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沙氏成像原理(也称沙姆定律)是一种光学成像原理,主要描述如何通过调整光学镜头相对于探测器平面的倾斜从而获得较大的成像景深。其原理可简述为:在成像系统的物面与光学透镜不平行的情况下,如果像面、物面及透镜所在平面三者相交于一条直线,则依然可对物面清晰成像。沙氏成像原理的优点在于采用大口径的光学成像系统的同时,依然可实现理论上的无穷远景深。这一重要特点对于探测微弱回波信号的激光雷达系统而言十分重要。
如图1所示,沙氏激光雷达一般以连续光(强度调制)二极管激光器作为系统发射光源,利用倾斜放置的面阵图像传感器探测待测目标的后向散射光。在满足沙氏成像原理的条件下,不同距离上的后向散射(反射)光会以不同入射角度会聚到图像传感器上,实现对探测目标(光束)的清晰成像。由于不同的像素对应着不同的入射光线及测量距离,沙氏激光雷达技术以角度分辨的方式实现了距离分辨的后向散射信号探测。由于采用半导体激光器作为光源及工业相机作为探测器,沙氏激光雷达具有系统光源和光电探测部分相对简单、稳定,探测盲区小及性价比高等特点。
图 1 沙氏成像原理示意图。f是接收望远镜的焦距
Figure 1. Optical layout of the Scheimpflug imaging principle.
$f$ is the focal length of the receiving telescope根据几何光学原理,可推导出沙氏激光雷达技术的图像传感器像素与测量距离(z)之间的关系式:
$${\textit{z}} = \frac{{L\left[ {{p_{\rm{I}}}(\sin \varTheta - \cos \varTheta \tan \varPhi ) + {L_{{\rm{IL}}}}} \right]}}{{{{{p}}_{\rm{I}}}(\cos \varTheta + \sin \varTheta \tan \varPhi ) + {L_{{\rm{IL}}}}\tan \varPhi }}$$ (1) 式中:
$\varTheta $ 为像面相对于成像透镜的倾角;$\varPhi $ 为接收望远镜的观测角;$L$ 为接收望远镜与发射望远镜光轴之间的间隔;${p_{\rm{I}}}$ 为每个像素单元在成像平面上的位置,可表示为${p_{\rm{I}}} = ({N_{\rm{p}}}/2 - {n_{\rm{p}}}){w_{\rm{p}}}$ ,其中${N_{\rm{p}}}$ 为总像素个数,${n_{\rm{p}}}$ 为每个像素元的索引,${w_{\rm{p}}}$ 为像素大小。对公式(1)进行微分,可得到距离分辨率与探测距离的关系表达式:$${\rm{d}}{\textit{z}} = - \frac{{{{\textit{z}}^2}\sin \varTheta \left( {1 + {{\tan }^2}\varPhi } \right)}}{{{{\left[ {{p_{\rm{I}}}(\sin \varTheta - \cos \varTheta \tan \varPhi ) + {L_{{\rm{IL}}}}} \right]}^2}}}{\rm{d}}{p_{\rm{I}}}$$ (2) 图2给出了不同系统配置参数的沙氏成像系统的像素-距离、距离-分辨率关系曲线。从图中不难看出,图像传感器的像素与测量距离之间存在着非线性关系,并且激光雷达系统的距离分辨率与测量距离的平方成线性关系。因此,沙氏激光雷达系统的近距离分辨非常高,而远距离分辨率则会迅速下降。如图2(b)所示,当接收望远镜焦距为800 mm,发射与接收系统之间的间隔约为806 mm时,沙氏激光雷达系统在1 km范围内的分辨率优于6 m,在100 m附近的距离分辨率甚至可达到cm量级。在实际应用时,由于观测角较小,难以直接测量,因而无法直接利用公式(2)求解像素-距离关系。因此,一般通过测量距离已知的目标物体回波信号像素位置来校准像素-距离关系[4]。
图 2 (a) 不同配置的沙氏成像系统像素与测量距离的对应关系;(b)距离分辨率与测量距离之间的关系。f为接收望远镜的焦距,separation表示发射端与接收端之间的间隔,相机参数:像素大小为5.5 μm,像元数量为 2048×1024,相机倾角为45˚
