Progress to coherent frequency transfer over a telecom fiber link at NICT

We performed the 1-GHz and 10-GHz transfer over the 114-km and 90-km urban optical fiber links in Tokyo, respectively. The phase noise in the free-run link was huge because almost 50 % of the optical fibers were not buried under the ground. Nevertheless, we achieved the phase-noise cancellation ratio of 45 dB by an electrical phase-noise compensation system using a VCXO and the stability of 6 times 10^-18 at 1 day in the 10-GHz 90-km transfer. Besides, the RF transfer over the 204-km link was performed by the cascade system. It proved that the transfer stability degrades only radic(N) times by the cascade of the N-th systems.