Timing jitter reduction over 20-km urban fiber by compensating harmonic phase difference of locked femtosecond comb

A novel timing jitter reduction system over a round trip 20-km urban fiber link is reported. The phase difference of the ninth harmonic of a high-repetition-rate mode-locked laser between the local and the returned signals is obtained. Based on the phase difference, the system uses an optical delay line (ODL) to compensate the optical fiber link. The root-mean-square (RMS) timing jitter is reduced from 50 to 8.9 ps in 80 min.