Abstract
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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> /2 radians, and an added phase of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.9\pi $ </tex-math></inline-formula> 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.
Highlights
B EAMFORMING is a key capability in millimeter-wave and submillimeter-wave imaging with active applications research in areas, including medical sensing, nondestructive testing, and personnel screening
We have demonstrated a dispersive hologram combined with a terahertz time-domain spectrometer operating at 0.1–2.0 THz and leveraged the dispersive field illumination to conduct spatial localization tasks [16]
The diffraction efficiency of a hologram is defined as the ratio of power over the region of interest (RoI) with the hologram present divided by the power incident on the hologram or η=
Summary
B EAMFORMING is a key capability in millimeter-wave (mm-wave) and submillimeter-wave imaging with active applications research in areas, including medical sensing, nondestructive testing, and personnel screening. A concept with increasing interest inside the computational imaging paradigm has been frequency-diverse imaging, as in [3]–[5] These methods rely on swept-frequency sources and engineered frequencydependent apertures to generate quasi-random radiation patterns. One logical progression of this approach is the “single-pixel camera” consisting of a single sensor that is multiplexed to different modes to acquire information from the target [6], [7] These different modes can be created using random masks or coded apertures that introduce spatio-temporal modulation to the target illumination. We present the design process, simulations, and experimental characterization of dual-band frequency-diverse phase holograms for distributed beamforming. The holograms served as a frequency-diverse aperture in the front end of a quasi-optical setup, modulating the phase of the incident collimated beam from a parabolic mirror.
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