Long-term stable frequency transfer over an urban fiber link using microwave phase stabilization.

We report a novel technique for highly stable transfer of a frequency comb over long optical fiber link. The technique implements an electronic compensation loop to cancel out the phase fluctuations that is introduced by the fiber. We utilized this technique to transfer a stable microwave frequency through a 20 km urban fiber link and an 80 km open air fiber link respectively. For the 20 km urban fiber link, the active compensation system reduced the phase fluctuation from 75 mrad (118 ps) to 4 mrad (6.3 ps) in 48 hours, and the frequency stability was improved by three orders of magnitude. For the 80 km open air fiber link, the active compensation system reduced the rms phase fluctuation from 580 mrad (914 ps) to 10 mrad (16 ps) in 24 hours, and the frequency stability was improved by two orders of magnitude.