The synchronisation of time and frequency between remote locations is crucial for many important applications. Conventional time and frequency dissemination often makes use of satellite links. Recently, the communication fibre network has become an attractive option for long-distance time and frequency dissemination. Here, we demonstrate accurate frequency transfer and time synchronisation via an 80 km fibre link between Tsinghua University (THU) and the National Institute of Metrology of China (NIM). Using a 9.1 GHz microwave modulation and a timing signal carried by two continuous-wave lasers and transferred across the same 80 km urban fibre link, frequency transfer stability at the level of 5×10−19/day was achieved. Time synchronisation at the 50 ps precision level was also demonstrated. The system is reliable and has operated continuously for several months. We further discuss the feasibility of using such frequency and time transfer over 1000 km and its applications to long-baseline radio astronomy.

We demonstrate a fiber-based multiple-access ultrastable frequency dissemination scheme over an 83 km fiber link. As a performance test, we reproduce the disseminated 9.1 GHz radio-frequency modulation signal at an arbitrary point in the dissemination channel. Relative frequency stability of 7×10(-14)/s and 5×10(-18)/day is obtained. Highly synchronized frequency signals can be regenerated along the entire fiber pathway and its applications are discussed.

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