I. Introduction
Precise time synchronization is crucial and has led to significant advances in capability in varies of scientific and industrial applications [1]–[2]. Fiber-optic time transfer is an ideal solution due to the advantages of low loss, high reliability, wide bandwidth and high stability. To suppress the effect of backscattered noises, the bidirectional wavelength division multiplexing (BWDM) based and the bidirectional time division multiplexing (BTDM) based fiber-optic time transfer scheme has been proposed and demonstrated [3]–[4]. However, for BWDM based fiber-optic time transfer scheme, the bidirectional propagation delay is not symmetrical because of the chromatic dispersion of fiber-links, which will require the complicated and expensive link calibration. For BTDM based scheme, the link calibration is not requiring, however, the special bidirectional optical amplifier and/or repeater are introduced for the long-distance transmission, which requires a dedicated control procedure [5]–[6].