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In this paper, we propose a fiber-optic time synchronization technique based on the bidirectional wavelength division multiplexing (WDM) transmission over a fiber link. The effect of fiber dispersion on the uncertainty of the system is effectively suppressed by properly allocating the forward and backward wavelength maps. Thus, only an initial easy-to-implement calibration in a back-to-back configuration instead of multiple complicated link calibrations is required. To compensate the propagation delay fluctuations, a large-dynamic-range time pre-compensator with high precision is implemented at the local site, enabling a time signal at the remote site is accurately synchronized with the clock at the local site. Our experiments demonstrate that stabilities in terms of time deviation (TDEV) of less than 29.8 ps at 1s and 5.2 ps at 10 4 s and the clock difference of less than 28 ps can be achieved for the fiber link up to 200 km. INDEX TERMS Optical fiber, time synchronization, wavelength division multiplexing, metrology.

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We propose a high-precision joint time and frequency transmission scheme based on bidirectional wavelength division multiplexing transmission over an optical fiber link. The time signal is generated based on the phase-stable frequency signal with employment of a dedicated designed low-jitter event generator. Time synchronization is realized by eliminating the time difference between the time signals of the master and slave stations, which is determined by accurate two-way time comparison. In this way, thanks to the high stability of the frequency transmission, low jitter of the dedicated designed event generator, and the high accuracy of the two-way time comparison, a high precision time signal with enhanced time stability and accuracy can be obtained at the slave station, which is synchronized to the master station. Experimentally, a joint time and frequency transmission system is demonstrated over a 62-km urban fiber link. The results show a time stability in terms of time deviation (TDEV) of 3.5 ps/s and 430 fs/10,000 s, and an accuracy of better than 20 ps can be realized.

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