Figure 2. (a) Relationships between the pixel and the measurement distance with different optical configurations; (b) Relationship between the range resolution and the measurement distance. System parameters: f is the focus of the receiving telescope, separation refers to the distance between the transmitter and the receiver, pixel width: 5.5 µm, pixel number: 2048×1024,
$\varTheta = {45^ \circ }$ 需要强调的是,图2(b)所示的距离分辨率没有考虑激光光束的宽度,此时距离分辨率只受沙氏成像原理约束。在实际应用中,受光束准直系统等因素的限制,激光光束总是有一定宽度,从而会影响系统的有效距离分辨率,尤其是远距离探测性能[5]。根据实际情况,应尽可能地压缩发射光束的发散角,使得激光光斑在探测距离内尽可能的小。
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基于上述原理,通过对发射到大气中的激光光束(强度调制)在满足沙氏成像原理的条件下利用图像传感器进行成像,可实现从近地面到数公里范围内的大气后向散射信号的有效探测,从而获取大气气溶胶在不同距离(高度)的分布信息。根据传统脉冲式大气激光雷达方程以及距离分辨率与探测距离之间的关系,可推导出SLidar技术的大气激光雷达方程:
$$ P(\lambda ,{\textit{z}}) = K{P_0}(\lambda )O({\textit{z}})\beta (\lambda ,{\textit{z}})\exp \left[ { - 2\int_0^{\textit{z}} {\alpha (\lambda ,{\textit{z}}'){\rm{d}}{\textit{z}}'} } \right] $$ (3) 式中:
$\lambda $ 为激光发射波长;${\textit{z}}$ 为探测距离;K 为系统常数;${P_0}(\lambda )$ 为激光器的输出功率;$O({\textit{z}})$ 为几何重叠因子(在SLidar系统中,使用面阵图像传感器时通常为1);$\;\beta (\lambda ,{\textit{z}})$ 为大气的后向散射系数;$\alpha (\lambda ,{\textit{z}})$ 为大气消光系数;$P(\lambda ,{\textit{z}})$ 为SLidar系统探测的大气后向散射信号。从公式(3)可以看出,SLidar测量的激光雷达信号不随距离平方衰减。因此,虽然SLidar系统的远距离分辨率有所下降,但是远距离信号的信噪比不随距离平方衰减,在不使用单光子计数等高灵敏度探测技术的情况下依然可探测最远7 km左右的大气后向散射信号。典型SLidar系统如图3(a)所示。SLidar系统采用高功率、多模、连续波二极管激光器和面阵CMOS/CCD图像传感器作为光源和探测器。二极管激光器通过温控驱动和恒流驱动分别对激光器的工作温度和电流进行精确的控制。二极管激光器通过驱动电流进行开/关调制,该驱动电流与CMOS图像传感器的触发信号同步。因此,大气背景信号及激光光束的后向散射信号交替成像在CMOS图像传感器的探测区域中。通过对图像在垂直方向上的像素进行累加,扣除大气背景信号[5],并对多次测量得到的激光雷达信号进行中值平均,可获得像素-强度的大气回波曲线。通过参考目标校准测量并将图像传感器的像素转换成距离,最终可获得距离-强度的大气激光雷达信号。
图 3 (a)基于牛顿望远镜的沙氏大气激光雷达系统示意图;(b)典型的像素-强度的激光雷达信号;(c)经过像素-距离转换后的距离-强度的激光雷达信号
Figure 3. (a) Architecture of the Scheimpflug atmospheric lidar system based on a Newtonian telescope; (b) Typical pixel-intensity lidar signal; (c) Range-intensity lidar signal transferred by pixel-range lidar signal
