N231-063 TITLE: Additive Manufacturing for Graded-Index Lens Apertures

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): 5G; Networked C3

OBJECTIVE: Develop methods for additive manufacturing (AM) of dielectric materials with structurally varying densities, using a wider variety of materials to achieve a larger range of effective dielectric constants. Methods may include AM source materials with a higher natural dielectric constant, multiple different source materials, heterogeneous integration with planar printed circuit board or other antenna structures, or selective metallization patterning on AM dielectric structures, with the goal of higher performance, more compact lens and filtering components.

DESCRIPTION: AM of dielectrics and metals is a promising area for novel microwave and millimeter-wave components. As with other AM areas, it enables rapid iteration of antenna designs and components that would otherwise require significant time-consuming manufacturing steps. In some cases, such as graded-index (GRIN) lens structures, AM is the only way to achieve a particular design that subtractive methods cannot.

GRIN lens structures require a spatially varying index of refraction. A classic example is the Luneburg lens, which is ideally a sphere with a continuously increasing dielectric constant toward the center. This causes incident plane waves to focus down on the opposite side of the lens, which is often used to implement a high-gain antenna or a retrodirective radar cross-section (RCS) enhancement. Previous methods of producing a Luneburg lens relied on creating multiple concentric shells, each with a discrete dielectric constant. This produces a step-wise approximation of the ideal gradient index profile, and while a Luneburg lens with fewer shells is easier to produce, the beam sidelobes and aperture efficiency suffer when used as an antenna. A sliced Luneburg lens structure using punctured dielectrics to tailor the effective dielectric can more closely emulate the desired Luneburg lens profile [Ref 1].

Another downside of Luneburg lens structures is that they are bulky and protrude from the surface where they are installed. Planar GRIN lenses are better suited to conformal antenna applications and can still provide some degree of focusing. These designs are created using transformation optics to morph a Luneburg lens dielectric gradient into a planar design [Ref 2]. This requires significantly higher peak dielectric constant values to achieve focusing over a thinner, planar volume.

To reach these higher peak dielectric constants, planar GRIN lens structures can be fabricated using multiple slices of different circuit board laminate materials typically available for planar microwave circuits, similar to Rondineau et. al. [Ref 1]. These materials are available with adequate peak dielectric constants for planar GRIN designs. To achieve an effective dielectric constant, sheets are drilled out with specific hole patterns to remove material, such that certain frequencies see a lower effective dielectric constant if the wavelength is somewhat larger than the feature size.

Planar GRIN designs using punctured printed circuit layers have a few drawbacks, which include the significant number of drill holes required per layer and the number of layers. This increases the cost of a planar GRIN aperture designed using traditional planar printed circuit board (PCB) methods. Additionally, improving the operating frequency range requires additional layers to ensure good impedance matching. The planar design itself results in non-ideal focusing over the outer edges, leading to reduced aperture efficiency without additional corrective elements [Ref 3].

To produce novel lens designs quickly, new materials are becoming available that allow for the creation via AM of low-loss dielectric structures using photoresins. These allow for smaller feature sizes compared to other AM methods and potentially faster build speeds when batch printing. The Navy is seeking methods of designing and producing planar GRIN lenses that leverage these new materials, or a hybrid combination of these materials with other methods for developing microwave/mm-wave lensing and antenna structures, that can operate over large bandwidths and challenging environmental conditions.

PHASE I: Design and test GRIN lens structures that can conform to a flat outer profile, using a heterogeneous combination of ceramic photopolymer resins, other photopolymer dielectrics, and planar laminate dielectrics if needed to cover higher peak dielectric constants. Test apertures should cover all of K-band, with a scan loss exponent less than 4, peak sidelobes no more than 20-dB down from peak gain when steered at boresight, and no more than 15 dB when scanned to 50� off boresight. Other design objectives should focus on: minimizing weight of the lens, preferably below one pound or otherwise suitable for a large Group 1 or small Group 2 Unmanned Aerial System (UAS); maximizing aperture efficiency with a desired efficiency greater than 50%; increasing the bandwidth and highest frequency; and reducing the overall thickness of the lens between the outer conformal profile and the feed layer.

PHASE II: Design and test GRIN lens structures that can conform to a curving profile such as the fuselage of a small Group 2 UAS using a heterogeneous combination of ceramic photopolymer resins, other photopolymer dielectrics, planar laminate dielectrics if needed to cover higher peak dielectric constants, and metalized structures that aid in tailoring the operating frequency or required thickness of the aperture. Test apertures shall cover at least a 10-to-1 bandwidth with an objective of covering 2-40 GHz. Test apertures shall exhibit a scan loss exponent less than 2.5, peak sidelobes no more than 20-dB down from peak gain when steered at boresight, and no more than 15 dB when scanned to 50� off boresight. Other design objectives should focus on: minimizing weight of the lens; minimizing dielectric and other efficiency losses; improving thermal properties of the structure when supporting microwave power up to 10 kW; reducing the overall thickness of the lens between the outer conformal profile and the feed layer; minimizing production costs; and any potential performance or material issues that might arise in Naval maritime environments.

PHASE III DUAL USE APPLICATIONS: Design, build, and assist the Navy with integrating a set of broadband planar GRIN-lens apertures for a Naval communications, radar, or electronic surveillance application, which has similar specifications as the test components built up in Phase II. The effort will also focus on translating the design principles of these apertures to beamforming for terrestrial 5/6G or space-based communications.

REFERENCES:

1.       Rondineau, S.; Himdi, M. and Sorieux, J. "A sliced spherical Luneburg lens." in IEEE Antennas and Wireless Propagation Letters, vol. 2, pp. 163-166, 2003

2.       Mateo-Segura, C.; Dyke, A.; Dyke, H.; Haq, S. and Hao, Y. "Flat Luneburg Lens via Transformation Optics for Directive Antenna Applications." in IEEE Transactions on Antennas and Propagation, vol. 62, no. 4, pp. 1945-1953, April 2014

3.       Garcia, N.; Wang, W. and Chisum, J. "Feed corrective lenslets for enhanced beamscan in flat lens antenna systems", Optics Express 30.8 (2022)

 

KEYWORDS: Antennas; graded-index lens; GRIN; additive manufacturing

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