基于粒子谱的多波长激光雷达近场大气光学参数校正方法

非同轴激光雷达由于存在发射激光与接收望远镜之间的不完全重叠区, 造成近场回波信号与真实大气信号不一致. 对于多波长激光雷达, 这种不一致更为突出和复杂. 然而, 近场大气是人类活动最集中的区域, 因此对多波长激光雷达近场信号进行校正, 对于了解和探究边界层大气具有十分重要的意义. 提出了一种利用粒子谱仪测量近地层气溶胶尺度谱分布并运用Mie 散射理论和低层大气指数衰减规律, 进而直接校正多波长激光雷达消光系数廓线近场信号的新方法. 通过对晴天、多云天气和雾天多波长气溶胶消光系数廓线近场信号的校正, 证明了该方法的可行性和实用性. 该方法着重考虑了多波长激光雷达比的波长依赖性和气溶胶粒子谱分布的天气相关性, 将该方法用于近地层大气消光系数廓线校正, 减少了由于不考虑这两个因素带来的消光系数廓线反演和校正的不确定性. 该方法对于研究不同天气情况下边界层内的大气气溶胶物理、光学特性具有一定的实用价值和借鉴意义.     

Application of Geometrical Form Factor in Differential Absorption Lidar Measurement

The peak position for a lidar return signal is calculated and measured for the horizontal path with variation of the laser beam divergence angle (θ), and the inclination angle (δ) between the telescope and laser axes. This work shows that θ and δ are very important parameters to use in the design or alignment of a lidar system receiving a good lidar signal. This paper describes an experimental determination of geometrical form factors in the lidar equation. We receive the signals and determine the geometrical form factors by slope method in a homogeneous atmosphere. The differential absorption lidar equation is evaluated for the dual-pulse lidar system. A method using a geometrical form factor determined by the experiment is introduced to correct the error in C₂H₄ measurement. This method shows good correction of measurement error in lidar dual-pulse operation, especially in the short range.     

激光气象雷达多次后向散射信号特性分析

利用小角近似方法，分别给出了单次和多次后向散射激光雷达方程.通过引入的多次散射评价参量，分析了接收视场角和光学厚度等因素对多次后向散射信号的影响.理论研究表明:多次后向散射主要与接收视场角和光学厚度密切相关，当接收视场角比发射视场角大10倍，光学厚度超过3时, 多次后向散射信号逐渐增大并占主要优势，当接收视场角比发射视场角大100倍时，光学厚度超过1时，多次后向散射信号开始明显增强.     

Lidar Multiple Scattering in Water Droplet Clouds: Toward an Improved Treatment

Although it has long been recognized that the effects of photon multiple scattering generally need to be accounted for in the analysis of lidar cloud returns, this is a difficult problem and current approaches are still rudimentary. The multiple scattering process is controlled by the size of the lidar beamwidth and the distance to the cloud, which jointly determine the lidar footprint, but cloud microphysical content (i.e., particle size, concentration, and shape) exerts a strong influence on the range distribution and depolarization of the returned energy. Since clouds are inherently inhomogeneous with height, it is our premise that vertically homogeneous cloud simulations based on idealized particle size distributions lead to misleading results. We offer a more realistic approach based on the contents of growing water droplet clouds predicted by a sophisticated adiabatic cloud model, which are offered for use as new standard vertically-inhomogeneous cloud models. Lidar returned signal and depolarization profiles derived from our analytical double-scattering method are given for inter-comparison purposes.Presented at the 7th International Workshop on Multiple Scattering Lidar/Light Experiments (MUSCLE7), July 21–23 1994, Chiba, Japan.     

