脉冲波在强起伏湍流介质中的传播特征分析

提出分析脉冲波在强起伏湍流介质中传播的理论和数值方法,利用不同的波结构函数模型,计算了球面脉冲波在湍流介质中传播的双频互相关函数,给出了广延介质和薄屏结构的脉冲展宽和闪烁系数的数值结果,并与观测结果对比 关键词： 脉冲波 湍流介质 双频相关函数 脉冲展宽 闪烁系数     

等离子体光子晶体的FDTD分析

等离子体光子晶体是等离子体和介质（真空）构成的人工周期性结构.用分段线性电流密度 递归卷积时域有限差分（PLCDRC-FDTD）算法分析了等离子体光子晶体和缺陷等离子体光子 晶体.从时域的角度分析了高斯脉冲在等离子体光子晶体中的传播过程，给出了时域反射和 透射波形.然后，从频域的角度分析了等离子体光子晶体和带缺陷的等离子体光子晶体的电 磁反射系数和透射系数.计算表明，等离子体光子晶体对频率小于等离子体频率的低频电磁 波几乎完全反射，而透射的电磁波则为频率高于等离子体频率的电磁波.在高频，等离子体 光子晶体则出现类似一般光子晶体的光子带隙特性. 关键词： 等离子体 光子晶体 时域有限差分法 等离子体光子晶体     

填充等离子体的介质同轴波导中的切伦科夫辐射

 利用自洽线性场理论，导出了薄环形相对论电子注通过填充等离子体的介质同轴波导中的注波互作用色散方程，得到了注波互作用产生切伦科夫辐射的同步条件和波增长率。分析了填充等离子体后的波与电子注之间的能量交换及等离子体密度对色散特性、波增长率和注波能量交换的影响。分析结果表明：切伦科夫辐射是由沿介质同轴波导传播的慢波与沿薄环形相对论电子注传播的负能空间电荷波耦合所致，且其耦合强度与电子注的密度成正比；输出频率和波增长率随着填充等离子体密度的增大而提高；保持一定的输出频率，增大电子注的束流可得到高的微波输出功率。     

有限磁场中等离子体圆柱波导的传播特性

分析了有耗介质中等离子体圆柱波导在有限外加磁场中的传播特性.重点讨论了波导传播常数随等离子体参数、介质参数和外加磁场的变化.分析结果表明，有限强磁场中的等离子体波导的传播特性比无外磁场或外加磁场为无穷大时具有更强的控制能力. 关键词：     

随机粗糙面激光脉冲散射四阶统计特征

依据基尔霍夫近似,推导出一维高斯随机粗糙面四阶统计特性解析式,并获得脉冲波入射下粗糙面双点双频四阶矩互相关函数.数值计算不同入射角,不同高度起伏和相干长度,随机粗糙面双点双频四阶矩互相关函数随相干频谱带宽频差,散射角的变化情况.计算结果表明,高度起伏和相干长度对粗糙面散射四阶统计特的影响很大,粗糙面越光滑,在镜反射方向有最大的四阶矩散射峰值和小的相关带宽.随着粗糙度增加,随机粗糙面脉冲散射的四阶矩的相干分量减弱,展宽现象明显;而相干带宽频差越大,四阶矩衰减越快. 关键词： 四阶矩统计特征 双点双频互相关函数 随机粗糙面 脉冲波散射     

光纤放大器中非自治光畸波的传播控制研究

本文采用一个通用的理论,即用相似变换的方法,研究构建了(1+1)维变系数非线性薛定谔方程的精确畸形波解,首先讨论了一阶光畸波在光纤放大器中的传播问题.发现光学畸波的特性,如宽度、振幅和位置,可通过非线性光学介质特性和光脉冲的初始参量进行控制;然后在选择可控参数条件下,讨论了可控光畸波在非线性介质的传播行为,包括延迟激发、抑制、以及保持.这在理论和实际应用上具有启迪价值.     

