A 500-km Cascaded White Rabbit Link for High-Performance Frequency Dissemination

We perform experiments exploring the use of white rabbit precision time protocol (WR-PTP) for time and frequency dissemination over long-distance optical fiber links. We use unidirectional links, to ensure compatibility with active telecommunication networks, and White Rabbit equipment with modifications for improved performance. Using fiber spools, we realize a 500 km, four-span cascaded white rabbit link. We show short term fractional frequency stability of
$2\times 10^{-12}$
, averaging down to
$2\times 10^{-15}$
at one day of integration time, with no frequency shift within the statistical uncertainty. We demonstrate the impact of increasing the White Rabbit SoftPLL bandwidth and the PTP message rate. We show evidence of the effect of thermal fluctuations acting on the fiber, and finally discuss the limitations of the achieved performance. We show comparisons with experimental data acquired with commercial good quality global positioning system (GPS) receivers and show that the medium- and long- term stability and accuracy are more than one order of magnitude better with a WR-PTP link.