High-resolution microwave frequency transfer over an 86-km-long optical fiber network using a mode-locked laser

We demonstrate the transfer of an ultrastable microwave frequency by transmitting a 30-nm-wide optical frequency comb from a mode-locked laser over $86\mathrm{km}$ of installed optical fiber. The pulse train is returned to the transmitter via the same fiber for compensation of environmentally induced optical path length changes. The fractional transfer stability measured at the remote end reaches $4\times {10}^{-17}$ after $1600\mathrm{s}$, corresponding to a timing jitter of $64\mathrm{fs}$.