SPIE Journal Paper | 18 December 2018
KEYWORDS: Coating, Thin films, Laser induced damage, Deposition processes, Refractive index, Mirrors, Laser damage threshold, Polarization, Resistance, Hybrid fiber optics
The thin film damage competition series at the Boulder Damage Symposium provides an opportunity to observe general trends in laser damage behavior between different coating types (high reflector, antireflector, polarizer, and Fabry–Perot filter), wavelength ranges (193 to 1064 nm), and pulse length ranges (40 fs to 18 ns). Additionally, the impact of deposition process, coating material, cleaning process, and layer count can be studied within a single year or more broadly across the history of this competition. Although there are instances where participants attempted to isolate a single variable to better understand its impact on laser resistance, this series of competitions isolates the variable of the damage testing service and protocol for a wide variety of participants to enable the observation of general trends. In total, 275 samples from 58 different participants have been tested at four different laser damage testing facilities over the last 10 years. Hafnia was clearly the best high refractive index material except for ultraviolet (UV) applications, although a wide range of high refractive index materials performed well. The best deposition process varied significantly between the different competitions. The best deposition process was dependent on the coating type, wavelength, and pulse duration. For 1064-nm coatings with nanosecond scale pulse lengths, e-beam coatings tended to be the best performers. For short-pulse length NIR mirrors and nanosecond pulse length UV mirrors, densified coating processes, which all involved sputtering of the target material, were the best performers. For UV antireflector (AR) coatings and excimer mirrors, both tested at nanosecond pulse lengths, they tended to favor very low energetic deposition methods yielding soft coatings, such as sol gel dip coating for the AR and resistive heating of fluorides for the excimer mirrors. Finally, cleaning method and layer count had a less obvious correlation with laser resistance over the history of this thin film damage competition series.