Feasibility of a Lidar Utilizing the Glory for Measuring Particle Size of Water Clouds

A new lidar method for measuring water cloud particle size is proposed, and the feasibility of the measurement is discussed. The method utilizes the phenomenon known as the glory which is observed in open air. The proposed lidar consists of a multicolor laser transmitter and two receiver systems looking at the scattering from the target cloud with different scattering angles. Results of the theoretical study show that a system with five laser wavelengths (355, 532, 750, 1064 and 1500 nm) and two receivers located at scattering angles of 180 and 177.5–179 deg is useful for measuring particle size (mode radius of the size distribution) in a range of 4 to 12μm.     

Development of a Ti:Sapphire DIAL system for pollutant monitoring and meteorological applications

A mobile differential absorption lidar system with SHG of Nd-YAG pumped Ti : Sapphire laser has been developed and tested. Mixing between Ti : Sapphire and Nd-YAG fundamentals extends the wavelength range to UV–vis region (250–480 nm), which is ideal to monitor tropospheric pollutants. Preliminary water vapor profiles in the near IR region (in our case 800–900 nm) are presented and discussed.     

Experimental determination of the multiple-scattering effect on the lidar-signal polarization characteristics during liquid- and solid-phase precipitation

We present results of experimental investigations of the signal-polarization characteristics in the case of lidar sounding during precipitation. We show and discuss the lidar signals and the depolarization profiles along the sounding path for liquid- and solid-phase precipitation. In the former case we compare the signal characteristics at different degrees of precipitation rate. In the latter situation, we consider snowfall with particle shape close to that of Chebyshev particles. We also follow the lidar-signal changes depending on the field-of-view of the receiving optics. The experimental data are compared with results of theoretical estimates and models concerning the optical and microphysical characteristics of the rain and snow particles. In the case of liquid-phase precipitation – rain – the observed dependence of the lidar’s signal-polarization structure on the precipitation intensity has two aspects: on the one hand, the change of the raindrops’ shape, and, on the other, the multiple-scattering effects. The lidar data demonstrate that the signal depolarization, and, more specifically, its behavior along the sounding path, can be used as a criterion for the presence of multiple scattering. In the case of a snowfall consisting of Chebyshev particles, the simultaneous role is evident of two factors influencing the lidar-signal depolarization, namely, the non-spherical shape of the particles and the multiple-scattering effects. When the scattering takes place off particles with a large size and a shape strongly differing from spherical, we observed the predominant role of the non-sphericity of the scattering centers in the signal depolarization. Received: 6 December 2000 / Revised version: 11 July 2001 / Published online: 19 September 2001     

A Pseudo-Random Frequency Modulation Continuous Wave Coherent Lidar Using an Optical Field Correlation Detection Method

We propose a new detection method for a pseudo-random frequency modulation continuous wave (RM-CW) coherent lidar. The feature of this method is modulation of local beam with a time delayed pseudo-random sequence. Heterodyne detection and correlation detection between the received beam and the local beam are simultaneously carried out in an optical field. In the RM-CW coherent lidar using the optical field correlation detection method, the received equipment is greatly simplified. We carried out preliminary experiments and demonstrated that the new method is effective for detection of a CW coherent lidar.     

Signal Penetration into Thick Clouds Studied by Multi-Layer Data Observed with a Micro-Pulse Lidar

Lidar return signals are studied for a micro-pulse lidar under sky conditions with multi-layer clouds. From theoretical considerations on the lidar signal-to-noise ratio, it is estimated that the maximum cloud optical thickness detectable is about 3.7. This result reasonably agrees with the actual multi-layer cloud data obtained from observations in Sukhothai, Thailand. Deviations from theoretical prediction, however, are found for a geometrically thin but dense cloud, and for a moderately concentrated but geometrically thick cloud. The effect of multiple scattering is also discussed.     

Angularly resolved backscattering of light from turbid suspensions of dielectric spheres

An analytic expression for the angular dependency of light backscattered from turbid suspensions of dielectric spheres that accounts for multiple-scattering effects is developed. The multiple-scattering model expresses backscattered light as a convolution of the phase function and a kernel representing the effects of forward scattering, multiplied by the optical thickness of the slab. Excellent agreement between Monte Carlo simulations and the analytic expressions is demonstrated for optical thicknesses of 0.1-4. The results are important for goniometric sizing measurements of biological cells and cellular organelles using elastically backscattered light.     

