Cascaded Fiber-Optic Time Synchronization System with Different Modulation Format

High-precision time and frequency transfer plays a pivotal role in metrology, geodesy, deep-space exploration and other scientific applications. Based on current time synchronization research, extending point-to-point schemes into a wide-area network can significantly increase the range of applications. Therefore, we design and implement a cascaded fiber optic time synchronization system, which is the most basic form of networking. This paper considers two different modulation formats for time synchronization systems from the perspective of fiber nonlinear effects, namely the intensity modulation with direct detection (IMDD) scheme and the phase modulation with self-coherent detection (PMSCD) scheme. The analysis indicates that the constant-envelope characteristic of the PMSCD scheme provides superior transmission performance. Accordingly, we deployed the PMSCD system over a 500 km intercity fiber link and the IMDD system over a 68 km metropolitan fiber link, forming a 568 km cascaded system that achieves time synchronization precision better than 50 ps. This work offers a practical reference for the future development of high-precision fiber-optic time and frequency synchronization networks.