Traditional large aperture optical flat measurement methods are mainly interferometric measurement methods. The measurement accuracy of these methods is restricted by the interferometer’s reference surface accuracy; therefore, it is necessary to investigate more accurate methods for system calibration and the elimination of systematic error. The absolute measurement method can separate the reference surface error from the measurement result to obtain the absolute surface of the optical component, and surface shape measurements with sub-nanometer precision can be obtained.

Recently, the research group from Shanghai Institute of Optics and Fine Mechanics(SIOM), Chinese Academy of Sciences, had proposed a novel absolute measurement method based on multisurface interference for a 600 mm aperture interferometer. The result was published in Optics and Lasers in Engineering.

The researchers put forward a novel absolute measurement method based on multisurface interference for a 600 mm aperture interferometer. Compared to existing 3-flat testing methods, which generally require two transmission flats and one reflection flat, the proposed method does not require the replacement of the transmission flat with another flat and rotation of the reflection flat about the z-axis of the 600 mm aperture interferometer. Only by rotating an auxiliary parallel flat about the z-axis can we realize the in situ absolute measurement of the reference flat surface error of the interferometer without losing the medium-frequency error.

Repeatability experiments were carried out on the 600 mm aperture interferometer produced by Zygo. The experimental results showed that the RMS value of the repeatability precision was less than 2 nm. The wavefront residual RMS between our measurement results and the Zygo’s measurement results was better than 3nm.

The research results of this project can not only improve the measurement accuracy of existing 600mm aperture plane interferometer, but also provide theoretical and technical support for the establishment metrology of 600mm aperture standard flat in the industry and at the national level, which has certain scientific significance and practical value.

Schematic diagram of the absolute measurement principle. (Image by SIOM)

Measurement results of flats A (a), B (b), and C (c). (Image by SIOM)

Difference between new method and Zygo’s method for flats A (a) and B (b). (Image by SIOM)

Article website:
https://www.sciencedirect.com/science/article/pii/S0143816619314472?dgcid=author

Contact:
Mr. CAO Yong
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: caoyong@siom.ac.cn