Joint Frequency and Time Transfer over Optical Fiber with High-Precision Delay Variation Measurement Using a Phase-Locked Loop

We demonstrate a simultaneous frequency and time reference dissemination system over 80 km fiber with a phase-locked loop. The phase fluctuation of the transferred radio-frequency (RF) signal is compensated by adjusting the frequency of the voltage-controlled oscillator (VCO), ensuring a highly stable RF signal at the remote end. The high-precision fiber delay variation is simultaneously obtained with a resolution of sub-picosecond by monitoring the phase of the VCO. In order to break the limitation of phase ambiguity and obtain wide-range delay variation measurement, we employ an auxiliary frequency-divided phase measurement branch. A stable time reference transfer is realized after the accurate delay compensation. Experimentally, the frequency stability of the dissemination system with an Allan deviation achieves 1.2 × 10^-17 at 10 000 s averaging time, and the time stability with the time deviation reaches 0.97 ps at 4000 s averaging time. The root mean square jitter of the time distribution system is about 6.6 ps.