Fused Silica Transmission Gratings With High Aspect Ratio

As important instruments for space astronomy and deep space exploration,optical telescopes are effective tools to explore the origin of the universe,the formation and evolution of galaxies.To capture and measure farther,deeper and weaker celestial objects,the research of large aperture and super large aperture optical telescopes are being committed at home and abroad.Diffraction grating is the core spectroscopic element of telescope spectrograph.Volume phase holographic grating(VPHG)has become the preferred spectroscopic element due to the high peak diffraction efficiency.However,the grating has some disadvantages,such as narrow band width,obvious polarization effect at high resolution,high absorption of UV band materials,complex fabrication process and strict environmental requirements.In the past 20 years,the University of science and technology of China has been committed to the research of laser pulse compression grating in inertial confinement fusion and has developed holographic-ion beam etching technology.Based on the technology,to meet the requirements of high diffraction efficiency,high resolution and stable performance of gratings in astronomical telescope spectrometers,this paper proposes fused silica transmission grating with high aspect ratio,which makes up for the shortcomings of VPHG.The main work of this paper includes:1.Design and analyze the parameters of the fused silica gratings.Aiming at the wavelength range of the diffraction gratings of the astronomical telescope spectrograph,a non-polarized rectangular groove fused silica gratings with high diffraction efficiency are designed,and the theoretical peak diffraction efficiency of the-1st order can reach nearly 100%.The allowable manufacturing tolerance range in the process is determined.Taking various VPHGs used in the TMT telescope spectrograph as examples,the two kinds of gratings are compared theoretically and technologically.2.Technological exploration of fused silica gratings with high aspect ratioThe high-line-density gratings with large filling factor and high aspect ratio were fabricated by holography-ion beam etching technology,sputtering chromium plating,atomic layer deposition and other processes.The measured peak diffraction efficiency reaches 94%and results show that there is a 130 nm wavelength bandwidth and a 12°AOI(Angle of incident)bandwidth when the diffraction efficiency is higher than 70%.Due to the advantages of smooth and clean grating grooves and holographic exposure,the average stray light influence of the gratings in the band range is less than 0.5%.3.Exploration of reactive ion beam etching of fused silica gratings.Reactive ion beam etching technology is selected to transfer the mask shape to the substrate.By analyzing the influence of etching parameters on the etching morphology,comparing the deep etching results of gratings with different linear density and the morphological changes of photoresist mask during the downward transfer,the high aspect ratio etching technology of fused silica is systematically explored.Taking the defective grooves in the experiment as examples,the influence of etching error on diffraction efficiency is analyzed.