Picosecond-precision optical two-way time transfer in free space using flexible binary offset carrier modulation

Free-space optical time transfer that features high precision will act as a crucial role in near-future outdoor timing service and ground-to-satellite/inter-satellite clock networks. Here we propose a free-space optical two-way time transfer method using flexible binary offset carrier modulation. The alternative method could yield a comparative precision compared to optical binary phase-shift keyed modulation. For verification, a time transfer experiment with our home-built system between two sites separated by a 30-m free-space path outside the laboratory was conducted. Over a 15 h period, the time deviation is 2.3 ps at 1-s averaging time, and averages down to 1.1 ps until ∼30 s. The fractional frequency instability exhibits 4.0×10−12 at a gate time of 1 s, and approaches to 1.3×10−15 at 10000 s.