Uncertainty analysis of BTDM-SFSW based fiber-optic time transfer

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.