Wang, J.G. Liu, B. Wu, W. JACoW Publishing Geneva, Switzerland 2673-5350 978-3-95450-236-3 10.18429/JACoW-IBIC2023-TU2C03 English 151-153 laser timing FEM detector electron Contribution to a conference proceedings 2023 2023-12 https://doi.org/10.18429/JACoW-IBIC2023-TU2C03 https://jacow.org/ibic2023/papers/tu2c03.pdf The femtosecond synchronization and distribution system of the Shanghai soft X-ray free-electron laser facility (SXFEL) and Shanghai high repetition rate XFEL and extreme light facility (SHINE) are based on the optical pulse trains generated by passively mode-locked lasers. The passively mode-locked laser has ultralow noise in the high offset frequency (<5 fs, [1 kHz- 1 MHz]). In this paper, we report precise synchronization of the low-noise passively mode-locked laser to the radio frequency (RF) master oscillator. RF-based phase-locked loop scheme, the absolute jitter of the phase-locked passively mode-locked laser is less than 20 fs integrated from 10 Hz up to 1 MHz.