Influence of deposition parameters on laser-damage threshold of silica-tantala AR coatings.

A series of 4-layer silica-tantala antireflection coatings was deposited under 18 different combinations of substrate temperature (175, 250, and 325 degrees C), oxygen pressure (0.5,1.0, and 2.0 x 10(-4) Torr), and rate of deposition (1.5 and 5 A/sec). Measurements of laser-damage threshold for 1064-nm, 1-nsec pulses, average absorption, net stress, and reflectivity were then made on these coatings. Coatings deposited at the lowest temperature had the highest damage thresholds. Damage thresholds were found not to be directly related to average absorption or net stress. Coatings deposited on fused silica substrates which had been polished by a bowl-feed process had generally higher damage thresholds than coatings deposited on conventionally polished fused silica or on BK-7 glass polished by either conventional or bowl-feed processes. Baking coatings in air for 4 h at 400 degrees C generally reduced average absorption and net stress, changed the net stress from compression to tension and, in some cases, increased the damage threshold.