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

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.

Frequency transfer via a two-way optical phase comparison on a multiplexed fiber network

“…Phase-compensated optical links have proved to be reliable from this point of view and outperform state-ofthe-art satellite techniques by orders of magnitude [4]. The trasmission of RF and microwaves [5][6][7][8][9], optical frequencies [10][11][12][13], and of an optical comb [14] have been demonstrated, and time dissemination has recently been performed as well [15][16][17]. In coherent optical links the phase noise added by the fiber due to environmental noise is actively cancelled.…”

Frequency and time transfer for metrology and beyond using telecommunication network fibres

“…For this purpose, several methods are under development. One method relies on the stabilization of the propagation delay and a careful calibration procedure [18,52]. Another is based on the well-known two-way methods, introduced in the 80's by Hanson [53].…”

Delivering 10 Gb/s optical data with picosecond timing uncertainty over 75 km distance

“…The uncertainty in the clock offsets is primarily due to the estimated value of the one-way delay (OWD), which is determined from measured signal round trip delays (RTDs). To our knowledge, the values reported in Ref [7]. represent the most accurate estimates of one-way signal propagation delays over distances in the 50 -500 km range to date.…”

“…GNSS. Recent work on fiber-optical TWTT has resulted in observed clock offsets as small as 6 ps (with a standard uncertainty σ = 8 ps) after synchronization through a link of 62 km length, and 35 ps (σ = 20 ps) for a link of 480 km length [7]. The uncertainty in the clock offsets is primarily due to the estimated value of the one-way delay (OWD), which is determined from measured signal round trip delays (RTDs).…”

“…Apart from absolute time accuracy, an important figure of merit of any TWTT system is the time stability (TDEV), which is a measure of the averaging time needed to achieve a certain synchronization level [8]. For TWTT through the 62 km (480 km) link in Ref [7]. this equals about 2 ps after 100 s averaging, reaching a minimum of 0.3 ps (0.6 ps) after 7 × 10 4 s (3 × 10 3 s).…”