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差分吸收激光雷达是利用大气分子和气溶胶对发射光束的吸收和散射进行测量的。假设P(R)为距离R处的激光雷达回波信号,波长为λ的发射光束功率为P0(λ),在距离R处大气后向散射系数和消光系数分别为β(λ, R)和α(λ, R),待测气体分子在波长λ处的吸收截面为σ(λ),在距离R处的分子数密度为N(R),系统的接收效率为η,望远镜的接收面积为A,ΔR是空间取样距离,则回波信号功率为[14]:
$ \begin{align} &P\left( R \right)={{P}_{0}}\left( \lambda \right)\eta \beta (\lambda, R)\Delta R(A/{{R}^{2}})\cdot \\ &\text{exp}-\left[2\int_{0}^{R}{\left[N\left( z \right)\sigma \left( \lambda \right)+\alpha \left( \lambda, z \right) \right]}\text{d}z \right] \\ \end{align} $
(1) 在探测路径上同时或交替发射两束波长非常接近的激光,一束波长位于待测气体分子的吸收峰上,对待测气体有强烈的吸收作用,记为λon,另一个波长位于待测气体分子的吸收谷或吸收峰外,记为λoff,这两束光的回波分别记为Pon(R)和Poff(R),由(1)式可得传播路径上不同距离R处待测气体的分子数密度为:
$ \begin{align} &N\left( R \right)=-\frac{1}{2\Delta \sigma }~\frac{\text{d}}{\text{d}R}\text{ln}\frac{P({{\lambda }_{\text{on}}}, R)}{P({{\lambda }_{\text{off}}}, R)}+ \\ &\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ B-{{E}_{\text{a}}}\text{-}{{E}_{\text{m}}}~ \\ \end{align} $
(2) 式中,
$ \Delta \sigma =\sigma ({{\lambda }_{\text{on}}})-\sigma ({{\lambda }_{\text{off}}}) $
(3) $ B=\frac{1}{2\Delta \sigma }\frac{\text{d}}{\text{d}R}\left[\text{ln}\frac{\beta ({{\lambda }_{\text{on}}}, R)}{\beta ({{\lambda }_{\text{off}}}, R)} \right] $
(4) $ {{E}_{\text{a}}}=\frac{1}{\Delta \sigma }\text{ }[{{\alpha }_{\text{a}}}({{\lambda }_{\text{on}}}, R)-{{\alpha }_{\text{a}}}({{\lambda }_{\text{off}}}, R)] $
(5) $ {{E}_{\text{m}}}=\frac{1}{\Delta \sigma }[{{\alpha }_{\text{m}}}({{\lambda }_{\text{on}}}, R)-{{\alpha }_{\text{m}}}({{\lambda }_{\text{off}}}, R)] $
(6) 式中,Δσ为待测气体分子在波长λon和λoff处的吸收截面差,B, Ea及Em分别为大气后向散射作用项、大气气溶胶消光作用项及大气分子消光作用项,统称为修正项;αa (λon, z)和αa (λoff, z)为距离R处λon和λoff波长大气气溶胶的消光系数;αm (λon, z)和αm (λoff, z)为距离R处λon和λoff波长大气分子的消光系数。
若λon和λoff相差很小,则修正项B, Ea及Em可以忽略不计。对(2)式进行差分运算,可得R到R+ΔR之间的平均值:
$ N\left( R \right)=-\frac{1}{2\Delta R\Delta \sigma }\text{ln}\frac{P({{\lambda }_{\text{on}}}, R+\Delta R)P({{\lambda }_{\text{off}}}, R)}{P({{\lambda }_{\text{off}}}, R+\Delta R)P({{\lambda }_{\text{on}}}, R)} $
(7) 式中,ΔR为差分距离。由(2)式~(7)式可知,只有当λon和λoff越接近时,修正项B, Ea及Em的影响才越小。因此,λon和λoff要尽可能选取波长间隔小且吸收截面差Δσ大的波长对,并尽量避开待测气体以外其它气体的吸收干扰。图 1是NO2在230nm~630nm之间的光谱吸收截面[15]。在450nm附近有几个吸收峰可用于差分吸收激光雷达测量,本文中选择λon(448.10nm)和λoff(446.80nm)作为探测波长,而不选择λon左侧更高的吸收峰(447.92nm),这是因为它的左侧0.05nm范围内谱线变化剧烈,如果激光器的波长略为漂移,吸收截面就会造成较大的误差。
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按照APSOS项目中探测大气NO2的测量指标,要求探测精度为10μg/m3,探测范围3.0km。参照AML系列车载式测污激光雷达的设计及探测结果[11-13],接收望远镜为直径350mm牛顿式望远镜,干涉滤光片滤波(中心波长447.50nm,带宽3.0nm,带外扼制比为OD5@200~1100nm,峰值透过率为70%),光电倍增管选用日本滨松公司的H10426,数据采集卡为ADLINK公司的PCI-9826型(四通道,16bit,采样速率20MHz)。由(7)式可得差分吸收激光雷达的探测极限为:
