Study on immersion grating for methane detection spectrometer

Methane is the second largest greenhouse gas after carbon dioxide, and the measurement and monitoring of methane is essential for regional and city-scale emission reduction strategies. Atmospheric methane concentration observations are also important for predicting regional emission trends. Spectrometers are important optical instruments for monitoring the content of greenhouse gases in the atmosphere, and most spectrometers choose a grating as the spectroscopic element. Due to the higher resolution of the immersion grating, it reduces its volume while improving the performance of the spectrometer and reducing the weight of the whole machine, so it is of great significance to study the immersion grating for the practical application of the spectrometer. In this paper, the quartz immersed grating was designed for weak CH4 band. The rectangular groove structure with high refractive index dielectric film was designed in consideration of possible fabrication errors. In the 2.275-2.325μm band, the duty cycle of the rectangular groove is 0.3~0.45, the thickness of TiO2 is 165~170nm and the groove depth is in the 800~980nm region, the diffraction efficiency of the grating is higher than 70%. When the thickness of TiO2 is 170nm and the groove depth is in the 800~950 nm region, the diffraction efficiency of the grating is higher than 80%. When the TiO2 thickness is 175~180 nm, the overall polarization degree is less than 0.1. For trapezoidal grooves, when the duty cycle is 0.3~0.35, the bottom angle of the trapezoid is 80°~86°, the thickness of TiO2 is 195 nm, the depth of the grating groove is 820~900nm, and the diffraction efficiency can be above 80%. When the thickness of TiO2 is 177~195nm, the polarization degree is less than 0.1. The TiO2 film will be deposited by atomic layer deposition.