Contamination Induced Continuous-Wave Laser Damage of Optical Elements

Abstract

Continuous-Wave laser-induced optical breakdown affects anyone whose work requires tightly focused light, high power sources, or delicate materials. It often occurs unexpectedly and seemingly randomly at optical intensities far lower than those predicted by ultra-short pulse laser experiments. Further complicating the issue is that the majority of laser damage experiments use carefully controlled laboratory conditions with short-pulsed lasers focused to small spots on clean, pristine materials. Continuous-Wave laser damage is usually attributed to contamination, and occurs under radically different conditions. To determine the origin of contamination-induced breakdown, microparticle contaminated optics were stressed using a 17 kW continuous-wave laser. Contamination-induced breakdown occurred at intensity levels many orders of magnitude lower than expected in clean, pristine materials. For both half-wave and high reflectivity coatings, damage thresholds were found to strongly follow the bandgap energy of the film. It is theorized that surface contamination heated by the laser thermally generates free carriers in the films. If the free carrier concentration exceeds a certain threshold, runaway absorption and breakdown will occur. A thermal model incorporating the particle absorption, interfacial heat transfer, and free carrier absorption was developed, and it explains the observed data. The bandgap of the film, the absorption and thermal contact of the contaminant, and the evaporation time of the particle, all determine whether a material can survive. The observed bandgap dependence is in direct contrast to the behavior observed for clean samples under continuous wave and long-pulse illumination, and, unexpectedly, has similarities to ultra-short pulse breakdown for clean samples, albeit with a substantially different physical mechanism. These findings strongly suggest that low bandgap materials are a liability in optics exposed to environmental contamination. Laser conditioning was examined as a means of preventing damage by removing contamination without initiating damage. Absorption measurements taken using photo thermal common-path interferometry show up to a 90% absorption reduction with conditioned samples. Regular laser conditioning at low irradiances can prolong the life of optics that must operate in difficult environmental conditions.