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Ultra-Compact, Lightweight MWIR Zoom Imaging Optics Based on Flat Lens Technology

Description:

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Integrated Sensing and Cyber; Advanced Materials OBJECTIVE: Develop ultra-compact, lightweight Mid-wave infrared (MWIR) zoom imaging optics based on flat lens technology and with 7 times improvement in SWaP. DESCRIPTION: MWIR imaging systems are essential to intelligence, surveillance, and reconnaissance (ISR) missions. Conventional IR optics are thick, heavy, costly, and often require bulky mechanical mounting system to keep their multiple optical elements stable and well-aligned. Current systems weigh 400 g or more, often necessitating heavy-duty gimbals for aiming. Heavy optics require tens of minutes to reach thermal equilibrium during takeoff and landing of air platforms such as unmanned aircraft systems (UASs), limiting ISR operations. Flat lens technologies have the potential to provide extremely compact and lightweight imaging capability in the MWIR, including reduced costs and added functionality such as zoom or variable focus [Refs 1 & 2]. Microfabricated flat lens optics can be only 1 mm thick or less, reducing overall lens system length and supporting more compact imaging systems. Their extremely low mass allows them to reach thermal equilibrium in less than one minute in typical airborne scenarios. Two promising flat lens technologies are metasurface optics (or meta-optics) [Refs 3–5], and multilevel diffractive lenses (MLD) [Ref 6]. Both have challenges related to scalable fabrication and with chromatic aberration for broadband applications. The MLD approach has challenges associated with large numerical apertures (small f-numbers) and high throughput, especially for oblique angles of incidence, which gives rise to significant vignetting, that is, reduction of the image's brightness toward the sensor periphery compared to center. Meta-optics can achieve extremely low-aberration imaging, often reducing the number of optical elements required. As the design space and device function complexity of flat lenses scales up, limitations of the conventional human-driven forward design of meta-optics have necessitated a breakthrough in design philosophy. Recent work using inverse design with appropriate artificial intelligence (AI) tools positions this method to transform meta-optics design [Refs 7–9]. The inverse design approach explores the physics of nanophotonics using advanced mathematical tools, iterating until an optimal solution is achieved. Inverse design has significant advantages over conventional forward design: it does not need a priori knowledge of physics; it can be used for complex designs that have no analytical solution; and it can automatically balance the trade-offs among multiple device functions given the design constraints while minimizing crosstalk. Design robustness can also be considered using the inverse design techniques. The development of a novel lightweight, low-aberration flat lens-based MWIR zoom optics must achieve certain minimum specifications in performance and size, weight, and power (SWaP) to accelerate its adoption for next-generation ISR missions. For example, the aperture size and f/# must be large enough to be useful for low-light imaging. For this effort, the target specifications include, but are not limited to, the follows: (a) total weight, including housing, mount, and motorized control: 100 g or less (b) axial length of lens elements: 2 cm or less (i.e., > 7 times improvement over conventional zoom lens), (c) focal length range: f/2 for wide field of view (FoV), variable up to f/8 for narrow FoV, (d) clear aperture: 2 cm or more, with a scalable fabrication process that can achieve larger sizes, (e) spot size: within 20% of diffraction limit, (f) in-band transmittance: 85% or better, including for all incident angles up to ±25°, (g) electro-mechanical zoom time: 3s or less, and zoom range up to 5X. The goal is to dramatically reduce size and weight versus comparable state-of-the-art conventional zoom lens systems, without reducing performance. Weight will be reduced to 25% or less, and axial length of the optical elements (excluding back focal length standoff distance) will be reduced to 15% or less than that of the conventional zoom lens set-up. The reduced weight and length should reduce time required for zoom adjustments. Initially, focal length tuning can be performed manually, but tuning speed should achieve the above target specification by the end of this effort. Longer-term goals