新型LED光源气溶胶探测雷达系统优化设计与初步实验

为了实现近地面、多波长气溶胶时空探测,克服激光波长可选择性不足的问题,利用LED丰富的光谱特性,设计了一种新型发光二极管(LED)光源雷达。LED能量小且发散角大,其出光能量、发散角、接收视场角和几何重叠因子都会对LED光源雷达探测距离产生影响。针对共轴LED光源雷达系统的特点,分析了几何重叠因子的特征和计算方法,详细讨论了光源发散角和接收视场角对几何重叠因子的影响。结合LED光源的特点,利用美国标准大气模型对雷达系统的探测能力进行了仿真,优化了光源发散角和接收视场角,在LED能量一定的情况下获得了最大的探测距离。搭建了LED光源雷达系统并完成了初步实验观测,结果表明该雷达夜晚能够接收到240m远处的大气回波,验证了雷达探测低层大气气溶胶的能力。     

新型遥感探测系统——白光激光雷达

 设计了一种新型的大气遥感探测系统——白光激光雷达，发展了一种雷达模式的宽谱范围长程差分吸收光谱探测方法。介绍了白光激光雷达系统结构和时间分辨光谱探测技术；阐述了飞秒激光在大气中自导光丝传输和产生的超连续白光的特性；利用该白光激光雷达系统对大气中氧气成分在685~694 nm和759~769 nm范围的长程差分吸收光谱进行了探测，并与Hitran数据库拟合计算的标准值进行比对，结果一致，验证了白光激光雷达遥感探测系统的正确性和实验方法的可行性。     

基于角度-多普勒分辨的反射层析激光成像雷达研究

设计了一种基于角度-多普勒分辨的接发同轴反射层析激光成像雷达系统,给出了基本成像原理和数学表达,相应分析了目标横向距离分辨率和单个角度采样时间的关系。并在实验室平台上模拟远场衍射传输,获取探测目标的角度-多普勒分辨的反射投影图,采用滤波反投影算法实现目标横截面图像重建。此方法采用相干外差探测,可以大大提高雷达接收信号灵敏度,同时不涉及光频空间相位和时间相位,成像中没有相位匹配过程,降低了实施技术难度,具有一定的实际意义和使用价值。     

车载探地雷达信号在分层介质中的散射特性

建立了多层分层介质散射的物理模型,基于雷达方程和斯涅尔定律给出了多层分层介质散射场强度计算公式.分析研究了分层介质的电特性参数(介电常数和损耗正切)对雷达系统的探测距离和散射场的影响.以安装于探测车上的探地雷达系统为应用背景,利用时域有限差分方法对整个车载探地雷达系统进行了建模和仿真计算,对接收到的散射回波信号进行了研究.最后将系统数值模拟结果和解析计算结果进行了对比,并对结果做出了分析.     

多波长发光二极管光源雷达系统与近地面低层大气气溶胶探测

设计并研制了一台多波长发光二极管(LED)光源雷达系统,用于探测近地面低层大气气溶胶特性.介绍了LED光源雷达系统的组成及工作原理,计算分析了系统几何重叠因子,从而确定了LED光源雷达系统的最低探测高度为60 m.研究了LED光源雷达散射回波信号的数据反演方法,根据LED光源雷达适合近距离探测的特点,采用了Fernald前向积分反演算法,并以地面能见度仪数据为基础,确定了气溶胶消光系数的边界值.利用所设计的475, 530和625 nm三个波长的LED光源雷达系统,分别在轻度污染、中度污染和重度污染天气情况下,对西安夜晚城区上空低层大气气溶胶进行了探测,获得了近300 m高度内三个波长的大气气溶胶消光系数高度分布曲线,并对近地面低层大气气溶胶的垂直分布与变化特征进行了探讨.     

