Thomas Fromenteze

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.

Single-Shot Compressive Multiple-Inputs Multiple-Outputs Radar Imaging Using a Two-Port Passive Device

This paper presents a compressive technique that simplifies the architecture of multiple-input multiple-output (MIMO) radar imaging systems. By means of a passive device connected to an MIMO array of antennae, a novel approach is introduced to extract the interaction between antennae from a compressed signal measured between two ports. This technique relies on a multiplexing principle that exploits the frequency diversity and it is thus particularly suitable for ultrawideband imaging systems. This paper presents the theoretical principle underlying this compressive technique, defining the key parameters affecting the information retrieval from a measured frequency signal and the prior characterization of the compressive devices transfer functions. Simulations are performed to demonstrate the potential of the technique for MIMO ultrawideband imaging systems and its compatibility with fast range migration algorithms based on fast Fourier transforms. Finally, an experimental validation is performed with a two-dimensional radiating aperture connected to a compressive device, allowing for the three-dimensional near-field imaging of different targets.

Waveform Coding for Passive Multiplexing: Application to Microwave Imaging

This paper proposes a novel passive technique for the collection of microwave images. A compact component is developed that passively codes and sums the waves received by an antenna array to which it is connected, and produces a unique signal that contains all of the scene information. This technique of passive multiplexing simplifies the microwave reception chains for radar and beamforming systems (whose complexity and cost highly increase with the number of antennas) and does not require any active elements to achieve beamsteering. The preservation of the waveforms is ensured using orthogonal codes supplied by the propagation through the components uncorrelated channels. Here we show a multiplexing technique in the physical layer that, besides being compact and passive, is compatible with all ultrawideband antennas, enabling its implementation in various fields.

Application of range migration algorithms to imaging with a dynamic metasurface antenna

Dynamic metasurface antennas are planar structures that exhibit remarkable capabilities in controlling electromagnetic wavefronts, advantages that are particularly attractive for microwave imaging. These antennas exhibit strong frequency dispersion and produce rapidly varying radiation patterns. Such behavior presents unique challenges for integration with conventional imaging algorithms. We adapt the range migration algorithm (RMA) for use with dynamic metasurfaces and propose a preprocessing step that ultimately allows for expression of measurements in the spatial frequency domain, from which the fast Fourier transform can efficiently reconstruct the scene. Numerical studies illustrate imaging performance using conventional methods and the adapted RMA, demonstrating that the RMA can reconstruct images with comparable quality in a fraction of the time. The algorithm can be extended to a broad class of complex antennas for application in synthetic aperture radar and MIMO imaging.

Synthetic aperture radar with dynamic metasurface antennas: a conceptual development

We investigate the application of dynamic metasurface antennas (DMAs) to synthetic aperture radar (SAR) systems. Metasurface antennas can generate a multitude of tailored electromagnetic waveforms from a physical platform that is low-cost, lightweight, and planar; these characteristics are not readily available with traditional SAR technologies, such as phased arrays and mechanically steered systems. We show that electronically tuned DMAs can generate steerable, directive beams for traditional stripmap and spotlight SAR imaging modes. This capability eliminates the need for mechanical gimbals and phase shifters, simplifying the hardware architecture of a SAR system. Additionally, we discuss alternative imaging modalities, including enhanced resolution stripmap and diverse pattern stripmap, which can achieve resolution on par with spotlight, while maintaining a large region-of-interest, as possible with stripmap. Further consideration is given to strategies for integrating metasurfaces with chirped pulse RF sources. DMAs are poised to propel SAR systems forward by offering a vast range of capabilities from a significantly improved physical platform.