$ \begin{align} &{{N}_{\text{min}}}=\frac{1}{2\Delta R\Delta \sigma }[~\text{ln}\frac{P({{\lambda }_{\text{on}}}, R)}{P({{\lambda }_{\text{off}}}, R)}-\\ &\text{ln}~\frac{P({{\lambda }_{\text{on}}}, R+\Delta R)}{P({{\lambda }_{\text{off}}}, R+\Delta R)}]{{~}_{\text{min}}}=~\frac{\Delta }{~2\Delta R\Delta \sigma } \\ \end{align} $
(8) 由(8)式可知,当探测波长确定后,探测极限可通过空间分辨率和回波的功率比来调整,按所选元器件的性能,回波比的对数差值Δ≈1.2×10-4,探测极限为10μg/m3。为确定激光器的能量,将从气溶胶消光系数、大气NO2含量、激光雷达的几何因子和探测距离等因素来确定。回波的信噪比定义为[16]:
$ {{R}_{\text{SNR}}}=~\frac{{{N}_{\text{s}}}\sqrt{M}}{\sqrt{{{N}_{\text{s}}}+2({{N}_{\text{b}}}+{{N}_{\text{d}}})~}} $
(9) 式中,Ns为回波光子数,Nb为背景辐射光子数,Nd为光电倍增管的暗计数,M是激光累计脉冲数。设大气消光系数α对λon为0.6km-1,而λoff的消光系数按指数关系求出。NO2水平均匀分布,浓度为30μg/m3,激光器的发射能量为5mJ,重复频率为10Hz。由(1)式和(9)式可计算出回波的信噪比(signal-to-noise ratio, SNR),其中,激光累计脉冲数M=6000(对应测量时间10min),图 2是其回波的信噪比。由图可知,当波长λon和λoff两个回波随着距离的增大而衰减,λon衰减较快,信噪比大于10的范围约为0km~4.7km,信号的动态范围为6个数量级。发射系统采用与接收望远镜同轴的结构,盲区短,也可对近端信号进行压缩。图 3是几何因子及λon回波信号。经过系统几何因子对回波的压缩,把近距离的300m内强回波信号压下来,而后面的信号几乎不受影响,动态范围为4个数量级,方便光电倍增管的接收。
图 4是信噪比大于10时,最远探测距离随激光输入能量变化的情况。在消光系数α=0.6km-1条件下,当输入能量从2mJ增大到4mJ时,最远探测距离增加300m;从4mJ增大到6mJ时,最远探测距离增加200m。由曲线可知,在满足探测距离的条件下,通过增大能量的方法来增大探测距离并不明显。比较两条曲线可知,在相同的激光能量下,消光系数系数对探测距离影响较大。
图 5是在信噪比大于10、测量距离大于3.0km条件下,不同消光系数下所需的能量。可以看出,消光系数越大时,所需的能量越高。消光系数α>0.6km-1时所需的能量增长率远大于α < 0.6km-1时。图 6是在信噪比大于10、测量距离大于3.0km条件下,大气中不同的NO2含量所需的激光发射能量。可以看出,当大气中NO2含量越高时,所需的激光发射能量大,但与气溶胶增量(消光系数)相比,所需发射能量的增量较为缓慢。为满足测量NO2浓度的垂直廓线,以合肥科学岛上空气溶胶的典型分布为参考[17],假设NO2的垂直分布为指数分布,大气分子的分布为中纬度模式。假定激光发射能量为5mJ,图 7是消光系数廓线及对应的回波信噪比。由于受气溶胶不均匀性的影响,回波有明显的起伏,信噪比大于10的高度为4.5km。如果消光系数采用中纬度模式分布,信噪比大于10的高度为6.5km。综合气溶胶在水平和垂直方向上的分布、NO2含量、信噪比、几何因子和探测距离等因素的影响,确定激光器的输出能量为5mJ能满足要求。
Figure 5. Relationship between the required energy and extinction coefficients (RSNR>10, Rmax>3.0km)
由于探测NO2所用两个波长为λon(448.10nm), λoff(446.80nm),选用染料激光器作为光源比较方便。采用两台Nd: YAG激光器(美国Continnue公司的PL8010)的354.7nm波长分别抽运两台染料激光器(德国Radiant Dyes公司的NarrowScanK)的方式来实现。选用乙醇作溶剂,染料为香豆素(C450),可产生这两个波长,转换效率约为15%。系统结构如图 8所示,主要参量如表 1所示。从两台染料激光器输出的这两束光,先用合束棱镜合为一束,再用望远镜扩束及调整发散角,最后通过3维扫描头射向大气。依据各个元件的传输效率,并预留20%的能量裕量,确定染料激光器的输出能量为10mJ,抽运用的Nd: YAG激光器354.7nm波长的能量不小于90mJ,而354.7nm波长是由1064nm波长的三倍频获得,这里不再赘述。
Table 1. Specifications of DIAL system
transmitter laser class Nd:YAG laser dye-laser laser type Continnum PL8010 radiant narrowscan dyes-laser waelength/nm 354.7 448.1/446.8 energy/mJ 100 10 repetition/Hz 10 10 divergence/ mrad ≤0.45 ≤2.0 receiver telescope type:near newtonian;focus length:750mm;diameter:350mm;field of vision:(0.2~2.0)mrad optical fibre numerical aperture:0.22;diameter:1.5mm detector H10426;aperture:25mm;185nm~650nm;rise time < 50ns;gains: 105~106 optical filter central wavelength:447.50nm;bandwidth:3.0nm;block optical density 5:(200~1100)nm;aperture:25.4mm signal acquisition A/D PCI-9826H;20M samples/s;channel number 4(16bit)
