Time synchronization over a free-space optical communication channel

Future high-precision free-space optical (FSO) clock networks will require laser links to transmit time-frequency between sites and, with this information, to synchronize the times across widely separated clocks. FSO communication networks already use laser links between remote sites to transmit high-speed data. Here we repurpose a FSO digital communication system and use it directly for two-way time-frequency transfer. We demonstrate synchronization of the time between two sites separated by a turbulent air path of 4 km using binary-phase-modulated continuous-wave laser light. Under synchronization, the two sites exhibit a fractional frequency deviation below 10−15 at 1500 s averaging time and a time deviation below 1 ps at averaging times of seconds to hours. Over an 8 h period, the peak-to-peak wander is 16 ps. This method should be applicable to future ground-to-space and intra-satellite links and could lead to an improved global navigation and satellite system.