Design and Fabrication of a Radio Frequency Grin Lens Using 3D Printing

Microwave lenses are used in a variety of applications such as electromagnetic wave collection and imaging.

Engineered electromagnetic materials and meta-materials have been researched to explore devices that enable access to electromagnetic properties that are not available in nature. This new class of devices can not only open the door to new functionality, but also be effectively utilized to improve the overall performance of existing systems with respect to electromagnetic performance, cost, size, weight, and repeatability.

This work focuses on the design, fabrication, and characterization of a 3D meta-material implementation of a focusing GRadient INdex (GRIN) lens with an operational frequency of 12 GHz that is capable of focusing a uniform plane wave to a point outside the lens.

In the microwave bands, GRIN lenses are often made of polystyrene. Currently, focusing lenses with homogenous refractive index are curved. Their size and weight can make them prohibitive in applications such as airborne systems where these constraints are crucial to efficient operation. These issues can be addressed by designing a lens based on meta-material structures and manufactured with 3D printing. The GRIN lens operates at radio frequency (RF) frequencies, and is not polarization constrained.

In contrast to lenses designed for operation in the optical regime, where lens size is many magnitudes larger than the wavelength, lenses that operate at RF wavelengths are physically only a few times larger than the wavelength.

A 3D rapid prototyping printer was used to fabricate the GRIN lens shown in the figure. 3D printers can be used to print a diverse set of materials, from di-electrics to metals. The materials can be used either individually or in combination (mixed during fabrication) to obtain anisotropic permittivity and permeability values with a range wider than the basic materials used in their neat form. This range of available material parameters can be further enhanced through careful design of geometric features in the unit cells. For example, to lower the limit on available refractive index range, holes/voids were included in this lens design. Since these voids can be defined in the computer-aided design (CAD) file, the printing is seamless and does not require an additional milling step used in conventional processes.

The lens was measured on a modified microwave Gaussian beam system that is used to fully illuminate the lens with a wave and measure the performance of the manufactured lens. The measurement system emits a Gaussian-like wave that does not have uniform amplitude or phase. This has to be accounted for because it has a large effect on the output of the lens. The electric field is measured down the centerline of the lens from 10 cm to 40 cm from the back face of the lens in a similar fashion to the data obtained in simulations.

Microwave lenses such as GRIN lenses are used in a variety of applications such as electromagnetic wave collection and imaging. These lenses are major contributors to system size, weight, and cost, which forces tradeoffs between system parameters such as focal length, field of view, resolution, bandwidth, reflectivity, and range.

The 3D printing approach was chosen to show how such methods can be used to efficiently and accurately fabricate such devices and electromagnetic materials. This allows for building truly 3D electromagnetic structures, as opposed to stacked layer approaches. This provides the freedom to leverage geometrical anisotropy as well as 3D material variation in designs, which in turn increases the degrees of freedom available to optimize the unit cells for a given application.

This work was done by Jeffrey W. Allen and Bae-Ian Wu of the Air Force Research Laboratory. AFRL-0229

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This article first appeared in the June, 2014 issue of Aerospace & Defense Technology Magazine.

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