Okan Yurduseven

Computational imaging using a mode-mixing cavity at microwave frequencies

We present a 3D computational imaging system based on a mode-mixing cavity at microwave frequencies. The core component of this system is an electrically large rectangular cavity with one corner re-shaped to catalyze mode mixing, often called a Sinai Billiard. The front side of the cavity is perforated with a grid of periodic apertures that sample the cavity modes and project them into the imaging scene. The radiated fields are scattered by the scene and are measured by low gain probe antennas. The complex radiation patterns generated by the cavity thus encode the scene information onto a set of frequency modes. Assuming the first Born approximation for scattering dynamics, the received signal is processed using computational methods to reconstruct a 3D image of the scene with resolution determined by the diffraction limit. The proposed mode-mixing cavity is simple to fabricate, exhibits low losses, and can generate highly diverse measurement modes. The imaging system demonstrated in this letter can find application in security screening and medical diagnostic imaging.

Resolution of the Frequency Diverse Metamaterial Aperture Imager

The resolution of a frequency diverse compressive metamaterial aperture imager is investigated. The aperture consists of a parallel plate waveguide, in which an array of complementary, resonant metamaterial elements is patterned into one of the plates. Microwaves injected into the waveguide leak out through the resonant metamaterial elements, forming a spatially diverse waveform at the scene. As the frequency is scanned, the waveforms change, such that scene information can be encoded onto a set of frequency measurements. The compressive nature of the metamaterial imager enables image reconstruction from a significantly reduced number of measurements. We characterize the resolution of this complex aperture by studying the simulated point spread function (PSF) computed using different image reconstruction techniques. We compare the imaging performance of the system with that expected from synthetic aperture radar (SAR) limits.

Comprehensive simulation platform for a metamaterial imaging system

Recently, a frequency-diverse, metamaterial-based aperture has been introduced in the context of microwave and millimeter wave imaging. The generic form of the aperture is that of a parallel plate waveguide, in which complementary metamaterial elements patterned into the upper plate couple energy from the waveguide mode to the scene. To reliably predict the imaging performance of such an aperture prior to fabrication and experiments, it is necessary to have an accurate forward model that predicts radiation from the aperture, a model for scattering from an arbitrary target in the scene, and a set of image reconstruction approaches that allow scene estimation from an arbitrary set of measurements. Here, we introduce a forward model in which the metamaterial elements are approximated as polarizable magnetic dipoles, excited by the fields propagating within the waveguide. The dipoles used in the model can have arbitrarily assigned polarizability characteristics. Alternatively, fields measured from actual metamaterial samples can be decomposed into a set of effective dipole radiators, allowing the performance of actual samples to be quantitatively modeled and compared with simulated apertures. To confirm the validity of our model, we simulate measurements and scene reconstructions with a virtual multiaperture imaging system operating in the K-band spectrum (18-26.5 GHz) and compare its performance with an experimental system.

Large Metasurface Aperture for Millimeter Wave Computational Imaging at the Human-Scale

We demonstrate a low-profile holographic imaging system at millimeter wavelengths based on an aperture composed of frequency-diverse metasurfaces. Utilizing measurements of spatially-diverse field patterns, diffraction-limited images of human-sized subjects are reconstructed. The system is driven by a single microwave source swept over a band of frequencies (17.5–26.5 GHz) and switched between a collection of transmit and receive metasurface panels. High fidelity image reconstruction requires a precise model for each field pattern generated by the aperture, as well as the manner in which the field scatters from objects in the scene. This constraint makes scaling of computational imaging systems inherently challenging for electrically large, coherent apertures. To meet the demanding requirements, we introduce computational methods and calibration approaches that enable rapid and accurate imaging performance.

Printed Aperiodic Cavity for Computational and Microwave Imaging

We demonstrate a frequency-diverse aperture for microwave imaging based on a planar cavity at K-band frequencies (18-26.5 GHz). The structure consists of an array of radiating circular irises patterned into the front surface of a double-sided printed circuit board. The irises are distributed in a Fibonacci pattern to maximize spatial diversity at the scene. The printed cavity is a phase-diverse system and encodes imaged scene information onto a set of frequencies that span the K-band. Similar to recently reported metamaterial apertures, the printed cavity imager does not require any mechanically moving parts or complex phase shifting networks. Imaging of a number of targets is shown; these reconstructed images demonstrate the ability of the system to perform imaging at the diffraction limit. The proposed printed cavity imager possesses a relatively large quality factor that can be traded off to achieve higher radiation efficiency. The general mode characteristics of the printed cavity suggest advantages when used in computational imaging scenarios.

