Fig. 1: A photonics-enabled radar system for contactless vital sign detection.
a, Conceptual drawing of the vital sign radar with distributed sensing access points (APs) enabled by low-loss fibre and a centralized, photonics-assisted radar platform. b, Schematic of the demonstrated photonic radar for vital sign detection based on a frequency-shifting (FS) fibre cavity with its optical input and output signals shown on the top. A seed pulse at a single frequency gated by a temporal square envelope is optically injected to the FS fibre loop. The output signal unveils the resultant optically synthesized stepped-frequency signal. E(t) denotes the electric field of the optical signals in the time domain. c, The optical coherent ranging principle is realized by mixing transmitted (solid lines depicted in the time–frequency domain) and received (dashed lines) optical SF signals in a photodetector (PD). The demodulated RF signals after the PD are illustrated in the bottom panels, demonstrating the range at two distances, d1 and d2. Δτ1 and Δτ2 are the time delays introduced by the radar signal propagation with a speed of c. I(t) denotes the demodulated signals (the photocurrent) after the photodetection in the time domain. d, Illustration of the use of the demonstrated photonic radar with millimetre resolution for detecting respiratory activities from multiple targets. OS, optical switch; FS, frequency-shifting. Δf, FS introduced by the AOFS.