激光诱导生物气溶胶荧光雷达的设计与数值仿真

为了评估激光诱导荧光雷达对生物气溶胶粒子的有效探测距离及随生物气溶胶浓度变化的敏感性,在阐述生物气溶胶探测原理的基础上,设计了一台激光诱导荧光雷达。该雷达选用波长为355 nm的二极管抽运的Nd:YAG固体激光器作为激励光源,基于脉冲能量、脉冲数量、滤光片带宽、望远镜口径、接收视场角以及生物气溶胶粒子荧光非弹性散射截面积等主要参数,对生物气溶胶荧光回波信号的信噪比进行数值仿真。仿真结果表明,生物气溶胶粒子的质量分数为10-12时,在探测误差小于10%的情况下,系统在白天和夜晚的有效探测距离分别可达1.0 km和7.8 km;而在探测距离定义为0.5 km时,系统对生物气溶胶质量分数的最小分辨能力,白天和夜间分别为1.8×10-13和1.0×10-14。仿真结果有利于了解激光诱导荧光雷达系统的最优参数设定和最佳的实验环境,进而实现对生物气溶胶的有效探测。     

室内紫外光散射特性的研究

设计了一个以波长275nm的发光二极管为光源、光电二极管为探测器的紫外光传输实验系统。首先测试了该系统在视距和非视距两种传输方式不同距离下的传输特性,然后分别测试了不同光源仰角、探测器仰角下的传输特性,并把实验结果和理论结果进行了对比。结果表明,在非视距方式下,接收功率与光源仰角成反比关系,且距离越远光源仰角对本系统传输特性的影响越小,探测器仰角在30°～60°对本系统传输特性的影响较小。本研究结果给紫外光通信系统的设计提供了一定的参考。     

太赫兹波探测光子晶体涂层覆盖目标的可行性

覆盖于高温目标表面的光子晶体红外涂层可实现对目标红外辐射的抑制,而太赫兹波所具有的强穿透特性使其对该类目标的探测成为可能。以相关文献中设计的光子晶体涂层为例,采用特征矩阵理论对0.3~3 THz频率范围内的太赫兹波在该类涂层中的传输特性进行理论计算和分析,重点研究了不同入射角度的太赫兹波在该类涂层中的传输特性。研究发现,上述太赫兹波段处于光子晶体的带隙之外,0.3~0.5 THz频率范围内的太赫兹波对该类红外涂层具有较强的穿透特性,其光谱透过率大于90%;而在2.4~3 THz范围内,其在涂层上具有较强的反射,且整个波段内的吸收率小于0.2%。当入射角小于60°时,其对太赫兹波的传输特性影响较小;进一步增大入射角,其透过率逐渐降低,而反射率逐渐增大。研究结果证明了利用太赫兹波进行涂层覆盖目标探测的可行性,有望利用太赫兹雷达探测弥补红外探测系统的不足。     

多波长激光探测微小摄像头机理研究北大核心CSCD

基于激光不同波段的探测传输特点,通过分析典型微小摄像头的回波传输特性,提出了一种针对多波长激光特征检测微小摄像头的研究方案。利用几何光学和波动光学理论,分析微小摄像头结构及其反射光谱特征,计算并仿真了一定探测距离下的回波光场,在此基础上搭建多波长激光检测系统。实验结果表明,在一定的景深范围内,具有红外截止滤光片的微小摄像头对可见光的回波光斑衍射环特征明显、条纹对比度高且可探测距离远;近红外波段的目标回波功率低、后向散射干扰严重,可探测距离近;短波红外波段几乎不受红外截止滤光片影响,且1550 nm处于人眼安全波段。实验结果与数值分析、理论仿真结果一致,验证了短波红外激光用于探测微小摄像头的可行性。     

大气传输中红外偏振探测误差机理及校正

在红外偏振遥感探测系统中,目标的红外偏振特性受到大气环境因素的影响,其反演场景偏振参量会出现误差,因此,提出基于红外偏振传输理论的红外光谱偏振参量校正算法模型。通过分析大气辐射和传输特性,建立了大气有效透射率模型,研究了不同波段范围、大气环境、传输路径、温度等因素对有效透射率的影响,依据大气有效透射率模型校正偏振参量。实验分析表明,该校正算法模型有效抑制了大气传输对探测目标的红外偏振参量测量的影响,使得偏振参量能更准确地反映场景偏振特性,提高了红外遥感偏振的探测识别能力。     

