基于菱形光纤布拉格光栅传感阵列的声发射定位技术

声发射技术是结构损伤检测的重要手段,声发射源定位是损伤检测的首要环节。时差定位技术具有快速、高效、精确的特点,以此设计了由菱形阵列光纤布拉格光栅（FBG）传感器构成的声发射定位系统。采用小波变换和传统阈值法提取特征信号,结合互相关法获得传感器间的信号到达时差,然后根据几何定位模型求解非线性方程组得到声源可能存在的位置,最后根据时差的正负特性进一步确定声源的准确位置,有效避免了伪声源的情况。在铝合金板上,以对角线为48 cm48 cm的监测区域进行了10组测试实验验证,平均误差为1.29 cm。     

在时域内计算色散媒质中光脉冲的传输

得到了时域内色散媒质中光脉冲传输的计算公式，并提出了时域内色散媒质中显式的光束传播法。计算了短脉冲在具有二阶色散效应的定向耦合器内的传输、计算结果同参考文献中的一致，但本文的计算方法简单、方便、实用。     

On pulse compression in dispersive media

A comparison between three methods used for the synthesis of suitable signals which result in pulse compression, when transmitted through dispersive media, is presented. These are the equalization of group-time delay method, the space-time rays method, and the matched-signal method. The three methods are shown to be equivalent if the signals are restricted to be frequency-modulated with uniform envelopes. A generalized expression for the requited frequency modulation law for pulse compression in an inhomogeneous dispersive medium is obtained. Considerations for lossy dispersive media and additional optimization of the signal envelope are also discussed.     

Wave packets in strongly dispersive media

An analytical study of the evolution of slowly varying wave pulses in strongly dispersive media which takes into account dispersive correction terms involving higher derivatives of the group velocity is given. A higher order differential equation for the envelope function is derived and solved recursively and by means of a procedure based on an analogy with the Schrödinger equation. The equation for the envelope function is used to obtain generalizations of the velocity of the pulse defined as the velocity of the center of inertia, and expressions are derived which determine the spreading of the pulse. Finally, we discuss how the presence of other wave modes affects the primary mode in the multimode case.     

Propagation of transients in dispersive dielectric media

The propagation of transient electromagnetic fields in dispersive dielectric media is studied. The dielectric medium is assumed to be linear, isotropic, and homogeneous, and is described by the Debye model. Incident fields are assumed to be transverse electromagnetic plane wave pulses. The dielectric body can assume the form of infinite half space or an infinite circular cylinder, either of which may be homogeneous or stratified. The electric fields induced in the dielectric are calculated from time-domain Maxwell's equations using the finite-difference time-domain method.<>     

High-order compact schemes for dispersive media

Staggered high-order compact (HOC) finite difference schemes are developed for modelling electromagnetic wave propagation in dispersive media. The main advantage of HOC schemes is their very low dispersion error, which is dominant in low-order methods. The high accuracy of HOC schemes is demonstrated by examples of wave propagation through first-order Debye and Lorentz media in one dimension.     

Time-domain finite-element modeling of dispersive media

A general formulation is described for time-domain finite-element modeling of electromagnetic fields in a general dispersive medium. The formulation is based on the second-order vector wave equation and incorporates the dispersion effect of a medium via a recursively evaluated convolution integral. This evaluation is kept to second order in accuracy using linear interpolation within each time step. Numerical examples are given to validate the proposed formulation     

Energy relations for waves in strongly dispersive media

Expressions for the averaged stored energy, energy flow, and energy velocity of quasi-monochromatic waves in temporally and spatially dispersive media have been extended to take into account broad-band signals in strongly dispersive media.     

Parameterization of media dispersive properties for FDTD

Convenient means of obtaining a Debye model of the dielectric dispersion are given for media whose properties are given either by the Cole-Cole model or numerical data in the frequency domain     

A novel FDTD formulation for dispersive media

A novel FDTD formulation for dispersive media called piecewise linear JE recursive convolution (PLJERC) finite-different time-domain (FDTD) method is derived using the piecewise linear approximation and the recursive convolution relationship between the current density J and the electric field E. The high accuracy and efficiency of the PLJERC method is confirmed by computing the reflection coefficients of an electromagnetic wave through a collision plasma slab in one dimension.     

色散媒质二阶时域参量的演绎

为了实现色散媒质二阶时域参量的变换, 提出将色散媒质二阶参量演绎为时域参量的方法.以德拜媒质水为对象, 依据其本构参量和平面波在空气-水分界面上频域反射和透射系数, 先建立其频域超越方程式; 再依据易于得到的本构时域参量和待求的时域参量建立起其相应的时域超越方程式, 对其用迭代方法可演绎出二阶时域参量.通过时-频变换技术已证明方法的有效性.给出演绎过程和技术要点, 并对所得结果进行了讨论, 展示了因果关系.本演绎技术具有拓展延伸的潜力.     

Transients in dispersive media, part I: Theory

The asymptotic evaluation of complex frequency integrals arising in the theory of radiation from transient sources in dispersive media is reviewed and extended to accommodate phenomena observed at distant observation points at any observation time. Helpful graphical methods are emphasized and illustrated for a plasma medium discussed in detail in Part II of this paper. Results are interpreted in terms of space-time rays whose properties are reviewed, and these rays, together with the (k, omega) dispersion surfaces for the medium, are utilized for the study of reflection and refraction processes at an interface.     

时域多分辨率分析在色散介质中的应用

将基于Daubechies尺度函数的时域多分辨率分析(Muti-Resolution Time-Domain ,MRTD)方法应用到色散介质的分析中.采用梯形递归卷积（Trapezoidal Recursive Convolution ,TRC）方法,推导出了适合Drude-Lorentz模型的差分方程.通过对一维等离子体和金属铝板反射系数的仿真分析,证明了该方法的正确性和有效性.与基于辅助方程（Auxiliary Difference Equation ,ADE）的MRTD方法相比,该方法可以有效地节约内存和计算时间,并且获得更高的计算精度.     

Propagation in linear dispersive media: finite differencetime-domain methodologies

Finite difference time-domain (FDTD) methodologies are presented for electromagnetic wave propagation in two different kinds of linear dispersive media: an Nth order Lorentz and an Mth order Debye medium. The temporal discretization is accomplished by invoking the central difference approximation for the temporal derivatives that appear in the first-order differential equations. From this, the final equations are temporally advanced using the classical leapfrog method. One-dimensional scattering from a dielectric slab is chosen for a test case. Provided that the maximum operating frequency times the time step is small and that the wave is adequately resolved in space, as shown in the error analysis, the agreement between the computed and exact solutions will be excellent. The attached data, which are associated with the four pole Lorentz dielectric and the five pole Debye medium, verify this assertion     

Acoustic microscopy with noncoherent source

A noise generator was used as a noncoherent source to image with the acoustic microscope. Depending on the bandwidth of the transducers it was possible to reduce the interference patterns and artefacts due to multiple transits of the acoustic waves, especially when focusing inside thick samples.     

Acoustic diffraction of light in anisotropic media

The Bragg formalism which describes the diffraction of optical radiation by traveling acoustic waves is generalized to include diffraction in anisotropic media. When the diffracted and incident optical polarizations are different, the Bragg relations require important modification. The correctness of the suggested modifications is confirmed by experiments using optical diffraction from longitudinal and transverse acoustic waves in crystal quartz and in sapphire. A special case of diffraction from longitudinal waves in an anisotropic medium is discussed as a simple example of a collinear parametric interaction.