Design and Fabrication of a Non-Polarized Transmission Grating with High Diffraction Efficiency

Objective Spectroscopy is a technique for measuring light intensity across ultraviolet, visible, near- infrared, and infrared bands. In astronomical telescope systems, spectrographs operating in the visible light band enable the study of individual stars in neighboring galaxies, exoplanets in the Milky Way, black holes, and neutron stars. The core component of a spectrograph is the diffraction grating, which should possess high diffraction efficiency, wide bandwidth, and low polarization sensitivity. Current diffraction gratings used in large astronomical telescopes include volume phase holographic gratings (VPHGs) and surface relief gratings (SRGs). While VPHGs offer compact design and high diffraction efficiency, they are significantly affected by environmental factors such as temperature and humidity. They also have limitations such as narrow bandwidth, reduced efficiency in non- polarized modes, and complex fabrication processes. To address these issues, we design and fabricate a stable fused silica encapsulated grating that exhibits high diffraction efficiency and low polarization sensitivity for spectral detection in the visible light band. Methods For the design and fabrication of encapsulated gratings, we first use the modal method to determine the optimal grating groove depth and duty cycle. We then design an anti- reflective film, model the encapsulated grating structure using finite element software, and calculate the diffraction efficiency and polarization sensitivity of the optimal structure. A TiO2 material encapsulated grating is fabricated using holographic ion beam etching combined with atomic layer deposition coating technology. Finally, we conduct efficiency testing on the encapsulated grating, using finite element software to model the actual morphology and calculate the theoretical diffraction efficiency, comparing these results with experimental data to analyze error sources. Results and Discussions We propose a fused silica encapsulated grating for visible light spectral observation that offers high diffraction efficiency, low polarization sensitivity, and wide bandwidth. The grating features rectangular grooves, TiO2 material encapsulated within the grooves, and Al2O3 and SiO2 film layers, enhancing transmittance. The results show that the non- polarized peak diffraction efficiency of the theoretical encapsulated grating is 96.8 degrees o, with a bandwidth of 47 nm where diffraction efficiency exceeds 90 degrees o, and polarization sensitivity less than 5 degrees o. The actual non- polarized peak diffraction efficiency of the fabricated grating is approximately 91 degrees o, with a bandwidth of 37 nm where diffraction efficiency exceeds 85 degrees o, and polarization sensitivity less than 7 degrees o. Conclusions We present a grating structure design that encapsulates TiO2 transmission gratings to achieve high diffraction efficiency while simplifying fabrication. The fabricated grating's experimental values align well with theoretical predictions. Utilizing the modal method, we design the grating groove structure parameters and use finite element analysis to obtain optimal grating parameters, focusing photon energy on the-1 order. Our design also shows that TiO2 encapsulated gratings reduce the groove depth and production complexity. The resulting grating has high efficiency, large bandwidth, and low polarization sensitivity, providing valuable insights for the future development of surface relief gratings.