Guided Reflectometry Imaging Unit Using Millimeter Wave FMCW Radars

Abstract

<p>This article introduces a novel and simplified implementation of a <span><strong>guided terahertz reflectometry system</strong></span> that leverages <span><strong>Frequency Modulated Continuous Wave (FMCW) radar technology</strong></span> for imaging and sensing applications. The innovation lies in the use of a <span><strong>single hollow-core dielectric waveguide</strong></span> to directly connect the radar transceiver to the sample, eliminating the need for bulky and alignment-sensitive optical components typically required in quasi-optical setups.</p> <p> </p> <p>The study demonstrates that by combining FMCW radar systems—known for their phase-sensitive distance measurement capabilities—with dielectric waveguides, it becomes possible to differentiate between <span><strong>parasitic reflections</strong></span> along the waveguide and <span><strong>true sensing signals</strong></span> at the probing end. This significantly improves the <span><strong>signal-to-noise ratio (SNR)</strong></span> and simplifies the system architecture, paving the way for <span><strong>compact, portable, and cost-effective terahertz sensing solutions</strong></span>.</p> <p> </p> <p>Two radar architectures are tested:</p> <p><span> 1. A </span><strong>high-performance III-V semiconductor-based 100 GHz SynView radar unit</strong><span>, which serves as the reference, and</span></p> <p><span> 2. A </span><strong>compact, low-cost 122 GHz SiGe radar chip from Silicon Radar GmbH</strong><span>, illustrating the versatility of the concept.</span></p> <p> </p> <p>The study incorporates <span><strong>3D full-wave electromagnetic simulations</strong></span> to evaluate key aspects such as:</p> <p>• Power coupling efficiency between the radar and waveguide</p> <p>• Beam propagation and mode profiles within the waveguide</p> <p>• Influence of waveguide dimensions on performance</p> <p>• Imaging resolution and artefacts</p> <p> </p> <p>The use of a <span><strong>thin-walled polypropylene hollow-core waveguide</strong></span> is central to this approach. Its design is based on anti-resonant reflection guiding, enabling low-loss propagation in the air core. Coupling efficiencies are carefully examined through simulations and experiments, revealing an efficiency of ~70% with the horn-based SynView setup and ~18% with the compact patch-antenna chip.</p> <p> </p> <p><span><strong>Imaging capabilities</strong></span> of both configurations are validated through raster scans of standard test targets. Results show that the simpler low-cost system can achieve resolutions close to that of the more advanced setup, albeit with some artefacts. To improve resolution, a <span><strong>solid immersion lens</strong></span> is introduced at the waveguide’s output, enabling better beam focusing and minimizing ghost artefacts. With the lens, the resolution improves significantly—from ~4.5 mm to 2 mm for the SynView setup and down to 1.4 mm for the Silicon Radar system.</p> <p> </p> <p>Key performance metrics evaluated include:</p> <p>• Maximum achievable dynamic range (~27 dB)</p> <p>• Beam profile uniformity and imaging artefacts</p> <p>• Working distance and lateral resolution trade-offs</p> <p> </p> <p>Ultimately, the work <span><strong>proves the feasibility of a low-cost, guided terahertz FMCW reflectometry unit</strong></span>. It highlights its potential for scalable applications in <span><strong>non-destructive testing (NDT), remote sensing, and imaging</strong></span> across a wide range of industries. The modularity of the system—allowing different radar front-ends and waveguide configurations—adds flexibility and broadens its applicability.</p>