| Summary: | This paper demonstrates a fiber-optic time transfer over a 6000 km noncalibrated fiber link using proposed single-fiber bidirectional-transmission unidirectional optical amplifiers (SFBT-UOAs). The proposed SFBT-UOA employs a 2 × 2 optical switch to enable the time-division-multiplexed time signals for both directions to pass through the same UOA in different time slots. The bidirectional delay symmetry is guaranteed to the maximum extent by allowing the time signals to be transmitted back and forth using the same wavelength over a single fiber. Meanwhile, the noises suffered in the systems employing single fiber bidirectional optical amplifiers (SF-BOAs), such as double Rayleigh backscattering, are effectively suppressed. Consequently, high-precision fiber-optic time transfer can be implemented over ultralong distance by adopting the proposed SFBT-UOAs. Moreover, the SFBT-UOA can support the conventional network services via wavelength division multiplexings (WDMs) and, therefore, offers the potential to perform high-precision time transfer over long-haul commercial fiber links. Over 6000 km, the measured time deviations of fiber-optic time transfer in the laboratory are less than 190 ps/s and 61 ps/10<sup>5</sup> s, respectively. The calculated combined uncertainty is not beyond 70 ps, which agrees well with the experimental verification over the noncalibrated fiber link.
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