Objective Affected by the difference of albedo in different regions of the surface and non-uniformity of atmospheric radiation, the physical slits in the satellite- borne imaging spectrometer are illuminated non- uniformly to result in linear distortion of the spectral response function of the instrument. This is manifested as the superposition of random errors in the acquired spectral signals, affecting the inversion accuracy of the imaging spectrometer. Meanwhile, a single greenhouse gas imaging spectrometer with a very wide spatial field of view and extremely high spectral resolution emerges to improve the temporal resolution and accuracy of atmospheric greenhouse gas concentration detection, with the influence of radiation non-uniformity being more obvious than that of ordinary imaging spectrometers. We design a two-dimensional spatial homogenizer for greenhouse gas monitoring imaging spectrometers to homogenize scene radiation in two dimensions of across- track and along-track. The two-dimensional spatial homogenizer consists of a rectangular fiber array, the width of a single fiber along the track is determined by the integration time, and the size of a single fiber across the track indicates the spatial sampling width of the instrument. The unique advantages of the two-dimensional spatial homogenizer can ensure not only the stability of the spectral response function of the spectrometer in the inhomogeneous scene of sharp contrast but also the spatial coregistration between different spectral channels. Methods To ensure the stability of the spectral response function of the spectrometer in non-uniform scenarios, we should consider the homogenization performance of the rectangular fiber in the design stage. Additionally, due to the influence of the internal microstructure and macroscopic bending of the optical fiber, the optical fiber inevitably has a focal ratio degradation effect, which will influence the quality of the outgoing beam and the design of the subsequent optical system. Therefore, we study the stability of the rectangular optical fiber composed of the two-dimensional spatial homogenizer by simulation, compare the uniformity of the optical fiber exit spot under different fiber lengths, different lighting scenarios, and different roughness conditions, and research the focal ratio degradation characteristics of the rectangular fiber by simulation. Results and Discussions The uniformity of the outgoing spot increases with the rising fiber length, although the energy loss increases correspondingly (Fig. 4). The uniformity of the optical fiber outgoing spot in the non-uniform illumination scene is the same as that in the uniform illumination scene (Table 1). This indicates that the rectangular optical fiber has sound homogenization properties and that the two-dimensional spatial homogenizer can reduce the sensitivity of the spectrometer to inhomogeneous radiation of the earth scene. Meanwhile, the focal ratio degradation characteristics of rectangular optical fiber are also studied, and the results show that the optical efficiency can reach more than 95% when the input F number is 4 and the optical system of F/ 3.5 is employed at the output end (Fig. 9). Conclusions The influence of inhomogeneous radiation on the greenhouse gas monitoring imaging spectrometer should be solved and the temporal resolution and accuracy of atmospheric greenhouse gas concentration detection should be improved. To this end, a two- dimensional spatial homogenizer for greenhouse gas monitoring imaging spectrometer is studied to achieve two-dimensional homogenization of scene radiation across and along orbits. Our study simulates and studies the homogenization performance and focal ratio degradation characteristics of the rectangular optical fiber composed of the two-dimensional space homogenizer. It is found that if the rectangular fiber length increases, the uniformity also rises, but the energy loss increases accordingly. Meanwhile, the rectangular fiber can realize the homogenization of the nonuniform scene and reduce the sensitivity of the spectrometer to the non-uniform earth scene, and the optical efficiency can reach more than 95% when the optical system of F/ 3.5 is adopted at the output end when the input F number of the optical fiber is 4. The simulation results reveal that the two-dimensional spatial homogenizer can provide an ideal solution to the influence of non-uniform earth scenes on the spectrometer accuracy.
