Optical and environmental performance of nonradioactive infrared optical coatings

Abstract

Infra-red (IR) optical systems tend to make use of IR lens and window materials with high refractive indices, such as germanium (n = 4), zinc selenide (n = 2.45) and zinc suiphide (n = 2.25). Consequent Fresnel reflection is sufficient to significantly degrade effective transmission and JR imaging capability.To maximise JR optical performance in such systems, Barr & Stroud has for over 20 years produced high efficiency JR anti reflection (AR) coatings for lenses and windows. Typically these coatings provide average reflectance below 0.25 % per surface with sufficient durability to meet internal surface environmental conditions. Design constraints on these AR coatings require a low refractive index outer layer material, with current technology and practice relying heavily on thorium fluoride as the low loss material. However, a primary disadvantage of thorium fluoride usage is its associated radioactivity and high toxicity. Moreover, future IR coating requirements insist upon replacement of thorium fluoride with a non-radioactive, low toxicity, low index film material.A recent programme at Pilkington Optronics (Barr & Stroud) has resulted in the identification of a non-radioactive replacement film material with reduced toxicity. As described in this paper, the material has been incorporated into high efficiency single and dual band AR coatings for germanium, zinc selenide and zinc sulphide, without compromising optical and durability performance.