水面目标复合电磁散射的并行迭代快速计算

采用快速远场近似的降秩电磁流迭代法计算目标与粗糙水面的复合散射, 并进一步采用粗糙面分区的多进程并行迭代算法提高其计算速度, 分析了降秩电磁流迭代法对散射结果的影响, 表明该方法可用于近场复合散射问题的计算; 对平板目标与水面间的复合散射进行了计算, 计算结果与快速多极子方法(MLFMM)结果符合较好, 验证了该算法的有效性. 同时, 利用造波水池开展了目标与水面近场复合散射的模拟实测, 测试与计算结果趋势较为一致.     

二维导体微粗糙面与其上方金属平板的复合电磁散射研究

研究了二维导体微粗糙面与其上方金属平板复合电磁散射特征.应用互易性原理使求解二次散射场简化为求解包含平板上的极化电流和微粗糙面散射场的积分方程，从而降低了求解难度.应用物理光学近似和微扰法分别求解了平板上的极化电流和粗糙面的电磁散射场，得到了复合散射截面计算公式并进行了数值计算.尤其对该复合模型后向耦合电磁散射结果进行了详细分析和讨论. 关键词： 互易性原理 复合电磁散射 微粗糙面 平板     

三维随机粗糙面上导体目标散射的解析-数值混合算法

提出三维导体目标与导体粗糙面复合散射的解析-数值混合迭代算法,推导出三维目标与粗糙面的耦合积分方程,以及粗糙面散射的Kirchhoff近似(KA)计算式.粗糙面的KA解析计算大大降低了粗糙面求解的复杂度,与目标矩量法的混合迭代保证了计算结果的精度,使得三维体-面目标复合散射计算变得可行.由于体-面两者的高阶耦合作用明显减小,保证了该混合迭代算法的收敛性.与镜像Green函数方法的比较表明该混合算法的有效性,并讨论了粗糙面长度选择对计算结果的影响.结合Monte-Carlo方法,数值分析了理想导体Gauss 关键词： 复合散射 Kirchhoff近似 共轭梯度法 互耦迭代     

三维随机粗糙面上导体目标散射的解析-数值混合算法

提出三维导体目标与导体粗糙面复合散射的解析-数值混合迭代算法，推导出三维目标与粗糙面的耦合积分方程，以及粗糙面散射的Kirchhoff近似(KA)计算式.粗糙面的KA解析计算大大降低了粗糙面求解的复杂度，与目标矩量法的混合迭代保证了计算结果的精度，使得三维体-面目标复合散射计算变得可行.由于体-面两者的高阶耦合作用明显减小，保证了该混合迭代算法的收敛性.与镜像Green函数方法的比较表明该混合算法的有效性，并讨论了粗糙面长度选择对计算结果的影响.结合Monte-Carlo方法，数值分析了理想导体Gauss     

一维粗糙地表面下方埋藏目标SAR成像

提出一种快速实现一维粗糙地表面下方埋藏金属目标的成像算法.用层内波传播算法结合谱加速的前后向迭代算法(PILE+FB-SA)计算一维粗糙面下方埋藏金属目标的后向散射;用得到的后向散射数据结合后向投影算法实现二维SAR(synthetic aperture radar)成像,研究窗函数的选取,讨论目标位置及尺寸大小变化的影响.用具有高斯谱的粗糙面模拟实际地表面,并采用锥形波入射以减小人为截断粗糙面引起的边缘效应.由于散射数据由快速数值算法得到,该算法不受地面和目标参数限制,可以实现任意粗糙地表面和复杂目标成像,对地下目标探测具有重要的应用价值.     

