Time Synchronization over a Free-Space Optical Communication Channel

TL;DR

This paper demonstrates real-time two-way time synchronization over a 4 km turbulent free-space optical channel using a repurposed digital communication system, achieving sub-picosecond accuracy.

Contribution

It introduces a novel application of FSO communication for precise time transfer, achieving high accuracy over turbulent air paths.

Findings

Fractional frequency deviation below 10^-15

Time deviation below 1 picosecond

Peak-to-peak wander of 16 ps over 8 hours

Abstract

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.