Abstract

If a diffraction-limited wave front is to be produced by an optical train, the requirements on the optical figure of individual mirror components may be very stringent. Values of λ/8 visible equivalent are common for infrared systems, and λ/100 visible equivalent has been suggested for some ultraviolet systems. The optical figure of multilayer-coated optics is determined not only by the geometrical perfection of the optical surface of the mirror but also by the uniformity in thickness of the multilayer coating. The question then arises of how to tell if a multilayer coating is uniform enough not to degrade the mirror surface on which it is coated to below optical figure specifications. The peak mirror reflectance is also of interest and is often difficult to accurately measure directly. It varies far more slowly with variations in film thickness than does the optical figure, as seen in Fig. 1, but is very sensitive to variations in film absorption coefficients. The optical figure of a coated part can be determined interferometrically, but the interferometric measurement must be made at the wavelength at which the coating is to be used. Alternate techniques are to measure the secondary structure surrounding the reflectance maximum or to use ellipsometry at the design wavelength. Typically, the ellipsometric parameter ψ is insensitive to variations in film thickness but quite sensitive to absorption in the film. The converse holds for Δ, as seen in Fig. 2. For a representative infrared high-reflectance multilayer coating, a measurable change in ψ corresponds to a change in peak reflectance of 0.001, and a measurable change in Δ corresponds to film thickness nonuniformities of less than 0.01%. An automatic ellipsometer could be readily used to scan the surface of even a large mirror. Ellipsometry may thus provide the optimum technique for testing mirror optics for reflectance uniformity and multilayer-film- induced changes in apparent optical figure.

© 1981 Optical Society of America