Research On Key Technologies Of Thin Film Element Based On Diffractive Imaging System

Optical remote sensing camera with high resolution is the current hot research areas of space optics in the world.Geostationary orbit based observation camera with high time resolution and ultra-width observation is one of the important tools to obtain spatial information in the field of aerospace remote sensing of earth.Though geostationary orbit optical remote sensing can realize continuous observation of the ground about 1/3 of the earth's surface,the ground pixel resolution will be greatly reduced.The most effective way to improve the resolution of space camera is to increase the focal length and enlarge the aperture.To achieve 1 m pixel resolution in geostationary orbit,the caliber of the camera needs to reach 20 m.With the increase of caliber,the support and control structure is essential because its weight and diameter is proportional to the square of.As a result,it has become increasingly difficult for traditional reflective space telescope system whether manufacturing technology,or the ability to launch and space expansion,adjustment technology.With the increase of the diameter,the control becomes more and more complex and the cost of the traditional optical system including hardware,software,and system integration and testing is increasing rapidly.Compared with the reflective optical system,diffraction imaging system based on the thin film material as the system of the objective lens can achieve large diameter,high resolution,lightweight structure,space expandable,low surface error requirement,easy to copy and so on;not only can save Launch costs,but also can reduce manufacturing costs significantly,diffraction imaging technology has great potential applications in the field of geostationary orbit high-resolution imaging.In this paper,a thin film based diffraction imaging system,from design to component processing is carried out in-depth study;the main contents are divided into the following aspects:1.The diffractive telescope system was designed and optimized by using the Zemax optical design software.Based on the Schupmann achromatism principle,the chromatic aberration of the diffractive objective lens of the calibration mirror group was designed and the waveband of the system was broadened.The diffractive telescope with aperture with 5 meter is designed and the imaging performance of the system is close to the diffraction limit.At the same time,a diffractive telescope prototype with 300 millimeter aperture is designed to verify the imaging performance of the diffractive telescope system.The machining tolerances,position tolerances and stitching tolerances in the diffractive imaging system are analyzed to verify the feasibility of the diffraction element applied to the large diameter telescope system.2.A processing platform based on polyimide film material is constructed which can used to manufacture large aperture membrane diffractive element.The process flow of membrane diffraction elements with lightweight and high diffraction efficiency is established and optimized.The fabrication process of the replication template of fused silica glass substrate was studied by combining the laser direct writing and Ar ion beam physical etching.We use the transparent organic material—polyimide as the substrate materia.The design structure was transferred to the surface of the polyimide film by glass template.According to the process of exploration,two diffractive elements with diffrerent substrate of 300 millimeter effective aperture is fabricated.3.The fabrication errors of the multi-step diffractive membrane elements in the whole process is analyzed and improved.The fabrication errors are divided into two part: one is introduced in the process of replicate substrate fabrication and the other on is introduced by the film transfer process.The fabrication errors of replicate substrate can be devided as etching depth errors,alignment errors and linewidth errors.The fabrication errors of transfer errors can be devided as linewidth transfer errors and depth transfer errors.The relationship between the fabrication errors and the diffraction efficiency of the membrane diffractive elements are analyzed by using the Matlab software simulation.4.The diffractive elements with different substrate are tested to evaluate their optical performance.The diffraction path is designed and constructed to test the diffraction efficiency of the thin film diffraction element.The diffractive efficiency are tested and the model of the relationship between the fabrication errors and the diffractive efficiency is verified.The diffractive telescope system is built and the imaging performace of the system is tested by the star image and the resolution plate.The work of this paper shows that the diffraction imaging system based on membrane provides a new idea for the future space-based telescopes in geostationary orbit with large diameter and high resolution.The diffractive membrane produced by the template replication method has the characteristics of lightweight,stable performance,excellent optical performance and high diffraction efficiency.The work of this paper provides a theoretical and technical basis for the study of future membrane diffraction imaging system with large-scale aperture.