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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。
Parameter Value Lidar wavelength/nm 1064 Array 200×200 Pixel resolution 0.5 mrad@Spot duty ratio 0.9 Scanning range 5.75°×5.75° Frame rate of lidar/fps 2 Spectral range of visible imaging/nm 450-700 Pixel resolution of visible imaging/ mrad·pixel−1 0.10 FOV of visible camera 5.75°×5.75° Frame rate of visible imaging/fps 45 Table 1. Parameters of lidar optical system
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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。
Number Radius/mm Thickness/mm Glass 1 Inf 20 MEMS Inf −10 3 92.642 −2 N-SK10 4 −14.622 −6.039 5 −235.659 −2.018 N-SK10 6 −385.957 −9.855 7 28.620 −2.322 N-SK10 8 27.605 −3.316 9 52.799 −4.45 N-SK10 10 20.867 −10 Table 2. Parameter of emission optical system
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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。
Parameter Value DMD mirror size/mm2 8.66×6.93 DMD array 1140×912 Micro-mirror size/μm2 7.6×7.6 Refresh rate (1 bit)/Hz 22727 Detector pixel size/μm2 6.45×6.45 Detector array scale 1384×1036 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。
Stop Inf Thickness Glass 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
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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。
Parameter Value Data sources Peak power of laser pulse (PL) 8.9 kW Average 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 mmReflection 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。
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
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Key words:
- optical design /
- lidar /
- MEMS mirror /
- digital mirror device /
- background noise
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.