Analytical and finite element prediction of Lamb wave scattering at delaminations in quasi-isotropic composite laminates

This paper presents a theoretical and finite element (FE) investigation of the scattering characteristics of the fundamental anti-symmetric (A₀) Lamb wave at delaminations in a quasi-isotropic (QI) composite laminate. Analytical models based on the Mindlin plate theory and Born approximation are presented to predict the A₀ Lamb wave scattering at a delamination, which is modelled as an inhomogeneity, in an equivalent isotropic model of the QI composite laminate. The results are compared with FE predictions, in which the delamination is modelled as a volume split. The equivalent isotropic model and QI composite laminate are used to investigate the feasibility of the common theoretical approach of modelling the delamination as the inhomogeneity. A good correlation is observed between the theoretical solutions and FE results in the forward scattering amplitudes, but there exists a larger discrepancy in the backward scattering amplitudes. The FE results also show that the fibre direction of the outer laminae has a pronounced influence on the forward and backward scattering amplitudes, which is not predicted by the analytical models.     

The creation of supra-thermal densities of high-energy phonons in He II

We report the new phenomenon that high-energy phonons can be created from low-energy phonons. This arises because the dynamics of phonons in propagating pulses are quite different to those in isotropic phonon distributions. A pulse of low-energy phonons rapidly thermalises by three-phonon processes. On a much longer time scale four-phonon processes occur within this phonon cloud which create high-energy (10 K) phonons that cannot spontaneously decay. These phonons have a lower velocity and so are lost from the back of phonon cloud; their deficit is restored continuously by four-phonon processes. These now isolated high-energy phonons are very stable and propagate ballistically behind the low-energy phonons, so giving the two pulses which are detected in experiments. For long pulses the high-energy phonons may also decay within the cloud, however the available low-energy phonons for scattering are confined to a narrow-angle cone, so the decay probability is very low because the four phonon process requires large angle scattering. A supra-thermal density of these high-energy phonons is predicted.     

Temperature response in participating media with anisotropic scattering caused by pulsed lasers

It is not by isotropic scattering but by anisotropic scattering that radiant energy is redistributed in some materials containing real particles, fibers, or impurities. In some instances, great difference can be caused in transient thermal behavior between isotropic scattering and anisotropic scattering media. Ray tracing method combined with Hottel's zonal method is introduced to deduce thermal radiative source term for various optical boundary conditions induced by collimated incidence passing through translucent boundary. Temperature response caused by laser pulse at non-incident side of participating and anisotropic scattering media is examined. We investigate effects of scattering albedo, scattering phase function, initial temperature of media and thickness of media on temperature response. Results obtained for anisotropic scattering media are compared with those for isotropic scattering one and show that anisotropic scattering must be considered in the simulating measurement of thermophysical properties by the laser flash method for some materials with big scattering albedo which behave anisotropically, or big error will be introduced; forward scattering can increase excess temperature and backward scattering can decrease it at non-incident side of the considered sample irradiated by laser pulse.     

Conductivity tensor and scattering of carriers in isotropic crystals with parabolic bands

The conductivity tensor for isotropic crystals with parabolic bands is evaluated for mixed scattering by phonons and charged centers and for various values of the reduced Fermi level. The evaluated tensor elements are shown as a function of magnetic field in reduced scales, which makes it easy to compare widely different characteristics of any crystal. The effect of mixing is important in discussing the scattering mechanism-the galvanomagnetic effects for mixed scattering by acoustic phonons and charged centers resemble, in a certain range of mixing, those for pure optical phonon scattering rather than for acoustic.     

Impulsive stimulated Raman scattering: comparison between phase-sensitive and spectrally filtered techniques

Impulsive stimulated Raman scattering is used to generate and control coherent phonons and other low-frequency modes. In transparent materials, pump-probe experiments are usually performed by spectrally resolving the probe beam and measuring the spectral shift as a function of pump-probe time delay. By measuring the optical phase of the probe pulse as a function of time delay, we find that the phonon signal can be increased by a factor alpha(omegadelta)(-1), where omega is the phonon frequency and delta is the pulse duration.     

