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Liu Zhuang, Wang Chao, Jiang Lun, Shi Haodong. Design of low altitude high resolution lidar optical system[J]. Infrared and Laser Engineering, 2021, 50(1): 20200117. doi: 10.3788/IRLA20200117
Citation: Liu Zhuang, Wang Chao, Jiang Lun, Shi Haodong. Design of low altitude high resolution lidar optical system[J]. Infrared and Laser Engineering, 2021, 50(1): 20200117. doi: 10.3788/IRLA20200117

Design of low altitude high resolution lidar optical system

doi: 10.3788/IRLA20200117
  • Received Date: 2020-04-08
  • Rev Recd Date: 2020-07-23
  • Available Online: 2021-01-22
  • Publish Date: 2021-01-22
  • Aiming at the problems of optical imaging for low altitude, low speed and small target recognition ability and low signal-to-noise ratio, a low altitude and high resolution lidar optical system was designed. The MEMS mirror was used by the scanning device of the transmitting optical system, and the beam expanding system was designed to ensure the beam quality of the laser emitted at different scanning angles. The digital mirror device combine with objective lens, polarizing device were used for receiving optical system, the background noise was greatly lower than laser receiving system using single-point detector and could realize laser echo receiving and visible light imaging at the same time. The structure parameters of the optical system were given, an optical system was designed using optical design software. The optical system has a spatial resolution of 0.5 mrad/pixel and an array scale of 200×200. The simulation results show that design method is feasible. The detection distance can reach 1000 m, and the background noise can be reduced by about 22162 times compared to the single-point detector receiving system.ystem.
  • [1] 李小路, 曾晶晶, 王皓, 等. 三维扫描激光雷达系统设计及实时成像技术[J]. 红外与激光工程, 2019, 48(5): 0503004.

    Li Xiaolu, Zeng Jingjing, Wang Hao, et al. Design and real-time imaging technology of three-dimensional scanning LiDAR [J]. Infrared and Laser Engineering, 2019, 48(5): 0503004. (in Chinese)
    [2] 陈剑彪, 孙华燕, 赵融生, 等. 基于距离分辨的激光雷达技术研究进展[J]. 红外与激光工程, 2019, 48(8): 0805007.

    Chen Jianbiao, Sun Huayan, Zhao Rongsheng, et al. Research development of range-resolved laser radar technology [J]. Infrared and Laser Engineering, 2019, 48(8): 0805007. (in Chinese)
    [3] 李小宝, 王春晖, 曲扬, 等. 三维成像激光雷达系统中的微电子机械系统扫描:扫描视场角与扩束[J]. 激光与光电子学进展, 2015, 52(11): 111501.

    Li Xiaobao, Wang Chunhui, Qu Yang, et al. Micro-electromechanical systems scan in three-dimensional imaging Lidar system: Scanning field angle and beam expender [J]. Laser & Optoelectronics Progress, 2015, 52(11): 111501. (in Chinese)
    [4] Lee Xiaobao, Wang Chunhui, Luo Zhaoxu, et al. Optical design of a new folding scanning system in MEMS-based lidar [J]. Optics and Laser Technology, 2019, 125: 106013.
    [5] Zhou Jingkun, Qian Keyuan. Low-voltage wide-field-of-view lidar scanning system based on a MEMS mirror [J]. Applied Optics, 2019, 58(5): A283.
    [6] Li Yunxi, Li Qingyan, Zhang Bin, et al. The effect of closed-loop optimization enhances the MEMS lidar for rapid scanning [J]. Optik, 2019, 208: 164097.
    [7] 孙剑峰, 姜鹏, 张秀川, 等. 32×32面阵InGaAs Gm-APD激光主动成像实验[J]. 红外与激光工程, 2016, 45(12): 1206006.

    Sun Jianfeng, Jiang Peng, Zhang Xiuchuan, et al. Experimental research of 32×32 InGaAs Gm-APD arrays laser active imaging [J]. Infrared and Laser Engineering, 2016, 45(12): 1206006. (in Chinese)
    [8] 陈德章, 张华, 冷杰, 等. 基于APD面阵探测器的非扫描激光主动成像雷达[J]. 激光技术, 2017, 41(6): 775-778.

