Submillimeter non-line-of-sight ranging and imaging via cost-effective FMCW interferometry

Non-line-of-sight (NLOS) imaging technology reconstructs the spatial structure of hidden objects by analyzing scattered light field information. However, existing methods primarily rely on time-of-flight (ToF) measurements, which are limited by imaging resolution and environmental robustness. Here, we propose a submillimeter NLOS ranging and imaging method based on cost-effective frequency-modulated continuous wave (FMCW) interferometry. By deriving a beat signal model for NLOS scenarios, we design a dual-path interferometric system incorporating a Mach–Zehnder calibration optical path constructed with polarization-maintaining optical delay fiber (PM-ODF). A transformation model is established to convert beat signals from the time domain to the transform domain, followed by the development of a dynamic temporal phase subdivision (DTPS) algorithm. This algorithm utilizes the fixed calibration optical path, rather than an optical frequency comb, to monitor the instantaneous frequency of the swept-frequency laser in real-time and compensate for the sweep nonlinearity effect at arbitrary detection distances. Our method achieves a temporal resolution of approximately 1.8 ps over an all-optical fiber link, approaching the theoretical limit determined by the 546 GHz sweep bandwidth. In free space conditions, we demonstrate NLOS ranging and 3D imaging with a ranging resolution of 450 μm and a ranging error of 23 μm, an axial imaging resolving capability of 1 mm with an imaging error of 0.26 mm, a lateral imaging resolving capability of 2.4 mm, and a non-visible imaging distance of 110 cm, which is consistent with theoretical predictions. We demonstrate the capability for high-fidelity reconstruction under intense ambient illumination conditions exceeding 8.0 klux. Notably, our method maintains accurate reconstruction capabilities for 3D hidden objects. These results provide a 3D visualization with submillimeter accuracy for NLOS detection in challenging environmental conditions.