Statistics of a coherent lidar signal in a turbulent atmosphere

Absolute and conditional statistical properties of a pulse coherent Doppler lidar signal in a turbulent atmosphere are studied. Upon coherent receiving of optical fields scattered by a large number of particles, the lidar signal is shown to be a nonstationary non-Gaussian random process with Gaussian conditional statistical characteristics. The appearance of non-Gaussian properties of the signal is caused by correlation of turbulent fluctuations of the wind velocity field within the scattering volume. For the considered signal model, which corresponds to the single scattering approximation and is a sum of a large number of random variables, the central limit theorem is found to be untrue due to the statistical dependence of particles’ positions in a turbulent atmosphere. The results of numerical calculations show that, for a homogeneous and isotropic turbulence, the behavior of the signal statistics significantly depends on the size of the scattering volume and on the state of atmospheric turbulence. A Gaussian statistics is observed at small heights; with an increase in height, the non-Gaussian component becomes considerable in fluctuations of the lidar signal.     

陡前沿脉冲产生的受激布里渊散射稳定性研究

受激布里渊散射转换效率及相位共轭保真度的稳定性与泵浦脉冲的前沿上升时间有关，对前沿上升时间与介质声学声子寿命接近的泵浦脉冲，在泵浦能量超过受激布里渊散射阈值较多时，会造成严重的相位共轭保真度不稳定。本文针对上升前沿时间为２ｎｓ的泵浦脉冲及声学声子寿命的１ｎｓ和ＣＣｌ４介质，在泵浦脉冲前沿根部引入缓慢上升的小信号后，采用振放双池受激布里渊散射相位共轭镜，在泵浦激光能量达到５００倍受激布里渊散射阈值时     

Scattering response of a phonon damped harmonic oscillator

We calculate the response for scattering off a particle bound by an isotropic harmonic force and damped through hybridization with a phonon bath. An exact expression for the scattering response is studied numerically to provide a detailed picture of its dependence on wave vector, frequency, temperature and phonon hybridization strength.     

海洋波导中刚性球及旋转椭球前向散射场时频特征的畸变

利用波导中目标声散射理论的简正波方法,数值模拟了自由空间和浅海均匀波导中刚性球、旋转椭球的散射场,分析了波导中刚性体前向散射时频特征的畸变规律。仿真结果表明:波导中刚性体的前向散射时域波形与频谱特征受波导制约;目标散射场的固有特征受波导色散特征的调制,时频谱上呈现出由于色散的简正波间的耦合而导致各简正波能量条纹界限模糊的现象。时频特征的畸变程度与组成散射共振系统的波导、目标的尺度以及散射体布放深度有关,随波导底质透声能力和深度增加、散射体散射强度(信混比)增强而减弱。      

Symmetrization of the Coulomb pairing potential by electron-phonon interaction

It is shown that the Coulomb superconducting pairing in systems with the Fermi contour nesting can be described by a quasi-one-dimensional potential oscillating in real space. The supplement of this repulsive potential with an isotropic pairing attraction corresponding to the phonon superconductivity mechanism and including the effect of predominant forward scattering upon electron-phonon interaction leads to symmetrization of this potential and a considerable increase in the superconducting transition temperature.     

Effects of non-equilibrium polar optic phonons and the band non-parabolicity on small signal high-frequency hot electrons ac mobility in narrow gap semiconductors in high magnetic fields

The small signal high-frequency ac mobility of hot electrons in n-HgCdTe in the extreme quantum limit at low and high temperatures have been calculated considering the non-equilibrium phonon distribution as well as the thermal phonon distribution .The energy loss rate has been calculated considering only optical phonon scattering while the momentum loss rate has been calculated considering acoustic phonon scattering and piezoelectric scattering together with polar optical phonon scattering and separately considering only the polar optical scattering. The results have been discussed and compared. It has been observed that at 20 K, the normalized mobility considering all the three scattering mechanisms differs appreciably from that considering only the polar optical phonon scattering. However, at 77 K, there is no difference in the normalized mobility. This establishes the fact that at higher temperature, the effect of acoustic phonon scattering and piezoelectric coupling is negligible, compared to the polar optical phonon scattering. So the ac mobility considering only polar optical phonon scattering has been studied at 77 and 20 K. The ac mobility is found to remain constant up to 100 GHz and thereafter it started decreasing at higher frequencies at 77 K whereas the ac mobility reduces at much lower frequencies at lower temperature at lower field. The non-parabolicity of the band structure enhances the normalized mobility.     

