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    Sensitive Technique for Measuring Apparent Optical Figure Error Caused by Coating Nonuniformity

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    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. Peak-to-valley values of λ/8 visible equivalent are found for some infrared systems, λ/20 for visible systems, and λ/100 visible equivalent has been suggested for some ultraviolet systems. These mirrors often have high reflectance multilayer coatings, in which case the optical figure is determined not only by the geometrical perfection of the optical surface but also by the uniformity of the multilayer coating. Although this uniformity can be determined interferometrically or by measuring the secondary structure surrounding the reflectance maxima, the most sensitive technique for determining film nonuniformities appears to be ellipsometry. It should be performed at the wavelength at which the mirror is to be used. Typically, the ellipsometric parameter Δ is found to be very sensitive to variations in film thickness but quite insensitive to absorption in the film; the converse holds for Ψ. For a representative infrared, high reflectance multilayer coating, a measurable change in Δ corresponds to film thickness nonuniformities of less than 0.01%, and a measurable change in Ψ corresponds to a change in peak reflectance of 0.001. An automated 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 multilayer-film-induced changes in apparent optical figure. It also provides a sensitive technique for measuring the uniformity of reflectance of large, multilayer-coated mirrors.


    ellipsometry, film thickness nonuniformity, multilayer dielectric films, optical figure measurement, wave front distortion

    Author Information:

    Bennett, HE
    Michelson Laboratory, Physics Division Naval Weapons Center, China Lake, California

    Burge, DK
    Michelson Laboratory, Physics Division Naval Weapons Center, China Lake, California

    Committee/Subcommittee: F01.02

    DOI: 10.1520/STP37265S