RAR Nano-Texturing For High Performance Super-Hydrophobic Beam Director Head Windows

The U.S. military has a long history in developing directed energy weapons, which offer a variety of advantages over conventional kinetic weapons including precision, low cost per shot, and adjustable dose for both non-lethal and lethal targets. In 2014, the Navy deployed the 30kW Laser Weapon System (LaWS), and with the need to target larger and faster objects at greater distances, the Navy is planning deployment of the High Energy Laser and Integrated Optical-dazzler with Surveillance system (HELIOS) in 2021. The Navy has recently shown interest in extending shipboard high energy laser (HEL) weapon capability to the undersea world of the submarine, to enable surprise attack and deterrent capability across the oceans. Early Navy designs for this aperture are to make it common for a laser rangefinders and sensors as well, specifying the need for broadband transmission from the visible through the mid-wave infrared (MWIR). Suitable materials for this requirement include sapphire, spinel, and ALON. While the technology of laser weapons has advanced in recent years, the undersea application presents the unique and potentially show-stopping problem that cannot be addressed with conventional coatings- how can the physical interface or exit aperture between the ocean and laser weapon be maintained to the durability, antireflection level, and cleanliness specifications that are required laser optics?    An innovative and rugged method for providing extreme bandwidth, self-cleaning properties, and laser damage resistance involves the fabrication of a nanometer scale surface relief texture into the Beam Director Head Windows.  A texture consisting of randomly distributed features, the RAR Nano-Texture, imparts a graded index function that has been shown to be far superior to thin film coatings with respect to transmission, bandwidth, and off-axis performance; as well as exhibiting ultra-hydrophobic and self-cleaning properties.  As a comprehensive Phase I proof of concept for extending the advances made in the nano-texturing of fused silica, TelAztec will design, model, and fabricate RAR nano-textured apertures in relevant visible thru MWIR transparent materials to meet the Navy’s desire for a submarine based HEL exit aperture. Particular modeling effort will focus on producing 99.9% transmission at the HEL wavelength near 1µm. Tradeoffs in aperture materials, design, complexity, and manufacturability will be discussed. To address the need for self-cleaning properties, the super-hydrophobic nature of RAR nano-textured surfaces will be demonstrated, attaining a contact angle of greater than 150°.  With a look forward to Phase II work, the Phase I option will include exposing high quality, low absorption RAR nano-textured windows to prolonged sea water exposure, followed by CW laser damage testing of the samples at the laser weapon line of 1070nm.