A spectral domain approach to modelling of EM scattering for Synthetic Aperture Radar target recognition

A Fourier-based technique for electromagnetic (EM) wave reconstruction with application to polarimetric airborne and spaceborne radar data exploitation is presented. The method is different from conventional modelling techniques for Synthetic Aperture Radar (SAR) applications as a result of the full electromagnetic treatment of field interactions with the scatterer, the possibility of introducing new and controllable feature classes for target classification, and accurate decomposition of the source impulse response function that avoids potential errors (e.g. loss of coherent information) associated with the spherical phase approximations. The capability of extracting scatterer information such as the coherent radar cross section (RCS) is explored.     

A spectral domain approach to modelling of EM scattering for Synthetic Aperture Radar target recognition

A Fourier-based technique for electromagnetic (EM) wave reconstruction with application to polarimetric airborne and spaceborne radar data exploitation is presented. The method is different from conventional modelling techniques for Synthetic Aperture Radar (SAR) applications as a result of the full electromagnetic treatment of field interactions with the scatterer, the possibility of introducing new and controllable feature classes for target classification, and accurate decomposition of the source impulse response function that avoids potential errors (e.g. loss of coherent information) associated with the spherical phase approximations. The capability of extracting scatterer information such as the coherent radar cross section (RCS) is explored.     

Optical design of retro-reflectors by coordinate transformation

Transformation optics based on the form-invariant coordinate transformation of Maxwell’s equations offers an unconventional approach for designing devices with unprecedented electromagnetic (EM) behaviors. In this paper, we expand the coordinate transformation method to design omni-directional (OD) retro-reflectors by extending the conventional Luneberg lens reflectors to work in a much wider angular width. The constitutive medium tensors of the transformation devices are derived. Based on full wave simulations combined with Huygen’s principle, the flat and efficient backscattering responses of the resultant devices have been evaluated quantitatively. We confirm that the proposed structures are highly visible to incident EM waves from all directions, and they can be used as good OD radar cross section (RCS) enhancers in radar applications, such as the reference target for RCS measurements, range finding and position identification.     

Electromagnetic backscattering from one-dimensional drifting fractal sea surface Ⅱ:Electromagnetic backscattering model

Sea surface current has a significant influence on electromagnetic(EM) backscattering signals and may constitute a dominant synthetic aperture radar(SAR) imaging mechanism. An effective EM backscattering model for a one-dimensional drifting fractal sea surface is presented in this paper. This model is used to simulate EM backscattering signals from the drifting sea surface. Numerical results show that ocean currents have a significant influence on EM backscattering signals from the sea surface. The normalized radar cross section(NRCS) discrepancies between the model for a coupled wavecurrent fractal sea surface and the model for an uncoupled fractal sea surface increase with the increase of incidence angle,as well as with increasing ocean currents. Ocean currents that are parallel to the direction of the wave can weaken the EM backscattering signal intensity, while the EM backscattering signal is intensified by ocean currents propagating oppositely to the wave direction. The model presented in this paper can be used to study the SAR imaging mechanism for a drifting sea surface.     

Numerical analysis of the reflection and absorption of electromagnetic wave in nonuniform magnetized plasma slab

In this paper, a model for calculating the reflection and absorption powers of electromagnetic wave (EM wave) in nonuniform magnetized plasma slab is given out based on layer propagation theory. The effects of various plasma parameters and different values of magnetic field intensity on the reflected and absorbed powers are discussed. The results illustrate that the thickness of plasma seldom affects the reflection of radar wave, but it can broaden or reduce the absorption width. Meanwhile, the background magnetic field intensity has an influence upon the results, and it could change the resonance spectrum of magnetized plasma. We also find out that, with appropriate plasma density, collision frequency and magnetic field intensity, more than 90% of radar wave power can be absorbed and the resonant absorption band is about 2 GHz.     

