Feed-forward digital phase compensation for long-distance precise frequency dissemination via fiber network

We demonstrate precise microwave frequency dissemination of a hydrogen maser synchronized frequency comb over a 120 km commercial fiber link. The phase noise was compensated by a feed-forward digital compensation scheme, where the value of locally detected phase noise was first transmitted to the remote user end via a wavelength division multiplexing channel in the same fiber link and then compensated directly at the user end. The fractional frequency instability was measured to be at 5.28×10(-16)/s.