Study On Immersion Grating For Atmospheric Carbon Dioxide Detection Spectroment

The absorption characteristic spectral lines of atmospheric CO2 are mainly 15 μm,4.3 μm,2.7 μm,2.06 μm,and 1.61 μm.High-resolution imaging spectrometer is an optical instrument for monitoring atmospheric CO2 content,and the immersion grating,as a key dispersion element of high-resolution imaging spectrometer,can reduce its size and weight while maintaining the same spectral resolution.Therefore,it is important to study the immersion grating.In this paper,we designed and fabricated a immersion grating for the 1.61 μm band of CO2 to satisfy the requirements of small size,compact structure and high diffraction efficiency.The main work includes the following aspects:1.Introduces the background and development process of the grating,and introduces the current status of domestic and international research on immersion gratings,taking into account the needs of atmospheric CO2 detection.2.The study compares the dispersion characteristics of immersion gratings and conventional gratings,and illustrates that immersion gratings can significantly reduce the size and weight of spectrometers while achieving high spectral resolution.The finite difference in time domain(FDTD)method is introduced.The simulation analysis of the immersion grating is based on the need of 1.61 μm band detection,and it is concluded that it is more advantageous under the condition that the metal reflective film is Ag.Rectangular and trapezoidal groove structures are analyzed to obtain high diffraction efficiency and low polarization sensitivity for each parameter range.For the rectangular groove immersion grating,with the duty cycle of 0.3～0.45,titanium oxide thickness of 110·115 nm,and the height of the grating teeth of 580～650 nm,the diffraction efficiency of the grating is above 70%and the polarization sensitivity is below 0.1.For the trapezoidal groove immersion grating,with the trapezoidal base angle of 82°～86°,the duty cycle of 0.3～0.35,the thickness of titanium oxide of 115～125 nm,and the height of the grating teeth of 550～640 nm,the diffraction efficiency of the grating is above 70%and the polarization sensitivity is below 0.1.the duty cycle of 0.4～0.45,the thickness of titanium oxide of 100～105 nm,and the height of the grating teeth of 570～670 nm,the diffraction efficiency of the grating is above 70%and the polarization sensitivity is below 0.1.3.The commonly used grating preparation methods are introduced,and the advantages and disadvantages of these methods are analyzed.The holographic ion beam etching combined with atomic layer deposition technique is selected for the development of the immersion grating,taking into account the characteristics of the immersion grating and the laboratory conditions,and the process of preparing the grating by the holographic ion beam etching process and the atomic layer deposition technique are introduced,and the atomic layer deposit has good conformal property.4.Based on the optimization results,a grating mask was prepared by holographic exposure,and then an etched grating was prepared on a quartz substrate using ion beam etching.The optimal titanium oxide thickness was optimized according to the specific grating groove shape,which can achieve more than 80%diffraction efficiency and polarization sensitivity less than 0.1.Finally,a prismatic immersion grating was fabricated for the 1.61 μm band.