Objective A variety of infrared spectrometers have been designed in China and abroad to achieve sensitive and high-resolution measurement of the absorption spectrum of trace components in the atmosphere, but the spectral resolution is relatively low, which seriously hinders the detection of tiny species in the atmosphere. Because of its special dual dispersion structure, the echelle grating can realize both wide-band and high-resolution spectral measurement. However, the spectrum order overlap of the echelle grating is serious and should be combined with secondary dispersion elements to eliminate the order overlap. As a result, the optical system structure is complicated, with large weight and too high processing cost. Therefore, it is necessary to improve the optical system structure to make the spectrometer easier to employ on the spacecraft and ensure imaging quality. Methods We improve the traditional echelle grating spectrometer system, and study and design a small, sensitive, and ultra-high spectral resolution optical system structure. The structure adopts an acousto-optic tunable filter (AOTF) combined with echelle grating to achieve spectral order separation. AOTF is made according to the acousto-optic diffraction principle of birefringent crystal. Compared with the prism or grating in the traditional spectral analysis system, AOTF features small volume, light weight, arbitrary wavelength selection, and high diffraction efficiency. Due to the aperture limitation of AOTF, the beam enters AOTF after being collimated by the beam reduction system, and the spectral information of each spectrum segment is obtained in the image plane after the beam is spliced through the echelle grating. Finally, the optimized telescopic system and the spectrometer are interfaced to obtain the overall optical structure of the hyperspectral resolution imaging spectrometer, and the slit overlaps with the image plane of the telescopic system. According to the grating equation, different diffraction levels need to be adopted to make the corresponding wavelength of each spectrum segment shine. By optimizing the grooving spacing of the echelle grating, the grating order using 50 to 100 orders in the working band can be obtained. The combination of echelle grating and AOTF technology will produce imaging of longer spectral lines on the detector, resulting in a higher signal-to-noise ratio. Results and Discussions By utilizing optical design software to optimize the initial structure of the imaging spectrometer, we obtain a miniaturized ultra-high spectral resolution imaging spectrometer based on the combination of AOTF and echelle grating. The wavelength of the output light is quickly and randomly changed by changing the RF signal. The technical index of the optical system is greatly improved compared with that of the ordinary echelle grating spectrometer. Finally, this solves the overlapping problem of the echelle grating and makes up for the defects of the large volume and heavy mass of the echelle grating spectrometer. The imaging quality of the optical system is sound when the operating band of the optical system is 2320-4250 nm, with the F of the system less than 1. 8 and the ultra-high spectral resolution better than 0. 15 nm. At a Nyquist frequency of 17 lp/mm, the overall modulation transfer function (MTF) is greater than 0. 7 (Fig. 10), and the root mean square (RMS) radius of the diffuse spot in all fields of view is less than 11 mu m (Fig. 11). Conclusions The imaging spectrometer adopts the scheme of combining AOTF with echelle grating, solving the overlapping problem of echelle grating and greatly improving the spectral resolution of the optical system. Meanwhile, it can achieve high-precision and high-sensitivity measurement of atmospheric trace components and has smaller volume and higher signal-to-noise ratio compared to other high-resolution infrared spectrometers, making it easier to be installed on planets or interstellar spacecraft with limited space resources. Additionally, the spectral resolution is greatly improved to help obtain more refined spectral signals.
