High-precision long-haul fiber-optic time transfer between multi stations

To achieve high-precision fiber-optic time transfer, the method of two-way transmission is usually used. Therefore in this paper we propose to develop a high-precision long-haul fiber-optic time transfer between multi stations by simultaneously transferring the 1 pluse per second signal, time code signal and 10 MHz frequency signal over single fiber with the same wavelength, and adopting the time division multi address (TDMA) as well as the purification and regeneration method at individual station. In this proposal, the equipment at each remote station has its own address, and the equipment at the local station can establish the periodic two-way time transfer with any remote station by using the TDMA method, therefore each remote station is synchronized with the local station. To avoid the superimposed effect of optical noises during propagation in fiber, the optical-electro-optical relay amplifiers are utilized. In the meantime the propagation delay of the fiber link is compensated for at each remote station. With the self-developed engineering prototypes, the experimental verifications are subsequently conducted both in laboratory and real field. In the laboratory, the experimental setup is built by cascading 11 rolls of 50 km-long fiber coils, and locating three monitoring devices at different fiber distances of 50, 300, and 550 km from the local station. The stabilities of the time transfer at these three points are achieved to be 16.7, 16.8, and 18.4 ps in standard deviation, and the time deviations are 1.78, 2.09, and 2.92 ps at an averaging time of 1000 s respectively. In the real field test, a field fiber link of 871.6 km in length is utilized, along which 11 self-developed time-frequency transceivers are set at the cascaded fiber-optic stations. Since only the 11th remote station is co-located at the local station, the performance and the time transfer between the 11th remote station and the local station are measured accurately. The time transfer is experimentally demonstrated with the time standard deviation of 29.8 ps and the time deviations of 3.85 ps/1000 s. The timing uncertainty on the field fiber link is also checked and gives a value of 25.4 ps. To further improve the long-term stability of time transfer, the more accurate thermal control of the lasers used in the system should be adopted to reduce the optical wavelength drift. By compressing the bandwidth of the phase locked loop module in each remote device, the short-term stability of time synchronization can also be better. This proposal can also be extended to the fiber networks with star-shaped and chain-shaped connections. Therefore time synchronization in even larger areas and more stations can be realized.