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Free-space optical time transfer that features high precision and flexibilitywill act a crucial role in near-future ground-to-satellite/inter-satelliteclock networks and outdoor timing services. Here we propose a free-spaceoptical flexible-binary-offset-carrier-modulated (FlexBOC-modulated) timetransfer method. The utilized FlexBOC modulation could yield a comparativeprecision, although its occupied bandwidth is tremendously reduced by at least97.5% compared to optical binary phase modulation. Meanwhile, the adoption ofoptical techniques eliminates the multi-path effect that is major limit in thecurrent microwave satellite time transfer system. What's more, the timeinterval measurement avoids a continuous link that may be routinely broken byphysical obstructions. For verification, a time transfer experiment with ourhome-built system between two sites separated by a 30-m free-space path outsidethe laboratory was conducted. Over a 15 h period, the time deviation is 2.3 psin a 1-s averaging time, and averages down to 1.0 ps until ~60 s. Thefractional frequency instability exhibits 4.0E-12 at a gate time of 1 s, andapproaches to 2.6E10-15 at 10000 s.

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