Dissemination of time and RF frequency via a stabilized fibre optic link over a distance of 420 km

TLDR

The paper presents long‑distance fibre‑optic dissemination of 1 PPS time and 10 MHz frequency signals from atomic sources such as caesium clocks, hydrogen masers, and caesium fountains. The system uses a fibre‑optic link with active delay stabilization, bidirectional amplifiers, and a calibration procedure, and was deployed over a 421.4 km route between GUM in Warsaw and AOS near Poznań. Laboratory tests over up to 480 km achieved Allan deviation ≈ 4 × 10⁻¹⁷, sub‑1 ps time deviation, and picosecond‑level calibration; field tests over 421.4 km matched GPS‑receiver calibration accuracy with lower noise, confirming full functionality.

Abstract

In this paper we present the results of our work concerning the long-distance fibre optic dissemination of time (1 PPS) and frequency (10 MHz) signals generated by atomic sources, such as caesium clocks, hydrogen masers or caesium fountains. For these purposes we developed dedicated hardware (a fibre optic system with active stabilization of the propagation delay and bidirectional fibre optic amplifiers) together with a procedure to enable calibration of the time transfer. Our laboratory measurements performed over fibre lengths of up to 480 km showed an Allan deviation of the order of 4 × 10−17, time deviation below 1 ps (both at one-day averaging) and the possibility of calibration with picosecond accuracy even for the longest from evaluated links. After successful laboratory evaluation the system was next installed on a 421.4 km long route between the Central Office of Measures (GUM) in Warsaw, Poland, and the Astrogeodynamic Observatory (AOS) in Borowiec near Poznań, Poland. Experiments comparing the UTC(PL) and UTC(AOS) atomic timescales using the fibre optic link and TTS-4 dual-frequency GNSS time transfer receivers showed that the consistency of the results is within the calibration accuracy of the GPS receivers and with much better noise performance. The field operation of the system proved its full functionality and confirmed our previous laboratory evaluation to the maximum extent possible using the methods for comparing distant clocks available at GUM and AOS.