磁化等离子体光子晶体的FDTD分析

磁化等离子体光子晶体是磁化等离子体和介质(真空)构成的人工周期性结构.本文用磁化等离子体的分段线形电流密度卷积(PLCDRC)时域有限差分(FDTD)算法分析了磁化等离子体光子晶体特性.分析了磁化等离子体参数对电磁带隙的影响.从时域的角度分析了高斯脉冲在磁化等离子体光子晶体中的传播过程，给出了时域反射和透射波形.从频域的角度给出了磁化等离子体光子晶体的电磁反射系数和透射系数，并对结果进行了分析. 关键词： 磁化等离子体 光子晶体 时域有限差分法     

TM模式下二维非磁化等离子体光子晶体的禁带调制特性分析

采用时域有限差分法(FDTD)中的分段线性电流密度卷积(PLCDRC)算法研究了TM波入射时二维非磁化等离子体光子晶体的禁带特性.从频域角度分析得到微分高斯脉冲的透射系数,并讨论该光子晶体的介质圆柱的介电常数、晶格常数、介质圆柱半径,周期常数和等离子体参数对其禁带特性的影响.结果表明,增加周期常数和等离子体碰撞频率不会改变禁带宽度,增加介质圆柱的相对介电常数和等离子体频率可以展宽禁带的宽度. 当填充率一定时,减小介质圆柱的半径和晶格常数可以实现禁带的拓展. 关键词： 等离子体 光子晶体 禁带 PLCDRC算法     

沙尘天气下能见度对光脉冲时延和展宽的影响

针对激光脉冲在沙尘天气中的时延和展宽问题,以光信号在离散介质中的传输理论为依据,求解了脉冲信号在传播过程中的双频互相关函数,推导了沙尘天气下的脉冲响应。通过分析沙尘天气中不同能见度下激光脉冲时延和展宽的变化情况,得到了激光脉冲的时延量和展宽量与沙尘能见度间的关系,并分别研究了波长为3.8、7.6和10.6 mm的激光脉冲的时延和展宽与沙尘天气中大气能见度间的关系。结果表明:在波长一定的情况下,激光脉冲的时延量和展宽量随能见度的增加呈负指数形式减小,并逐渐趋于稳定值。同时,在能见度一定的情况下,激光脉冲的时延量和展宽量会随信号波长的增加而增加。     

时变磁化等离子体的LTJEC-FDTD方法研究

基于拉普拉斯变换的电流密度卷积技术（LTJEC）,构造了时变磁化等离子体的新型时域有限差分方法（LTJEC-FDTD）。借助于高斯脉冲在磁化等离子体中的传播实例,验证了LTJEC-FDTD算法的准确性及高效性。进一步,研究了Whistler波在一维时变磁化等离子体中的具体传播特性。结果表明,当离子体频率随时间指数衰减后,输出波的频率上升、极化方式不变,而电场增强、磁场减弱。同时,通过优化磁化等离子体参数,可进一步提高Whistler波的输出频率,获得了频率为300 GHz的圆极化太赫兹波。研究结果可为利用磁化等离子体产生太赫兹波源提供相关的技术支持。     

Temporal broadening of pulsed waves propagating through turbulent media

Pulse signals, propagating through a turbulent medium such as the ionosphere, can be distorted by dispersion and scattering from both the background medium and irregularities embedded in. Thus, the mean square pulse width is changed, and temporal broadening is introduced. We carry out a study on the temporal broadening with theoretical analyses and numerical simulations by using an analytical solution of two-frequency mutual coherence function obtained recently by iteration. As a case of study, pulse broadening is investigated in detail in trans-ionospheric propagation. Results show that most contributions are mainly from the dispersion of the background ionosphere and scattering effects of electron density irregularities in most cases.     

The reference wave solution for a two-frequency wave propagating in a random medium

In this work a new reference wave method for solving parabolic-type equations is proposed. The performance of the method is demonstrated by applying it to the equation governing the propagation of the two-frequency mutual coherence function in a random medium. An analytic solution is presented for arbitrary correlation properties of the medium. It is shown that when approximating the transverse structure function of the medium by a quadratic form, the solution reduces to the exact result derived previously. Extensions to more general types of media are considered.     