A Thin Self-Feeding Janus Metasurface for Manipulating Incident Waves and Emitting Radiation Waves Simultaneously

A thin metasurface has shown powerful capabilities in controlling either incident electromagnetic (EM) waves or radiation waves, but is difficult for both. Here, a self-feeding Janus metasurface (SFJ-MS) is proposed to manipulate the incident EM waves and emit the radiated waves simultaneously, which can realize the polarization conversion of incident waves, scattering control, EM wave radiation, and radiation-beam steering. On the upper of SFJ-MS, a diagonal-split square ring and a rectangular patch with rotation for radiation are designed to introduce anisotropy in the meta-atom for converting the polarization of incident EM waves. On the bottom of SFJ-MS, a self-feeding microstructure converts the alternating current into the excitation of SFJ-MS to emit the EM waves to free space. The multiple functions of SFJ-MS are comprehensively substantiated by measured results, which are in agreements with the stringent simulations. This SFJ-MS, with lightweight, compact, low profile, and power-efficient features, can find potential applications in phased array radar systems, wireless communication systems, polarimetric radar imaging systems, and target detection systems.     

A study on electromagnetic wave absorber for the collision-avoidance radar

In this paper, we designed and fabricated the EM wave absorber for eliminating false image and system-to-system interference in a collision-avoidance radar using Permalloy and CPE, the mixing ratio of which was Permalloy:CPE = 70:30 wt%. The EM wave absorption ability was simulated according to different thicknesses of the EM wave absorber using measured permittivity and permeability, and then the EM wave absorber was fabricated based on the simulated design. The simulated results agree well with the measured ones. As a result, we suggest Permalloy for use as a new EM wave absorber in W-band, and the fabricated EM wave absorber with the thickness of 1.4 mm has absorption ability more than 20 dB in frequency range of 76–77 GHz.     

等离子体覆盖立方散射体目标雷达散射截面的时域有限差分法分析

采用分段线性电流密度递归卷积时域有限差分（PLJERC-FDTD）算法计算了均匀非磁化等离子体覆盖三维立方体目标的散射特性.分析了等离子体厚度、密度和碰撞频率对雷达散射截面（RCS）的影响.计算结果表明：等离子体包层能有效地减小雷达目标的RCS，当等离子体频率比入射电磁波频率小得多时，主要靠增大等离子体的厚度使立方散射体目标的RCS值减小，增大等离子体碰撞频率对立方散射体目标的RCS值影响不大；当等离子体频率约为入射电磁波频率的一半时，增大等离子体厚度和碰撞频率都对立方散射体目标的RCS值减小有影响；当等 关键词： FDTD算法 电磁波 等离子体隐身 雷达散射截面     

Analysis of plasma sheath propagation attenuation based on ray tracing

Based on the complex Snell's law of lossy media, a ray tracing method is proposed to study the propagation attenuation characteristics of electromagnetic (EM) waves in plasma sheaths. The plasma sheath is modelled as layered media. This method considers the complex ray characteristics of inhomogeneous plane EM waves, tracks the propagation rays of EM waves in each layer of media, and calculates the propagation attenuation of EM waves in each layer of media according to the propagation direction of the complex rays. The attenuation during numerical cumulative propagation is the total attenuation of EM waves through the plasma sheath. By comparing the results with that obtained from the WKB method, the accuracy of the ray tracing algorithm is proved. The results of the propagation attenuation of a blunt cone model are calculated by the proposed method, and the effects of different parameters on the EM wave propagation attenuation in the plasma sheath are analysed at different heights, velocities, incident angles, and incident positions. Studying the propagation characteristics of EM waves in the plasma sheath is of importance in application for radar target tracking, blackout communication, and other issues.