The influence of greenhouse gases, industrial pollutant gases, and volatile organic compounds on the living environment and ecology is becoming increasingly severe. It is necessary to regularly monitor their concentrations in the atmosphere. Satellite remote sensing with high spectral resolution and high sensitivity can realize high accuracy determination of gas concentration, which has been an important technology for air pollution monitoring and global change research. Two main types of passive remote sensing payloads for atmospheric composition detection are Fourier transform spectrometer (FTS) and grating spectrometer. The former has wide spectral coverage and big throughput. However, the spatial continuous detection of gas concentration distribution is limited, and the system performance of FTS is better in the infrared band. The advantages of grating spectrometers are a big field of view (FOV), spatial continuous imaging, and high spectral resolution. The spatial and spectra information can be obtained at the same time by an area array detector. The development of atmospheric composition monitoring payload technology needs to realize the rapid change monitoring of gas composition with rapid chemical reaction and short life in the atmosphere, so as to meet the needs of atmospheric chemistry research and pollutant gas emission supervision and law enforcement. The quantitative detection of a variety of atmospheric components at the same time can provide a more effective means for analyzing and evaluating the situation and law of atmospheric compound pollution. This requires the payload to achieve wide spectral coverage, ultra-high spectral resolution, and high sensitivity while having larger swath and higher spatial resolution. The improvement of the performance will directly lead to an increase in the volume and weight of the spectrometer, increasing the cost of payload and satellite development and launch. The focus of our study is how to achieve a compact design of the spectrometer while meeting high-performance requirements.

Considering the application requirements and system performance, the spectrometer is designed to have four bands: ultraviolet (B1), ultraviolet-visible (B2), near-infrared (B3), and short-wave infrared (B4). Applying fully freeform optics, the telescope system with only two off-axis reflective mirrors realizes aberration correction along a large FOV. The design of large F-number and different focal lengths in meridian and sagittal directions is used to simplify the structure and effectively reduce the alignment tolerance. Through the processing of free-form aluminum mirrors and the assembly and adjustment test of the telescope system, the verification of the free-form telescope system with a large FOV is realized. A reflective slit of ultraviolet (UV) band to divide the FOV and the entrance slit of the other three bands are integrated into one assembly. For the ultraviolet band, especially before 320 nm, the spectral radiance after atmospheric absorption in orbit is very weak, so the dichroic is replaced by the FOV splitting method. The optical design is optimized to reduce the number of optical elements, so as to ensure the high transmittance of the ultraviolet band. According to the spectral line dispersion requirements and the grating equation, the focal length and grating parameters of the imaging system are determined. The nonlinear dispersion of the grating is corrected by selecting the appropriate prism parameters. Correction of aberrations using aspheric surfaces in the collimator and imager simplifies system architecture while achieving high image quality. Using silicon immersed grating, the short-wave infrared region (SWIR) spectrometer has high dispersive capability. Meanwhile, the aperture of grating diffraction surface and the volume are reduced considerably.

Both the primary mirror and the secondary mirror use the freeform surface to achieve 108.8° full-field aberration correction (Fig. 3). The volume of the telescope system is only about 1/5 of the volume of the off-axis three-mirror system with the same specifications. Our optical path design is more compact, and the transmittance of the UV band without a dichroic is higher due to the integrated slit assembly (Fig. 7). The volume of the SWIR spectral imaging system with silicon immersion grating is only about 1/40 of that of the ordinary reflection grating spectrometer (Fig. 10). The addition of immersion medium to the grating, which reduces the angle of incidence and diffraction, can further reduce the aberration in the focal plane. The spectral resolution of each band is better than 0.53 nm (B1), 0.54 nm (B2), 0.44 nm (B3), and 0.25 nm (B4) (Fig. 12 and Table 3). The signal-to-noise ratios of B1, B2, B3, and B4 bands are better than 120, 200‒1000 (Fig.13), 640, and 130, respectively.

To meet the urgent needs of atmospheric composition monitoring in orbit with high temporal resolution, we develop a spectrometer prototype with a spectral resolution of 0.25‒0.55 nm covering the range of 270‒2385 nm. With a large coverage of better than 2600 km, the instrument can monitor the daily emission variation of polluting gases and greenhouse gases. The hyper-spectral optical system, which includes multiple complex optics and four different channels is aligned, integrated, and tested. The spectral and radiometric performance of the instrument is validated, laying the foundation for engineering development. Our technical results can be directly applied to the development of a spaceborne spectrometer for large-swath, multi-channel spectrometer in low Earth orbit, and can also be used for monitoring atmospheric composition in geostationary Earth orbit.