Two-Way Optical Time and Frequency Transfer Over a 20-km Fiber Link Based on Optical Frequency Combs

Traditional high-precision long-distance time–frequency transmission requires a complex compensation structure and a continuous optical link, which make high-precision transmission difficult. In this study, we use two counter-propagating optical frequency combs to realize high-precision two-way time–frequency transfer over a 20-km fiber link. The transmission distance transmitted in this experiment is also farther greater than the current free-space transmission distances transmitted over free space. The two optical frequency combs are locked to the same radio frequency reference source. The method makes measurements more flexible and reduces interference. To suppress the effect of dispersion, we use an optical filter to narrow the bandwidth of the optical comb. Meanwhile, the bandwidth of the optical filter is chosen to ensure that the optical comb has the best signal-to-noise ratio at a remote site. We measure the timing difference over 24 h between the local site and remote site.