Fiber-Optic Time Transfer Based on Bidirectional FDM and Cross Correlation Processing

In this article, we proposed a high-precision fiber-optic time transfer (FOTT) scheme based on bidirectional frequency division multiplexing and cross correlation (BFDM-CC) processing. Time signals at different stations are encoded as different time-varying signals within different frequency passbands, respectively. The transferred time-varying signals are carried on the same wavelengths and transmitted to each other, which are precisely recovered at the receiving stations. The time differences between the received time-varying signals and the one generated according to the local time signals are measured by cross correlation processing. Since the transferred time-varying signals are nonoverlapping on spectrum, backscattering noises from fiber links can be effectively suppressed by simple electrical filtering. At the same time, the symmetry of bidirectional transmission can be guaranteed maximally to avoid the time-consuming and laborious link calibration and support fiber link switching without requiring link recalibration. The proposed scheme is experimentally demonstrated over 50-, 100-, and 150-km fiber links, respectively. The results show that the measured mean clock difference can be less than 3.55 ps over fibers with different lengths, and the stability in terms of time deviation can be less than 34.46 ps at 1 s and 1.26 ps at 1000 s over 150-km fiber link, respectively.