Optical coherence transfer over 50-km spooled fiber with frequency instability of 2×10^-17 at 1 s

We demonstrate coherent transfer of an ultra-stable optical frequency at 192.8 THz over 50-km spooled fiber. Random phase noise induced by environmental disturbance through fiber is detected and suppressed by feeding a correctional signal into an acousto-optic modulator. After being compensated, the fiber-induced frequency instability is 2×10^-17 at 1-s averaging time and reaches 8×10^-20 after 16 h. The noise floor of the compensation system could be as low as 2×10^-18 at 1-s averaging time.