Millimetre-Resolution Photonics-Assisted Radar

Abstract

Radar is essential in applications such as anti-collision systems for driving, airport security screening, and contactless vital sign detection. The demand for high-resolution and real-time recognition in radar applications is growing, driving the development of electronic radars with increased bandwidth, higher frequency, and improved reconfigurability. However, conventional electronic approaches are challenging due to limitations in synthesising radar signals, limiting performance. In contrast, microwave photonics-enabled radars have gained interest because they offer numerous benefits compared to traditional electronic methods. Photonics-assisted techniques provide a broad fractional bandwidth at the optical carrier frequency and enable spectrum manipulation, producing wideband and high-resolution radar signals in various formats. However, photonic-based methods face limitations like low time-frequency linearity due to the inherent nonlinearity of lasers, restricted RF bandwidth, limited stability of the photonic frequency multipliers, and difficulties in achieving extended sensing with dispersion-based techniques. In response to these challenges, this thesis presents approaches for generating broadband radar signals with high time-frequency linearity using recirculated unidirectional optical frequency-shifted modulation. The photonics-assisted system allows flexible bandwidth tuning from sub-GHz to over 30 GHz and requires only MHz-level electronics. Such a system offers millimetre-level range resolution and a high imaging refresh rate, detecting fast-moving objects using the ISAR technique. With millimetre-level resolution and micrometre accuracy, this system supports contactless vital sign detection, capturing precise respiratory patterns from simulators and a living body using a cane toad. In the end, we highlight the promise of merging radar and LiDAR, foreshadowing future advancements in sensor fusion for enhanced sensing performance and resilience.