非共轴激光雷达系统参数对几何因子的影响

介绍了非共轴激光雷达几何因子的物理意义,通过数值模拟分析了激光雷达各系统参数包括：激光发散角、接收视场角、光轴间距、光轴失调夹角对几何因子的影响。结果表明：光轴离轴失调对激光雷达影响最大,在系统设计时应极力避免。结合自行研制的CSY2型能见度激光雷达,将理论计算与实验测量值进行对比,实验结果表明两种计算结果基本一致。     

Raman激光雷达探测气溶胶光学特性

介绍了武汉大学自行研制的Raman多通道激光雷达系统,给出了整个系统的设计原理及主要技术参量.详细描述了利用Raman激光雷达原理反演大气气溶胶消光系数、后向散射系数和激光雷达比等光学特性的方法,并对求解消光系数过程中的关键部分做了讨论分析.同时对武汉上空对流层低空大气气溶胶、云以及边界层等光学特性进行了实时探测反演.实验结果表明:该Raman多通道激光雷达系统在夜晚对低空气溶胶的垂直分布特性具有较好的探测能力,工作性能可靠.     

Spaceborne Laser Rangefinder “LORA” Used as a Cloud Lidar

First, in this report an overview is given on the use of the Russian laser altimeter in space “LORA” as a lidar for cloud measurements. Secondly, the influence of multiple scattering is verified by analysis of the backscatter signals from clouds having the excellent range resolution of a rangefinder.Presented at the 7th International Workshop on Multiple Scattering Lidar/Light Experiments (MUSCLE7), July 21–23 1994, Chiba, Japan.     

Iterative Deconvolution Algorithm for Improving Range Resolution of a CO2-Laser Based Lidar System

An iterative deconvolution algorithm for improving range resolution of long-pulse lidars is proposed, and can be applied to the lidar data obtained with the typical pulse of a CO₂-laser which consists of a gain-switching peak and a long tail. The lidar signal itself with certain temporal shift is set to be the start profile for the unknown maximally resolved profile in the proposed technique, and then is corrected in proportion to the difference between the lidar return calculated with the assumption and the real one. The same process is repeated until the correction is smaller than tolerance. Simulations are made to test the performance of the proposed algorithm. We investigate the errors in the vicinity of data boundary in the retrieved profile when a part of lidar data is absent. The sensitivity of the iteration algorithm to noise in the lidar signals and the laser pulse profile is also numerically determined.     

Broadband amplitude-modulated laser source with high output power for FM/cw lidar transmitter

We are building a long-range FM/cw nonscanning imaging lidar breadboard. This lidar system achieves ranging based on a frequency modulation/continuous wave (FM/cw) technique, implemented by an amplitude modulated mid-IR diode laser transmitter with a linear frequency modulation (LFM) of the subcarrier. Firstly, various schemes of light beam modulation are analyzed. Secondly, we put forward a laser modulation scheme whose core was formed by a 1.55 μm electro-absorption modulated laser diode (EML) and an erbium-doped optical fiber amplifier (EDFA), then a corresponding experimental system architecture and components for light beam modulation and detection are established. Finally, a corresponding experiment of laser beam modulation is completed for the first time. In our experiment, the EML amplitude is modulated by a 200 MHz to 800 MHz LFM signal, whose amplitude value is 2.05 V. The average output power of the modulated laser obtained in the experiment is 10 W, peak power is 16.4 W, and the average modulation depth is 78%. The results of tests predict that this laser modulation scheme is likely to improve the imaging range of the FM/cw lidar.     