    Chen Dezhang, Zhang Hua, Leng Jie, et al. Non-scanning active imaging lidar based on APD planar array detector [J]. Laser Technology, 2017, 41(6): 775-778. (in Chinese)
    [9] 孙超. 背景光作用下激光雷达中Si-APD噪声特性研究[D]. 南京: 南京理工大学, 2014.

    Sun Chao. Study on noise characteristics of Si-APD in Lidar under background light[D]. Nanjing: Nanjing University of Science and Technology, 2014. (in Chinese)
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Design of low altitude high resolution lidar optical system

doi: 10.3788/IRLA20200117
  • Institute of Space Photo-electronics Technology, Changchun University of Science and Technology, Changchun 130022, China

Abstract: Aiming at the problems of optical imaging for low altitude, low speed and small target recognition ability and low signal-to-noise ratio, a low altitude and high resolution lidar optical system was designed. The MEMS mirror was used by the scanning device of the transmitting optical system, and the beam expanding system was designed to ensure the beam quality of the laser emitted at different scanning angles. The digital mirror device combine with objective lens, polarizing device were used for receiving optical system, the background noise was greatly lower than laser receiving system using single-point detector and could realize laser echo receiving and visible light imaging at the same time. The structure parameters of the optical system were given, an optical system was designed using optical design software. The optical system has a spatial resolution of 0.5 mrad/pixel and an array scale of 200×200. The simulation results show that design method is feasible. The detection distance can reach 1000 m, and the background noise can be reduced by about 22162 times compared to the single-point detector receiving system.ystem.

    • “低慢小”目标是指飞行高度低,速度较慢,雷达反射面积小的小型飞机、旋翼无人机、风筝等。因其威胁大、难以发现与识别,已成为现有雷达与光学技术亟待解决的难点。

      激光雷达技术来源于激光测距技术,能够通过逐点、逐线扫描或面阵探测器接收获得目标三维信息,相较于微波雷达具有空间分辨率高、测距精度高的优势。利用激光雷达结合可见光相机或者红外相机实现复合探测,可有效提高对“低慢小”目标识别概率[1]

      目前激光雷达扫描元件以振镜为主,存在体积大、质量重、功耗大等问题。参考文献[2-6]设计了基于MEMS反射镜的微型激光雷达,但是其空间分辨率均不足以满足远距离探测的需求;单点探测器构成的激光接收系统由于要接收视场内的全部入射光,易受视场内强背景光干扰造成探测信噪比低;参考文献[7-8]等设计了基于APD面阵探测器的激光雷达接收系统,大面阵APD探测器需要与其面阵规模对应的高压前置放大电路,存在成本高、体积大、可靠性低的缺点。

      提出了基于MEMS反射镜的激光发射光学系统与基于数字微反射镜器件(Digital Mirror Device,DMD)等器件的激光接收光学系统,根据MEMS反射镜、DMD器件在系统设计过程中存在的问题,设计了多角度扩束镜、物镜以及二次成像透镜组。

    • MEMS反射镜是指将微光反射镜与MEMS驱动器集成在一起的光学MEMS器件,其可实现平动和扭转两种机械运动,实现入射光束的二维快速偏转。

      将MEMS反射镜作为发射激光光束扫描器件需要解决激光束散角与偏转角度的优化选取问题,即需要在MEMS反射镜前将入射激光直径调整至适合反射镜的尺寸,在MEMS反射镜后加入多入射角度情况下均能保证光束质量的扩束镜(或者缩束镜),压缩(或增大)发射激光的束散角,同时缩小(或增大)发射激光偏转角度。保证在不同扫描角度的光束质量,使产生的图像畸变可控。

