With the development of ultrashort and ultra-intense laser, it has been revealed that when a femtosecond laser pulse propagates in air, filaments, referred to as “filament laser”, would occur owing to nonlinear effects. Traditional spaceborne air-pollution monitoring devices rely on spectral imaging and LiDAR technology, which cannot realize real-time monitoring of atmospheric multi-component pollutants, identify unknown pollutants, and detect the chemical composition of various pollutants. The filament laser system in orbit emits a femtosecond laser pulse into the atmosphere, and the intensity of the femtosecond laser pulse is sufficient to ionize molecules in the atmospheric environment. Ionization excites the fluorescence spectrum carrying the information of the material composition, which can determine the various material components, species, and content in the area of the filament laser. To aid the research on space-based filament lidar technology, this study explored the optical system design of a space-based filament lidar spectrometer for remote sensing applications and realized the optical system configuration design. The spectral range and resolution of the spectrometer are 320-950 nm and 2 nm, respectively, and it has applicability in high resolution spectral detection of atmospheric pollutant composition.

First, the application requirements of space-based filament lidar spectrometer were analyzed. On the basis of the characteristics of pollutants and the corresponding spectra of the substance elements, the working spectrum of the spectrometer and the spectral resolution were designed to be 320-950 nm and 2 nm, respectively. Using filament laser propagation simulation software, the filament laser diameter was found to be approximately 6 mm after 400 km orbital propagation. The filament laser diameter can be constrained to a small spatial scale after ultralong-distance propagation. To conform to the spectral range and resolution requirements, the size of CCD detector is 1024×1024, with 13 μm×13 μm pixel size. The minimum spectral sampling interval was designed to be 0.67 nm/pixel. Considering the signal-to-noise ratio requirements of the spectrometer, the relative aperture of the optical system was determined as D/f '=1/3.5, and the aperture of the spectrometer system was set as 0.5 m. Then, considering the requirements of engineering and the space environment, the optical design and optical-mechanical design of the spectrometer were performed so as to provide an effective load scheme for space-based filament laser atmospheric detection.

The optical system of the spectrometer mainly comprises a telescopic system, slit, collimating system, plane grating, and imaging system; the collimating system, dispersion element, and imaging system constitute the spectrometer. The front telescopic system adopts a total reflection Cassegrain structure without chromatic aberration correction, and the root mean square (RMS) value of the diffuse spot radius of its imaging point is within 14 μm. The spectrometer uses a reflective plane grating; the number of plane grating lines was determined to be approximately 263 lp/mm, and the grating aperture was 22 mm. A spectral resolution of 2 nm was achieved using first-order diffraction light. The maximum RMS diameter of the spectrometer system imaging slit is less than 17 μm. The modulation transfer function (MTF) is greater than 0.99@3.7 lp/mm, and the maximum color distortion is 1.1 μm. The energy concentration in three pixels is over 96%, which can be used for spaceborne high-resolution spectral detection of atmospheric components. The spectrometer system adopts a damping truss-unlocking mechanism for three-point support and is installed on the bottom plate of the satellite load compartment. The front lens tube is made of a carbon fiber composite material to ensure the thermal stability of the primary and secondary lens spacing. The main bearing frame and connecting plate are made of a titanium alloy. The design of stray light adopts the combination of “secondary mirror mask and primary mirror central hole baffle” with the simplest structure, which can ensure that the stray light coefficient of the camera is less than 0.5%. The statistics of various light paths that could reach the image surface were also obtained, and no abnormal stray light paths were found.

For space-based applications of the filament LiDAR spectrometer system, the optical system was designed and examined, and the main technical indicators of the optical system were determined. The designed spectrometer system can achieve a spectral resolution of 2 nm in the spectral range of 320-950 nm and thus provide a reference for the design and development of spectrometers used in filament LiDAR systems.