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小视场高重频激光雷达系统结构示意图如图1所示,该系统光学单元部分主要分为发射单元系统、接收单元系统和后继单元系统。从光路上看,首先由激光器出射激光,通过光束转折反射镜将激光正入射到扩束器中,再从扩束器出射到反射镜上,通过电动调整架调节反射镜姿态将激光反射到大气中,完成激光的发射。由大气返回的回波光信号通过望远镜接收,通过小孔光阑和目镜将回波光信号变成平行光,经后继光路将回波光信号分成532垂直和平行两探测通道。探测器将光信号转换成电信号,经过数据采集、滤波技术及数据分级技术将云和气溶胶垂直分布信息存储起来。在计算&控制平台中,主要控制对象有控制激光器电源及触发、控制扫描电机上的反射镜偏转等。小视场高重频激光雷达系统参数如表1所示。
图 1 小视场高重频激光雷达系统结构示意图
Figure 1. Schematic diagram of lidar system structure with small-field of view and high-repetition frequency
表 1 小视场高重频激光雷达系统参数
Table 1. System parameters of lidar with small-field and high-repetition frequency
Item Parameters Laser Wavelength/nm 532.18 Repetition rate/kHz 3 Output divergence 2 (full) Output beam energy 1 Beam expansion 20X Telescope Diameter/mm 125 Field of view 0.28(full) Focal length/mm 1430 Diffuse spots/mm <0.045 Wavefront difference <1/4λ(λ=632.8 nm) Focal length of ocular/mm 50 Reflector(532 nm) R:99% Filter bandwidth/nm 0.3 Extinction ratio of polarizing prism 3000:1 Detector PMT Capture card Photon -
设计发射单元发散角要求为0.1 mrad,选用某公司提供的20倍扩束器来压缩激光器出射光束发散角。利用Zemax软件进行模拟仿真,采用光束出射口径为1 mm,发散角为2 mrad来模拟激光器出射光源。通过折转反射镜将激光光束正入射到扩束器中,经过扩束后两折转反射镜反射到大气中,所设计的发射单元模块光路图如图2所示。通过计算获得发射单元发散角为0.106 mrad,与设计要求相差不到10%。
在实际安装过程中装校误差不可避免,使得激光光束成一定的角度入射到扩束器中,从而影响扩束效果。为了研究激光光束入射到扩束器的角度对扩束效果的影响,通过改变扩束器倾斜角度来模拟入射角度对扩束的影响。图3为不同入射角对扩束后的光束发散角影响。
图 3 入射角与扩束后发散角的关系曲线
Figure 3. Relation curve between incident angle and divergence angle after beam expansion
由图3可知,随着入射角的不断增大,其扩束后的发散角近似线性增加,因此采用折转镜调整激光光束方向进行补偿。一般采用三维调整架调节折转镜的位姿来进行角度补偿,但这种调整架只适合在实验室中进行,因为其结构不稳定的。设计新型的光束转向结构,可以实现垂直于光轴方向的平动及改变光轴方向的转动,其中平动可实现激光光轴落到反射镜的旋转中心,再通过转动可改变光轴方向来实现角度补偿。如图4所示,为所设计发射单元的光机结构图。
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望远镜光学系统设计时,确定中心遮拦比为0.272,由于缩比样机总体尺寸的严格限制,结合控制主次镜间距为200 mm。望远镜由某公司提供,光学参数如表2所示。
表 2 小型化高重频激光雷达望远镜光学参数
Table 2. Optical parameters of telescope of lidar with miniaturization and high-repetition frequency
Radius of curvature/mm Distance to the next side/mm Radius/mm Quadric coefficient −516 −200 68 0 −141.536 321.82 17 6.288 通常采用视场角乘以望远镜焦距来计算出小孔光阑孔径,所设计的接收视场角为0.28 mrad,望远镜焦距为1430 mm,计算出的小孔光阑孔径约为0.4 mm。如图5所示,为所设计的接收与后继单元光路图。
图 5 光机系统接收与后继单元光路图
Figure 5. Light path diagram of optical-mechanical system of the receiving and aft-optical unit
利用Zemax软件对接收视场角进行仿真,文中以平行光入射角0.14 mrad模拟接收视场角(半角)进行仿真模拟,获得探测器靶面的光斑,如图6所示。
图 6 0.14 mrad (半角)视场下探测器靶面上的光斑
Figure 6. Light spot on the target surface of the detector under the field of view of 0.14 mrad (half angle)
如图6所示,在0.14 mrad (半角)视场下,探测器靶面上的光斑偏心为1.398 mm,光斑半径为1.906 mm(无光阑),如图6(a)所示,完全被8 mm探测器靶面所探测;而在0.4 mm孔径光阑下,探测器靶面光斑不完整,如图6(b)所示,说明在0.4 mm孔径光阑下实际接收视场角小于0.28 mrad,原因没有考虑到望远镜焦点处弥散斑直径所导致。