Millimeter-wave spotlight imager using dynamic holographic metasurface antennas

Computational imaging systems leverage generalized measurements to produce high-fidelity images, enabling novel and often lower cost hardware platforms at the expense of increased processing. However, obtaining full resolution images across a large field-of-view (FOV) can lead to slow reconstruction times, limiting system performance where faster frame rates are desired. In many imaging scenarios, the highest resolution is needed only in smaller subdomains of interest within a scene, suggesting an aperture supporting multiple modalities of image capture with different resolutions can provide a path to system optimization. We explore this concept in the context of millimeter-wave imaging, presenting the design and simulation of a single frequency (75 GHz), multistatic, holographic spotlight aperture integrated into a K-band (17.5-26.5 GHz), frequency-diverse imager. The spotlight aperture - synthesized using an array of dynamically tuned, holographic, metasurface antennas - illuminates a constrained region-of-interest (ROI) identified from a low-resolution image, extracting a high-fidelity image of the constrained-ROI with a minimum number of measurement modes. The designs of both the static, frequency-diverse sub-aperture and the integrated dynamic spotlight aperture are evaluated using simulation techniques developed for large-scale synthetic apertures.

UWB passive beamforming for large antenna arrays

A passive technique is developed in this paper to achieve UWB beamforming using a 1 × N device able to code the waves received and transmitted by an antenna array, preventing the use of phase shifters, delay lines, or moving parts. The link between the array inter-element space and the beamforming performance is studied to show the possible application of this device for far-field high resolution beamforming, achieved in a passive way.

A Precorrection Method for Passive UWB Time-Reversal Beamformer

The passive time-reversal beamformer is a 1 × N ports device with N orthogonal channels. Beamforming is achieved using simultaneous time reversal focusing on the output ports with the only input waveform. Utilizing this method, an antenna array can be supplied with focused signals of controlled complex weights and achieve beam steering. A precorrection method is developed in this letter to normalize the weights of the output signals at the focusing time, avoiding a long optimization step of beamformer designing. The theory is developed in this letter, followed by experimental validation.

Computational polarimetric microwave imaging

We propose a polarimetric microwave imaging technique that exploits recent advances in computational imaging. We utilize a frequency-diverse cavity-backed metasurface, allowing us to demonstrate high-resolution polarimetric imaging using a single transceiver and frequency sweep over the operational microwave bandwidth. The frequency-diverse metasurface imager greatly simplifies the system architecture compared with active arrays and other conventional microwave imaging approaches. We further develop the theoretical framework for computational polarimetric imaging and validate the approach experimentally using a multi-modal leaky cavity. The scalar approximation for the interaction between the radiated waves and the target- often applied in microwave computational imaging schemes-is thus extended to retrieve the susceptibility tensors, and hence provides additional information about the targets. Computational polarimetry has relevance for existing systems in the field that extract polarimetric imagery, and particular for ground observation. A growing number of short-range microwave imaging applications can also notably benefit from computational polarimetry, particularly for imaging objects that are difficult to reconstruct when assuming scalar estimations.

Single-frequency microwave imaging with dynamic metasurface apertures

Conventional microwave imaging schemes, enabled by the ubiquity of coherent sources and detectors, have traditionally relied on frequency bandwidth to retrieve range information, while using mechanical or electronic beamsteering to obtain cross-range information. This approach has resulted in complex and expensive hardware when extended to large-scale systems with ultrawide bandwidth. Relying on bandwidth can create difficulties in calibration, alignment, and imaging of dispersive objects. We present an alternative approach using electrically large, dynamically reconfigurable, metasurface antennas that generate spatially distinct radiation patterns as a function of tuning state. The metasurface antenna consists of a waveguide feeding an array of metamaterial radiators, each with properties that can be modified by applying a voltage to diodes integrated into the element. By deploying two of these apertures, one as the transmitter and one as the receiver, we realize sufficient spatial diversity to alleviate the dependence on frequency bandwidth and obtain range and cross-range information using measurements at a single frequency. We experimentally demonstrate this proposal by using two 1D dynamic metasurface apertures and reconstructing various 2D scenes (range and cross-range). Furthermore, we modify a conventional reconstruction method—the range migration algorithm—to be compatible with such configurations, resulting in an imaging system that is efficient in software and hardware. The imaging scheme presented in this paper has broad application to radio frequency imaging, including security screening, through-wall imaging, biomedical diagnostics, and synthetic aperture radar.