      可用于MEMS反射镜后的扩束镜具有两种形式:伽利略结构与开普勒结构。伽利略结构适合于对宽度有限制对长度无限制的光学系统,开普勒结构适合于对长度有限制对宽度无限制的光学系统。

      设计的以MEMS反射镜为核心的激光发射光学系统包含光纤光源、光纤准直器、MEMS反射镜与扩束镜。扩束镜拟选择开普勒结构,其中第一个透镜为副透镜,实现光束的扩展,后面为多个正透镜用于校正多角度的像差。

      MEMS反射镜尺寸、扫描范围以及激光器参数为限制激光雷达性能的主要因素。其中,激光雷达扫描范围:

      式中:α为扩束镜扩束倍率;2ω为MEMS反射镜最大光学扫描角度范围。

      为保证空间分辨率满足远距离探测的需求,同样需要激光光源具有较高的光束质量,选择光源为单模光纤激光器。从单模光纤发出的光束为高斯光束,经过焦距为f的光纤准直镜后,出射光束直径与发散角θ分别为:

      式中:NA为光纤数值孔径;dm为光纤内光束模场直径。

      MEMS反射镜反射镜面直径为Φ,则入射镜面的光束直径:

      式中:i为入射角需要满足i≥30°,以免后续扩束系统会阻挡入射光,则d<0.87Φ。经扩束后的发散角为θ’,可实现激光扫描阵列值m为:

      式中:β为光斑占空比,为光斑1/e2能量占像元空间分辨率的比例。

    • DMD是一系列在半导体晶片上产生的二维矩形微反射镜,每个微反射镜能够旋转±12°,频率可达数千赫兹,入射光能够以固定角度被反射。每一个微反射镜可以作为一个光开关使用,当驱动电压在“开”状态时,微反射镜保持在0° (或者−12°,+12°)状态,光学系统会使入射光照在探测器上,当驱动信号在“关”状态时,微反射镜会旋转到±12° (或者0°),入射光会被微反射镜反射到远离探测器方向。

      当实现DMD与激光发射端的扫描角度同步控制时,能够做到只接收扫描角度的漫反射光,其他方向的背景光将被滤除,因此可以降低背景噪声,提高探测信噪比。

    • 采用DMD的光学系统设计难点是如何分离入射光与反射光,当采用±12°作为“开”状态时,需要后续光学系统具有较大的景深,进而提高了设计与装调的难度;当采用0°作为“开”状态时,入射光将以近原方向返回,更加难以分离。

      采用0°作为“开”状态方案,在物镜与DMD中间加入偏振分光棱镜(PBS)、1/4波片作为分光器件。原理如图1所示:入射光中的P光将透过PBS,经过1/4波片后将变成左旋圆偏振光,经过DMD反射后将变成右旋圆偏振光,再一次经过1/4波片后变成S光,被PBS反射向探测器,进而实现了入射光与反射光的分离。入射光中的S光将被PBS反射,汇聚于可见光相机感光面,同时实现目标方位的复合成像。

      Figure 1.  Splitting principle of receiving system

      设计的接收光学系统包含:物镜、偏振分光棱镜、1/4波片、DMD、窄带滤光片、二次成像透镜组、APD探测器以及可见光探测器,设计重点是DMD镜面与相机感光面成像质量要满足要求,同时视场内的反射光均要汇聚在APD探测器感光面。

      为限制二次成像透镜组的孔径,物镜不同视场角的近轴光线与物镜像面法线夹角要接近于0°,近轴光线在光阑处与像面法线夹角通常较大,光阑与像面之间需要多片透镜用以调整近轴光线角度,这些透镜足够校正物镜像差,所以最简单物镜结构为光阑在前,后置一组正负透镜组合与一组“正负正”透镜组合。

