Band Gap Dependence of Continuous-Wave Laser Induced Damage Threshold of Optics with Absorptive Contamination

The laser induced damage threshold of optical coatings with differing band gaps was measured using a high power 1070 nm continuous-wave laser. High reflectivity distributed Bragg reflectors of niobia-silica, tantala-silica, and hafnia-silica were tested in addition to half-wave coatings of titania, tantala, hafnia, and alumina. Absorbing contamination in the form of 20-50 μm carbon particles was added to the surface of the optics prior to exposure to test for particle induced damage. For both half-wave and high reflectivity coatings, the minimum damage thresholds were found to increase for larger band gap materials. Low band gap niobia reflectors and titania half wave coatings damaged at 20 kWcm-2 and 155 kWcm-2 respectively while larger bandgap hafnia reflectors failed at 1 MWcm-2 and alumina half-wave coatings failed at 9.7 MWcm-2 . Most laser induced damage was catastrophic with the film and underlying substrate being damaged and the optic uncontrollably hearting. Damage effects differed for high band gap half-wave coatings that did not damage the substrate or thermally runaway with additional laser exposure.