Dynamics of an optical signal in a multimode waveguide structure

A mathematical model of linear dynamic distortions of average signal power in a planar optical waveguide stemming from the intermodal coupling and dispersion of waveguide modes caused by macroscopic bends and irregularities of the optical waveguide surface has been developed on the basis of pulse intermodal scattering matrix. It is shown that the elements of this matrix are determined at a waveguide segment whose length greatly exceeds the correlation scale of the field of irregularities but is much smaller than the normalization length of waveguide modes and are described by rectangular functions with a duration proportional to the relative velocity of the corresponding modes. The dependences of the pulse intermodal scattering matrix on the intensities of absorption, Rayleigh scattering, and optical signal scattering from waveguide irregularities have been established. It is shown that, at waveguide lengths exceeding the normalization length, the optical signal shape becomes almost stable and can be expressed in terms of the pulse characteristic of an integrating circuit. It has been established that irregularities of the film in a bent optical waveguide lead not only to additional attenuation but also to enhancement of multimode distortion. A matrix method is proposed for taking into account these distortions, which is based on the effect of equalization (scrambling) of the signal energy over the spectrum of the bent waveguide modes. The model developed was used to process the experimental data on the dynamics of a signal in a weakly guiding multimode polymer optical fiber in order to estimate the parameters of attenuation and irregularities of the waveguide surface.     

The effects of medium phonon lifetime on pulse compression ratio in the process of stimulated Brillouin scattering

The performances of stimulated Brillouin scattering (SBS) media with short phonon lifetimes on temporal compression are studied in this paper. In the experiment, FC-72, HT-135 and FC-40 (phonon lifetimes are 1.2 ns, 0.4 ns and 0.2 ns, respectively) are used to study the influence of phonon lifetime on pulse compression in a compact two-cell SBS system. High pulse compression ratio is obtained by a medium with a short phonon lifetime when input energy is high. These results are useful in the generation of picoseconds pulse for Shock Ignition of thermonuclear fuel for ICF.     

Characteristics of perfluorinated amine media for stimulated Brillouin scattering in hundreds of picoseconds pulse compression at 532 nm

The characteristics of stimulated Brillouin scattering(SBS) in perfluorinated amine media and the experimental structure used in hundreds of picoseconds pulse compression at 532 nm are demonstrated. A two-stage SBS pulse compression structure is adopted for this work. The compact double-cell SBS compression structure and the scattering media FC-70 are chosen to compress the incident light from 9.5 to about 1 ns in the first stage. Then,the light is used as the pumping source for the second pulse compression. In the second stage, using a single-cell SBS structure in a pulse compression system, perfluorinated amine media with different phonon lifetimes, such as FC-3283, FC-40, FC-43, and FC-70, are chosen to run the comparative experimental study. The narrowest compressed pulse times obtained are 294, 274, 277, and 194 ps; they respectively correspond to the above listed media. The average width of the compressed pulse width is 320 ps for FC-3283, with a fluctuation range of87 ps. For FC-40, the average pulse width is 320 ps, with a fluctuation range of 72 ps. And for FC-43, the average pulse width is 335 ps, with a fluctuation range of 88 ps. However, the average pulse width is only 280 ps for FC-70, with a fluctuation range of 57 ps. The highest energy reflectivity is more than 80% for all of the media. The experimental results show that a two-stage SBS pulse compression system has lower pump energy requirements, thus making it easier to achieve a compressed pulse waveform. The results also show that the shorter the phonon lifetime of the medium, the narrower the obtained compressed pulse width.     

