High Threshold HR Coatings at 1064 nm

Abstract

We have conducted an extensive series of laser damage measurements on highly reflective (HR) dielectric coatings which have yielded 1064-nm thresholds as high as 40 J/cm2 for 8- to 10-ns pulses at pulse-repetition frequencies (PRF) of 10 Hz. Moreover, by laser conditioning these coatings with sub-threshold pulses, the thresholds of some coatings were raised to levels exceeding 70 J/cm2. These are the highest threshold dielectric HR coatings that we have tested in this regime. The coatings were originally developed to produce HR-overcoated metal mirrors for free-electron-laser (FED applicationsat high PRF. Our tests included coatings deposited on both dielectric substrates and molybdenum (Mo) substrates. In each category we also examined coatings with a pre-coat of Mo between the substrate and the HR stack. The improved dielectric HR stacks effectively shielded the Mo from the laser irradiation so that the thresholds of virtually all Mo samples exceeded levels of the best dielectric-enhanced and dielectric-HR-coated metal mirrors we have tested to date. In addition to the low PRF measurements, we also conducted 1064-nm damage tests at 6-kHz PRF using 65-ns pulses from the Kilroy damage test facility. The coatings survived thermal loading of fluences ranging from 2 to 10 J/cm2 with respective small spot sizes on the order of 1.2 to 0.3 mm (1/e2 diameter).