Design of encapsulated transmission grism with high diffraction efficiency and low polarization

In view of the problems that the existing subwavelength transmission grating structure parameters require high processing technologies and are difficult to prepare, this paper explores and investigates the design simulation of encapsulated grism grating based on the finite element method (FEM). The variation of diffraction efficiency and polarization sensitivity of encapsulated grism is explored for different groove structures, trench depth and duty cycle, according to the requirements of high diffraction efficiency and low polarization sensitivity. The grating surface etched onto a fused silica substrate is formed by binary structure of grooves and trenches filled by a high refractive index multi-layer coating, working at Littrow configuration in the SWIR-1 (1590-1625nm) and SWIR-2 (1635-1670nm). The simulation results show that the average diffraction efficiency exceeds 85% and the polarization sensitivity is less than 5% with a wide tolerance range when the depth-period ratio is about 2 and the duty cycle is around 0.6. The diffraction efficiency and polarization sensitivity meet the design requirements, substantially improving the efficiency of transmission grating design and processing. This enables compact optical design to achieve high signal-to-noise ratio and low stray light to meet the critical radiation measurement accuracy requirements.