High Precision Silicon Carbide Aspherical Mirrors

Not long ago, by the Changchun Institute of Optical Machinery undertaken by the national major scientific research equipment development project “4 meters scale high-precision silicon carbide aspheric mirror integrated manufacturing system” through the project acceptance. This is the world's largest publicly reported aperture silicon carbide single mirror. The achievement of the results, also marks China's optical system manufacturing capacity among the international advanced level, for China's large-caliber optoelectronic equipment across the upgrade has laid a solid foundation.

The diameter of 4 meters, weighing 1.6 tons of “big mirror” is like a huge ring-shaped disk - if it is mounted on the telescope, will greatly enhance its resolution.

“Modern large-caliber optical systems are used in reflective structures, in which the main mirror aperture directly determines the system's resolving power, the larger the caliber, the higher the resolution and precision of the optical telescope.” 4-meter silicon carbide reflector project acceptance experts, academician of the Chinese Academy of Engineering Jiang Huilin said.

It is not easy to build a large aperture reflector. “The upper limit of the reflector's aperture depends on the manufacturing and processing capabilities. At the same time, in order to ensure the resolution and imaging quality of the telescope, the reflector must have a very high surface precision.” Changchun Institute of Optical Machinery, deputy director of Zhang Xuejun introduction, visible wavelength observation, for example, the surface precision requirements of at least 1/30 above the wavelength, which is like a 4-meter reflector enlarged to the size of a city, land leveling, and requires the southeast corner and the northwest corner of the height difference between plus or minus a few millimeters, the land leveling should be less than 1 millimeter. This reflects the mirror mirror blank material and optical processing technology have put forward extremely stringent requirements.

For reflector mirror blanks, the specific stiffness and thermal stability of the material must be as large as possible, so that with the increase in aperture, the rigidity of the material can still ensure the stability of the face shape, less affected by the thermal environment, and is conducive to reducing the weight of the system.

Internationally commonly used mirror mirror blank materials are quartz glass, microcrystalline glass, silicon carbide, beryllium metal, etc. Silicon carbide with its excellent specific stiffness and thermal stability has become the favorite mirror alternative materials.

Although silicon carbide has excellent performance, fast thermal conductivity, heat deformation is small, but compared to other raw materials such as microcrystalline glass, its manufacturing difficulty is also greater - silicon carbide itself belongs to ceramics, once the caliber increases, the sintering is prone to cracks or even broken, once thought to be unable to break through the limit of the caliber of 1.5 meters.

“Large-caliber mirror mirror blank manufacturing and mirror processing technology has been mastered by a few Western countries, China has never had the ability to independently manufacture 4-meter-class large-caliber mirrors.” Zhang Xuejun frankly. In order to break the monopoly, in the late 1990s, Changchun Institute of Optical Machinery has laid out the optical grade silicon carbide ceramic materials research. The team spent more than 10 years, experienced hundreds of experimental exploration and process verification, has broken through 1 m, 2 m caliber silicon carbide mirror mirror blank, finally in 2016 successfully developed a 4 m caliber silicon carbide mirror blank.

Mirror blanks have, but the extremely high hardness of silicon carbide also brings new challenges to the processing method. In addition, silicon carbide has minor defects on the surface after optical rough polishing, which can affect optical properties such as reflectivity, and thus the surface properties need to be improved by subsequent processes.

The project team used computer-controlled optical surface forming technology, through the use of “stress disk” polishing, magnetorheological polishing and other combinations of processing technology, greatly improving the manufacturing accuracy and efficiency of the aspherical surface; while using the pendulum arm profilometer detection, optical zero compensation interferometry and other advanced detection techniques, to achieve the in-situ detection of 4-meter mirrors. Finally, after fine polishing of the mirror coating reflectivity reached more than 95%, with engineering application conditions, to achieve a 4-meter large-diameter aspheric mirror of high-precision optical processing.

“Only mastered 4 meters mirror manufacturing process, and is not considered independent mastery of the core technology.” Zhang Xuejun said, with the processing technology is equally important, there is a complete set of manufacturing equipment required to complete the 4-meter mirror manufacturing research and development. Around the mirror development process, the project completed three subsystems, more than 10 sets of processing and testing equipment development, all from independent intellectual property rights.

At present, by the Changchun Institute of optical research and development of 2-meter magnitude aperture mirrors have been applied in practical engineering; 2022, in the Chinese space station's multi-functional optical facilities, will use China's self-developed large aperture mirrors; in the near future, the 4-meter magnitude mirrors will be applied to a new generation of optoelectronic observation systems in China.

From now on, China's optical system manufacturing capacity among the international advanced level.

Changchun Bena is a quality supplier certified by Changchun Institute of Optical Machinery.