Applying feedback control to additive manufacturing of graded refractive index lenses with continuous-distributions in permittivity

Graded refractive index (GRIN) lenses focus and direct electromagnetic waves by making use of spatiallyvarying material properties to achieve compact, low profile devices in a range of technological applications. For microwave applications, GRIN lenses facilitate performance enhancement of key components such as transmitting and receiving horns used in communication devices. However, the fabrication of GRIN lenses with spatially varying permittivity and/or permeability in order to vary the local refractive index is highly challenging. Recently, additive manufacturing has been used to fabricate GRIN lenses with some success, but accurate control of the local refractive index to meet demanding lens designs is difficult. Here, a feedback control system is designed for improved additive manufacturing of GRIN microwave lenses fabricated from a mixture of both polylactic acid (PLA) and an acrylonitrile butadiene styrene/barium titanate (ABS/BaTiO3) composite. After each layer is printed, a controller measures the just-printed local permittivity distribution using a split ring resonator and automatically updates the composition of the next-to-print layer to ensure the resulting local material properties match the design, thus accounting for any printing inconsistencies or defects. The defect structure of lenses without and with the controller are examined using X-ray tomography, and lens performance assessed in the 12-18 GHz range. Overall, the controller is shown to deliver higher fidelity and performance GRIN lenses by ameliorating print defects in real-time during printing.