Transmission losses in infrared anti-reflection coatings due to water absorption

摘要

Infrared optical systems require a range of exotic materials, many with large indices of refraction. The mismatch in the index between the optical element and the surrounding medium can result in reflection losses that approach 50%. Anti-reflection (AR) coatings are applied to these elements in order to minimize "ghost" reflections and improve the optical transmission through a system. The coatings are designed to be highly transparent; however, significant infrared absorption has been observed in some AR coatings. Likely candidates for the loss mechanism are water trapped into the optical coatings during deposition and water being incorporated into coating voids or grain boundaries during exposure to ambient humidity. Five different AR coatings have been procured from four manufacturers to study the cause of the observed losses. Upon receipt of the coated samples, infrared transmission measurements were made which showed the presence of incorporated water/hydroxyl, as evidenced by reduced transmission around 2.9 μm. Four of the five sample types placed in laboratory air for two months continued to absorb water, whereas those placed in flowing dry nitrogen showed no change. Samples placed in a humid environment for one, three and ten days also showed additional water incorporation with the magnitude of the change in transmission on the order of that observed with the two-month air exposure.