  Optical frequency transfer provides the means for high-fidelity frequency
transfer across thousands of kilometers. A critical step in the further
developing optical frequency transfer is its capability to transfer a high
spectral purity feature from ultrastable lasers or optical clocks to any remote
locations and, at the same time, its adaptability to incorporate the optical
frequency transfer technique into any existing communication networks with
different topologies. Here we for the first time report a technique that
delivers optical-frequency signals to multiple independent remote hubs along a
ring optical-fiber network with passive phase stabilization. The technique
automatically corrects optical-fiber length fluctuations of arbitrary hubs
along the loop by mixing and shifting optical signals. Without the help of an
active phase tracker and a compensator, it could significantly mitigate some
technical problems such as the limited compensation speed and phase recovery
time, the phase jitter contamination caused by the servo bump in conventional
phase noise cancellation. Moreover, by transmitting optical signals along both
directions using the same optical source, it can improve the signal-to-noise
ratio at each hub. This technique maintains the same delay-limited phase noise
correction capability as in conventional techniques and, furthermore, improves
the phase jitter by a factor of 3, opening a way to a broad distribution of an
ultrastable frequency reference with high spectral purity and enabling a wide
range of applications beyond metrology over a ring fiber network with the
naturally impressive reliability and scalability.
