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    Instrumentation of a Variable Angle Scatterometer

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    The problem of light scatter from optical surfaces is amplified to a critical level for the optics used in Ring Laser Gyros (RLG). Preparation of these optics is slow, laborious and very expensive. Further, the resulting surfaces are very fragile and extremely difficult to handle without damage. These concerns have led to the development of a scatterometer at the RLG Lab, Wright-Patterson AFB, which can detect light scatter from the so-called “supersmooth” optics used in RLG's without first overcoating with metals. A HeNe laser is used to illuminate a 0.5mm diameter spot on the surface of the test mirror or mirror substrate. The incident angle can be varied to accommodate different mirror designs. The test piece can be maneuvered with five degrees of freedom, four of which are computer controlled to facilitate scanning. Scatter measurements are made with a photomultiplier detection system which has a sensitivity of ten parts per billion per steradian. For these optics the standard process for determining substrate surface quality prior to applying the dielectric coatings is to coat the surface with silver and make a scatter measurement. The silver is then removed by etching and the dielectric coatings are applied. Using the scatterometer we have been able to measure surface scatter without first coating the surface with silver. This equipment has yielded evidence that virtually all the techniques now used to characterize these surfaces damage them, including the technique of coating with silver. Subjecting these optics to vibration, thermal cycling, multiple cleaning steps of almost any type and some classes of shipping containers, all seem to attack the Beilby layer in a way that increases the scatter substantially.


    light scatter, scatter measurement, scatterometer, surface damage, surface quality, surface scatter

    Author Information:

    Stowell, WK
    Air Force Wright Aeronautical Laboratories,

    Orzio, FD
    Air Force Wright Aeronautical Laboratories,

    Silva, RM
    Air Force Wright Aeronautical Laboratories,

    Committee/Subcommittee: F01.02

    DOI: 10.1520/STP37243S