大气消光特性与机载激光雷达眼睛安全关系的研究

依据美国ANSI标准，定量分析了气溶胶、卷云和沙尘的垂直消光特性对机载激光雷达激光脉冲眼睛安全最大阈值能量的影响；以地基激光雷达探测得到的合肥上空沙尘暴为例，定量给出了大气中各成分消光特性对飞机眼睛安全最低高度的影响；模拟计算了各种大气成分的消光特性对激光脉冲眼睛安全系数的影响随高度的变化规律。计算结果表明：沙尘层对激光脉冲眼睛安全最大阈值能量的影响幅度可达几十毫焦。该结果为我国第一台机载大气探测激光雷达激光脉冲能量设置提供了基本依据。     

机载大气探测激光雷达人眼安全分析

 依据美国ANSI标准,模拟计算了0~12 km的高度范围内不同的激光束发散角和不同激光脉冲能量比例的532和1 064 nm激光脉冲人眼安全最大阈值能量。给出两种532和1 064 nm激光脉冲人眼安全最大阈值能量分配方案：(1)激光束发散角为0.3 mrad且532与1 064 nm的激光脉冲最大阈值能量之比为1∶2;（2）激光束发散角为0.4 mrad且532与1 064 nm的激光脉冲最大阈值能量之比为1∶1。分析了用这两种激光脉冲人眼安全最大阈值能量分配方案探测模式大气时所对应的信噪比。分析结果表明：这两种方案既能保证机载激光雷达对模式大气探测时地面人眼安全又能达到探测所要求的信噪比。     

双视场米散射激光雷达探测对流层气溶胶

介绍了自行研制的用来探测对流层大气气溶胶消光特性的双视场米散射激光雷达.该激光雷达采用两个具有独立接收视场的探测通道分别接收高低层532 nm的大气回波信号,可以兼顾低层大视场角低探测盲区和高层小视场角高探测高度的要求.叙述了该雷达系统的总体结构和技术参量以及数据处理方法,给出了合肥地区(东经117.16°,北纬31.90°)大气气溶胶消光系数廓线和对流层光学厚度的探测结果.测量结果表明,该雷达具备昼夜连续观测对流层大气气溶胶的能力,可以很好地反映气溶胶粒子的时间和空间分布特征.     

多波长激光雷达探测多种天气气溶胶光学特性与分析

设计和构建了波长为355,532和1064nm的多波长米散射激光雷达系统,并研究了多波长激光雷达信号数据处理和反演算法,实现了对地表气溶胶的探测;利用该激光雷达对2013年冬季西安市上空大气进行了探测,研究分析了雾霾天、晴天和有云天气的混合层高度、气溶胶消光特征和粒径分布特征.分析比较了不同波长探测到的混合层高度变化情况.在雾霾天,大气混合层高度与晴天和有云天相比明显偏低,在0.4 km附近,而晴天的混合层高度有0.5—0.8 km.利用长波(1064 nm/532 nm)段和短波(532 nm/355 nm)段两个ngstrm指数分析了不同天气情况下的粒径分布特征.对于近地层气溶胶,雾霾天的长波ngstrm(1064/532)指数小于短波ngstrm(532/355)指数,而晴天与之相反,说明在近地层雾霾有污染的大气中,存在有较多的粗粒子.在云层中,ngstrm指数明显减小,并且出现负值,说明云粒子半径比较大.     

探测对流层气溶胶的双波长米氏散射激光雷达

研制了一台双波长米氏散射激光雷达,用于532nm和1064nm两个波长对流层气深胶消光系数垂直廓线的长期探测.介绍了该激光雷达的技术参数和总体结构,叙述了各部分的结构和工作原理,给出了合肥地区对流层气溶胶测量的若干典型结果.     

光学器件性能参数对激光雷达探测气溶胶和云退偏比的影响

二向色镜及偏振分光棱镜作为常用的光学器件，近年来在大气探测偏振激光雷达中得到了广泛使用。但两种光学元件性能上的非理想特性与安装时存在的偏振误差夹角等问题在一定程度上会对大气探测后向散射光的退偏比造成影响。针对偏振激光雷达标定中通常只考虑偏振分光棱镜影响的问题，通过仿真模拟分别分析了二向色镜、偏振分光棱镜以及二者级联下对大气中气溶胶的退偏比影响，并给出了误差分析。以532 nm和1064 nm两种波长下的沙尘粒子与卷云的后向散射光作为输入进行模拟计算，结果显示，常用的长波通二向色镜对模拟输入光源在1064 nm透射通道下有7.111%的退偏比变化，在532 nm反射通道下有3.012%的退偏比变化。对偏振分光棱镜而言，输入为532 nm及1064 nm处探测的沙尘粒子退偏比会分别产生21.333%和27.3%的相对误差变化，532 nm处探测的卷云退偏比会产生14.2%的相对误差变化。两种光学元件在存在偏振误差夹角时均会带来额外的退偏比误差增量，在两种光学元件级联条件下，对模拟光源的退偏比也表现出累加性的误差增大。     

