<title>Coating requirements for the reference flat of a Fizeau interferometer used for measuring from uncoated to highly reflecting surfaces</title>

Abstract

Fizeau interferometers are traditionally used to determine the surface figure of uncoated optics. Since the reflectances from the uncoated reference and test surfaces are usually equal, the intensity of the interference fringe minimum is zero, so the fringe contrast is unity. If the same reference surface is used to measure the figure of a higher reflecting surface, the fringe minimum intensity is no longer zero and the fringe contrast decreases, eventually reaching zero for 100 percent reflecting optics. The problem can be overcome if the reference surface is coated with a lossy reflecting surface. We describe a coating to measure the figure of optical components having reflectance from 100 percent to 3.5 percent at 1064 nm. The spatial variations across the 150 mm working aperture in physical thickness of the two materials used to coat the reference surface were determined to an accuracy better than 1 nm using a single wavelength ellipsometer operating at 633 nm. Variations across the aperture of the phase changes on reflection from both sides of the reference flat substrate and on transmission were estimated. These results are presented together with calculations designed to determine the relative positions of the outside of the coating with respect to substrate surface as well as the position as seen optically. Substrate distortion due to stress in the coatings has also been measured.