Dispersive distortions of a radio-wave pulse in a double-resonance gaseous medium

The problem on dispersive distortions of an electromagnetic pulse in a gaseous medium with two isolated resonant frequencies is solved analytically. The solution is obtained directly in the time region and, thus, is not the result of calculations of the Fourier integral. Without introducing additional assumptions, it is possible to study the regularities and the features of the process of propagation of pulses caused by variations of both their initial characteristics and the parameters of the propagation medium. As an example, the solution is applied to describe the distortions of the two-frequency pulse of subnanosecond duration in the terrestrial atmosphere.     

Short pulse detection and imaging of objects behind obscuring random layers

This paper presents a theory of imaging objects behind layers of scattering media. The transmitter is a focused array or an aperture emitting a short pulse. The scattered pulse is received by a focused array or aperture. The received signal consists of two components: the pulse scattered from a random medium and from the target, and these two components can be distinguished by the use of ultra wide band (UWB) pulse. The second moment of the received signal includes the fourth-order moments of stochastic Green's functions, which are reduced to the second moments by the use of the circular complex Gaussian assumption, and of the generalized two-frequency mutual coherence function. This imaging theory is a generalization of optical coherence tomography (OCT), SAR and confocal imaging. It clarifies the relationships among resolution, coherence length, shower curtain effects and backscattering enhancement.     

Short pulse detection and imaging of objects behind obscuring random layers

This paper presents a theory of imaging objects behind layers of scattering media. The transmitter is a focused array or an aperture emitting a short pulse. The scattered pulse is received by a focused array or aperture. The received signal consists of two components: the pulse scattered from a random medium and from the target, and these two components can be distinguished by the use of ultra wide band (UWB) pulse. The second moment of the received signal includes the fourth-order moments of stochastic Green's functions, which are reduced to the second moments by the use of the circular complex Gaussian assumption, and of the generalized two-frequency mutual coherence function. This imaging theory is a generalization of optical coherence tomography (OCT), SAR and confocal imaging. It clarifies the relationships among resolution, coherence length, shower curtain effects and backscattering enhancement.     

Pulse broadening and two-frequency mutual coherence function of the scattered wave from rough surfaces

Analytical expressions for the two-frequency mutual coherence function and angular correlation function of the scattered wave from rough surfaces based on the Kirchhoff approximation are presented. The coherence bandwidth depends on the illumination area as well as on the incident and scattered angles and the surface characteristics. Scattered pulse shapes are calculated as the Fourier transform of the two-frequency mutual coherence function. Calculations based on analytical solutions are compared with millimetre wave experimental data and Monte Carlo simulations showing good agreement.     

Pulse broadening and two-frequency mutual coherence function of the scattered wave from rough surfaces

Abstract

Analytical expressions for the two-frequency mutual coherence function and angular correlation function of the scattered wave from rough surfaces based on the Kirchhoff approximation are presented. The coherence bandwidth depends on the illumination area as well as on the incident and scattered angles and the surface characteristics. Scattered pulse shapes are calculated as the Fourier transform of the two-frequency mutual coherence function. Calculations based on analytical solutions are compared with millimetre wave experimental data and Monte Carlo simulations showing good agreement.     

Scattering and propagation of UV pulses in soot aerosols

Scattering and propagation of a UV pulse in soot aerosols are studied using generalized multi-sphere Mie theory (GMM) and a two-frequency mutual coherence function. Soot aerosols are obtained by the diffusion-limited aggregation (DLA) model. Scattering characteristics of aggregate structures in soot aerosols are analyzed by GMM theory in detail. Scattering intensities versus scattering angles are given and discussed. The effects of different-positions of the aggregate on the scattering intensities, scattering cross section, extinction cross section, absorption cross section and asymmetry factor are computed and compared. The two-frequency mutual coherence functions of UV pulses in soot aerosols are simulated, and the effects of optical distance, frequency difference are analyzed.     

Modal theory for the two-frequency mutual coherence function in random media: beam waves

Pulse propagation in a random medium is mainly determined by the two-frequency mutual coherence function which satisfies the parabolic equation. It has recently been shown that this equation can be solved by separation of variables, thereby reducing the solution for any structure function to the solution of ordinary differential equations. In this paper, the method is applied for a beam-wave excitation in a random medium. The exact solution for a quadratic medium is derived. For non-quadratic power-law media an analytical expression at equal positions is presented.