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Autonomous navigation systems rely on the collection and processing of large datasets with advanced and memory-intensive algorithms. The requirements in terms of computing power and energy consumption can be significant. In this work, we propose a passive electromagnetic structure that can reduce the computational load by detecting edges (obstacles) in the far field. This is accomplished by probing the scene and processing its scattering on the physical layer, at the speed of light in the medium. By using a recently developed inverse design method, we present an edge detector able to detect obstacles at 15 different locations with an average efficiency of 97% and minimal crosstalk. We call this class of devices that can integrate a vast number of distinct optical functions with high efficiency symphotic.

We experimentally demonstrate a frequency-diverse, computational imaging system at W-band frequencies utilizing an array of cavity-fed metasurface antennas. Each metasurface antenna consists of a cavity milled from aluminum stock, with an upper plate patterned with a set of radiating slots. As a function of frequency, the metasurface cavities produce a set of spatially diverse radiation patterns that probe the reflectivity distribution of a scene. The antennas are designed to maximize the measurement diversity and hence imaging capacity of the system. The number and distribution of the radiating slots is optimized by balancing the cavity quality factor (Q) and Fourier space coverage. In the experimental realizations, the radiation patterns from each cavity-fed metasurface antenna is first measured using near-field scanning techniques, propagated over the imaging domain, and then stored for use in the image reconstruction step. Comprehensive alignment procedure is implemented to align the measured radiation patterns with regard to the physical position of the cavities. Using a modeling platform, we find excellent agreement between the simulation and experiment, indicating the validity of the calibration and alignment procedures. The scaling of the cavity-fed metasurface antenna represents a key step in the development of alternative high-frequency apertures for imaging and beam-forming applications.

Computational imaging at microwave frequencies has gained traction due to its potential for obtaining high-quality images with fast acquisition rates. Complex and diverse radiation patterns form the cornerstone of this approach. Electrically large antennas, such as mode-mixing cavities and metamaterial apertures, have proven to be effective platforms for generating such waveforms. Due to the complex nature of these antennas, near field scanning is often required to characterize their radiation patterns. However, accurate knowledge of the produced waveforms’ spatial distribution, with respect to the physical position of the antenna, is imperative. This relies on precise alignment between the antenna and the near field scan stage during the characterization process—a requirement that is especially cumbersome to achieve when operating at high frequencies. We present an effective method to address this problem; by introducing RF markers into the antenna the position of the antenna under test within the near field scanning setup can be obtained directly from the measurements. The proposed method is experimentally verified through comparison with measurements made using optical photogrammetry. The proposed process will find application in the alignment of computational and multistatic imaging systems, commonly used in security screening and threat detection, as well as in tiled electrically large antenna structures.

We present a novel method of rapid prototyping waveguide and antenna using plating on plastic technique. The part is created by high-precision three-dimensional printing and plated with copper using both electroless plating and electroplating. The performance is comparable to industry-made waveguides and antennas, but the time and cost for creating these parts are largely reduced.

Multiple-input-multiple-output (MIMO) radar image processing presents problems difficult to address by modifying conventional monostatic radar methods as Fourier range migration. When the distance between the transmitter and receiver is comparable to the target size, the single phase center approximation is not accurate. Furthermore, if the antenna radiation pattern significantly deviates from a spherical wave, the symmetries assumed in most range migration techniques are violated. We present a rapid Fourier-based MIMO reconstruction called Fourier accelerated multistatic imaging (FAMI) suitable for massively parallel computation that accounts for frequency-dependent radiation patterns, does not require the single phase center approximation, and is able to dynamically adapt to different target support volume shapes. FAMI is especially suitable for frequency-diversity antenna systems that use spectrally modulated coded spatial radiation patterns.

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.

We demonstrate a technique for calibrating a frequency-diverse, multistatic, computational imaging system. A frequency-diverse aperture enables an image to be reconstructed primarily from a set of scattered field measurements taken over a band of frequencies, avoiding mechanical scanning and active components. Since computational imaging systems crucially rely on the accuracy of a forward model that relates the measured and transmitted fields, deviations of the actual system from that model will rapidly degrade imaging performance. Here, we study the performance of a computational imaging system at microwave frequencies based on a set of frequency-diverse aperture antennas, or panels. We propose a calibration scheme that compares the measured versus simulated scattered field from a cylinder and calculates a compensating phase difference to be applied at each of the panels comprising the system. The calibration of the entire system needs be performed only once, avoiding a more laborious manual calibration step for each transmitting and receiving path. Imaging measurements performed using the system confirm the efficacy and importance of the calibration step.

Vibrational contrast imaging of the distribution of complex biological molecules requires the use of techniques that provide broadband spectra with sufficient resolution. Coherent anti-Stokes Raman scattering (CARS) microscopy is currently limited in meeting these requirements due to the presence of a nonresonant background and its inability to target multiple resonances simultaneously. We present nonlinear interferometric vibrational imaging (NIVI), a technique based on CARS that uses femtosecond pump and Stokes pulses to retrieve broadband vibrational spectra over 200 cm(-1) (full-width at half maximum). By chirping the pump and performing spectral interferometric detection, the anti-Stokes pulses are resolved in time. This phase-sensitive detection allows suppression of not only the nonresonant background, but also of the real part of the nonlinear susceptibility χ((3)), improving the spectral resolution and features to make them comparable to those acquired with spontaneous Raman microscopy, as shown for a material sample and mammary tissue.

Phaseless imaging approaches provide a significant advantage for systems where maintaining coherency during the acquisition time is difficult. Here, we demonstrate a phaseless, frequency-diverse, computational imaging system that operates at K-band frequencies (17.5-26.5 GHz). The system consists of a cavity-backed metasurface antenna producing spatially diverse radiation patterns that vary as a function of the driving frequency. The frequency-diverse metasurface antenna can be used to form images at microwave frequencies by collecting measurements at frequencies sampled over the operational bandwidth, obviating the need for either mechanically moving parts or phase-shifting circuits. We show that high-fidelity images can be obtained with the metasurface antenna using only the intensity of the measurements by leveraging a sparse variant of the Wirtinger Flow algorithm. In addition to the hardware simplification achieved by using a frequency-diverse approach, we demonstrate a significant reduction in the number of measurements required to reconstruct a given number of voxels for the phaseless imaging problem. This difference from conventional phase retrieval techniques is achieved by leveraging the sparsity concept, simplifying the complexity of the imaging problem.