SLidar在使用图像传感器对大气后向散射光进行探测时,不可避免地受到背景光散粒噪声以及图像传感器的暗电流噪声、读出噪声和固定模式噪声(FPN)的影响。研究发现,背景光散粒噪声和图像传感器的光子响应不均匀性(PRNU)噪声是测量信号的主要噪声来源。由于沙氏激光雷达像素-距离的非线性关系,一般通过分析近距离(100~150 m)连续200个像素点的信号来评估激光雷达信号的整体噪声水平。把近距离局部信号的多项式拟合残差作为噪声信号,该残差信号的标准差即为激光雷达信号的噪声水平。不同距离的信噪比可通过该距离上的信号强度与噪声的比值来估计,由此可评估激光雷达信号的质量。在白天强背景测量条件下,大气回波信号主要受太阳背景噪声影响。在空气质量良好的条件下,经过短曝光(比如20 ms)以及1~10 min的信号平均后,白天最远可获得7 km左右的探测距离(信噪比>10)。在夜间测量条件下,可通过增加曝光时间(比如500 ms),提升入射光通量,从而抑制PRNU噪声,经过同等时间的信号平均后,探测性能一般优于白天。为了进一步提升SLidar测量信号的信噪比,提高消光系数反演可靠性,可利用Savitzky-Golay等数字滤波器对激光雷达信号进行数字滤波,一般可获得3~10倍的信噪比提升效果[6]。为了避免过度滤波导致的信号失真(比如由云层或污染源导致的强回波失真),可通过傅里叶变换在频域分析滤波残差的方式动态优化Savitzky-Golay滤波器的窗口宽度[7],在保证信号无失真的情况下实现最大的信噪比改善效果。
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SLidar技术与脉冲式大气激光雷达技术的探测原理显著不同,为了研究SLidar技术测量的大气回波信号的特点,大连理工大学利用自主研制的SLidar系统和脉冲式大气激光雷达系统开展了详细对比研究[8]。图4是SLidar系统和脉冲激光雷达系统对比测量实验实物图。SLidar系统采用520 nm二极管激光器(功率约1 W),脉冲激光雷达系统采用工作波长为532 nm的Nd:YAG激光器。由于两套系统工作波长接近,在对比分析时可假定大气在两个波长的吸收和散射特性一致。在水平测量(系统仰角均为3˚)对比研究中,SLidar和脉冲激光雷达反演的消光系数表现出了相似的时间演变特征,平均消光系数的相关性达到0.99。在30˚仰角倾斜测量对比研究中(如图5所示),在0.5~2 km的范围内SLidar激光雷达曲线和脉冲激光雷达曲线之间的相对均方根误差小于5%。利用SLidar技术探测盲区小的特点,还可对脉冲式大气激光雷达的近距离几何重叠因子进行修正,可实现从近地面(90 m)到28 km的大气回波信号的有效探测[9]。
图 4 (a)对比测量示意图,(b)脉冲激光雷达系统的实物图,(c) SLidar系统实物图。防雨SLidar系统放置在楼顶,脉冲激光雷达系统位于实验室内部
Figure 4. (a) Measurement geometry, (b) physical diagram of the pulsed lidar system, and (c) physical diagram of the Scheimpflug lidar system. The rain-proofed SLidar system was placed on the rooftop while the pulsed lidar system was located inside a laboratory
在沙氏大气激光雷达技术中,为了满足沙氏成像原理从而获得最大的成像景深,一般会将图像传感器倾斜(比如45˚)放置。由于图像传感器表面微透镜及制造工艺等问题,大多数图像传感器的量子效率会随着入射光的入射角度增加而急剧下降[10-12],部分图像传感器45˚倾斜放置时的量子效率只有正入射时的20%左右。为了探讨沙氏成像与传统成像模式测量结果的差异,2019年大连理工大学课题组搭建了由两个型号一致的图像传感器(Lumenera, Lt225)构成的大气激光雷达系统[13]。该系统采用808 nm二极管激光器作为发射光源,采用200 mm口径的牛顿望远镜(f=800 mm)收集后向散射光,并通过0°和45°倾斜放置的CMOS图像传感器同时探测大气后向散射信号。研究结果表明,0°倾角探测时图像传感器具有较高的量子效率。因此,在同等测量时间内(45 s),白天测量时0˚倾角放置的图像传感器测得的激光雷达信号的信噪比显著高于45˚倾斜放置时回波信号信噪比(3.5~5.5倍)。然而,由于不满足沙氏成像原理,成像景深受到极大限制。0˚倾角图像传感器探测时,成像系统的失焦效应会引起不同距离的激光雷达信号的串扰,在近距离探测时尤为明显。通过对比发现,0˚倾角放置的图像传感器在100 m处测量得到的大气激光雷达信号及消光系数比45°倾斜放置时测量结果要高出约11%,这也说明了满足沙氏成像原理对高精度大气遥感探测十分重要。另一方面,增大接收望远镜的焦比(比如减小口径),可一定程度上降低0˚倾角探测时失焦效应的影响,但其改善效果还有待深入研究。
