Adaptive characterization of laser damage from sparse defects

Abstract

Standard techniques for characterizing laser damage are ill-suited to the regime in which sparse defects form the dominant damage mechanism. Previous work on this problem using REO’s automated laser damage threshold test system has included linking damage events in HfO₂/SiO₂ high reflector coatings with visible pre-existing defects, and using a probability per defect based on size and local fluence to generate predictions of damage events in subsequent coating runs. However, in all this work the test sites were always in a predefined array, and the association of defects with damage events was done only after the fact. In an effort to make this process both more efficient and less susceptible to uncertainties, we have now developed an adaptive test strategy that puts defect identification and analysis into the loop. A map of defect locations and sizes on a test surface is compiled, and a set of test sites and corresponding fluences based on that map is then generated. With defects of interest now centered on the damaging beam, the problem of higher-order spatial variation in the beam profile is greatly reduced. Test sites in zones with no detectable defects are also included. This technique allows for the test regimen to be tailored to the specific surface under consideration. We report on characterization of a variety of coating materials and designs with this adaptive method.