Impact of PLL Bandwidth on Stable Frequency Dissemination over a 2500 km Optical Fiber

Phase-locked loop (PLL) is a core component to achieve stable frequency dissemination over long-distance fiber links, because of its signal regeneration and noise filtering. Simply calculating the bandwidth of the PLL from the transmission delay exacerbates the asymmetry due to the dispersion in the fiber, which ultimately leads to reduced frequency instability. A theoretical analysis to describe the relationship of the PLL bandwidth and the frequency instability is proposed. Optimization strategy for the PLL bandwidth is proposed based on the fiber parameters, and an adaptive fourth-order PLL is designed to evolve the system. In experiment, we transmit a 2.4 GHz frequency signal over a 2500 km optical fiber. The frequency instability of
$4.16 \times {10^{ - 14}}$
@ 1 s and
$5.54 \times {10^{ - 17}}$
@ 10,000 s is demonstrated, which is eligible for extra long-distance atomic clock comparison and continental-scale frequency synchronization network construction.