Frequency-diverse phase holograms for millimeter- and submillimeterwave computational imaging

Millimeter- and submillimeter-wave imaging systems have gained significant attention due to their unique capabilities in penetrating many non-metallic materials and providing detailed spatial resolution, making them ideal for security screening, industrial inspection, and medical diagnostics applications. However, at these wavelengths, traditional imaging systems often rely on costly and complex hardware, including large detector arrays and mechanical or electronic beamsteering mechanisms, which limit their scalability and affordability. This thesis addresses these challenges by focusing on designing, optimizing, and applying frequency-diverse phase holograms for computational imaging systems at millimeter- and submillimeter-wave frequencies. The research aims to develop simple and cost-efficient diffractive optical elements capable of encoding spatial information from a region of interest into frequency-diverse field patterns over a wide bandwidth, thus enabling advanced computational imaging independent of conventional mechanical or electronic beamsteering methods. The research is motivated by the need to create practical imaging systems that combine high resolution with reduced hardware complexity, leveraging advances in hologram design and neural network-based image reconstruction. The first part of this thesis introduces the design process, optimization, fabrication, and characterization of frequency-diverse phase holograms. These holograms utilize discretized phase modulation through quasirandom surface profiles to create spatially varying field patterns across a wide bandwidth. The hologram synthesis process incorporates physical optics simulations to optimize parameters such as the operation bandwidth, diffraction efficiency, and frequency diversity. Two fabricated holograms are designed for dual-band operation at WR-15 (50–75 GHz) and WR-3.4 (220–330 GHz) and a third hologram for a custom frequency band of 325–355 GHz. The latter employs spatial filtering methods to further enhance frequency diversity by reducing field intensity outside the RoI. The fabricated holograms are characterized using near-field measurements, demonstrating close to 50% diffraction efficiency and sufficient frequency diversity. The second part of the thesis focuses on two imaging systems utilizing the designed holograms: a vector network analyzer (VNA)-based system and a frequency-modulated continuous-wave (FMCW) radar system. The VNA-based system, operating at waveguide bands of 50–75 GHz and 220–330 GHz, is used to demonstrate the ability to reconstruct images of targets using a trained neural network with close to diffraction-limited accuracy. The effects of available bandwidth and spatial characteristics of the illuminating field on imaging accuracy are studied. The FMCW radar system, operating at 325–355 GHz, integrates an optimized hologram with the fast sweep rate of the radar and on-the-fly neural network-assisted image reconstruction. The system represents a development from laboratory-grade equipment to a more compact and standalone imaging system. The imaging capability is demonstrated by imaging a rotating target in real-time with a 60-Hz frame rate and good accuracy.