra and the molecular weight distribution function, along with the relationship between the relaxation spectra and the frequency distribution of a polymer’s loss modulus.     

Dielectric relaxation measurements on nonlinear side-chain polymers

Dynamic dielectric relaxation measurements in the frequency range from 0.001 Hz to 5 kHz have been performed on an amorphous nonlinear optical side-chain polymer, containing the active molecule 4-dimethylamino-4-nitrostilbene as pendant group. The results yielded the relaxation times of the group in the temperature region from 20° below to 40° above the glass-transition temperature of the polymer. The temperature dependence could be described well by the Williams-Landel-Ferry (WLF) equation. It is shown that the relaxation times have a distribution, which could be fit to the form of Cole and Cole with a parameter =0.55. Consequently a substantial part of the active molecules will not be aligned when the polymer is subjected to a poling procedure.     

Chirped nonlinear pulse propagation in a dispersion-compensated system

We study nonlinear pulse propagation in an optical transmission system with dispersion compensation. A chirped nonlinear pulse can propagate in such a system, but eventually it decays into dispersive waves in a way similar to the tunneling effect in quantum mechanics. The pulse consists of a quadratic potential that is due to chirp in addition to the usual self-trapping potential and is responsible for the power enhancement and the decay.     

Propagation of a soliton-like pulse in a weakly nonlinear medium

Numerical methods are used to study the decay dynamics of a soliton-like signal in a medium with a cubic nonlinearity (using the nonlinear Shrödinger equation). We consider the influence of the initial pulse area, shape, and random modulations in its amplitude and phase on the decay process, as well as the effects of absorption and pumping in the medium.V. D. Kuznetsov Physico-Technical Institute, Tomsk State University, Siberia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 36–40, April, 1995.     

Transformation of a Gaussian pulse reflecting from a nonlinear medium

The transformation of the envelope of a Gaussian pulse incident on and reflecting from a nonlinear dispersive medium is studied. It is shown that the pulse envelope transforms considerably upon reflection near the optical resonance frequency and the nonlinearity of the medium may appreciably distort the reflected pulse. Away from the resonance frequency, conditions may arise when the shift of the reflected pulse does not lead to the loss of the Gaussian form. In this case, the influence of nonlinearity is insignificant.     

声学脉冲序列及全透声机理研究

本文研究一维周期性层状结构中材料弹性模量时变因素作用下的声透射临界现象, 探究了这种声学脉冲序列及全透声现象的内在机理. 考虑弹性模量时变因素的影响, 通过求解波动方程, 运用分离变量法解得特征指数, 进而得到临界现象. 当调制幅值低于或等于临界值时, 周期性结构能够将入射波转化为一系列周期的毫秒级的反射脉冲序列; 尤其在调制幅值等于临界值时, 当周期性结构的层数达到16层时, 反射脉冲序列有完整的0和1 的波形; 当大于临界值时, 反射系数在50 ms内迅速衰减至0, 此时的结构相当于完全透声, 入射波可以无损地完全透过, 产生调制透声现象. 这些特殊的现象可为声波的动态控制、声脉冲序列的产生和声波的定向传播提供重要的理论依据, 在超声换能器及调制透声方面具有实际的工程应用前景.