导电平板上任意缝隙填充各向异性介质时TM波散射和传输特性分析

简单介绍了用于分析任意口径问题的通用频域方法——广义网络原理,并利用边界积分法和广义网络原理分析导电平板上任意缝隙填充各向异性介质时TM波的散射及传输特性.由于缝隙填充各向异性介质的情形尚未见公开文献报道,作为验证,将本方法退化计算各向同性介质填充时缝隙的散射和传输特性,并与文献结果进行比较.最后,给出了缝隙填充各向异性介质时的算例.     

Physical model of acoustic forward scattering by cylindrical shell and its experimental validation

Research on the underwater target scattering can provide important theoretical support for target detection. The scattering model of cylindrical shell is established in this paper. It is found that the forward target strength is much stronger and varies with angles of incident wave less significantly than backward target strength. The received forward signal strength fluctuates with the target moving due to the interference between direct signal and scattering signal, which is most significant when target approaches the baseline. An experiment is carried out in an anechoic tank to validate the scattering model. The method of acquisiting forward scattering in the tank is proposed. The forward and the backward target strengths are achieved by using the pulse compression technology, and they are about 3dB less than the modeling results. The forward scattering phenomena of quiescent and moving target are measured, which are similar to modeling results with different target types.     

High power pulse amplification of ytterbium-doped double-clad fiber amplifier

By solving a set of time-dependent equations, the characteristics of the ytterbium-doped double-clad fiber amplifier are presented. Besides the steady state in the fiber of the upper-state population, pump power and amplified spontaneous emission without the input signal, the dynamic characteristics of the high power Gaussian pulse amplification like the evolution of pulse waveform distortion, upper-state population distribution and stored energy and pulse energy of the amplifier under the forward and backward pump,are simulated. The relations between the output pulse energy of the amplifier and the different input pulse peak power or pump power are also discussed. The models and results can provide important guide for the design and optimization of the high power pulse amplification.     

Sound scattering from partially water-filled elastic spherical shell with an internal elastic plate

According to the equation of motion in the elastic medium and integral equation of target scattering, the sound scattering from the partially water-filled elastic spherical shells with and without an inner plate is studied using the finite element and boundary element method, and the scattering normalized form functions of the shell filled with different volume of water are computed and the mechanism of resonance scattering is analyzed. The results show that the resonance of the shell with partially water-filled and without the plate is mainly related to the volume of water, and the resonance is produced by inner water and the spherical shell. The resonance characteristics of partially water-filled elastic shell with the plate are similar to that of empty structured elastic spherical shell, and the sound field in inner water is weaker which indicates the main resonance characteristics are decided by spherical shell and the plate. In addition, the scattering characteristics of spherical shell with plate and one side full water-filled are greatly different from the partially water-filled ones.     

Nonohmic behavior of semiconductors in a quantizing magnetic field

We have investigated the nonohmic resistivity of a nondegenerate semiconductor in quantizing magnetic fields for the case where acoustic phonons are the dominant scattering mechanism. The type of I-V characteristics found depends upon which of three mechanism are dominant. The three mechanisms are due to collisional broadening, inelasticities due to the finite phonon energy and phonon drag. When collistion broadening is important, the nonlinearities in the current voltage characteristic arise only from electron heating, while when the inelasticities are dominant, there is also an intrinsic nonlinearity in the characteristic. Finally, when phonon drag is dominant, high frequency acoustoelectric amplification will occur when the Hall velocity exceeds the sound velocity, i.e. V_H > S.For the case where inelasticities dominate, a region of negative differential resistance is obtained that should persist even when there is considerable optical phonon scattering.     

利用染料片吸收控制受激布里渊散射功率限幅波形

理论研究了受激布里渊散射过程中功率限幅响应特性.采用高斯型入射脉冲，数值模拟了抽运参数(抽运峰值功率密度、抽运脉冲宽度)，介质参数(增益系数、声子寿命)和结构参数(聚焦透镜焦距、介质池长)等物理参数影响限幅输出波形中剩余峰的特性规律.对如何控制剩余峰进行了理论分析.研究表明，声子寿命较小、增益系数大的布里渊介质光限幅响应较好，声子寿命较大、增益系数小的布里渊介质光限幅响应较差；无法单纯通过控制布里渊介质参数来完全消除剩余峰.实验上采用染料片吸收剩余峰，获得接近平顶的限幅输出波形.