Light Scattering And Absorption In Infrared Laser Mirrors

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Abstract
The demands on the performance of mirrors used in high energy laser applications are severe. They may be summarized as follows: (1) excellent optical figure even under high thermal load, (2) high catastrophic damage threshold, and (3) low scattering level. Light scattering, particularly backscattering, is important, and can result in cavity depumping or damage to components in the cavity. Scattering is a consequence of microirregularities, localized imperfections, and contamination at the mirror surface. For many surfaces total integrated scatter (TIS) for visible wavelengths follows a 1/λ2 dependence, as predicted theoretically for distributed microirregularities. However, at infrared wavelengths, TIS levels often are much larger than predicted by this theory. Evidence that this additional scatter is a consequence of dust and other localized imperfections rather than distributed microirregularities will be presented. Comparison of TIS levels and angular scattering distribution measurements for several surfaces with the results that a newly derived theory using directly measured FECO surface statistical information will be made. Low absorption under thermal load is also a very desirable characteristic of laser mirrors, both because mirror heating causes loss of optical figure and because the damage threshold of the mirror surface is related to its reflectance, R, as a function of temperature, T. Typically, R 1 so that even large percentage changes in absorption with increasing tem-perature result in very small changes in dR/dT. However a new instrument has been designed to measure these small changes and measurements on a number of mirror coatings made with a precision of better than 1 x 10-4 will be presented and interpreted.

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