图 5 2019年6月11日至6月12日,斜程测量时(a)脉冲激光雷达系统测量得到的大气回波信号时空演变图,(b) SLidar系统测量得到的大气回波信号时空演变图。斜程测量角度为30˚
Figure 5. Time-range evolution maps of range-corrected log-scale lidar signals measured by (a) the pulsed lidar system and (b) the SLidar system while slope measurement from 11th June 2019 to 12th June 2019. The elevation angle was about 30˚
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单波长激光雷达是大气遥感领域应用最为广泛的大气激光雷达技术。利用单波长大气激光雷达可实现大气气溶胶时空演变特征探测、大气消光系数、能见度反演研究等。2014年,瑞典隆德大学最早实现了单波长沙氏大气激光雷达技术并开展了大气气溶胶遥感探测实验研究工作[14]。该系统采用808 nm高功率二极管激光器作为光源,利用半高全宽(FWHM)为3 nm的窄带干涉滤光片滤除大气背景光,实现了大气回波信号的探测,并对SLidar技术的激光雷达方程及距离分辨率等问题进行了探讨,初步论证了SLidar技术在大气气溶胶探测方面的可行性。2017年,大连理工大学课题组利用输出功率约为4 W的808 nm二极管激光器和CMOS图像传感器搭建了一套808 nm全天时单波长米散射SLidar实验系统,并在近地面近似水平方向开展了24 h连续实验测量研究工作[4],如图6所示。该系统的探测盲区约为85 m,最远探测距离可达7 km (信噪比>10)。即使在中度污染天气条件下,大气消光系数的反演距离也能达3~4 km左右。由于远距离分辨率迅速下降,在反演大气消光系数时,边界值的求解一般只针对7 km以内的大气激光雷达信号。通过基于道格拉斯-普克算法的斜率法在远端求解消光系数边界值,并根据大气条件获得激光雷达比经验值,可实现大气消光系数时空分布反演。实验结果表明,利用SLidar技术测量得到的大气消光系数与传统点式监测站测量得到的PM2.5和PM10浓度之间存在较高的相关性。这一研究工作为沙氏大气激光雷达在大气环境监测领域的实际应用奠定了重要基础。
另一方面,得益于近年来迅速发展的半导体激光器技术,沙氏大气激光雷达的波长选择范围非常广。通过选择不同波长的二极管激光器,可实现从紫外到近红外1550 nm波段不同波长的沙氏大气激光雷达系统。近年来,大连理工大学课题组相继实现了407 nm[15]、450 nm[16]及520 nm[17]的SLidar系统,并在绝大多数测量条件下实现了全天时大气遥感探测,为不同应用场景的测量及多波长系统的实现提供了重要参考。在450 nm和520 nm波段目前采用的滤光片带宽为10 nm,因此如果采用1~3 nm的干涉滤光片还可大幅提升白天测量信号的信噪比。为了满足小范围大气遥感测量需求,中国科学院安徽光学精密机械研究所以及大连理工大学等单位开展了小型化沙氏大气激光雷达技术研究工作。2019年,中国科学院安徽光学精密机械研究所孙国栋等人提出了一种532 nm小型化沙氏激光雷达,该激光雷达能够检测到离地面1 km处的气溶胶消光系数和垂直大气透射率[18]。通过将测得的气溶胶消光系数与传统的脉冲式激光雷达进行比较,验证了小型化SLidar系统探测边界层内气溶胶的可行性,但该系统难以实现全天时大气遥感探测。2020年,大连理工大学课题组研制了一套可进行户外全天时大气测量的小型化808 nm沙氏激光雷达系统,并对系统性能进行了详细的评估[19]。
