Conventionally, resolution characterization of an imaging radar is performed by means of analyzing the diffraction limited point-spread-function (PSF) pattern of the radar. Such an analysis is straightforward and can easily be implemented at microwave and millimeter-wave frequencies using simple point-scatter targets. However, it poses significant challenges at submillimeter-wave (or THz) frequencies due to the strong scattering response of secondary objects that are used to align the PSF targets for imaging at these frequencies. As a result, the reconstructed PSF patterns suffer from artifacts caused by the secondary objects present in the background. In this work, we present the use of the acoustic levitation principle to obtain the PSF characterization of a 340 GHz stand-off imaging radar. We show that using a water droplet acoustically levitated at the focal point of the 340 GHz imaging radar, high-fidelity PSF characterization of the radar is achieved, revealing the resolution limits of the radar while exhibiting good signal-to-noise ratio (SNR).
In this paper, we review modern advances in microwave and millimeter-wave computational frequency-diverse imaging, and submillimeter-wave radar systems. We first present a frequency-diverse computational imaging system developed by Duke University for security-screening applications at K-band (17.5-26.5 GHz) frequencies. Following, we show a millimeter-wave spotlight imaging concept and its conceptual integration with the K-band system as interesting example of sensor fusion. We also demonstrate the application of computational frequency-diverse imaging for polarimetric imaging and phase retrieval problems. We show that using the concept of computational frequency-diverse imaging and quasi-random measurement bases, high-fidelity images of objects can be retrieved without the need for any mechanical scanning apparatus and phase shifting circuits. Increasing the frequency-band of operation, we also demonstrate a 340 GHz radar developed by the Jet Propulsion Laboratory and its application for standoff detection. We demonstrate a new technique to characterize the point-spread-function (PSF) of radars operating at submillimeter-wave frequencies.
In this paper, a spotlight imaging system integrated with a frequency-diverse aperture is presented for security-screening applications. The spotlight imager consists of holographic metasurface antennas that can dynamically be tuned to radiate spotlight patterns allowing the extraction of high-resolution images from a constrained field-of-view (FOV). The reconfigurable holographic metasurface antennas consist of a metasurface layer used to modulate the guided-mode reference to an aperture field of interest producing the desired radiated wavefronts. The reconfigurable operation is achieved in an all-electronic manner without the need for any mechanical moving apparatus or phase shifting circuits. The spotlight aperture operates at a single frequency, 75 GHz, within the W-band frequency regime (75 – 110 GHz) and is used for the high-resolution identification of threat objects while the frequency-diverse aperture operates at K-band frequencies (17.5 – 26.5 GHz) and is used for low-resolution detection purposes. The scene to be imaged is first interrogated using the K-band aperture at low resolution and the constrained-FOV is imaged using the W-band system to achieve the identification of threat objects.
Computational imaging is a proven strategy for obtaining high-quality images with fast acquisition rates and simpler hardware. Metasurfaces provide exquisite control over electromagnetic fields, enabling the radiated field to be molded into unique patterns. The fusion of these two concepts can bring about revolutionary advances in the design of imaging systems for security screening. In the context of computational imaging, each field pattern serves as a single measurement of a scene; imaging a scene can then be interpreted as estimating the reflectivity distribution of a target from a set of measurements. As with any computational imaging system, the key challenge is to arrive at a minimal set of measurements from which a diffraction-limited image can be resolved. Here, we show that the information content of a frequency-diverse metasurface aperture can be maximized by design, and used to construct a complete millimeter-wave imaging system spanning a 2 m by 2 m area, consisting of 96 metasurfaces, capable of producing diffraction-limited images of human-scale targets. The metasurfacebased frequency-diverse system presented in this work represents an inexpensive, but tremendously flexible alternative to traditional hardware paradigms, offering the possibility of low-cost, real-time, and ubiquitous screening platforms.
Conference Committee Involvement (1)
Passive and Active Millimeter-Wave Imaging XXIII
29 April 2020 | Anaheim, California, United States