米散射多普勒激光雷达性能改进

介绍了自行研制的米散射多普勒激光雷达的系统结构。针对系统存在的问题,对接收机子系统、工作时序、扫描方式等方面作了改造。实验结果表明:改造后系统光学鉴频器的频谱稳定性提高了2.8倍;消除了系统近距离的探测盲区;在0~2.5 km的高度范围内,当垂直距离分辨力为21.2 m、脉冲累积数为9 000发时,改造后的米散射多普勒激光雷达系统连续30 min测量的水平风速大小和方向的标准偏差的最大值分别小于1 m/s和18.3°,平均值分别小于0.43 m/s和7.7°;与探空仪的风场对比测量结果吻合得很好;当每个径向脉冲累积数为1 000发时,采用四波束扫描获得的2 km以下东西方向径向风速的相关系数为0.99,偏差为-0.038 m/s,标准偏差为1.34 m/s。     

大气水汽探测地基差分吸收激光雷达系统设计与性能仿真

为持续获得对流层低层高精度、高时空分辨率的水汽浓度分布数据,提出了一套改进的大气水汽探测地基差分吸收激光雷达系统方案.详细描述了系统主要组成部分,对主要误差进行了分析与估计,并提出了一种差分吸收截面实时测量装置用于补偿发射激光器带来的测量误差.针对该系统,并结合上海地区不同季节的水汽浓度状况,对935 nm水汽吸收带中四个水汽吸收峰的差分光学厚度、雪崩二极管的倍增系数M与回波信噪比的关系、水汽浓度随机测量误差等进行了详细的仿真与分析.仿真结果表明,根据不同的季节和天气状况,可以选择不同的吸收峰以达到最佳探测效果;在300—5000 m高度范围内,最大可以达到300 m的垂直分辨率与5 min的时间分辨率,水汽浓度随机测量误差不超过18%.     

双波长偏振测量激光雷达接收光路的设计

双波长偏振测量激光雷达在环境遥感、植被遥感、测绘以及军事侦察等方面具有重要的应用价值。双波长接收透射光路的设计对于提高激光雷达的应用效率、减小系统体积、降低系统的重量和成本有着重要的意义。应用ZE-MAX光学设计软件,设计了1064nm和532nm的双波长的激光雷达偏差测量接收光路,给出了该系统的最佳设计参数,对所设计光路的点扩散函数分析结果表明,该光路可以很好地获得双波长最佳成像探测质量,为实际工程应用具有良好的借鉴作用。     

基于舰船尾流激光雷达的Monte Carlo模型及方差消减方法研究

对舰船尾流激光散射机理及探测过程的数值模拟是研究舰船尾流激光探测与制导的重要基础.首先分析了Monte Carlo方法引入到尾流激光雷达数值模拟领域研究的可行性.结合自行研制的尾流激光雷达实际参数,建立了尾流激光探测的Monte Carlo数值仿真模型.通过对仿真结果进行统计,深入剖析了仿真结果方差大及光子利用率低等问题的成因.为解决该问题,基于有偏采样理论和分裂-轮盘赌基本原理提出了接收光学视场内光子强行碰撞方法和以光子自由程总长度为准则的光子分裂方法,并进行了两方法的融合.仿真与实验结果的对比分析表明,提出的模型仿真结果与实验数据符合较好,验证了模型的正确性;提出的接收光学视场内光子强行碰撞方法和光子分裂方法能有效减小方差并提高光子利用率;解决了Monte Carlo方法引入到尾流激光雷达模拟过程中的适用性问题.     

连续模式相干测风激光雷达性能优化分析及实验研究

基于风场探测的需求,采用1550 nm波长连续激光种子源搭建了一套全光纤结构多普勒相干测风雷达系统.从雷达方程出发,对连续激光相干雷达载噪比方程和不同雷达收发望远镜聚焦位置下风速合成权重进行了分析.针对测风雷达要求设计了5～200 m的变焦激光收发望远镜模块.扩束系统采用伽利略折射式结构,发射光束准直下扩束比为23,光...