Physical Surface Structures for Enhanced Transmission Performance at High Angles of Regard

ABSTRACT: The design of IR windows is dictated by the need for high optical performance while maintaining a low radar cross section (RCS). Distributed apertures and faceted window assemblies are used for increased situational awareness while maintaining acceptably low RCS. Unfortunately, the design requirements which allow for a low RCS also typically require looking through these windows at high angles of regard. The traditional method of reducing the reflection losses at the surfaces of IR windows is to apply anti-reflection (AR) coatings. Unfortunately, the reflections from the 2 dimensional surfaces of coated windows severely limit the usable range of angles. Structures physically etched into the surface of a window are an alternative approach to providing AR function. The 3D nature of the physical surface structures is predicted to reduce reflections at high angles. MWIR/LWIR sized features can be produced using widely available equipment; however, Vis-NIR sized features require UV lithography, which is not readily available. In the proposed effort, Surmet will demonstrate a more scalable and producible method for producing physical surface structures on IR transparent substrates with enhanced optical performance at high angles of regard. BENEFIT: The F-22 strike fighter employs a distributed aperture system (DAS, made by Northrop Grumman Electronic Systems) consisting of six silicon windows situated around the aircraft to provide the pilot with all-around coverage for full situational awareness, missile warning, aircraft warning, day/night pilot vision, and fire control capability. The silicon windows suffer from transmission loss at high angles of regard. Physical Surface Structures provide a good solution for these silicon EODAS windows. In addition, the Joint Strike Fighter is the next generation of strike aircraft weapon systems for the Air Force, Navy, and the Marines. The F-35 Joint Strike Fighter has an electro-optical targeting system employing a facetted sapphire IR window assembly whose performance is limited by low transmission at high angles of regard. Application of physical surface structures to the sapphire windows has the potential to increase system performance, and even decrease the number of window panels required for this window assembly.