Surface form error measurement for rough surfaces using an infrared lateral shearing interferometry. (May 2022)

• Record Type: Journal Article
• Authors: Song, In-Ung; Yang, Ho-Soon; Kim, Goeun; Kim, Sug-Whan
• Is Part Of: Optics and lasers in engineering. Volume 152(2022)
• Publication Date: 2022-05
• Subjects: Surface metrology -- Segmented mirror -- Quadri-wave lateral shearing interferometry -- Large aspheric mirror -- Rough surface measurement; Lasers in engineering -- Periodicals; Optical measurements -- Periodicals; Optics -- Periodicals; Lasers en ingénierie -- Périodiques; Mesures optiques -- Périodiques; Optique -- Périodiques; 621.36605
• DOI: 10.1016/j.optlaseng.2022.106947
• Languages: English
• ISSNs: 0143-8166

Abstract

Highlights: A new metrology tool for measurement of SFE for rough surfaces is proposed. The system was applied to the fabrication of an OAP mirror of 1.1-m-diameter. ISFEM offers solution for high throughput to make large/multiple segmented mirrors. Abstract: The fabrication of a large monolithic mirror or a segmented mirror requires a high throughput that satisfies the time-cost requirement. Particularly, the rough surface properties of grinding pose a challenge for accurate measurement of the surface figure error (SFE). If the SFE during grinding is as small as possible, the material removal volume in the figuring process gets reduced, and the total required time of the fabrication is greatly minimized. In this paper, we propose a new metrology tool called intermediate surface figure error measurement (ISFEM) that can be applied to measurement of SFE for rough surfaces. It consists of a CO2 laser of wavelength 10.6 µm as a light source, a lateral shearing interferometer as a wavefront sensor, and several optical components for beam shaping and imaging. The ISFEM demonstrated its ability to measure the microns to sub-micron rms SFE of the ground surface by testing the samples with a 0.59 µm -1.56 µm rms of surface roughness and a large SFE of about 3 µm rms. The system was applied to the fabrication of an off-axis parabolic mirror of diameter 1.1 m, where it successfully performed as an intermediate metrology between laser tracker measurement for the rough surface and interferometric measurement for the polished surface. The ISFEM obtained measurements from 3.88 µm rms SFE during the fine grinding process and down to 0.38 µm rms during the polishing process. Therefore, we confirmed that the ISFEM provides a unique solution for high throughput for the fabrication of large or multiple segmented mirrors.