    • 低空目标探测需求如下。(1)波长:高功率波长;(2)探测距离:50~1000 m;(3)像元分辨率:可发现最远距离1000 m处主要“低慢小”目标(如:风筝、旋翼无人机),投影尺寸约为0.5 m×0.5 m;(4)帧频:在最近工作距离(50 m)情况下,速度为10 m/s的目标,不能飞出视场范围,计算可得帧频≥2 fps;(5)扫描范围、阵列规模:要求器件可实现情况下应尽可能大,考虑到探测激光器平均功率,重复频率、单脉冲脉宽与峰值功率不可兼得,结合现有激光器能力(平均功率约为5 W,脉宽约为7 ns,根据第3节中计算,峰值功率需要约8.9 kW),重复频率为80 kHz;(6)可见光相机:视场与激光扫描视场相同,帧频与角分辨率要大于激光雷达。所得激光雷达与可见光相机光学系统的参数如表1所示。

      ParameterValue
      Lidar wavelength/nm1064
      Array200×200
      Pixel resolution0.5 mrad@Spot duty ratio 0.9
      Scanning range5.75°×5.75°
      Frame rate of lidar/fps2
      Spectral range of visible imaging/nm450-700
      Pixel resolution of visible imaging/ mrad·pixel−10.10
      FOV of visible camera5.75°×5.75°
      Frame rate of visible imaging/fps45

      Table 1.  Parameters of lidar optical system

    • 采用的MEMS反射镜为滨淞光子有限公司的器件,其镜面尺寸为2.6 mm,快、慢轴扫描角度均可达到15°,谐振频率可达到4 kHz,30°入射情况下,根据公式(3)则可计算出入射最小激光光束直径dmin=2.25 mm。

      选择单模光纤的模场直径dm=9.5 μm,数值孔径NA=0.13。为满足激光光束直径指标,可选择焦距f=8.18 mm的光纤准直器,准直后光束直径d=2.12 mm,发散角θ=1.16 mrad,则根据公式(4),反射镜需要的摆动角度为14.77°,小于MEMS反射镜的最大摆动角度。根据扫描范围指标,确定多角度扩束镜扩束倍率为2.32倍。

      利用光学设计软件建立发射光学系统模型,为防止光学元件间触碰,设置光纤准直器与MEMS反射镜间间距为20 mm,MEMS反射镜与多角度扩束器第一个表面的间距为8 mm,将激光经过MEMS后的波前差以及光束直径作为优化函数,优化后的发射光学系统参数如表2所示,光路图如图2所示,扩束后激光RMS波前差随着视场变化曲线关系如图3所示。从图3中可以看出,经过偏转与扩束后的光束波前差优于λ/30,光束质量良好。

      NumberRadius/mmThickness/mmGlass
      1Inf20
      MEMSInf−10
      392.642−2N-SK10
      4−14.622−6.039
      5−235.659−2.018N-SK10
      6−385.957−9.855
      728.620−2.322N-SK10
      827.605−3.316
      952.799−4.45N-SK10
      1020.867−10

      Table 2.  Parameter of emission optical system

      Figure 2.  Light path diagram of emission optical system

      Figure 3.  RMS wave-front error vs field of view

    • DMD器件选择德州仪器公司的Discovery 4100,可见光探测器选择Sony ICX825,2/3",参数如表3所示。所选近红外APD探测器感光面直径为3.0 μm。

      ParameterValue
      DMD mirror size/mm28.66×6.93
      DMD array1140×912
      Micro-mirror size/μm27.6×7.6
      Refresh rate (1 bit)/Hz22727
      Detector pixel size/μm26.45×6.45
      Detector array scale1384×1036

      Table 3.  Parameters of DMD and visible detector

      根据DMD反射面尺寸与扫描视场关系,确定物镜焦距f1≤69.3 mm,选择物镜焦距f1=65.0 mm。根据DMD反射面尺寸与探测器尺寸确定二次成像透镜组放大率≤0.323,确定二次成像透镜组放大率设为0.308。

