High precision silicon carbide aspheric mirror

Not long ago, the national major scientific research and equipment development project "4m level high precision silicon carbide aspheric mirror integrated manufacturing system" undertaken by Changchun Institute of Optics and machinery passed the project acceptance. This is the largest single SiC mirror in the world. The achievement also marks that China's optical system manufacturing capacity has reached the international advanced level, and has laid a solid foundation for the leapfrog upgrading of China's large-diameter optoelectronic equipment.

The 4-meter-diameter, 1.6-ton "big mirror" is like a huge circular disk - if it is installed in a telescope, its resolution will be greatly improved.

"Modern large aperture optical systems all adopt reflective structure, in which the aperture of the primary mirror directly determines the resolution of the system. The larger the aperture, the higher the resolution and accuracy of the optical telescope." Jiang Huilin, a project acceptance expert of 4-meter silicon carbide mirror and academician of Chinese Academy of engineering, said.
It is not easy to build large aperture mirrors[ The upper limit of mirror diameter depends on the manufacturing capacity. At the same time, in order to ensure the resolution and imaging quality of the telescope, the mirror must have a very high surface accuracy. " Zhang Xuejun, deputy director of Changchun Institute of Optics and mechanics, said that taking the visible light band observation as an example, the surface accuracy is required to be at least 1 / 30 of the wavelength, which is like magnifying a 4-meter mirror to the size of a city for land leveling, and requiring the height difference between the southeast corner and the northwest corner to be within a few millimeters of plus or minus zero, and the land flatness to be less than 1 mm. This puts forward very strict requirements for mirror blank materials and optical processing technology.
For the mirror blank, the specific stiffness and thermal stability of the material must be as large as possible, so with the increase of the diameter, the rigidity of the material can still ensure the stability of the surface, the influence of the heating environment is small, and it is conducive to reducing the weight of the system.
Quartz glass, glass ceramics, silicon carbide and beryllium are commonly used as blank materials for mirrors in the world. Silicon carbide has become a favorite candidate material for mirrors because of its excellent specific stiffness and thermal stability.
Although silicon carbide has excellent performance, fast heat conduction and small thermal deformation, it is more difficult to manufacture than other raw materials such as glass ceramics. Silicon carbide itself belongs to ceramics. Once its diameter increases, it is easy to crack or even break during sintering. It was once thought that it could not break through the 1.5m diameter limit.
"The manufacturing technology of large-diameter mirror blank and mirror processing technology have been mastered by a few Western countries, and China has never been able to independently manufacture 4-meter-sized large-diameter mirrors." Zhang Xuejun said frankly. In order to break the monopoly, Changchun Institute of Optics and mechanics has already arranged the research on optical grade silicon carbide ceramic materials in the late 1990s. The team spent more than 10 years, experienced hundreds of experimental exploration and process validation, successively broke through the 1-meter and 2-meter silicon carbide mirror blank, and finally successfully developed the 4-meter silicon carbide mirror blank in 2016.
However, the high hardness of silicon carbide also brings new challenges to the processing methods. In addition, there are fine defects on the surface of silicon carbide after optical rough polishing, which will affect the optical properties such as reflectivity, so it is necessary to improve the surface characteristics through subsequent processes.
The project team used computer-controlled optical surface forming technology, and combined processing technologies such as "stress disk" polishing and magnetorheological polishing, which greatly improved the manufacturing accuracy and efficiency of aspheric surfaces; At the same time, advanced detection technologies such as swing arm profilometer detection and optical zero position compensation interferometry are used to realize the in-situ detection of 4m mirror. Finally, the reflectance of the mirror after polishing reaches more than 95%, which meets the engineering application conditions, and realizes the high-precision optical processing of 4m large aperture aspheric mirror.
"Only mastering the manufacturing process of the 4m mirror is not an independent mastery of the core technology." Zhang Xuejun said that as important as the processing technology is the research and development of a full set of manufacturing equipment needed to complete the manufacturing of the 4m mirror. Around the mirror development process, the project has completed the development of three subsystems and more than 10 sets of processing and testing equipment, all from independent intellectual property rights.
At present, the 2m mirror developed by Changchun Institute of Optics and machinery has been applied in practical engineering; In 2022, large aperture mirrors independently developed by China will be used in the multifunctional optical facilities of China's space station; In the near future, the 4m mirror will also be used in the new generation of photoelectric observation system in China.
Since then, China's optical system manufacturing capacity has reached the international advanced level.
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