Microwave-to-optical phase synchronization techniques have attracted growing research interests in recent years. Here, we demonstrate tight, real-time phase synchronization of an optical frequency comb to a rubidium atomic clock. A detailed mathematical model of the phase locking system is developed to optimize its built-in parameters. Based on the model, we fabricate a phase locking circuit with high integration. Once synchronized, the fractional frequency instability of the repetition rate agrees to 6.35×10^-12 at 1 s and the standard deviation is 1.5 mHz, which indicates the phase synchronization system can implement high-precision stabilization. This integrated stable laser comb should enable a wide range of applications beyond the laboratory.

An ultra-highly precise and long-term stable frequency transmission system over 120 km commercial fiber link has been proposed and experimentally demonstrated. This system is based on digital output compensation technique to suppress phase fluctuations during the frequency transmission process. A mode-locked erbium-doped fiber laser driven by a hydrogen maser serves as an optical transmitter. Moreover, a dense wavelength division multiplexing system is able to separate forward and backward signals with reflection effect excluded. The ultimate fractional frequency insta-bilities for the long-distance frequency distributed system are up to 3.14×10-15 at 1 s and 2.96×10-19 at 10 000 s, re-spectively.

Time synchronization techniques, especially on the pulse per second (PPS) temporal basis, have attracted growing research interests in recent years. In this paper, we have proposed and experimentally demonstrated a high-precision two-way time transfer (TWTT) system to realize long-distance dissemination of 1 PPS signal generated by a hydrogen maser. A dense-wavelength-division-multiplexing (DWDM) system and bi-directional erbium-doped fiber amplifiers (Bi-EDFAs) have also been adopted to suppress the impact of Rayleigh backscattering and optimize the signal to noise ratio (SNR) as well. We have theoretically analyzed the systematic delay in detail. The ultimate root mean square (RMS) variation of time synchronization accuracy is sub-26 ps and the time deviation can be reduced to as low as 1.2 ps at 100 s and 0.253 ps at 12 000 s, respectively.