Objective Because of its high orbit and wide observation range, a space-borne lidar can accurately and quickly obtain large-scale 3D spatial 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 3D imaging lidar is small, and the resolution of equivalent ground pixel is low. In order to further expand the observation width and improve the 3D imaging 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 2 m diffractive aperture dual-wavelength space-borne lidar is studied. Methods In order to meet the requirements of 3D imaging and ocean depth detection, the space-borne dual-wavelength land-sea lidar with a 2 m aperture harmonic diffractive optical system at 500 km orbit height is analyzed. Firstly, due to the difficulty of direct processing of large aperture thin film mirrors at present, it is divided into 12 small 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 of 1.55 mu m are as follows: the ground pixel resolution is 4 m, the instantaneous width in the cross-track direction is 4 km, 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. 516 mu m 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 3D imaging processing on the echo signals corresponding to the 12 sub-mirrors, the angular resolution of the system can be increased to 8 mu rad, and the corresponding ground pixel resolution can be increased to 4 m. After the 12 sub-mirror signals are processed by coherent synthesis, the signal-to-noise ratio can be increased by 10.8 dB theoretically, basically equivalent to achieving the image signal-to-noise ratio corresponding to a 2 m aperture. The effectiveness of the proposed method is verified by performing 3D imaging processing simulation on the pixel echo signals corresponding to the detector's normal direction and the 4 mrad deviation from the normal direction (Figs. 7 and 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 12 sub-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) x 1000 (cross-track direction) is used. When the laser emits a narrow pulse of 5 ns, the signal-to-noise ratio per pixel is about 0 dB. The sliding window processing realizes the incoherent accumulation of 7 pulses, which can increase the signal-to-noise ratio to about 4 dB, which can basically meet the requirements of 3D imaging. Using a laser local oscillator array detector, the signal-to-noise ratio per pixel of a single pulse is about 0 dB when the laser emits a wide pulse of 100 mu s and adopts pulse compression technology, and the sliding window processing realizes the incoherent accumulation of 7 pulses, which can increase the signal-to-noise ratio to 8.4 dB.
