Vibration-resistant interferometric measurement of optical surface figure and roughness

Abstract

Interferometer is a powerful tool for optical surface measurement, including figure and roughness, due to its nanometer accuracy and non-contact manner. Traditional phase-shifting interferometry (PSI) is much sensitive to environmental vibration that impairs its application in measurement in workshop or on machine. Based on the iterative algorithm that is tolerant to phase-shifting error caused by vibration, two interferometers are developed to measure the optical surface figure and roughness respectively. A laser interferometer, of which the aperture size is 150mm, has been built to measure the surface figure. Practical test demonstrates that the laser interferometer achieves accuracy better than 5nm under vibration of 0.4 micron-amplitude over a large frequency range, 0-35Hz. And an interferometric microscope has been proposed to measure the surface roughness and verified to be effective. The measuring area of the microscope depends on the employed interference objective, and a typical value is about 1 squared millimeter. The error of measured roughness (Sq) under vibration, 0.4 micron-amplitude and over 0-20Hz frequency range, is less than 0.5nm. The developed method and instruments could be applied to optical surface measurement in vibration. The study relaxes the requirement of interferometers on environment and predicates an in-workshop or on-machine solution for optical surface measurement.