Objective The concentration of greenhouse gases in the earth's atmosphere is increasing year by year under the influence of fuel burning, deforestation, and industrial development. The continuous emission of greenhouse gases will result in increased global temperature and extreme weather such as heavy rainfall and sea level rise. Remote sensing of greenhouse gases is an important method for tracking greenhouse gas emissions and understanding the earth's climate evolution. As one of the most important optical payloads for spaceborne greenhouse gas monitoring, the grating imaging spectrometer features high resolution, high signal- to-noise ratio, and nearly linear dispersion. The immersed grating can achieve higher spectral resolution and more compact structural size and has been employed as the dispersion elements in imaging spectrometer for remote sensing of greenhouse gases. Currently, immersed gratings with higher performance are required to fulfill the requirements for more accurate greenhouse gas monitoring. For conventional reflective immersed gratings, metallic coatings are adopted to reflect the incident light. However, there are many disadvantages for the metallic coating. Firstly, this coating may cause resonance absorption due to the plasmon effect. The resonance absorption will decrease the diffraction efficiency and increase the polarization sensitivity of the grating. Secondly, it is difficult to deposit metal materials on the grating groove, which will also cause decreased diffraction efficiency. To this end, we propose and design a total internal reflection immersed grating whose grating groove is coated with nano laminate. It has high diffraction efficiency and low polarization sensitivity and can be utilized in the O-2-A channel for the imaging spectrometer of greenhouse gas monitoring. Methods According to the monitoring requirements of greenhouse gases, the design of the immersed grating is as follows. Firstly, the grating structure is modeled by the finite element software, and the diffraction efficiency and polarization sensitivity of the initial structure are calculated. Then, the parameters such as the duty cycle of the grating, the thickness of the nano laminate, the groove depth, and the refractive index of the nano laminate are optimized in turn with the controlled variable method. According to the optical film theory and the actual coating method, the thickness and stacking sequence of the nano laminate are optimized, and the multi-layer film structure is obtained. Finally, the manufacturing tolerance of the designed immersed grating is analyzed, and the tolerance of the grating with diffraction efficiency greater than 90% and polarization sensitivity less than 1% is presented. Results and Discussions Based on the introduced design method, an immersed grating working under the total internal reflection and coated with the nano laminate is designed, and it has high diffraction efficiency and low polarization sensitivity. Benefiting from the advantages of total internal reflection, the designed immersed grating has no transmission order, and the diffraction light energy is concentrated on the reflection diffraction order, which is helpful to improve the diffraction efficiency. Additionally, the coating on the grating groove is the nano laminate structure, which is alternately stacked with Al2O3 and TiO2 materials (Fig. 7). The nano laminate can improve the diffraction efficiency of the grating in the transverse electricity (TE) and transverse magnetism (TM) directions, and reduce the polarization sensitivity. The results show that the average diffraction efficiency of the design immersed grating at the - 1 order is higher than 92%, and the polarization sensitivity is lower than 1% in the working band of 750 - 770 nm (Fig. 8). Conclusions Our paper provides the spectral resolution formula of immersed gratings based on the principle of immersed gratings and shows that the immersed grating can reduce the size of the optical system and achieve high spectral resolution. According to the working conditions and grating parameters given by the optical design, fused silica is selected as the grating substrate, and a total internal reflection immersed diffraction grating with high diffraction efficiency and low polarization sensitivity is designed. The corresponding manufacturing tolerances are analyzed by considering the diffraction efficiency and polarization sensitivity requirements and the manufacturing method. The presented reflective immersed grating in our paper has the advantages of high diffraction efficiency and low polarization, and the grating groove tolerance is feasible for manufacturing. Therefore, our study lays a basis for the design of high-performance reflective immersed gratings.
