Demonstration of Ranging-Based Phase Encryption for Enhanced Free-Space Optical Security

Free-space optical (FSO) links in satellite networks, unmanned aerial vehicle swarms and dense indoor networks suffer rising interception threats as beam divergence and platform motion undermine traditional narrow-beam security. To counter these risks, we develop a ranging-based phase-encryption scheme that embeds ranging measurements as the physical layer key for secure FSO transmissions. The encryption integrates ranging‑derived phase rotations, an artificial‑perturbation layer, and a chip sequence to achieve uniform phase distribution and a tunable decoding range governed by phase resolution. Sub‑symbol‑level ranging is enabled by an encrypted preamble and square timing recovery within the receiver's digital signal processor. A unidirectional proof-of-concept demonstration is conducted in a controlled laboratory environment at 5 Gbps using 10- and 20-GBaud spread-coded symbol rates over variable link lengths. During the laboratory validation, the ranging reference is obtained from a transmitter-side digitized copy of the AWG waveform, and the proposed scheme delivers millimeter-level ranging precision with a root-mean-square error of 0.206 mm on 1-mm steps while achieving processing gains against both linear and nonlinear distortions along with improved synchronization performance. Comprehensive security tests based on phase statistics, time frequency analysis and autocorrelation intervals confirm that only signals within the configured decoding window can be recovered.