Free space optical (FSO) communication channels are typically used to transmit high-speed data between sites over the air. Here we repurpose an FSO digital communication system and use it directly for two-way time transfer. We demonstrate real-time synchronization between two sites over a turbulent air path of 4 km using binary phase modulated CW laser light. Under synchronization, the two sites exhibit a fractional frequency deviation below 10−15 and a time deviation below 1 picosecond at averaging times of seconds to hours. Over an 8-hour period the peak to peak wander is 16 ps.
@article{arxiv.1808.06710,
title = {Time Synchronization over a Free-Space Optical Communication Channel},
author = {Isaac Khader and Hugo Bergeron and Laura C. Sinclair and William C. Swann and Nathan R. Newbury and Jean-Daniel Deschênes},
journal= {arXiv preprint arXiv:1808.06710},
year = {2018}
}