Description:
OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Integrated Sensing and Cyber
The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.
OBJECTIVE: Develop and demonstrate an anti-reflection treatment for tungstate laser materials with a surface optical damage threshold exceeding 50 Joules per square cm in 10 nanosecond pulses over three separate wavebands (500-700 nm, 1000-1500 nm, 2000-5000 nm)
DESCRIPTION: Nonlinear optical materials are key components in modern high-intensity laser systems which require very high damage threshold anti-reflection treatments. Conventional multi-layer dielectric coatings are frequently prone to surface damage, especially when required to be effective over a wide spectral range. Stress within multilayer dielectrics often leads to failure during temperature cycling, especially over an extended timeframe. These issues are accentuated when higher-order optical nonlinearities are needed, as these require increased optical intensities to drive the nonlinearity. Third-order nonlinearities, such as direct third harmonic generation, Raman generation, and Stimulated Brillouin Scattering (SBS) are frequently difficult to employ in real-world laser systems due to the excessive pump intensities needed to drive the nonlinearity with high efficiency without incurring surface optical damage to the laser components. The frequently required broad bandwidths needed to encompass pump and emission wavelengths at high intensities is extremely challenging for conventional dielectric coating technology. A preferable option would be to develop anti-reflection treatments based on surface texturing. This approach provides a technical path to achieving surface optical damage thresholds that approach those of the internal bulk material. Anti-reflection treatments based on surface texturing comprise a dense “forest” of microscopic rod or cone-like structures which are etched into the surface of an optical component. The morphology of these structures varies and may be regularly spaced structures of identical size (often referred to a “moth eye” structures) or may be random in size and density within prescribed dimension bounds (called Random Anti-Reflection, or “RAR” structures). The process for creating these structures varies considerably according to the specific optical materials involved. This topic seeks to develop surface anti-reflection treatments specifically for tungstate based third order nonlinear optical media, such as potassium gadolinium tungstate (KGW). The goal is to demonstrate surface optical damage thresholds exceeding 50 Joules per square cm for laser pulse widths in the nanosecond regime (e.g. 10 ns). Three separate spectral regions are sought; 500-700 nm, 1000-1500 nm, 2000-5000 nm, each designed for normal incidence operation. The spectral coverage may be three separate surface treatment designs, but a single design to span all three spectral regions would also be acceptable. The reflectivity at normal incidence should not exceed 1% at any wavelength of interest, with a goal of less that 0.5%.
PHASE I: As this is a Direct-to-Phase-II (D2P2) topic, no Phase I awards will be made as a result of this topic. To qualify for this D2P2 topic, the Government expects the applicant to demonstrate feasibility by means of a prior “Phase I-type” effort that does not constitute work undertaken as part of a prior SBIR/STTR funding agreement. Qualifying “Phase I-type” efforts would include the prior design, development, and demonstration of surface textured anti-reflection treatments for broadly transparent solid state laser media with a damage threshold of 10 Joules per square cm, or greater, for nanosecond type pulse durations. Examples of appropriate laser media include yttrium aluminum garnet (YAG), yttrium lithium fluoride (YLF), potassium gadolinium tungstate (KGW), and zinc selenide (ZnSe), with broad band reflection values of less than 1 % peak across an optical bandwidth of at least 20% of the design center wavelength. For example, 200 nm bandwidth of anti-reflection treatment with less than 1% reflectivity at normal incidence from 900 nm to 1100 nm with a damage threshold of 10 Joules per square cm, or greater, would qualify as a “Phase I-type” effort.
PHASE II: This topic seeks to develop surface anti-reflection treatments specifically for tungstate based third order nonlinear optical media, such as potassium gadolinium tungstate (KGW). The goal is to demonstrate surface optical damage thresholds exceeding 50 Joules per square cm for laser pulse widths in the nanosecond regime (e.g. 10 ns). Three separate spectral regions are sought; 500-700 nm, 1000-1500 nm, 2000-5000 nm, each designed for normal incidence operation. The spectral coverage may be three separate surface treatment designs, but a single design to span all three spectral regions would be acceptable. The reflectivity at normal incidence should not exceed 1% at any wavelength of interest, with a goal of less than 0.5%. The demonstration should be initially characterized using small scale samples of KGW (to be sourced by the awardee), and then demonstrated at both ends of KGW rods, 5 mm square by at least 50 mm long. Demonstrate that all topic goals are met and develop a plan to scale to 25 mm or larger diameter rods. Deliver five rods of AR treated KGW at all three wavebands of interest.
PHASE III DUAL USE APPLICATIONS: Develop scaling and manufacturing capability for anti-reflection treatments in KGW with crystal apertures exceeding 25mm in diameter and at least 50mm in length. Identify and procure samples of KGW, or similar, in sufficient sizes to meet these requirements, and verify optical damage threshold of the completed crystals exceeds 50 J per square cm in 10 ns pulses across each of the three wavebands of interest (500-700 nm, 1000-1500 nm, 2000-5000 nm).
REFERENCES:
1. US Patent 8187481B, “Random texture anti-reflection optical surface treatment”; Stefaan Vandendriessche, Gregg Fales, Jim Nole, “Laser Optics: Antireflection nanotextures for laser optics go commercial”, Laser Focus World, Article 16546961, September 2016;
2. B Mousavi, A Mousavu, T Busani, M Zadeh, S Brueck, “Nanostructured Anti-Reflection Coatings for Enhancing Transmission of Light”, Journal of Applied Mathematics and Physics, 7, 3083-3100, (2019); S McDaniel, D Hobbs, B MacLeod, E Sabatino, P Berry, K Schepler, W Mitchell, and G Cook, "Cr:ZnSe laser incorporating anti-reflection microstructures exhibiting low-loss, damage-resistant lasing at near quantum limit efficiency," Opt. Mater. Express 4, 2225-2232 (2014);
3. https://www.laserfocusworld.com/optics/article/16546961/laser-optics-antireflection-nanotextures-for-laser-optics-go-commercial;
4. https://www.newport.com/medias/sys_master/images/images/hf3/hcf/9134343913502/DS-041801-Nano-Texture-Optics.pdf
KEYWORDS: Motheye; random anti-reflection; SBS; Raman; nonlinear; high laser damage threshold
