Design of optical system for short-wave infrared silicon immersion grating imaging spectrometer

To obtain the fine characteristics of the atmospheric carbon dioxide absorption spectrum, the imaging spectrometer must have high spectral resolution, but the volume of the instrument will increase simultaneously which is not conducive to remote sensing detection on satellite platforms. To solve this problem, we used a silicon immersion grating as dispersion element that diffraction occurs inside the silicon grating medium, the dispersion and spectral resolution are increased by 3.4 times respect to the conventional grating, and the volume of the spectrometer can be reduced to 1/40 at most. Smile is introduced because of the dispersion difference of light between the non-principal section and the principal section, which affects the spectral fidelity, data retrieval and detection accuracy. We established a model to analyze the smile by the silicon immersion grating and used the distortion of off-axis lens to correct. To miniaturize the instrument, optical system adopted the Littrow structure that the dimension is 410 mm×130 mm×110 mm. The optical system works in the short-wave infrared carbon dioxide weak absorption band 1.594-1.624 μm with a spectral resolution of 0.08 nm, entrance slit length is 16 mm and F number is 2. The optimized MTF is greater than 0.87 at the Nyquist frequency of the detector, the RMS radius of the spot diagram is within the Airy disk, and the maximum smile is less than 0.1 m, which is less than 1% of a single detector pixel. The optical system has excellent imaging performance, meeting the requirements of atmospheric carbon dioxide detection.