Diffractively Structured Multi-Spectral GaP EO Seeker Windows for Kill Vehicles

Strategic missile defense will benefit directly by increasing the operational IR bandwidth from the mid-wavelength to a multi-bandwidth capability. Increasing bandwidth performance will allow on-board processing (target acquisition and discrimination) to improve range performance, tracking, and hit-to-kill. Increasing bandwidth to include the near IR to the long wavelength IR also allows for incorporation of active sensing (LADAR) in addition to passive sensing. Sub-wavelength diffractive structures etched onto gallium phosphide (GaP) present tremendous performance enhancement potential for missile applications. Employing gray scale lithography to etch the sub-wavelength diffractive structures onto both surfaces of the GaP eliminates the need for anti-reflective coatings and allows for discreet sub-bandwidths of operation over a broadband operational window with very high performance, improved field-of-regard, and increased fracture toughness. Employing GaP as the bulk IR window opens the bandwidth from the mid-wavelength region (sapphire) to include the near IR to the far long wavelength IR. It also provides a cutoff temperature of nearly 400°C. GaP and the sub-wavelength diffractive structures present a significant opportunity to improve spectral performance of the THAAD system or any kill vehicle utilizing the IR part of the spectrum. It is anticipated that GaP as an IR window material which incorporates the sub-wavelength diffractive structures in place of conventional anti-reflection coatings will significantly reduce the cost of IR windows for tactical, strategic, and commercial systems, by as much as a factor 4. Additionally, using GaP with the sub-wavelength diffractive structures will increase the available field-of-regard of the optical system since the IR window will no longer be restricted to a relatively narrow instantaneous field-of-view due to the physics of the planar coating.