Stabilized Time Transfer via a 1000-km Optical Fiber Link Using High-Precision Delay Compensation System

Abstract

Variations in optical fiber length and refractive index are induced by environmental perturbation, resulting in an additional dynamic propagation delay in fiber-based time synchronization systems, which deteriorate their transfer stability. This disadvantage can be significantly reduced by transmitting the time signal in both directions through fiber and constructing a feedback loop to compensate the propagation delay at the remote end of the link. This paper proposes an analog-digital hybrid proportional integral derivative (PID) control compensation system based on the time-frequency phase-locked loop (TF-PLL). The system is designed to keep the merits of wide servo bandwidth, servo accuracy, and a large dynamic delay compensation range up to 1 s, which is much greater than that reported in previous studies. For proving the validity of this proposed scheme, a self-developed optical fiber time synchronization equipment based on the delay compensation system is applied. The delay compensation system is used on a 1100-km long laboratory optical fiber, and the results show that the time synchronization stability in terms of time deviation (TDEV) is less than 5.92 ps/1 s and 2.56 ps/10,000 s. After successful laboratory evaluation, the proposed system is installed on a real 988.48-km line between the Xi’an Lintong branch of the National Time Service Center (NTSC) and Linfen City, Shanxi Province, realizing the time synchronization of 10 stations along the optical fiber link. The experimental results in the 988.48-km link illustrate that the measured time difference with a peak-to-peak value of 176 ps, the standard deviation of 19.3 ps, and a TDEV of less than 10.49 ps/1 s and 2.31 ps/40,000 s is achieved. The high-precision time delay compensation system proposed in this paper is simple, reliable, and accurate; has a wide range of compensation; and opens up a feasible scheme for providing synchronized time signals to multiple users over the long-distance field optical fiber networks.

Hollow-Core Optical Fibers for Telecommunications and Data Transmission

“…Electronic Monitoring and Stabilization of Link Latency Since about 2005, there has been intense work carried out on the monitoring and stabilization of fiber path latency by sending a test signal to a remote site and back on a pair of closely spaced wavelengths (≈0.8 nm) via the same fiber to minimize the influence of chromatic dispersion and strain. The stability of time transfer over a 1000 km long SMF link improved to few ps was reported by several teams [125][126][127], and a 0.3 ps stability during 3 h was achieved during time transfer via a 96 km fiber link in a metro network [128]. This method has been adopted for long-and medium-distance time distribution, where the cost of equipment is compensated for by use of cheap, and usually already installed, SMF.…”

Simultaneous transmission of time-frequency and data with co-amplification over urban fiber links

“…With active compensation based on an optical delay line, the frequency instability 5:9 × 10 −17 at 10,000 s over a 106-km urban fiber link is obtained. Much excellent research has been done on long distance optical frequency [22] , radio frequency [23,24] , time [25,26] , and simultaneous transmission [27][28][29][30][31] . All of the present research is focused on the joint transmission of metrological signals over longer or more complex optical fiber links.…”

A high-precision bidirectional time-transfer system over a single fiber based on wavelength-division multiplexing and time-division multiplexing

“…The development of optical fiber time-transfer equipment will provide time signals and technical support for major scientific and technological facilities and also meet the needs of rapid economic and social development. The application range of high-precision time and frequency has penetrated from basic research to the engineering technology application field, which is related to many important departments of the national economy and the people's livelihood [5][6][7].…”