为了进一步研究在0.28 mrad视场下,0.4 mm孔径光阑下挡光的原因,利用Zemax软件对望远镜进行仿真模拟,获得望远镜在0.14 mrad(半角)视场下望远镜系统弥散斑,如图7(a)所示。
图 7 0.14 mrad (半角)视场望远镜系统弥散斑(a)及能量集中度(b)
Figure 7. Diffuse spots (a) and energy concentration (b) of telescope system under the field of view of 0.14 mrad (half angle)
由图7(a)可知,在0.14 mrad (半角)视场下望远镜系统弥散斑偏心0.199 mm,以主光线偏心位置为坐标原点,获得望远镜系统能量集中度,如图7(b)所示。根据光斑直径的测量原理,圈入能量分数为86.5%的圆半径作为弥散斑半径。由图7(b)可得,圈入能量分数为86.5%时,在视场0.14 mrad (半角)下的半径为20.9 μm,因此小孔光阑孔径为0.4398 mm才能满足视场0.28 mrad的探测功能,而实际的小孔光阑孔径只有0.4 mm,故利用Zemax软件仿真计算出不同视场下望远镜系统弥散斑偏心及半径,如图8所示。
图 8 不同视场下望远镜系统弥散斑偏心(a)及半径(b)
Figure 8. Eccentricity(a) and radius(b) of diffuse spot of the telescope system under different fields of view
由图8可以看出,望远镜的视场角与弥散斑偏心和半径近似成线性关系,可线性拟合推导出其关系式为:
$$\left\{ {\begin{array}{*{20}{c}} {{{e}} = 0.7{\theta _r} + 0.003} \\ {{r_1} = 20{\theta _r} + 15.3} \\ {d = 2\times(e + {r_1})} \end{array}} \right.$$ (2.1) 式中:θr为接收视场角(全角),单位为mrad;e为弥散斑偏心值,mm;
${r_1}$ 为弥散斑半径,μm。当小孔光阑孔径d为0.4 mm时,由公式(1)计算出望远镜的视场角为0.25 mrad,较理想视场角0.28 mrad减小10.7%。根据工程经验,一般设计接收视场角是发射光束发散角的两三倍,因此文中所设计的接收视场角不得小于0.212 mrad。通过公式(1)计算出所需小孔光阑孔径为0.34188 mm,故0.4 mm孔径的小孔光阑在焦平面处偏心不得超过29 μm,因此设计高精度的三维调整架来精确定位小孔光阑的位置,整个接收与后继单元光机结构如图9所示。
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如图10所示,总体结构方案以方形框架为基准,首先将望远镜固定在方形框架所在的基板上,将小孔光阑、目镜以及光束转向反射镜固定在基板的另一侧;然后将发射单元模块、后继单元模块以及电控单元模块分别固定在方形框架的三个侧面,可有效地将电学部分与后继单元隔离开来,避免相互干扰。数据采集单元与数据存储与发送模块与后继单元一起固定在一个工作平板上。在发射单元装调过程中,将发射单元模块固定在一个工作平板上,并把光路装调好,然后将整个发射单元系统装备到方形框架中,只要通过程序自动控制调整反射镜达到与接收光轴平行的效果可有利于发射单元的装调,大大缩短整机的装调时间,互换性好。
Optical-mechanical system design, installation and performance test of lidar with small-field and high-repetition frequency
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摘要: 为了后期研制星载高重频激光雷达提供数据校正及仿真,设计研制了一套小视场高重频激光雷达验证系统。对该激光雷达进行详细的光机系统结构设计,利用Zemax软件模拟发射、接收与后继单元光路图。精确计算出出射光束发散角为0.106 mrad,设计新型的光束转向结构确保正入射到扩束器中。在0.4 mm小孔光阑下,接收单元视场角0.25 mrad,在系统焦平面上的小孔光阑偏心不得超过29 μm,选择高精度三维调整结构对小孔光阑精确定位。整机结构设计采用模块化设计方法,以方形框架为基准,不同单元模块安装在其不同位置,高度集成在尺寸为390 mm×390 mm×550 mm以内。对发射单元进行装校,并检测出发散角为0.11 mrad,与仿真结果相比,相对误差为4.1%;对接收与后继单元进行装校,采用平行光管出射的平行光正入射到接收望远镜,获得系统焦点精确位置,完成高精度的装校。通过对系统增益比进行标定实验,得到系统增益比为1.15,并对系统进行探测实验,探测结果:系统在夜晚气溶胶探测距离可达22 km,退偏振比可达10 km。在白天探测距离可达10 km,退偏振比可达6 km,并与太阳光度计比较,光学厚度相对误差为7.1%。整机性能满足设计要求,为后期做飞行实验打好基础。
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关键词:
- 小视场高重频激光雷达 /
- 光机系统设计 /
- 装校 /
- 性能测试