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作为大气激光雷达的一个分支,偏振激光雷达在云的相位识别(水云、冰云)、气溶胶分类、光学特性反演等方面占有重要地位。目前,脉冲式双通道偏振激光雷达是大气偏振特性遥感探测最主要的方式之一。如图7(a)所示,通过向大气中发射一束线偏振激光,经大气粒子散射后,大气回波信号经过偏振分光器件分离出垂直和平行偏振分量,从而可得到大气退偏振比。不同于传统的脉冲式偏振激光雷达系统采用单光源、双偏振探测器的技术方案,大连理工大学提出了一种基于时分复用原理的新型偏振沙氏大气激光雷达探测技术,并实验验证了其可行性[20]。该研究工作采用正交偏振的两个二极管激光器,通过交替发射正交偏振激光光束,并利用单个图像传感器通过时分复用的方式实现了水平和垂直偏振的后向散射光信号的交替探测,系统工作原理如图7(b)所示。然而,该方案测量的垂直偏振大气激光雷达信号信噪比偏低,尤其是在白天强背景光的条件下,信号平均1.5 min测量得到的原始垂直偏振大气回波信号的信噪比仅为30左右。利用更高功率的二极管激光器或提升图像传感器的量子效率有望进一步提高偏振沙氏大气激光雷达系统在白天工作时的信噪比。
图 7 大气偏振激光雷达测量原理示意图。(a)双通道脉冲式偏振激光雷达,(b)基于时分复用的偏振沙式激光雷达,(c)基于偏振相机的偏振成像激光雷达
Figure 7. Measurement principles of atmospheric polarization lidar. (a) Dual-channel pulsed polarization lidar, (b) Polarization Scheimpflug lidar based on the time-division multiplexing scheme, (c) Polarization-sensitive imaging lidar utilizing a polarized image sensor
虽然双通道偏振激光雷达发展得非常成熟,然而其测量精度始终受到增益比、激光偏振纯度、偏振失配角、光学元件的非理想偏振特性等因素的影响。接收通道增益比定标问题几乎贯穿了整个偏振大气激光雷达的发展历程,成为该方案面临的主要挑战。2019年,大连理工大学提出了一种新型的偏振成像大气激光雷达技术[21]。如图7(c)所示,该技术使用线偏振连续波450 nm二极管激光器作为光源,使用具有四种偏振通道(0˚、45˚、90˚、135˚) CMOS偏振图像传感器作为探测器,发射端与接收端在系统结构满足沙氏成像原理的前提下实现大气的偏振测量。该技术无需添加额外的光学元件和复杂的增益比定标,可以实时获取体退偏比和偏振失配角。垂直大气测量实验表明偏振成像激光雷达可实现大气退偏振特性的高精度探测(图8)。此外,大连理工大学激光雷达课题组利用Stokes-Mueller理论的数学模型详细评估了成像激光雷达系统的测量误差[22]。基于该技术方案,有望利用45˚或135˚偏振方向的后向散射信号开展定向散射体(例如水平定向冰晶等)探测研究[23]。
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近年来,大气激光雷达技术被广泛应用于大气污染源的扫描探测,在大气污染监测与监管方面发挥着重要作用。2018年,大连理工大学激光雷达课题组自主设计了两套808 nm可携式沙氏大气激光雷达系统,分别采用折射式望远镜和牛顿反射式望远镜作为接收望远镜,如图9所示[11]。系统主要包括激光发射单元、大气回波信号接收单元、系统控制与数据采集单元,相机倾角均为45˚。为了提高信噪比,两套系统均采用3 nm干涉滤光片(808 nm)和高通(LG780)滤光片抑制背景光。CMOS相机探测到大气后向散射图像后,将数据传输到计算机进行后期数据预处理,最终获得大气激光雷达信号及背景信号。两套可携式SLidar系统均可固定在重载云台上,从而实现大气污染源扫描测量、大气边界层高度测量等[24]。
图 9 (a)基于折射式望远镜的可携式沙氏大气激光雷达系统。发射端:口径100 mm,焦距600 mm;接收端口径150 mm,焦距750 mm;发射端与接收端的间隔756 mm。(b)基于牛顿望远镜的可携式沙氏大气激光雷达系统。发射端:口径100 mm,焦距600 mm;接收端口径200 mm,焦距800 mm;发射端与接收端的间隔806 mm
Figure 9. (a) Portable SLidar atmospheric system based on a refracting telescope. Transmitter: Φ=100 mm, f=600 mm; Refracting telescope receiver: Φ=150 mm, f=750 mm; Separation between transmitter and receiver is 756 mm. (b) Portable SLidar atmospheric system based on a Newtonian telescope. Transmitter: Φ=100 mm, f=600 mm; Newtonian telescope receiver: Φ=200 mm, f=800 mm; Separation between transmitter and receiver is 806 mm
