Time Synchronization Over a Free-Space Optical Communication Channel

Free space optical communication channels can transmit high-speed data between sites over the air. We show here that a FSO digital optical communication channel can be re-purposed and used 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 have a sub-3-ps time deviation below the synchronization bandwidth and a fractional frequency deviation below 10-
¹⁵
at one hour averaging time. Over an 8-hour period the peak-to-peak wander is 16 ps. Work of the US government not subject to copyright.