Time Synchronization over a Free-Space Optical Communication Channel

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.