Study on immersion grating used in weak carbon dioxide band detection spectrometer

Abstract

The imaging spectrometer monitoring the CO₂ content in the atmosphere is mainly divided into the weak CO₂ band and the strong CO₂ band. Compared with the strong CO₂ band, the weak CO₂ band has a relatively clean spectrum, less interference from water vapor and other gases, and weaker CO₂ absorption, and the instrument can receive stronger groundreflected solar radiation signals. However, the concentration of CO₂ in the atmosphere is low. To use this band to detect changes in CO₂ concentration, it is necessary to ensure that the detection instrument has a sufficiently high spectral resolution. Using the immersion grating as the dispersive element in the spectrometer can greatly reduce the size and weight of the instrument while achieving high spectral resolution. In this paper, the quartz immersion grating used in the weak CO₂ detector is optimized and designed, the groove parameters of the grating are designed and optimized according to the requirements of the weak CO₂ detector used, and the rectangle and different bottom angles are designed and optimized in consideration of the actual production error. In the 1.595-1.625 μm band, the duty cycle of the rectangular groove is 0.3-0.35, the groove depth is 560-630 nm, and the TiO₂ thickness is 110-120 nm, the diffraction efficiency of the grating is higher than 80%; the duty ratio is 0.4-0.45, When the groove depth is 590-660 nm and the TiO₂ thickness is 100-105 nm, the diffraction efficiency of the grating is higher than 80%. When the duty ratio of the trapezoidal groove is 0.3-0.35, the bottom angle of the trapezoid is 82°-86°, the groove depth is 590-630 nm, and the TiO₂ thickness is 120-125 nm, the diffraction efficiency of the grating is higher than 80%. When the duty ratio of the trapezoidal groove is 0.4-0.45, the bottom angle of the trapezoid is 82°-86°, the groove depth is 620-670 nm, and the thickness of TiO₂ is 105 nm, the diffraction efficiency of the grating is higher than 80%.