Ultra Low Loss Coatings of KCl Laser Windows.

In the last few years, several low optical absorption 10.6 micrometer AR coatings for KCl have been developed at Hughes Research Laboratories (HRL) and at other laboratories. The objective of this program was the development of 9.27 micrometer AR coatings which have a film absorption loss of less than 0.01% and a reflection of less than 0.1% per surface. The achievement of the program objective will permit the development of a 9.27 micrometer AR coating for a 32.5 cm diameter polycrystalline KCl window. Because previous studies had shown that castings deposited under ultra-high vacuum (UHV) conditions had lower absorption losses than did coatings deposited in conventional vacuum systems, the present study was limited to UHV-deposited films. Mass spectrometer analysis had shown that impurities were evolved occasionally during the evaporation of film materials. As a consequence, residual gas analysis (RGA) was performed to ensure that impurities were not present in the system. Also if impurities were located in the film materials, either they were removed by reactive atmosphere processing (RAP) or techniques were developed to prepare clean material.