      优化后的接收光学系统参数如表4所示,二维图如图4所示,DMD面视场分别为(0°, 0°),(0°, 1.43°),(0°, 2.84°)与(0°, 4.86°)的点列图与几何光能量分布如图5所示。从图5中可以看出成像质量良好,扫描范围内反射光光斑能量均集中在3×3个微反射镜以内。可见光相机的MTF曲线如图6所示,从图6中可以看出,奈奎斯特频率内MTF曲线均优于45%,成像质量满足使用要求。APD探测器面视场分别为(2.875°, 2.875°),(2.875°, −2.875°),(−2.875°, 2.875°)与(−2.875°, −2.875°)的足迹图如图7所示,从图7中可以看出,视场内的光斑能量均集中在探测器感光面内。

      StopInfThicknessGlass
      2 51.421 14.917 Sk14
      3 −114.620 5.37
      4 −59.084 5.08 SF10
      5 −1461.183 18.225
      6 44.932 7.039 N-LAK7
      7 −48.425 3.151
      8 −36.412 3.318 SF10
      9 −96.467 1.000
      10 18.653 5.907 H-BAK3
      11 11.868 3.704
      12 Inf 25.400 BK7
      13 Inf 0.500
      14 Inf 0.500 BK7
      15 Inf 1.306(1.0)
      16 Inf 2.969 Mirror(Image)
      17 −30.862 3.708 SF59
      18 −19.941 41.412
      19 415.994 2.949 SF59
      20 −135.375 12.000
      21 33.911 3.948 SF59
      22 135.465 5.286
      23 −21.172 8 SF59
      24 −30.314 18.080
      25 43.745 3.997 SF59
      26 −90.478 1
      27 10.144 9.527 SF59
      28 7.992 3

      Table 4.  Parameter of receiving optical system

      Figure 4.  Layout of receiving optical system

      Figure 5.  Spot diagram of DMD’s mirror

      Figure 6.  MTF curve of visible detector photo-surface

      Figure 7.  Edge footprint diagram of APD detector photo-surface

    • 由于设计的激光雷达分辨率较高,其光斑大部分均可在其照射目标内,可认为探测目标为扩展目标,扩展目标的探测距离与可接收到光功率关系公式为:

      式中各个影响因素的含义、估值与其来源如表5所示。

      ParameterValueData sources
      Peak power of laser pulse (PL)8.9 kWAverage power of fiber laser: 5 W; The repetition frequency: 80 kHz;
      Pulse width: ≤7 ns;
      Transmittance of transmitting terminal (TE)82%Fiber-collimator coupling efficiency: 99%; Transmittance of fiber collimator: 99%;
      Reflectance of MEMS mirror: 98%; Transmittance of single lens
      surface: 98%;Transmittance of 8 surfaces: 85%;
      Reflectivity of target (ρ)

      Aperture of receiving terminal (AR)
      65%

      30 mm
      Reflection of hard plastic surface commonly used in rotor UAV : 60%-70%;

      Design value of diameter: 30 mm;
      Transmittance of receiving terminal (TR)24.6%Transmittance of single surface of lens is 98%, Total transmittance of 31 surfaces: 53%;
      Transmittance of PBS: 50%; Transmittance of
      narrowband filter: 97%; Reflective of DMD: 95%;
      Atmospheric transmittance (Tα)≈90%Simulation results based on atmospheric transmittance software.

      Table 5.  All influence factors and source for detection range

      经计算,在1000 m距离可接收到光功率PR为3.5×10−8 W,根据参考文献[9]中探测器的噪声等效功率约为1.0×10−8 W。此时信噪比为3,可以满足虚警率为1%,探测概率≥90%。

      该接收系统仅接收发射激光方向的入射光,在将DMD的3×3个单元作为光斑接收点时,其相较于单点探测器的全视场接收,背景噪声可降低:

      式中:TR为单点探测器接收系统透过率,估计为90%;dDMD为DMD单元尺寸。所以设计中背景噪声的最终可降低22162倍。

    • 设计了基于MEMS反射镜与DMD等器件的激光雷达光学系统,解决了MEMS反射镜、DMD器件系统中存在的问题。设计的低空高分辨率激光雷达光学系统与可见光相机复合后可用于“低慢小”目标探测,更改结构参数后也可用于激光雷达其他应用。

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