include reduction of f/# (the f number which is a measure of the FoV of the lens set up) and increase in mechanical robustness to withstand vibrations, acceleration forces, and mechanical shocks associated with unmanned aerial system (UAS) takeoff and landing. PHASE I: Determine the feasibility of, detail, model, and simulate an innovative approach for an ultra-compact lightweight, MWIR zoom imaging optic based on flat lens technology. Design, fabricate, and test in the laboratory a proof of concept to demonstrate the flat lens technology and its varifocal capability. Characterize performance based on the specifications outlined in the topic description. The proof of concept should have a clear aperture diameter of at least 4 mm, a zoom range of 5x, a wide FoV of f/2, in-band transmittance of 60% or better, and 96% polarization insensitivity. Design a compact MWIR zoom imaging optic to be fabricated and tested in Phase II. Use modeling and simulation to estimate its performance, including size, weight, and power. Develop a test plan and test procedures to be developed in Phase II. The Phase I effort will include prototype plans to be developed under Phase II. PHASE II: Optimize the design of the prototype flat lens MWIR zoom optic based on the design from Phase I. Develop modifications that can improve performance and perform modeling and simulations. Design electro-mechanical lens housing and motorized control software for the zoom system. Fabricate the zoom optic and test it in the laboratory to demonstrate all the performance specification targets listed in the Description. Detail a scalable fabrication process that provides a roadmap toward cost-effective production and larger clear aperture systems. Study numerically the trade-offs of zoom optic performance between aperture size, numerical aperture (f/#), working bandwidth, transmittance, and FoV. Also investigate theoretically and experimentally the feasibility of extending the zoom range from 5X to 10X. PHASE III DUAL USE APPLICATIONS: Transition the technology for U.S. Government use. Fully develop and transition the technology and methodology based on the research and development results developed for DoD applications in the various areas of anomaly detection, surveillance, and reconnaissance applications. The commercial sector can also benefit from this innovative flat lens with very low SWaP in the areas of detection of toxic gases, environmental monitoring, and noninvasive structural materials monitoring and sensing. REFERENCES: 1. Chen, W. T., Zhu, A. Y., Sanjeev, V., Khorasaninejad, M., Shi, Z., Lee, E., & Capasso, F. (2018). A broadband achromatic metalens for focusing and imaging in the visible. Nature nanotechnology, 13(3), 220-226. https://doi.org/10.1038/s41565-017-0034-6 2. Bosch, M., Shcherbakov, M. R., Won, K., Lee, H. S., Kim, Y., & Shvets, G. (2021). Electrically actuated varifocal lens based on liquid-crystal-embedded dielectric metasurfaces. Nano Letters, 21(9), 3849-3856. https://doi.org/10.1021/acs.nanolett.1c00356 3. Cotrufo, M., Guo, S., Overvig, A., & Alù, A. (2021, March). Nanostructured metasurfaces for optical wavefront manipulation. In Advanced Fabrication Technologies for Micro/Nano Optics and Photonics XIV (Vol. 11696, p. 1169609). International Society for Optics and Photonics. https://doi.org/10.1117/12.2584024 4. Presutti, F., & Monticone, F. (2020). Focusing on bandwidth: achromatic metalens limits. Optica, 7(6), 624-631. https://doi.org/10.1364/OPTICA.389404 5. Banerji, S., Meem, M., Majumder, A., Vasquez, F. G., Sensale-Rodriguez, B., & Menon, R. (2019). Imaging with flat optics: metalenses or diffractive lenses?. Optica, 6(6), 805-810. https://doi.org/10.1364/OPTICA.6.000805 6. Molesky, S., Lin, Z., Piggott, A. Y., Jin, W., Vuckovic, J., & Rodriguez, A. W. (2018). Inverse design in nanophotonics. Nature Photonics, 12(11), 659-670. https://doi.org/10.1038/s41566-018-0246-9 7. Li, Z., Lin, P., Huang, Y. W., Park, J. S., Chen, W. T., Shi, Z., Qui, C. –W., Cheng, J. –X., & Capasso, F. (2021). Meta-optics achieves RGB-achromatic focusing for virtual reality. Science Advances, 7(5), eabe4458. https://doi.org/10.1126/sciadv.abe4458 8. Shi, Z., Zhu, A. Y., Li, Z., Huang, Y. W., Chen, W. T., Qiu, C. W., & Capasso, F. (2020). Continuous angle-tunable birefringence with freeform metasurfaces for arbitrary polarization conversion. Science Advances, 6(23), eaba3367. https://doi.org/10.1126/sciadv.aba3367 KEYWORDS: Low-aberration; Imaging optics; Flat lens; Zoom capabilities; Mid-wave infrared; MWIR; Metamaterials
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