通过将扫描式沙氏大气激光雷达系统安装于城市至高点,并对城市大气环境进行全区域扫描探测,可实现对垃圾焚烧、路边烧烤、居民散乱燃烧、工地扬尘、渣土车/建筑垃圾倾倒以及工业园区排放的有效监测。图10展示了在秦皇岛市昌黎县夜间扫描测量得到的大气回波信号强度与消光系数反演结果示意图,分析消光系数时空分布可实现污染源的精准定位。通过在环保部门的安装应用表明,沙氏大气激光雷达技术在大气污染监测、监管及评估等方面具有重要应用价值。
图 10 沙氏激光雷达系统扫描测量结果示意图。(a)大气回波信号强度分布,(b)大气消光系数反演结果。SLidar系统安装于秦皇岛市昌黎县某商场大厦天台上
Figure 10. (a) Atmospheric backscattering intensity distribution map and (b) atmospheric extinction coefficient retrieving map measured by a scanning SLidar system. The SLidar system was placed on the roof of a shopping mall in Changli, Qinhuangdao City, Hebei Province
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气溶胶粒子的回波散射信号特征依赖于发射激光波长,利用多波长激光雷达可获得不同波长的气溶胶消光系数,并可根据不同波长消光系数之间的关系得到表征粒子尺寸的波长指数等参量,为进一步分析气溶胶的微物理特征提供了重要信息。另一方面,大气回波信号的退偏振效应反应了大气气溶胶的形态,与气溶胶的来源和种类密切相关。因此,国内外学者均开展了多波长偏振大气激光雷达技术和应用研究。现有脉冲式多波长偏振激光雷达系统大多利用Nd:YAG激光器的基频(1064 nm)、倍频(532 nm)以及三倍频(355 nm)激光作为光源。沙氏大气激光雷达技术利用半导体激光器作为光源,具备广泛的波长选择性,工作范围可覆盖整个可见光及近红外波段,在多波长激光雷达技术研究及应用方面具有重要潜力。
2018年,大连理工大学课题组首先实现了基于双二极管激光器、双图像传感器的双波长(407 nm和808 nm)全天时沙氏大气激光雷达系统,并将该系统用于大气颗粒物吸湿性增长研究、波长指数对比分析等[12]。此外,瑞典隆德大学课题组利用808 nm和405 nm二极管激光器作为光源,利用时分复用的结构对双波长SLidar系统进行了初步研究,但仅能在夜间及短距离范围内进行测量[25]。2019年,大连理工大学设计了三波长偏振沙氏大气激光雷达系统,如图11所示[10]。三波长偏振激光雷达系统采用405 nm、520 nm和808 nm三个通道,其中808 nm有垂直偏振和水平偏振两个通道。系统的发射单元采用两个808 nm、405 nm和520 nm激光器作为光源。其中,在808 nm激光器(LD Ⅱ)前端放置半波片将其偏振态旋转90˚。两个正交偏振的808 nm激光束由偏振分束器耦合,而405 nm和520 nm激光束通过二向色镜进行光束耦合,最终由准直透镜准直之后发射到大气之中。大气后向散射信号由牛顿望远镜来收集,接收单元采用两个CMOS相机作为探测器,其中一个探测808 nm的水平和垂直偏振信号,另外一个相机则用来探测405 nm和520 nm,每个相机均采用时分复用的方式实现信号的探测。由于没有采用干涉滤光片抑制太阳光,因此激光雷达系统只能在夜间工作。2019年,研究人员在典型大气条件下进行了近似水平大气测量,并对气溶胶消光系数、体退偏比和波长指数进行了反演。图12展示了808 nm的体退偏比时空演变图和波长指数与体退偏比随时间变化的曲线。不同大气条件下的体退偏比和波长指数如表1所示。
图 11 大连理工大学设计的(a)三波长偏振沙氏激光雷达系统原理图和(b)系统实物图
Figure 11. (a) Schematic and (b) photograph of the three-wavelength polarization SLidar system developed at Dalian University of Technology