Abstract: In order to provide data correction and simulation for later development of space-borne lidar with high-repetition frequency, the lidar verification system with small-field of view and high-repetition frequency was designed and developed. Its opto-mechanical system was designed in detail, and the light path diagram of the transmitting, receiving and aft optical units was simulated by Zemax. The divergence angle of the outgoing beam was accurately calculated to be 0.106 mrad, and the new beam steering structure was designed to ensure the normal incidence into the beam expander. The field of view of receiving unit was 0.25 mrad with the iris diameter of 0.4 mm, the eccentricity of the diaphragm on the focal plane of the system should not exceed 29 μm, so the high-precision three-dimensional steering structure was selected to accurately position the iris. The design of the whole machine structure adopted the modular design concept, the different unit modules were installed in the different positions of the square frame. The structure was highly integrated, and the overall size was 390 mm×390 mm ×550 mm.The system transmitting unit was installed and calibrated, its divergence angle was detected to be 0.11 mrad. Compared with the simulation calculation, the relative error was 4.1%. The receiving and aft optic units were installed and calibrated, and the parallel light emitted by the collimator tube was used to enter the receiving telescope to obtain the precise position of the focal point of the system to complete the high-precision installation and calibration. Through the calibration experiment of the gain ratio of the system, the gain ration is 1.15. The detection experiments on the system show that the system can achieve aerosol detection distance of up to 22 km, and the depolarization ratio can reach 10 km at night. In the daytime, the detection distance can reach 10 km and the depolarization ratio can reach 6 km. Compared with the solar photometer, the relative error of optical thickness is 7.1%. The performance of the whole machine meets the design requirements, which lays a good foundation for the later experiments on the boat-borne. -
表 1 小视场高重频激光雷达系统参数
Table 1. System parameters of lidar with small-field and high-repetition frequency
Item Parameters Laser Wavelength/nm 532.18 Repetition rate/kHz 3 Output divergence 2 (full) Output beam energy 1 Beam expansion 20X Telescope Diameter/mm 125 Field of view 0.28(full) Focal length/mm 1430 Diffuse spots/mm <0.045 Wavefront difference <1/4λ(λ=632.8 nm) Focal length of ocular/mm 50 Reflector(532 nm) R:99% Filter bandwidth/nm 0.3 Extinction ratio of polarizing prism 3000:1 Detector PMT Capture card Photon 表 2 小型化高重频激光雷达望远镜光学参数
Table 2. Optical parameters of telescope of lidar with miniaturization and high-repetition frequency
Radius of curvature/mm Distance to the next side/mm Radius/mm Quadric coefficient −516 −200 68 0 −141.536 321.82 17 6.288 -
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