双视场米散射激光雷达探测对流层气溶胶

介绍了自行研制的用来探测对流层大气气溶胶消光特性的双视场米散射激光雷达.该激光雷达采用两个具有独立接收视场的探测通道分别接收高低层532 nm的大气回波信号,可以兼顾低层大视场角低探测盲区和高层小视场角高探测高度的要求.叙述了该雷达系统的总体结构和技术参量以及数据处理方法,给出了合肥地区(东经117.16°,北纬31.90°)大气气溶胶消光系数廓线和对流层光学厚度的探测结果.测量结果表明,该雷达具备昼夜连续观测对流层大气气溶胶的能力,可以很好地反映气溶胶粒子的时间和空间分布特征.      

双波长米散射激光雷达探测对流层气溶胶消光特性

新近研制了一台基于532和1 064 nm的双波长米散射激光雷达(dual-wavelength lidar,简称DWL),用于探测对流层大气气溶胶可见和红外波段的消光特性及其时空分布,同时用于粒子尺度谱垂直分布特征的研究。系统采用4个通道分别用于接收对流层下部和中上部532及1 064 nm的大气回波信号,有效地缩短了获取大气信息的时间。采用窄带滤光片,并借助光阑,将接收的激光大气回波信号谱线(米散射和瑞利散射光谱)从天空太阳背景噪声中分离,提高系统的白天探测能力。叙述了雷达系统的总体结构和技术参数以及数据处理方法。利用该雷达对合肥地区(117.16°E, 31.90°N)上空的气溶胶进行了探测。给出了对流层大气气溶胶532及1 064 nm消光系数的垂直廓线及其时空分布典型探测结果。分析了气溶胶波长依赖指数的空间垂直分布。讨论了对流层大气气溶胶光学厚度月变化。观测和分析结果表明,双波长具备昼夜连续观测对流层大气气溶胶的能力,可以很好的反映气溶胶粒子的时间和空间分布特征。     

全天时紫外高光谱瑞利测温激光雷达系统EI北大核心CSCD

紫外高光谱瑞利测温激光雷达是一种探测大气温度廓线的有效工具。目前,紫外高光谱瑞利测温激光雷达通常采用355nm波长的光,然而白天太阳背景光辐射会影响雷达系统的信噪比(SNR),进而制约温度探测的距离和精度。针对大气温度的全天时探测,提出了基于法布里-珀罗标准具的266nm紫外高光谱瑞利测温激光雷达系统。由于到达地面的太阳背景光辐射不包含266nm波长的光,只需考虑臭氧对266nm波长光吸收的影响,进而实现全天时大气温度的探测。基于脉冲能量、望远镜直径、望远镜接收视场角、臭氧浓度以及太阳背景光强度等主要影响参数,对266nm和355nm两个波长紫外高光谱瑞利测温激光雷达系统的谱宽、透过率、回波信号SNR以及温度偏差参数进行数值仿真和对比分析。结果表明,大气分子和气溶胶散射对266nm波长光的影响远大于对355nm波长光的影响。白天266nm紫外高光谱瑞利测温激光雷达系统的有效探测距离为4km左右,比355nm紫外高光谱瑞利测温激光雷达系统的有效探测距离远2.9km;夜间266nm紫外高光谱瑞利测温激光雷达系统有效探测距离为6km。探测距离小于5km时,白天266nm紫外高光谱瑞利测温激光雷达的探测温度偏差比355nm紫外高光谱瑞利测温激光雷达的探测温度偏差小10K。266nm紫外高光谱瑞利测温激光雷达可实现全天时大气温度的探测。     

全天时紫外高光谱瑞利测温激光雷达系统

紫外高光谱瑞利测温激光雷达是一种探测大气温度廓线的有效工具。目前,紫外高光谱瑞利测温激光雷达通常采用355nm波长的光,然而白天太阳背景光辐射会影响雷达系统的信噪比(SNR),进而制约温度探测的距离和精度。针对大气温度的全天时探测,提出了基于法布里-珀罗标准具的266nm紫外高光谱瑞利测温激光雷达系统。由于到达地面的太阳背景光辐射不包含266nm波长的光,只需考虑臭氧对266nm波长光吸收的影响,进而实现全天时大气温度的探测。基于脉冲能量、望远镜直径、望远镜接收视场角、臭氧浓度以及太阳背景光强度等主要影响参数,对266nm和355nm两个波长紫外高光谱瑞利测温激光雷达系统的谱宽、透过率、回波信号SNR以及温度偏差参数进行数值仿真和对比分析。结果表明,大气分子和气溶胶散射对266nm波长光的影响远大于对355nm波长光的影响。白天266nm紫外高光谱瑞利测温激光雷达系统的有效探测距离为4km左右,比355nm紫外高光谱瑞利测温激光雷达系统的有效探测距离远2.9km;夜间266nm紫外高光谱瑞利测温激光雷达系统有效探测距离为6km。探测距离小于5km时,白天266nm紫外高光谱瑞利测温激光雷达的探测温度偏差比355nm紫外高光谱瑞利测温激光雷达的探测温度偏差小10K。266nm紫外高光谱瑞利测温激光雷达可实现全天时大气温度的探测。     

