Manuscript received September 28, 2015; revised October 26, 2015; accepted November 5, 2015. Date of publication November 10, 2015; date of current version November 20, 2015. This work was supported in part by the National Natural Science Foundation of China under Grant 61127016 and Grant 61107041 and in part by Specialized Research Fund for the Doctoral Program of Higher Education of Ministry of Education under Grant 20130073130005.

In this paper, the uncertainty of time transfer based on a bidirectional time division multiplexing transmission over a single fiber with the same wavelength (BTDM-SFSW) is investigated via theoretical models and experimental measurements. According to the principle and system architecture, the uncertainty evaluation schemes for BTDM-SFSW based time transfer are presented and the uncertainty sources are identified accordingly. In order to avoid the effect of the temperature-dependent fiber delay, the measured time intervals at two sites are regarded as a whole to obtain an overall uncertainty of time interval measurements. For the uncertainty of time transfer modem calibration, aside from the type A uncertainty obtained under the applied calibration scheme, the system reproducibility against practical operation and the contribution of optical power-dependent receiving delays are also included. A mathematical model considering fiber dispersion, polarization mode dispersion (PMD) and the Sagnac effect is established to evaluate the uncertainty from the fiber link. The characteristics of the uncertainty sources in a long-distance fiber-optic time transfer testbed are then explored in detail. The combined expanded uncertainties with a coverage factor of 2 are calculated and experimentally validated over various non-calibrated fiber extensions.

In this paper, a fiber-optic signal access method is proposed for stable wideband microwave frequency distribution. Incorporating with the tapped counter-propagating reference signals and variable optical delay lines (VODLs), a stable wideband microwave frequency signal can be obtained at any access point (AP) along the main fiber link. Moreover, the phases of received wideband microwave frequency can be aligned. In this way, the whole system complexity can be dispersed as the central station only stabilizes the main fiber link delay, and the access ends (ANs) operate independently. A multi-access wideband microwave frequency distribution is experimentally demonstrated over a 50 km dispersion-compensated fiber link. A system figure of merit is investigated in detail. The results show that the fiber-optic wideband microwave frequency transmission system can operate for the frequency over 40 GHz. The additive phase noises of -125 dBc/Hz for the remote end (RN), and -131 dBc/Hz for the AN, respectively, are achieved at the 1 kHz offset frequency for a 1 GHz frequency signal. Thanks to the active delay stabilization, the Allan deviation (ADEV) of $1.3 \times 10^{-13} / \text{s}$ and $2.4 \times 10^{-15} /100 \text{ s}$ for the remote end (RN), and the one of $8.8 \times 10^{-14} / \text{s}$ and $4.4 \times 10^{-15} /100 \text{ s}$ for the AN, respectively, are acquired.

Optical amplification, which is necessary for extending the distance of fiber-optic time transfer, has significant impacts on the time transfer performance. In this paper, we investigate and optimize the optical amplification for the time transfer based on bidirectional time division multiplexing transmission over a single fiber with the same wavelength (BTDM-SFSW). Three optical amplification schemes, namely single fiber bidirectional amplifier (SFBA), single fiber bidirectional amplifier with magneto-optical switch (SFBA-MOS), and single-fiber bidirectional-transmission unidirectional optical amplifier (SFBT-UOA) are considered. For each optical amplification scheme over different fiber links, the timing jitter and uncertainty of time transfer are theoretically and experimentally analyzed. A simple timing jitter optimization method for the BTDM-SFSW links is proposed. Further evaluations are performed from the point of view of system complexity, operation cost, and compatibility with telecom optical networks. Results show that SFBA is more suitable over shorter fiber links for its lower complexity and cost, while SFBA-MOS or SFBT-UOA should be employed for longer fiber links to achieve the accepted time transfer performance.

The system performance for wideband microwave frequency distribution is investigated and demonstrated for harsh ambient environment in this paper. A theoretical model for noise property, suffering from the temperature variation and mechanical vibration, is established. Via experimental measurements, the performance deterioration of a typical startopology distribution system is investigated and analyzed in detail. To enhance the capability of the environmental adaptation, phase stabilization and vibration absorption are taken in the optical transmitting end. The temperature dependent fiber propagation delay is stabilized to suppress the long-term drift. For the mechanical vibration, phase stabilization seems to have no obvious improvement on the fiber link, while vibration absorption can significantly suppress the high vibration frequency in the transmitting end. The results obtained reveals that high-performance wideband microwave frequency can still be achieved with fiber-optic distribution in the harsh ambient environment, which is full of significance for the field implementation.