Ultra-long haul high-precision fiber-optic two-way time transfer is demonstrated by adopting the proposed scheme called bidirectional time division multiplexing transmission over single fiber with the same wavelength (BTDM-SFSW), which can effectively eliminate the impact of Rayleigh backscattering and reach the maximum bidirectional symmetry at the same time. The inter-range instrumentation group (IRIG-B) time code is modified by increasing bit rate and defining new fields for the proposed BTDM-SFSW time transfer scheme. A dedicated codec (encoder and decoder) with low delay fluctuation and self-synchronization is developed by using a mask technique and combinational logic circuit. A BTDM-SFSW based time transfer testbed is built with the employment of loop configuration in laboratory to evaluate the performance of fiber-optic time transfer over thousands of kilometers. Stabilities of less than 89 ps/s and 23 ps/10⁵ s are achieved for the time transfer over a 2000 km fiber link with single fiber bidirectional amplifiers (SFBA). In order to overcome the issues of using the SFBA including the impact of Rayleigh backscattering and incompatibility with commercial fiber links, a novel time transfer scheme of single-fiber bidirectional-transmission with unidirectional optical amplifiers (SFBT-UOA) is proposed. Transmission over 6000 km is experimentally demonstrated with the time deviations of 190 ps/s and 61 ps/10⁵ s, respectively. A full uncertainty budget for the BTDM-SFSW based time transfer is provided, which considers the contributions of different factors, including the precision and stability of transmitted wavelengths, power dependence of transceivers'receiving delays, Sagnac effect, etc. The calculated theoretic combined uncertainty is not beyond 70 ps without the requirement of fiber link calibration, which well agrees with the experimental verification over different lengths of non-calibrated fiber link.

time transfer  /  optical fiber transmission  /  bidirectional optical amplification  /  uncertainty