Abstract

ThF₄ is one of the most important low-index materials used in the infrared BBAR coatings, but its use is restricted because of radiation safety regulations. Therefore, the replacement of ThF₄ by a nonradioactive material is badly needed in the coating industry. In this work YF₄ is chosen as the candidate of the low-index nonradioactive material because the refractive index (1.35 at 10.6 μm) of YF₄ is the same as that of ThF₄. A series of five broadband antireflection coatings on germanium and zinc selenide substrates was deposited with or without ion-assisted deposition at a substrate temperature of 200°C. Measurements of laser damage threshold for 10.6-μm 0.1-μs pulses were made on these coatings. Significant improvements have been made in increasing the durability and laser damage threshold of broadband AR coatings (YF₄/Ge/YF₄/Ge/YF₄ and YF₄/ZnSe/YF₄/ZnSe/YF₄) by using the ion-assisted deposition of the germanium and zinc selenide films. Coatings prepared using this technique consistently passed the environmental tests for adhesion, 24-h salt spray, and 10-day humidity tests specified by MIL-C-675 and MIL-M-13508.

© 1991 Optical Society of America