图 12 (a) 808 nm体退偏比时空演变图,(b)体退偏比中值与波长指数中值随时间变化曲线
Figure 12. (a) Time-range evolution map of the linear volume depolarization ratio at 808 nm,(b) Temporal evolution curves of the median of linear volume depolarization ratio and the median of Ångström exponent
表 1 不同大气条件下,三波长偏振沙氏激光雷达系统测量的体退偏比(808 nm)和波长指数
Table 1. LVDR at 808 nm and the Ångström exponents measured by the three-wavelength polarization SLidar system under different atmospheric conditions
Recent advancements of the lidar technique based on the Scheimpflug imaging principle (Invited)
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摘要: 当成像系统的物面与光学透镜所在平面不平行时,如果像面、物面及透镜所在平面三者相交于一条直线—即满足沙氏成像原理(也称沙姆定律),则成像系统依然可对物面清晰成像,从而实现理论上的无穷远景深。基于沙氏成像原理而发展起来的沙氏激光雷达可采用连续波二极管激光器作为光源以及图像传感器作为探测器,因而具备近距离探测盲区小、结构紧凑、低维护、高性价比等特色和优势。近年来,沙氏激光雷达技术逐渐应用于大气环境监测、三维目标成像、荧光(高光谱)激光雷达探测、生态学研究、燃烧诊断、水体光学测量等领域。文中将系统性地阐述沙氏激光雷达技术的基本原理,详细探讨其在相关领域取得的最新研究进展,并对未来研究工作进行展望。Abstract: When the object plane is not parallel to the lens plane in an imaging system, if the image plane, the object plane and the lens plane intersect into a straight line—satisfying the Scheimpflug imaging principle, the imaging system can still clearly image the object and achieve infinite depth-of-focus (DoF). The newly developed Scheimpflug lidar (SLidar) technique based on the Scheimpflug imaging principle can thus utilize continuous wave diode lasers as light sources and image sensors as detectors, featuring short blind range, compact structure, low maintenance and high cost performance, etc. In recent years, the SLidar technique has gradually been applied to the fields of atmospheric environment monitoring, three-dimensional (3D) target imaging, fluorescence(hyperspectral) lidar detection, ecological studies, combustion diagnosis, and water-body optical measurements, etc. This article will thoroughly explain the basic principles of the SLidar technique, discuss latest progresses in these fields and present its perspectives.
