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RESEARCH INTO THE CAUSES OF LASER DAMAGE TO OPTICAL COMPONENTS.
Final rept. 1 Dec 64-28 Feb 65,
PERKIN-ELMER CORP NORWALK CT ELECTRO-OPTICAL DIV
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The dielectric mirrors forming the resonators of high power laser devices are often damaged by the laser radiation. Coatings of zinc sulfide and thorium oxifluoride, the most common type of resonator mirrors used, have a threshold for damage of about 5 joules sq. cm. for a 50 nanosecond pulse. We think these coatings fail because they absorb light at 6943A. We have measured an absorbtion coefficient of 65cm from the bulk zinc sulfide used in these coatings. Using this absorbtion coefficient, we have calculated that the temperature in a thin film should rise several hundred degrees in a single pulse and cause sudden local mechanical stress. When the sum of this stress and the residual stress exceeds the surface tension of film, the coating fractures. Experiments made to check this calculation, show an increase durability as the residual stress is reduced. The factors controlled to reduce the residual stress were the use of soft as opposed to hard materials, evaporation rate, order of deposition of materials and temperature during test. Of the soft coatings we expected would show an improvement, cryolite, chiolite or thorium oxifluoride combined with lead fluoride were found to be the most durable. The damage threshold of these coatings was between 25 and 50 joulessq. cm. This is a 5 to 10 time improvement in the durable coating art. Author
APPROVED FOR PUBLIC RELEASE