Design and Characterization of Phase Holograms for Standoff Localization at Millimeter and Submillimeter Waves

We present design, simulation, and experimental characterization of dual-band frequency-diverse holograms for distributed beamforming. The holograms operate in the 50–75 GHz (WR-15) and 220–330 GHz (WR-3.4) bands for millimeter- and submillimeter-wave imaging. The holograms are designed to create a dispersive field in the region of interest (RoI) located 600 mm from the aperture. The holograms lie in the front end of an imaging setup and modulate the phase of the incident collimated beam from a parabolic mirror. The distributed beamforming enables interrogation of the RoI so that the measured reflection through the dispersive propagation path conveys the spatial information of the target. Different phase modulation schemes are evaluated, and two prototype holograms are manufactured. The dispersive operation and efficiency of the hologram are characterized with both simulations and measurements. The frequency diversity of the holograms is quantified using singular-value decomposition and spatial-spectral correlation coefficient methods. The results identified a design frequency of 120 GHz, a phase quantization step of
$\pi $
/2 radians, and an added phase of
$1.9\pi $
radians as a good dispersion-efficiency compromise. A fully connected neural network is trained to localize a corner-cube reflector in the RoI illuminated by the hologram. The localization accuracy follows the diffraction-limited resolution and confirms the best performance for the hologram considered optimal in the design metrics.