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.

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In this paper, we present an overview of the electronically stabilized (thus named ELSTAB) fiber-optic time and frequency (T&F) distribution system based on our idea of using variable electronic delay lines as compensating elements. Various extensions of the basic system, allowing building a long-haul, multiuser network are described. The fundamental limitations of the method arising from fiber chromatic dispersion and system dynamics are discussed. We briefly characterize the main hardware challenge of the system, which is the design of a pair of low-noise, precisely matched delay lines. Finally, we present experimental results with T&F distribution over up to 615 km of fiber, where we demonstrate frequency stability in the range of 1-7 ×10(-17) for 10(5) s averaging and time calibration with accuracy well below 50 ps. Also, practical implementation of the ELSTAB in the Polish T&F distribution network is shown.

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In the paper we analyze the fundamental accuracy limits of the time/frequency transfer in fiber-optic transmission systems based on intensity modulation and direct detection (IM-DD) of the light signal. The unidirectional and bidirectional time/frequency transfer schemes are considered, and their main limitations are pointed out. In particular, the impact of the fiber backscattering and the temperature dependence of the chromatic dispersion are examined in the context of bidirectional transfer. Finally, experimental results are presented and related to the preceding considerations. The experiments performed with a 60 km long fiber demonstrate single-picosecond accuracy of the time transfer. Our measurements suggest that it should be possible to obtain better accuracy of time/frequency transfer than that reported in the literature for systems based on the IM-DD principle.

In this paper we describe a new solution of active delay stabilization for fibre-optic distribution of time and RF-frequency signals, which allows one to obtain both high precision and a potentially unlimited range of compensation of the fibre delay fluctuations. The solution is based on a hybrid system exploiting a pair of continuously tuned electronic variable delay lines, and a set of switched optical delays. We present a fully operational prototype of the time and frequency distribution setup based on this idea, which is capable of compensating more than 1 µs of the fiber delay fluctuations, and thus may be used in very long-haul links up to about 1000 km, without the need for any seasonal maintenance. We also report measurements of the time and frequency distribution stability, and the verification of the time transfer calibration.

We report a stability below 7 × 10−17 of two independent optical lattice clocks operating with bosonic 88Sr isotope. The value (429 228 066 418 008.3(1.9)syst (0.9)stat Hz) of the absolute frequency of the 1S0 – 3P0 transition was measured with an optical frequency comb referenced to the local representation of the UTC by the 330 km-long stabilized fibre optical link. The result was verified by series of measurements on two independent optical lattice clocks and agrees with recommendation of Bureau International des Poids et Mesures.

The quality of Very Long Baseline Interferometry (VLBI) radio observations predominantly relies on precise and ultra-stable time and frequency (T&F) standards, usually hydrogen masers (HM), maintained locally at each VLBI station. Here, we present an operational solution in which the VLBI observations are routinely carried out without use of a local HM, but using remote synchronization via a stabilized, long-distance fibre-optic link. The T&F reference signals, traceable to international atomic timescale (TAI), are delivered to the VLBI station from a dedicated timekeeping laboratory. Moreover, we describe a proof-of-concept experiment where the VLBI station is synchronized to a remote strontium optical lattice clock during the observation.