差分吸收NO2激光雷达光源的设计与实现
Design and implementation of NO2 differential absorption lidar sources
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摘要: 为研制一台探测距离3km、分辨率10μg/m3的大气NO2廓线差分吸收激光雷达,以NO2的吸收光谱和激光雷达方程为基础,通过数值仿真分析了回波信噪比与水平和垂直方向上大气中气溶胶、NO2含量的分布、探测距离和几何因子的关系;搭建探测大气NO2实验系统,开展了大气NO2浓度实验观测,获得水平及垂直高度0.4km~3.0km内的NO2浓度实时分布,探测分辨率可达4.717μg/m3,系统稳定可靠。结果表明,采用两台波长为354.7nm、能量不小于100mJ的Nd:YAG激光器分别抽运两台染料激光器的方式,并以C450为染料,可满足差分吸收探测所需的两束波长为λon(448.10nm)和λoff(446.80nm)、能量为8mJ的输出光束。该方法为实用化NO2差分吸收激光雷达光源的设计及应用提供了理论依据及技术支持。Abstract: To develop an atmospheric NO2 differential absorption lidar (DIAL) with detection range of 3km and resolution of 10μg/m3, based on NO2 absorption spectrum and lidar equation, the relationships among echo signal-to-noise ratio (SNR), aerosol of the horizontal and vertical direction, NO2 concentration, detection distance and geometric factor were analyzed and simulated. The atmospheric NO2 experiment system was built, and the atmospheric NO2 concentration experiment was carried out. The NO2 concentration in horizontal and vertical height of 0.4km~3.0km was obtained in real time, and the resolution was up to 4.717μg/m3. The system was stable and reliable. The results show that, with two Nd:YAG lasers with wavelength of 354.7nm and laser energy not less than 100mJ to pump two dye lasers with C450 as the dye, two output light beams for differential absorption detection can be obtained with λon of 448.10nm, λoff of 446.80nm, and energy of 8mJ. This method provides theoretical basis and technical support for the design and application of practical NO2 differential absorption lidar light sources.
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Key words:
- atmospheric optics /
- differential absorption lidar /
- NO2 /
- wavelength and energy
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Table 1. Specifications of DIAL system
transmitter laser class Nd:YAG laser dye-laser laser type Continnum PL8010 radiant narrowscan dyes-laser waelength/nm 354.7 448.1/446.8 energy/mJ 100 10 repetition/Hz 10 10 divergence/ mrad ≤0.45 ≤2.0 receiver telescope type:near newtonian;focus length:750mm;diameter:350mm;field of vision:(0.2~2.0)mrad optical fibre numerical aperture:0.22;diameter:1.5mm detector H10426;aperture:25mm;185nm~650nm;rise time < 50ns;gains: 105~106 optical filter central wavelength:447.50nm;bandwidth:3.0nm;block optical density 5:(200~1100)nm;aperture:25.4mm signal acquisition A/D PCI-9826H;20M samples/s;channel number 4(16bit) -
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