Metal aspheric mirror processing technology

Metal mirrors can be fully lightweight designed, and optical-mechanical integration installation and adjustment can be designed to simplify the structure of the back support of the mirror. 

Metal materials are generally obtained by casting. Metal materials have good thermal conductivity, relatively large coefficient of expansion, and are easily affected by temperature. However, metal materials have the advantages of good processing technology and short procurement time. Commonly we use in the Optical System are Aluminum Aspheric Mirror and Ni-P Alloy aspheric mirror.

The key points of processing technology：

1.Single-point diamond turning is an indispensable and important process for metal mirror processing. Ultra-precision turning technology (STDP technology) uses diamond turning tools to perform deterministic material removal on rotating metal mirrors. After turning, the surface accuracy can be reached better than λ/10 (λ=632.8nm), and the surface roughness can be better than 2nm. As the below picture shows.

2.Although the metal mirror can obtain better surface accuracy and roughness after turning, due to the principle of turning, there will be turning tool marks on the surface of the metal mirror, even if it is invisible to the naked eye, it will cause certain damage to the optical system. Therefore, after turning, the surface needs to be polished. Some metal mirrors will be coated layers on their surfaces (such as Be-Al alloy mirrors, domestic 6061 mirrors, etc.), and then polish after coating. As the below process shows.

3.The hardness of metal mirrors is relatively low, and scratches are easy to occur on the surface during the polishing process, which can also accelerate surface oxidation. It is difficult to achieve a good mirror surface, and a common solution is to do metal coating on the surface. The difference in thermal expansion coefficient between the mirror substrate and the modified layer should be taken into account to avoid the bimetallic effect (after coating, the mirror surface accuracy changes with temperature). The commonly used metal mirror surface coating process is Ni-P layer which has uniform density and high hardness. After polishing, high-precision surface accuracy and roughness can be achieved.

4.After single-point diamond turning of the metal mirror, the surface accuracy and  roughness are relatively high, and it can directly enter the interferometric testing stage. According to the design of the testing optical path, the laser interferometer, computer-generated hologram (CGH), and adjustment frame are arranged for interferometric testing. After the surface accuracy meets the requirements (for example, λ1/50, λ=632.8nm), the mirror can be coated and delivered.