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图 2 (a) 不同配置的沙氏成像系统像素与测量距离的对应关系;(b)距离分辨率与测量距离之间的关系。f为接收望远镜的焦距,separation表示发射端与接收端之间的间隔,相机参数:像素大小为5.5 μm,像元数量为 2048×1024,相机倾角为45˚
Figure 2. (a) Relationships between the pixel and the measurement distance with different optical configurations; (b) Relationship between the range resolution and the measurement distance. System parameters: f is the focus of the receiving telescope, separation refers to the distance between the transmitter and the receiver, pixel width: 5.5 µm, pixel number: 2048×1024,
$\varTheta = {45^ \circ }$ 图 4 (a)对比测量示意图,(b)脉冲激光雷达系统的实物图,(c) SLidar系统实物图。防雨SLidar系统放置在楼顶,脉冲激光雷达系统位于实验室内部
Figure 4. (a) Measurement geometry, (b) physical diagram of the pulsed lidar system, and (c) physical diagram of the Scheimpflug lidar system. The rain-proofed SLidar system was placed on the rooftop while the pulsed lidar system was located inside a laboratory
图 5 2019年6月11日至6月12日,斜程测量时(a)脉冲激光雷达系统测量得到的大气回波信号时空演变图,(b) SLidar系统测量得到的大气回波信号时空演变图。斜程测量角度为30˚
Figure 5. Time-range evolution maps of range-corrected log-scale lidar signals measured by (a) the pulsed lidar system and (b) the SLidar system while slope measurement from 11th June 2019 to 12th June 2019. The elevation angle was about 30˚
图 7 大气偏振激光雷达测量原理示意图。(a)双通道脉冲式偏振激光雷达,(b)基于时分复用的偏振沙式激光雷达,(c)基于偏振相机的偏振成像激光雷达
Figure 7. Measurement principles of atmospheric polarization lidar. (a) Dual-channel pulsed polarization lidar, (b) Polarization Scheimpflug lidar based on the time-division multiplexing scheme, (c) Polarization-sensitive imaging lidar utilizing a polarized image sensor
图 9 (a)基于折射式望远镜的可携式沙氏大气激光雷达系统。发射端:口径100 mm,焦距600 mm;接收端口径150 mm,焦距750 mm;发射端与接收端的间隔756 mm。(b)基于牛顿望远镜的可携式沙氏大气激光雷达系统。发射端:口径100 mm,焦距600 mm;接收端口径200 mm,焦距800 mm;发射端与接收端的间隔806 mm
Figure 9. (a) Portable SLidar atmospheric system based on a refracting telescope. Transmitter: Φ=100 mm, f=600 mm; Refracting telescope receiver: Φ=150 mm, f=750 mm; Separation between transmitter and receiver is 756 mm. (b) Portable SLidar atmospheric system based on a Newtonian telescope. Transmitter: Φ=100 mm, f=600 mm; Newtonian telescope receiver: Φ=200 mm, f=800 mm; Separation between transmitter and receiver is 806 mm
图 10 沙氏激光雷达系统扫描测量结果示意图。(a)大气回波信号强度分布,(b)大气消光系数反演结果。SLidar系统安装于秦皇岛市昌黎县某商场大厦天台上
Figure 10. (a) Atmospheric backscattering intensity distribution map and (b) atmospheric extinction coefficient retrieving map measured by a scanning SLidar system. The SLidar system was placed on the roof of a shopping mall in Changli, Qinhuangdao City, Hebei Province
图 14 (a)二维(荧光)沙氏激光雷达原理示意图;(b)利用二维荧光沙氏激光雷达对5 m外柚子树的数据云重构结果,点云强度为图像传感器红色通道与蓝色通道信号强度比值
Figure 14. (a) Architecture of the 2D fluorescence SLidar principle; (b) Data cloud reconstruction result of a grapefruit tree with the 2D fluorescence SLidar 5 m away, the dot cloud intensity represents the signal intensity ratio of red channel to blue channel of image sensor
表 1 不同大气条件下,三波长偏振沙氏激光雷达系统测量的体退偏比(808 nm)和波长指数
Table 1. LVDR at 808 nm and the Ångström exponents measured by the three-wavelength polarization SLidar system under different atmospheric conditions
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