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Nowadays, the monitoring of CO2 concentration has gradually become the focus of scientists all over the world. In order to study the effects of CO2 on climate change and global ecosystems, hyperspectral and high spatial resolution CO2 detectors are necessary. Holographic grating is the core element of that CO2 detectors, and the immersion holographic grating can greatly improve the resolution of grating and reduce the volume of spectrometer. Therefore, it is necessary to take research on immersion grating. In this paper, we have designed and optimized a quartz immersion grating used in the CO2 detectors. We have designed and optimized the parameters of the grating, such as the width ratio and groove depth, according to the requirements of the spectrometer used and the actual fabrication errors, we designed and optimized rectangular and trapezoidal groove with different bottom angles to obtain high efficiency and low polarization-dependent immersion gratings. In the 2.04-2.08 μm band, with rectangular groove, the groove depth of the quartz immersion grating is 950 nm, the duty cycle is 0.7, the -1 order diffraction efficiency is over 82%, and the degree of polarization is below 12%. When the groove is trapezoidal and the bottom angle is 85 degrees, the -1 order diffraction efficiency is over 79% and the degree of polarization is below 10% at duty cycle of 0.63 and groove depth of 1050 nm. Then under the trapezoidal groove with a bottom angle of 80 degrees, when the duty cycle is 0.62 and the groove depth is 1100 nm, the -1 order diffraction efficiency is over 81% and the degree of polarization is less than 9%. Finally, we will fabricate a sample of immersion grating with a period of 1117 nm on a quartz substrate by holographic ion beam etching in late 2019.
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