  Free-space optical time transfer that features high precision and flexibility
will act a crucial role in near-future ground-to-satellite/inter-satellite
clock networks and outdoor timing services. Here we propose a free-space
optical flexible-binary-offset-carrier-modulated (FlexBOC-modulated) time
transfer method. The utilized FlexBOC modulation could yield a comparative
precision, although its occupied bandwidth is tremendously reduced by at least
97.5% compared to optical binary phase modulation. Meanwhile, the adoption of
optical techniques eliminates the multi-path effect that is major limit in the
current microwave satellite time transfer system. What's more, the time
interval measurement avoids a continuous link that may be routinely broken by
physical obstructions. 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
in a 1-s averaging time, and averages down to 1.0 ps until ~60 s. The
fractional frequency instability exhibits 4.0E-12 at a gate time of 1 s, and
approaches to 2.6E10-15 at 10000 s.
