Extending European access to high precision time and frequency information
Challenge
Time and frequency (T&F) reference signals underpin electricity grids, global navigation and the internet. Most applications use signals from orbiting satellites, but their precision is insufficient for advanced research into such things as fundamental physics and quantum technologies.
One alternative is to deliver clock signals via established fibre optic telecommunication networks but there are problems limiting this approach. Optical signals can fade over long distances due to losses or reflections from fibre joins and are affected by “propagation noise” caused by temperature fluctuations or mechanical vibrations in the fibre. These signals also require isolation from network traffic to prevent interference.
Many problems were addressed in EURAMET projects NEAT-FT (2012-2015), OFTEN (2016-2019) and the French REFIMEVE+ project (2012-2024) performed by the Laboratoire de Physique des Lasers (USPN, CNRS), LNE-SYRTE (Observatoire de Paris, CNRS, SU) and RENATER, the French Network for Research and Education (NREN). Ultra-stable T&F dissemination from LNE-SYRTE’s atomic clock was demonstrated over a 540 km telecommunication fibre. Together with PTB, the problem of signal degradation, when T&F is extracted from a link to one or more user, was also addressed. However, the solution was not compatible with automated signal distribution and regeneration.
Solution
REFIMEVE+ established a ring topology network between several universities in Paris using both dedicated fibres and RENATER telecommunication fibres. The loop, of around 2 x 20 km, connected LNE-SYRTE to Sorbonne Université, to Université Paris Cité and back to LNE-SYRTE. A multibranch station fed by an ultra-stable laser at LNE-SYRTE sent the T&F signal to the loop and simultaneously to other loops or links for global dissemination over France. The signal was mixed with telecommunication traffic using a dedicated wavelength channel (1542.14 nm).
During the TiFOON project extraction signals, from hub branches, were improved by partner Muquans allowing signal regeneration with an enhanced compensation for noise. A major development was the upgrade on an interferometer, to compare the signal from the input and output ends of the main link, allowing detection and compensation for noise on the output signal of each extraction station. This was designed to prevent main link perturbations and to be insensitive to signal variations. Results indicated that SI traceable T&F data could be sent to multiple users automatically, with a frequency instability below 10−15 (1s).
Impact
Muquans, who helped develop the extraction stations, are now part of Exail - a leading high-tech industrial group specialising in cuttingedge robotics, maritime, navigation, aerospace and photonics technologies. The company acknowledge that working with TiFOON allowed them to test their components and technology on a daily basis over several years in a “real world” setting – knowledge they can now pass onto their customers.
T&F signals from LNE-SYRTE are now provided to over 30 laboratories in France and the resulting network has been labelled as national research infrastructure REFIMEVE. One of its fundings is a new ANR project T-REFIMEVE (2021-2028) to provide SI traceable T&F signals over extended scientific communities. It will allow cutting edge research into such things as tests for special relativity, the search for dark matter, and timing in geodesy or astrophysics networks. The system now extends thousands of km across France with links to Italy, Germany, the UK and Switzerland with a measurement uncertainty typically below 10-19. This precision allowed an international comparison of 11 optical clocks from 7 different countries during EURAMET project ROCIT, greatly augmenting the high-accuracy clock comparison data available worldwide - essential information for the planned future redefinition of the SI second.
- Category
- EMPIR,
- SI Broader Scope / Integrated European Metrology,
- Industry,