High-precision time and frequency transfer plays an important role in frontier scientific research and major science and technology infrastructure projects, such as astronomy and geodetic surveying. At present, two-way satellite time and frequency transfer (TWSTFT) and satellite common-view (CV) are primarily applied for the transfer and synchronization of time and frequency signals from different time-keeping clocks. However, these conventional methods, which are based on radio transmission, are unable to satisfy some customers' unique requirements for ultra-high accuracy, stability, and security. Compared with the global navigation satellite system (GNSS)-based time transfer and long-wave time transfer, optical fiber time transfer has the advantages of low loss, low noise, and anti-electromagnetic interference. The bidirectional signal transfer characteristic of single optical fiber ensures a highly symmetric time delay of a bidirectional signal. For this reason, the optical fiber time delay can be compensated to pave the way for picosecond time transfer and frequency transfer. This research is expected to achieve the high-precision common optical fiber transfer of a 1PPS time signal and a 10 MHz frequency signal and ultimately meet the requirements of long-range comparison among time and frequency standards with hydrogen atomic clocks as time-keeping clocks.

To fulfill the engineering application requirements of atomic clock time and frequency comparison, this paper designs an optical fiber time and frequency transfer system based on a wavelength division multiplexing scheme. The methods of dual-wavelength bidirectional comparison and remote site compensation are applied to the time transfer. The time delay of the optical fiber link and its change are measured in real time, and a time-delay phase controller is used at the remote site to compensate for the time delay of the optical fiber link. In this way, the 1PPS signal output at the remote site is accurately synchronized with the reference 1PPS signal at the local site. The information, such as the comparison data, is loaded onto the optical carrier by the encoding technology to achieve information transfer. The transfer of the comparison data by additional links is thereby avoided. The single-wavelength pre-compensation method is employed for the frequency transfer, and phase detection and compensation are carried out at the local site. The phase of the transmitted frequency signal is controlled by a phase-locked loop to compensate for the time delay shift and phase noise caused by the optical fiber link. On this basis, the remote site outputs a 10 MHz frequency signal that is stable relative to the reference frequency signal. The remote-site equipment is connected with the local-site equipment by the standard single-mode optical fiber. The high-precision common optical fiber transfer of the 1PPS time signal and the 10 MHz frequency signal is achieved by wavelength division multiplexing. The fixed time delay of the equipment is further corrected to ensure the high-precision synchronization between the input and output time signals of the system.

To test the noise floor of the equipment, this study presents a test system constructed in the laboratory, which uses short optical fiber to connect the local-site equipment with the remote-site equipment. The test results show that the time transfer stability can reach 4.2 ps@1 s, 1.6 ps@10 s, 0.84 ps@100 s and 1.2 ps@10⁴ s. The stability of 10 MHz frequency transfer can reach 1.9×10^-14@1 s、4.2×10^-15@10 s and 4.8×10^-16@10⁴ s and is thus much better than that of the hydrogen atomic clocks, which reaches 1×10^-13@1 s. Finally, an optical fiber time and frequency transfer test is carried out on a 102-km field optical fiber link, and the stability of 10 MHz frequency transfer is 3.4×10^-14@1 s and 1.5×10^-15@10⁴ s. Time transfer with the stability of 15.7 ps@1s and 3.9 ps@1000 s, and the uncertainty of 25.3 ps is accomplished by correcting the time delay and dispersion of the equipment.

In the present study, high-precision transfer of a 1PPS time signal and a 10 MHz frequency signal on a field optical fiber link is achieved by dense wavelength division multiplexing (DWDM). The methods of dual-wavelength bidirectional comparison and remote-site compensation are applied to the time transfer, which raises transfer precision to more than 30 ps on a 100-km optical fiber link. The single-wavelength pre-compensation method is employed for the frequency transfer, and the stability of the 10 MHz frequency transfer achieved is superior to that of a hydrogen atomic clock. Finally, the development of the equipment is completed to satisfy the requirements of long-range comparison among time and frequency standards with hydrogen atomic clocks as time-keeping clocks.