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    Multilayer Thickness Uniformities Required to Meet Wave Front Error Tolerances in Laser Mirrors

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    The wave front error tolerance per mirror in typical laser optical trains is quite severe. Expressed in terms of an equivalent figure error in the visible region, values of an eighth wave are not uncommon for mirrors in infrared systems, and values of one hundredth of a wave have been suggested for mirrors in some ultraviolet laser optical trains. The effect of figure error, i.e., error in the geometrical shape of the mirror, is well known and for a metal-coated mirror is the only one of significance. However, if the mirror has a multilayer dielectric coating, an additional apparent figure error may be introduced by the physical thickness and phase change differences produced by coating thickness nonuniformities. This source of wave front error has been largely unrecognized but can be significant. For example, a 2% variation in thickness uniformity for a typical infrared multilayer coating may create a wave front error at the operating wavelength equal to that produced by a mirror whose optical figure is in error by an eighth wave in the visible. The entire wave front error tolerance for the mirror may thus be used up by a slightly nonuniform coating. Such wave front errors usually cannot be determined unambiguously by interferometric measurements made in the visible region. If multilayer dielectric mirrors are used, the wave front error should thus ideally be measured at the operating wavelength, often a difficult task. A discussion is given of thickness uniformity requirements for dielectric multilayer films and possible ways to achieve this uniformity.


    Film thickness nonuniformity, high power laser mirrors, multilayer dielectric films, optical figure measurement, vacuum deposition techniques, 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/STP37027S