Microwave imaging using indirect holographic techniques

This work describes how indirect holographic techniques, previously applied to the determination of antenna radiation patterns, can be adapted for the imaging of passive objects. It provides details of how complex scattered field values can be obtained in a simple and inexpensive manner from sampled scalar intensity measurements taken over a single scanning aperture. This work provides a brief outline of the basic theory of indirect microwave holography, and how the transformation of the holographic intensity pattern into the Fourier domain enables the isolation of the terms required for complex field reconstruction to be isolated from the remaining terms. The work is supported by a range of experimental results, illustrating the reconstructed complex fields for a number of simple test objects. Back-propagation techniques have also been included to reconstruct complex fields at the position of the scattering objects.

Frequency-diverse microwave imaging using planar Mills-Cross cavity apertures

We demonstrate a frequency diverse, multistatic microwave imaging system based on a set of transmit and receive, radiating, planar cavity apertures. The cavities consist of double-sided, copper-clad circuit boards, with a series of circular radiating irises patterned into the upper conducting plate. The iris arrangement is such that for any given transmitting and receiving aperture pair, a Mills-Cross pattern is formed from the overlapped patterns. The Mills-Cross distribution provides optimum coverage of the imaging scene in the spatial Fourier domain (k-space). The Mills-Cross configuration of the apertures produces measurement modes that are diverse and consistent with the computational imaging approach used for frequency-diverse apertures, yet significantly minimizes the redundancy of information received from the scene. We present a detailed analysis of the Mills-Cross aperture design, with numerical simulations that predict the performance of the apertures as part of an imaging system. Images reconstructed using fabricated apertures are presented, confirming the anticipated performance.

Indirect Microwave Holographic Imaging of Concealed Ordnance for Airport Security Imaging Systems

In this paper, indirect microwave holographic imaging of concealed ordnance is demonstrated. The proposed imaging technique difiers from conventional microwave imaging methods in that it does not require the direct measurement of the complex fleld scattered from the imaged object but mathematically recovers it from intensity-only scalar microwave measurements. This brings the advantages of simplifying the hardware implementation and signiflcantly reducing the cost of the imaging system. In order to demonstrate the ability of the proposed technique to reconstruct good quality images of concealed ordnance, indirect microwave holographic imaging of a metallic gun concealed in a pouch is carried out for airport security imaging applications. It is demonstrated that good resolution amplitude and phase images of concealed objects can be recovered when back-propagation is applied.

Design and Simulation of a Frequency-Diverse Aperture for Imaging of Human-Scale Targets

We present the design and simulation of a frequency-diverse aperture for imaging of human-size targets at microwave wavelengths. Predominantly relying on a frequency sweep to produce diverse radiation patterns, the frequency-diverse aperture provides a path to all-electronic operation, sampling a scene without the requirement for mechanical scanning or expensive active components. Similar to other computational imaging schemes, the frequency diverse aperture removes many hardware constraints by placing an increased burden on processing and analysis. While proof-of-concept simulations of scaled-down versions of the frequency-diverse imager and simple targets can be performed with relative ease, the end-to-end modeling of a full-size aperture capable of fully resolving human-size targets presents many challenges, particularly if parametric studies need to be performed during a design or optimization phase. Here, we show that an in-house developed simulation code can be adapted and parallelized for the rapid design and optimization of a full-size, frequency-diverse aperture. Using files of human models in stereolithography format, the software can model the entire imaging scenario in seconds, including mode generation and propagation, scattering from the human model, and measured backscatter. We illustrate the performance of several frequency-diverse aperture designs using images of human-scale targets reconstructed with various algorithms and compare with a conventional synthetic aperture radar approach. We demonstrate the potential of one aperture for threat object detection in security-screening applications.

Compact parabolic reflector antenna design with cosecant-squared radiation pattern

This paper deals with the parametric analysis and proper design of parabolic reflector antennas to obtain pencil-beam, cosecant-squared and inverse cosecant-squared radiation patterns for air and coastal surveillance radars. A novel design is introduced to obtain both pencil-beam and cosecant-squared radiation patterns by using the same modified parabolic reflector antenna structure fed by an H-plane horn feeder which can be adjusted as symmetric or asymmetric feeder by changing the bottom flare angle. The analytical regularization method (ARM) is used as a fast and accurate way to solve the problem of E-polarized wave diffraction by parabolic shaped perfectly electrical conductive (PEC) cylindrical reflector with finite thickness. The numerical procedure is initially verified by the analytical and numerical methods, and the calculated radiation characteristics are presented for the proposed antenna configurations.