Microwave frequency transfer over 3000-km fiber based on optical frequency combs and active noise cancellation

Time and frequency transfer plays an important role in the fields of navigation and positioning, remote sensing, and fundamental physics. The performance of long-haul frequency transfer is ultimately limited by accumulated phase and amplitude noise in fiber propagation. In this work we overcome these limitations and demonstrate a frequency transfer system over 3000 km of indoor spooled fibers via repetition-frequency-locked frequency combs, which benefit from the extremely high signal-to-noise ratio. With the help of the digital phase discrimination and compensation method and active power stabilization, the dramatic phase and amplitude variations are sufficiently suppressed, respectively, which results in the residual instability 7.5×10^-14 at 1 s and 1.0×10^-16 at 10 000 s, without optical-electrical-optical recovery by the regeneration system. Our results mark a breakthrough in building large-scale, high-precision synchronization networks.