  High-precision optical pulse trains distribution via fibre links has made
huge impacts in many fields. In most published works, the accuracies are still
fundamentally limited by some unavoidable noises, such as thermal and shot
noise from conventional photodiodes, thermal noise from mixers. Here, we
demonstrate a new high-precision timing distribution system by using
highly-precision phase detector to overcome the limitations. Instead of using
photodiodes and microwave mixers, we use several fibre Sagnac-loop-based
optical-microwave phase detectors to realize optical-electrical conversion and
phase measurements, for suppressing the noises and achieving ultra-high
accuracy. A 10-km fibre link distribution experiment shows our system provides
a residual instability at the level of 4.6*10-15@1-s and 6.1*10-18@10000-s,
with an integrated timing jitter as low as 3.8 fs in a bandwidth of 1 Hz to 100
KHz. This low instability and timing jitter makes it possible that our system
can be used in the optical clock distribution or the applications for the
facilities which require extremely accuracy frequency time synchronization.
