Research into the Causes of Laser Damage to Optical Components

摘要

The dielectric mirrors forming the resonators of high power laser devices are obten 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. It is thought that these coatings fail because they absorb light at 6943A. An absorption coefficient of 65/cm was measured from the bulk zinc sulfide used in these coatings. Using this absorption coefficient, it was 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 the film, the coating fractures. Experiments made to check this calculation, show an increase durability as the residual stress is reduced. (Author)