Plastic Open-Waveguide Probe for Compact Terahertz FMCW Radar Imaging

Terahertz (THz) imaging has become an attractive technique for non-destructive testing (NDT) owing to its non-ionizing nature and its sensitivity to dielectric properties. It enables the inspection of polymers, composites, foams, coatings, and other low-loss materials while providing access to sub-surface information that is difficult to obtain using conventional optical techniques. Recent advances in silicon technologies have led to the emergence of compact frequency-modulated continuous-wave (FMCW) radars operating in the D-band and beyond. These sensors offer a low-cost, highly integrated, and portable alternative to conventional THz systems, making them particularly attractive for industrial inspection.

However, despite the compactness of the radar electronics, practical imaging systems still rely on bulky dielectric lenses or quasi-optical assemblies to focus the electromagnetic beam. These optical components increase the overall system size, require careful alignment, and limit the development of ergonomic handheld inspection devices.

This work presents a compact THz imaging approach based on a tapered plastic open waveguide directly coupled to a silicon D-band FMCW radar. The waveguide guides and concentrates the electromagnetic field toward the sample, replacing conventional free-space optics with a lightweight probe that can easily access confined or complex geometries. The proposed architecture enables a simple and practical imaging configuration while preserving the range discrimination capabilities of FMCW radar.

The proposed probe was designed and optimized using full-wave electromagnetic simulations performed with CST Microwave Studio. A prototype was then fabricated and evaluated in a reflection-mode raster-scanning imaging setup. Experimental imaging results are presented and compared with the numerical predictions. The proposed approach demonstrates that plastic open-waveguide probes constitute a promising solution for developing compact, low-cost, and ergonomic THz FMCW imaging systems dedicated to industrial non-destructive evaluation.