粗糙海面及其上方导体目标复合电磁散射的混合算法研究

利用基于电流计算的矩量法结合高频算法基尔霍夫近似的混合算法,分析了一维PM谱粗糙海面及其上方二维无限长任意截面导体目标的双站复合电磁散射特性.混合算法将粗糙面和目标分别划分到KA区域和MOM区域,由于无需数值求解粗糙海面区域的表面极化电流,该算法的运算时间和对计算机内存的需求主要取决于粗糙面上方目标的网格划分情况.数值结果以无限长导体圆柱为例计算了其与一维下垫PM谱粗糙海面的复合双站散射截面,并将计算结果与经典MOM结果进行了比对和验证,结果表明混合方法具有较高的计算效率.最后应用混合方法讨论了不同极化状态、海上不同风速以及目标不同尺寸和位置对复合散射截面的影响. 关键词： 粗糙海面 电磁散射 混合算法 矩量法     

三维目标与粗糙面复合散射的广义稀疏矩阵平面迭代及规范网格算法

提出了快速计算二维导体粗糙面与面上金属目标复合散射的广义稀疏矩阵平面迭代及规范网格法(G-SMFSIA/CAG).推导了二维导体粗糙面与面上目标相互作用的耦合积分方程,用稀疏矩阵平面迭代及规范网格法(SMFSIA/CAG)求解粗糙面部分的表面积分方程,而用基于RWG基函数的矩量法(MOM)计算目标部分的表面积分方程,并通过更新方程的激励项迭代求解目标与粗糙面的相互耦合作用.结合Monte-Carlo方法产生具有PM(Pierson-Moskowitz)海浪谱的随机海洋粗糙面,数值分析了海面上不同形状导体目 关键词： 复合散射 广义稀疏矩阵平面迭代及规范网格法 随机海洋粗糙面 双站散射系数     

二维海面上舰船目标电磁散射及合成孔径雷达成像技术研究

研究了二维海面上三维金属目标的电磁散射计算以及合成 孔径雷达(SAR)成像技术. 基于物理光学法、几何光学法和射线弹跳法计算了海 面与目标镜面反射及相互耦合作用; 并用等效电流法计算了目标的棱边绕射作用, 该方法考虑了阴影效应,同时为了消除人为截断引起的边缘衍射, 采用锥形入射波入射. 应用蒙特卡罗法生成的Pierson-Moskowitz谱粗糙面模拟实际海洋面, 计算海面上立方体及舰船的双站雷达散射截面, 通过与数值算法的结果相比较, 验证了该算法的正确性. 运用该解析法快速获取不同频率、 不同角度入射波照射下海面与目标的复合后向散射场数组, 结合SAR成像技术, 得到海面上立方体以及不同姿态舰船目标的SAR成像结果. 该研究成果在实际海洋遥感、海面上军事目标的探测与识别 等领域中具有重要的应用价值.     

目标激光脉冲一维距离成像研究

根据激光雷达方程及粗糙面脉冲波束散射理论,提出了目标激光脉冲后向散射回波功率即激光一维距离像的计算表达式,并获得斜板、球和圆锥一维距离像的具体形式.数值计算结果与粗糙平板和圆锥激光脉冲回波强度实验数据比较,两者符合良好.数据分析表明,激光一维距离像曲线能够较好反映目标的纵向几何外形信息.我们着重讨论了入射激光脉冲宽度与目标径向尺寸对距离像的影响.上述研究可为激光波段的目标特征提取和识别提供理论依据.     

二维随机粗糙面上导体目标复合瞬态散射的混合算法

提出用时域积分方程法(TDIE)与时域基尔霍夫近似法(TDKA)的混合算法来求解二维导体随机粗糙面及其上方二维导体目标的复合瞬态散射,推导出了在TM波入射情形下显式及隐式格式的时间步进方程.将粗糙面与目标分别进行TDKA和TDIE计算,并考虑目标与粗糙面之间的耦合,对TDKA和TDIE进行混合迭代,既大大降低了粗糙面求解的复杂度,又保证了计算精度.数值算例中,考虑了角反射器(开放体)和圆柱(封闭体)两种目标,分别计算了目标表面电流响应和电场远场响应.计算结果表明,和单纯TDIE法相比,本文混合方法计算效率 关键词： 随机粗糙面 复合瞬态散射 时域积分方程法和时域基尔霍夫近似法 混合算法     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.