2m衍射口径星载双波长陆海激光雷达系统研究

Research on Space-Borne Dual-Wavelength Land-Sea LiDAR System with 2 m Diffractive Aperture

针对对地三维成像和海洋水深测量的需求,对轨道高度为500 km、口径为2 m的谐衍射光学系统星载双波长陆海激光雷达系统进行了分析。基于单光子阵列探测器,分析了大口径衍射光学系统的光学合成孔径实现方式,并设计了系统参数。波长为1.55μm的陆地观测激光雷达的主要性能指标为:地面像元分辨率4 m,交轨瞬时幅宽4 km,高程测量精度0.3 m。波长为0.516μm的海洋观测激光雷达的可探测水深达30 m。分析了激光本振阵列探测器的结构,提出了基于相干探测的光学合成孔径技术,有望采用计算成像的方式,利用多个子口径的低分辨率复图像信号相干合成高分辨率图像,同时提高图像的信噪比。相同系统参数下的对比分析表明,波长为1.55μm的陆地观测激光雷达采用相干探测体制后,探测性能优于传统的直接探测。采用子口径结构,可降低衍射光学系统的加工难度,同时子口径结构焦距短的特点使得光学系统的轴向尺寸和重量大幅减小。

Objective Because of its high orbit and wide observation range,a space-borne lidar can accurately and quickly obtain large-scale 3Dspatial information.In recent years,it has been widely used in ocean remote sensing,topographic surveying and mapping,and atmospheric environment detection.At present,the observation width of a satellite-to-ground 3Dimaging lidar is small,and the resolution of equivalent ground pixel is low.In order to further expand the observation width and improve the 3Dimaging efficiency,it is necessary to develop a new type of spaceborne lidar.The high monochromaticity of a laser makes lidar particularly suitable for diffractive optical systems.In order to solve the problem of limited transmission power of a space-borne lidar,the use of large-aperture and lightweight diffractive film mirrors should be considered for echo signal reception.Based on the diffractive optical system,single-photon array detector and laser local oscillator array detector,the realization of the 2mdiffractive aperture dual-wavelength space-borne lidar is studied.Methods In order to meet the requirements of 3Dimaging and ocean depth detection,the space-borne dualwavelength land-sea lidar with a 2maperture harmonic diffractive optical system at 500km orbit height is analyzed.Firstly,due to the difficulty of direct processing of large aperture thin film mirrors at present,it is divided into 12small aperture sub-mirrors,which are processed separately,and subsequently a large aperture is formed by the optical synthetic aperture technology.Second,based on the single photon array detector,the realization of an optical synthetic aperture of a large aperture diffractive optical system is analyzed,the system parameters are designed,and the detection performance is calculated.Third,the structure of the laser local oscillator array detector is analyzed,and finally an optical synthetic aperture method based on coherent detection is proposed.It is expected to use multiple sub-aperture low-resolution complex image signals to coherently synthesize a high-resolution image by computational imaging and improve the image signal-to-noise ratio.Results and Discussions The main performance indicators of the land observation lidar with the wavelength of1.55μm are as follows:the ground pixel resolution is 4m,the instantaneous width in the cross-track direction is4km,and the elevation measurement accuracy is 0.3 m.The detection depth of clear ocean water by an ocean observation lidar with the wavelength of 0.516μm can reach 30 m.In order to further improve the detection performance,an optical synthetic aperture method based on a coherent detection subarray structure is proposed,which is expected to reduce the volume and weight of the system by greatly reducing the axial size of the system while achieving computational imaging.The comparison under the same system parameters shows that the performance of the coherent detection system is better than that of traditional direct detection system.After performing 3Dimaging processing on the echo signals corresponding to the 12sub-mirrors,the angular resolution of the system can be increased to 8μrad,and the corresponding ground pixel resolution can be increased to 4m.After the 12sub-mirror signals are processed by coherent synthesis,the signal-to-noise ratio can be increased by 10.8dB theoretically,basically equivalent to achieving the image signal-to-noise ratio corresponding to a 2maperture.The effectiveness of the proposed method is verified by performing 3Dimaging processing simulation on the pixel echo signals corresponding to the detector’s normal direction and the 4mrad deviation from the normal direction(Figs.7and 8).Conclusions In the space-borne lidar,the large-aperture dual-wavelength harmonic diffractive optical system is used to achieve light-weight,and the along-track direction multi-pulse sliding window processing is adopted to improve the detection performance of the system.The 12sub-mirrors of the optical synthetic aperture can well replace the large-aperture main mirror(Fig.3).A single-photon array detector with a scale of 10(along-track direction)×1000(cross-track direction)is used.When the laser emits a narrow pulse of 5ns,the signal-to-noise ratio per pixel is about 0dB.The sliding window processing realizes the incoherent accumulation of 7pulses,which can increase the signal-to-noise ratio to about 4dB,which can basically meet the requirements of 3Dimaging.Using a laser local oscillator array detector,the signal-to-noise ratio per pixel of a single pulse is about 0dB when the laser emits a wide pulse of 100μs and adopts pulse compression technology,and the sliding window processing realizes the incoherent accumulation of 7pulses,which can increase the signal-to-noise ratio to 8.4dB.