Dual-wavelength interferometry based on dispersion

Abstract

Interferometry is commonly used for an optical element surface accurate test. But the testing dynamic range is affected by the ambiguity of 2π. To solve this problem, dual-wavelength interferometry testing method has been proposed. The current dual-wavelength interference system usually uses two different wavelength monochromatic lights to work time-sharing and obtain their interference patterns respectively, which makes the system complex and measurement time-consuming. In this paper, we put forward a dual-wavelength testing method based on Michelson interference system. It enables simple and efficient extraction of the phase distribution of the tested optical element surface to be realized in a sub-millimeter scale dynamic range with a nanometer accuracy. A sodium lamp has two different wavelengths, 589 nm and 589.6 nm, it is selected as the light of our interference measurement system, so the equivalent wavelength is 0.579 mm. A dispersion element is adopted to make the interference patterns which correspond to 589 nm and 589.6 nm can be separated. Furthermore, in order to eliminate the influence of background light intensity on the interference patterns processing, we do Fourier transform for the patterns recorded by CCD to extract the spectral component related to the tested phase. And then, an inverse Fourier transform for this component is done to obtain the phase distribution. Finally, the tested optical element surface can be obtained from the phase distribution. Simulations have been done to validate the feasibility of the method. The test error of the surface profile is 0.252 nmRMS. The simulations prove that this method can guarantee high accuracy and expand the detection range.