To realize high-precision time-frequency transmission within a territory, the performance of ultra-long distance field links needs to be verified. In the case of the fiber time-frequency transmission of ultra-long links, optical power attenuation and cumulative phase noise increase with increasing link length, resulting in a decrease in compensation bandwidth and noise rejection ratio. Multi-stage cascade compensation is an effective solution, and introducing more cascaded transmission is the key to building a wide-area optical fiber time-frequency transmission network. For many applications that require a high time synchronization accuracy, it is necessary to know the exact delay value between the two sides to be synchronized while obtaining a stable time signal. Therefore, delay calibration and evaluation of the delay uncertainty of the optical fiber time-frequency transmission system are essential. This paper reports the realization of time-frequency cascade transmission and accurate delay calibration over a 515.66 km metro fiber link.

A two-stage cascade design is adopted for the high-precision time-frequency simultaneous transmission system. The system is based on a dual-wavelength noise suppression scheme with optical compensation as the core, in which the time and frequency signals are co-transmitted by a wavelength division multiplexing (WDM) scheme. The structure and implementation of each level of synchronous transmission system are the same. Therefore, we consider the first level as an example. The 100 MHz frequency signal reaches 1 GHz frequency signal after 10 frequency doubling, and then is modulated by the light wave signal with wavelength of fiber channel C35 through the transmitter. The signal-to-noise ratio of transmission can be effectively improved by adopting a higher frequency. Concurrently, 1 PPS (pulse per second) time signal at the transmitting end is modulated by the optical wave signal of fiber channel C34 and entered into the optical fiber link using WDM technology. After reaching the receiving end, the frequency and time signals were obtained by demodulation of the detector output. The time-frequency signal is divided into two channels, with the frequency signal from one channel reduced by the frequency reducer to obtain a 100 MHz signal and the time signal reproduced to obtain a 1 PPS signal, output at the node as a time-frequency signal, which is input into the subsequent cascade system for transmission. The frequency signal from the other channel is modulated by the light wave signal of fiber channel C37, and the time signal is modulated by the light wave signal of fiber channel C36. The light wave signal is returned to the transmitting end through the same fiber link loop using WDM technology to suppress noise.

The frequency instabilities at the user end with compensation are 6.49×10^-14 at 1 s and 5.22×10^-17 at 10⁴ s on average. The time instabilities are 2.97×10^-11 at 1 s and 2.46×10^-12 at 400 s on average (Fig. 4). The discrepancy between the calibrated and measured values of the cascaded transmission delay for a 1 PPS signal in a 515.66 km field link is merely 29.90 ps (Table 2), whereas the uncertainty of unidirectional delay in the transmission link is 9.49 ps (Table 3). This demonstrates that achieving a time synchronization accuracy of better than 50.00 ps is feasible for a time-frequency transmission system over a field fiber link of more than 500 km. Consequently, this study presents an effective solution for accurate delay calibration and high-precision time synchronization in long-distance fiber optic links.

In this paper, we report the results of our work concerning high-precision frequency and time transfer in a partial Beijing-Langfang-Baoding optical fiber backbone network of 515.66 km using the cascaded method. Through optical compensation, transmission at each stage of the system is stabilized. The final additional frequency instability of the entire cascade system is 6.49×10^-14 at 1 s and 5.22×10^-17 at 10⁴ s on average. The final time-attached instability is 2.97×10^-11 at 1 s and 2.46×10^-12 at 400 s on average. The difference between calibration and measured values for delay calibration of the cascade system is 29.90 ps. By analyzing the source of the uncertainty, the calculated uncertainty of the one-way transmission delay is 9.49 ps. Lossless transmission of high-precision time-frequency signals in long-distance commercial fiber optic networks and delay calibration of different stations in the network are realized. This lays a foundation for the future construction of ultra-long distance optical fiber time-frequency transmission networks and multi-point time synchronization. This research has important application prospects in the fields of large-scale atomic clock comparison and ultra-long baseline interferometry.