Frequency Dissemination in the SWITCHlan network

In collaboration with METAS, University of Basel and ETH Zurich, SWITCH is very happy to announce the successful implementation of the first ultra-stable SI-traceable frequency dissemination in Switzerland, on top of the SWITCHlan network, between Bern, Basel and Zurich and parallel to the existing data traffic. The project, started in 2019, has been progressing methodically over the past year, successfully meeting the milestone of the first transmission of a frequency signal by the end of August 2020. Since then, the transmission parameters have been optimised and the overall performance evaluated. These results have now been published.

Many applications in physics research require a precise measurement of fundamental quantities. One of the most important quantities in the SI-system is the second, which can also be derived from a frequency signal. While there exist many technologies to obtain this quantity, they offer varying degrees of accuracy. The most accurate option currently available in Switzerland is the FoCS-2 primary frequency standard at METAS. This project thus aims to disseminate this frequency standard to the labs at the University of Basel and the ETH Zurich, in order to be available for physics research, e.g. for molecular spectroscopy.

To achieve this, the researchers in Basel and Zurich teamed up with METAS and SWITCH in a Swiss National Science Foundation (SNSF) Sinergia project to realize this frequency dissemination. To transmit a frequency, we do not use a data transmission, but the physical properties of the laser itself. In simplified terms, a narrow linewidth laser is made extremely accurate using an ultra low expansion (ULE) cavity. This laser might be very stable, but its absolute frequency is not predetermined. Instead, it is compared to the primary reference which is itself part of the international system for defining the SI units and the international atomic time (TAI). The laser light is then sent over the fiber to the first lab. However, the signal arriving at the remote site is no longer stable, for the fiber is subject to mechanical vibrations leading to small changes in the frequency; an effect known as doppler shift. To compensate for this, the laser is sent straight back using a mirror. At the originating side, this return light is compared to the original laser, which is then shifted in frequency to precompensated for the error, such that the light will arrive at the remote site with the correct frequency. For all the fine details, please refer to the publication.

While the technology is quite complex, it is not a new invention and it has been implemented by multiple metrology institutes in different countries. By drawing on the experience of the respective implementers, especially INRIM and Cesnet, we were able to adapt it on the SWITCHlan network. In order to save the prohibitively expensive fiber lease costs for this project, the frequency signal had to be multiplexed with the other optical channels used for data traffic. To avoid any issues with the spectrum management of the data traffic signals (for the frequency signal will be fixed), especially in anticipation of future data transmission technologies which will use the spectrum much more dynamically, the frequency signal was moved outside the C-band used for the data traffic. The final selection of 190.7 THz (1572.06nm) was mainly chosen based on parts availability, as the implementation requred custom made lasers and other components.

The results now available show a very good performance of the frequency dissemination. With an accuracy of $2*10^{-18}$, the error added by the transmission is 3 orders of magnitude below what the frequency standard provides, enabling an almost perfect reference at the remote site. This shows readiness for the next generation of frequency standards expected in the future. At the same time, we also measured the effect on the data transmissions, comparing the forward error correction (FEC) performance used in 100Gbit and 200Gbit signals with the frequency signal enabled and disabled. These measurements showed no correlation to the frequency signals confirming that multiplexed transmission is possible without any impact on the data traffic.

Now that this implementation has been successfully demonstrated we are looking forward to extending the frequency dissemination network to other locations. One of the prime candidates will be CERN, where a number of projects could benefit from this, and where there is an opportunity to compare the FoCS-2 frequency reference to other frequency standards, aiding in the international definition of TAI.

This project it funded by the Swiss National Science Foundation (SNSF), Sinergia grant nr. CRSII5 183579.

FM (Text)

P.S.: Also read the SWITCH Story High-precision frequency for research.