CALIPSO validation using ground-based lidar in Hefei (31.9°N, 117.2°E), China

Validation measurement in collaboration with existing lidar sites is a very important part of CALIPSO validation program, lidar site in Hefei is invited to collaborate in the CALIPSO validation program. In this paper, ground-based lidar measurements in Hefei performed in coincidence with CALIPSO overpass are presented, attenuated backscatter profiles at 532 nm and 1064 nm, as well as volume depolarization ratio profile at 532 nm measured by CALIPSO are compared with the ones measured by ground-based lidar. The comparisons indicate that CALIPSO measurements are consistent with the ground-based lidar measurements. However, due to the fact that horizontal distributions of aerosols in the lower troposphere and clouds are in most cases inhomogeneous, there are some differences between two lidar measurements in the boundary layer and clouds. The aerosol layer below the semi-transparent thick cloud can be detected by the 532 nm channel of CALIPSO in daytime.     

Laser-induced incandescence for soot particle size and volume fraction measurements using on-line extinction calibration

A novel technique for two-dimensional measurements of soot volume fraction and particle size has been developed. It is based on a combined measurement of extinction and laser-induced incandescence using Nd:YAG laser wavelengths of 532 nm and 1064 nm. A low-energy laser pulse at 532 nm was used for extinction measurements and was followed by a more intense pulse at 1064 nm, delayed by 15 ns, for LII measurements. The 532-nm beam was split into a signal beam passing the flame and a reference beam, both of which were directed to a dye cell. The resulting fluorescence signals, from which the extinction was deduced, together with the LII signal, were registered on a single CCD detector. Thus the two-dimensional LII image could be converted to a soot volume fraction map through a calibration procedure during the same laser shot. The soot particle sizes were evaluated from the ratio of the temporal LII signals at two gate time positions. The uncertainty in the particle sizing arose mainly from the low signal for small particles at long gate times and the uncertainty in the flame temperature. The technique was applied to a well-characterized premixed flat flame, the soot properties of which had been previously thoroughly investigated. Received: 21 June 2000 / Revised version: 11 September 2000 / Published online: 7 February 2001     

Determination of the ratio of soot refractive index function E(m) at the two wavelengths 532 and 1064 nm by laser induced incandescence

A new method is proposed to measure the ratio of the refractive index function of soot particles E(m) at the two fixed wavelengths: 532 and 1064 nm. Using a non-intrusive, in-situ laser based technique, the ratio E(m,1064 nm)/E(m,532 nm) can be determined by comparing laser induced incandescence (LII) intensities at 532 and 1064 nm excitation wavelengths. The method consists of selecting laser energies that insure the equality of the LII signals in the low fluence regime under given conditions. Such equality is consistent with the fact that the soot particle will have reached the same temperature independently of the laser wavelength, i.e. the soot particle has absorbed the same energy. As the absorbed energy is proportional to the laser irradiance times E(m), the measurement of the laser energies required to insure perfect concordance of the LII intensities (spatially and temporally) serves to deduce the ratio E(m,1064 nm)/E(m,532 nm). The method is demonstrated in an acetylene/air flame, validated against extinction measurements performed by cavity ring-down spectroscopy (CRDS) by using laser radiations at 532 nm and 1064 nm and finally applied to different flame conditions. PACS 78.20.Ci; 78.90.+t; 81.05.Uw; 42.62.-b     

车载式1 064 nm和532 nm双波长米散射激光雷达

 新近研制的车载式双波长米散射激光雷达可用于1 064 nm 和532 nm两个波长对白天与夜晚对流层气溶胶消光系数垂直分布进行的探测。该激光雷达由激光发射单元、接收光学和后继光学单元、信号探测和采集单元以及系统运行控制单元组成,后继光路之间采用光纤导光、高低层分层探测等关键技术。该激光雷达使用1 064 nm和532 nm的两个波长,其单发脉冲能量分别为400和300 mJ,重复频率都为20 Hz,光束发散角小于0.5 mrad ;望远镜接收视场为1～3 mrad,滤光片的中心波长为1 064 nm和532 nm,带宽1 nm。分别使用R3236及H7680的PMT和VT120及Phillips777的放大器对两个波长的信号进行探测;对532 nm波长用3 A/D采集卡、1 064 nm波长用了光子计数卡。给出了双波长测量对流层气溶胶消光系数垂直分布的结果,该激光雷达可以探测10 ^-5～1之间的消光系数,探测高度可达10 km以上。     

