Abstract
The laser damage thresholds of optical coatings can degrade over time due to a variety of factors, including contamination and aging. Optical coatings deposited using electron beam evaporation are particularly susceptible to degradation due to their porous structure. In a previous study, the laser damage thresholds of optical coatings were reduced by roughly a factor of 2 from the years 2013 to 2017. The coatings in question were high reflectors for 1054 nm that contained SiO2 and HfO2 and/or TiO2 layers, and they were stored in sealed PETG containers in a class 100 clean room with temperature control. At the time, it was not certain whether contamination or thin-film aging effects were responsible for the reduced laser damage thresholds. Therefore, to better understand the role of contamination, the coatings were recleaned and the laser damage thresholds were measured again in 2018. The results indicate that the contamination played the most dominant role in reducing the laser damage thresholds of these optical coatings, even though they were stored in an environment that was presumed to be clean.
Highlights
Optical coatings with high resistance to laser damage are integral to the operation of the Z-Backlighter laser facility[1] at Sandia National Laboratories
TiO2 has a lower bandgap and exhibits lower laser-induced damage thresholds (LIDTs) compared to HfO2.5 We discovered that the LIDTs of both HfO2- and TiO2-based mirror coatings from 2013 degraded even more after the coatings were stored for 4 years in sealed polyethylene terephthalate-G (PETG) containers in a temperature-controlled, class 100 clean room environment.[6]
The coating containing 21 TiO2 layers (0 HfO2 layers) has an high reflection (HR) bandwidth of 232 nm, whereas the coating containing 0 TiO2 layers (21 HfO2 layers) has an HR bandwidth of 77 nm (HR bandwidth is taken as interval, where transmission is
Summary
Optical coatings with high resistance to laser damage are integral to the operation of the Z-Backlighter laser facility[1] at Sandia National Laboratories. Our large optics coating system[2] utilizes electron beam evaporation to produce optical coatings that usually consist of HfO2 and SiO2 layers to achieve high laser-induced damage thresholds (LIDTs). This was accomplished by cleaning the aged coatings and measuring the LIDTs again to determine if laser damage resistance was improved by the cleaning
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