Challenges and strategies in high-accuracy manufacture of the world's largest SiC aspheric mirror

Abstract

In consideration of the difficulties in the high-accuracy manufacture of the world's largest aperture SiC (silicon carbide) aspheric mirror, such as the difficulty of processing complex mirror blanks, low accuracy and efficiency of conventional processing, print-through effect, inconsistent processing and testing states, high stress/poor denseness of conventional cladding, the insufficient dynamic range of conventional measurement, and air turbulence of long optical path testing, we proposed the water-soluble room temperature vanishing mold and gel casting technology, homogeneous microstructure reaction-formed joint technology, nano-accuracy efficient convergence processing technology, gravity unloading technology, high-denseness low-defect PVD (Physical Vapor Deposition) cladding process, data fusion testing technology, time-domain averaging method, etc. On the basis of the proposed key technologies and methods, we have currently developed the world's largest SiC aspheric mirror with the size of ⌀4.03m. The impressive performance of the SiC aspheric mirror is verified as follows. The area density of the SiC blank is less than 120kg/m2; surface error testing accuracy is less than 6nm RMS; the thickness inhomogeneity of cladding layer is less than 5%; the final surface error and roughness are 15.2nm RMS and 0.78nm RMS, respectively.

Modeling the wavefront degeneration and optimizing the CGH interferometric null test

“…Interferometric null test is the most effective way to reach ultra-high accuracy of measuring large and complex optical surfaces [1]. To test the free-form or other complex optical surface, people usually design a computer-generated holograms (CGH) as null correctors to generate wavefront matching the surface under test (SUT).…”