Effective lidar ratios of dense dust layers over North Africa derived from the CALIOP measurements

Lidar ratio (i.e., extinction-to-backscatter ratio) is a key parameter required for retrieving extinction profiles and optical depths from elastic backscatter lidar measurements, and the quality of any extinction retrieval depends critically on the accuracy of the assumed or measured lidar ratio. In this study, we analyze the first two and a half years of the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data acquired during nighttime. Distributions of the effective lidar ratio (ELR), which is the product of the lidar ratio and an instrument-dependent multiple scattering factor, are derived for opaque dust layers observed by CALIOP over the North Africa. The median and mean ELR values are, respectively, 36.4 and 38.5 sr at 532 nm and 47.7 and 50.3 sr at 1064 nm. For these opaque dust layers, the derived ELR decreases as the volume depolarization ratio (VDR) increases, reflecting the impact of multiple scattering within the dense layers. The particulate depolarization ratio is typically ∼0.3 at 532 nm for African dust observed by CALIOP. This ratio can increase to ∼0.4 in the presence of significant multiple scattering. Correspondingly, the calculated ELR will decrease to ∼20 sr at 532 nm and to ∼30 sr at 1064 nm. The median and mean effective lidar ratio values approach, respectively, to 38 and 40 sr at 532 nm and 52 and 55 sr at 1064 nm for smaller VDR values measured in less dense layers where the multiple scattering is relatively insignificant. These values are very close to those derived in previous case studies for moderately dense dust. Case studies are also performed to examine the impacts of multiple scattering. The results obtained are generally consistent with Monte-Carlo simulations.     

紫外域激光雷达探测西安城区上空大气气溶胶时空剖面

开发了一套紫外域波长的米散射激光雷达系统,探测西安城区上空大气气溶胶污染物质的光学特性及时空变化.系统选用对人眼较为安全的355 nm波长激光作光源,采用高光谱分辨率光栅,并借助光阑,将接收到的主要大气回波信号谱线（米散射和瑞利散射光谱与白天太阳背景光）从空间分离,剔除大部分太阳背景噪音,提高系统的白天探测能力.通过对西安城区上空的气溶胶时空变化特性进行24 h连续观测,采用Klett方法反演得到气溶胶消光系数,首次测得西安城区不同时刻消光系数的高度分布剖面图以及24 h内气溶胶相对质量密度的时空变化特性.     

Generation of flat-top waveform in the time domain based on stimulated Brillouin scattering

A method of generating a flat-top waveform in the time domain based on stimulated Brillouin scattering (SBS) is proposed. The transmitted pulses are simulated with pump wavelength at 1064 nm and 532 nm, respectively, and validated in the experiment performed with the Nd:YAG seed-injected laser. The experimental results agree well with the theoretical simulation. With 532 nm pump wavelength, the top of the transmitted pulse is almost a platform, while there is a peak in the front and a platform in the back with 1064 nm pump wavelength. The mechanism behind the generation of flat-top waveform with 532 nm pump wavelength is analyzed in details. PACS 42.65-k; 42.65.Es; 42.65.Hw     

Quantitative studies of copper plasma using laser induced breakdown spectroscopy

In the present work, we present the spatial evolution of the copper plasma produced by the fundamental harmonic (1064 nm) and second harmonic (532 nm) of a Q-switched Nd:YAG laser. The experimentally observed line profiles of neutral copper have been used to extract the electron temperature using the Boltzmann plot method, whereas, the electron number density has been determined from the Stark broadening. Besides we have studied the variation of electron temperature and electron number density as a function of laser energy at atmospheric pressure. The Cu I lines at 333.78, 406.26, 465.11 and 515.32 nm are used for the determination of electron temperature. The relative uncertainty in the determination of electron temperature is ≈10%. The electron temperature calculated for the fundamental harmonic (1064 nm) of Nd:YAG laser is 10500–15600 K, and that for the second harmonic (532 nm) of Nd:YAG laser is 11500–14700 K at a Q Switch delay of 40 μs. The electron temperature has also been calculated as a function of laser energy from the target surface for both modes of the laser. We have also studied the spatial behavior of the electron number density in the plume. The electron number densities close to the target surface (0.05 mm), in the case of fundamental harmonic (1064 nm) of Nd:YAG laser having pulse energy 135 mJ and second harmonic (532 nm) of Nd:YAG laser with pulse energy 80 mJ are 2.50×10¹⁶ and 2.60×10¹⁶ cm⁻³, respectively.