High-precision frequency for research

In addition to transporting data on its network, SWITCH will soon also deliver a high-precision frequency from METAS in Bern to research laboratories at the University of Basel and ETH Zurich.

Text: Ernst Heiri, published on 27.09.2019

The good old church clock has a pendulum that swings to and fro and the modern wristwatch has a crystal oscillator. But Switzerland’s reference clock, the FoCS-2 developed by the Swiss Federal Institute of Metrology (METAS), uses the oscillations of caesium atoms as a basis for measuring time. The precision of this measurement is so high that a time deviation of one second would take 30 million years to appear.

Reference for new experiments

Physicists want to use this high-precision oscillation – or frequency – as a reference for new experiments. The frequency will now be transmitted to the University of Basel and ETH Zurich as part of a project by the Swiss National Science Foundation (SNSF). The researchers hope that this will give them new insight into the field of molecular spectroscopy, and assist them with the development of new laser technologies and the further development of frequency measurement technology for metrology.

Fibre optics enables precise transmission

But how will METAS’ precise frequency get from Bern to the research labs in Basel and Zurich? The required precision can only be achieved using fibre optics. Of course, 200 km of extra optical fibre could be rented for this. But the researchers would prefer to use the limited funds they have for expensive lab equipment.

This is where SWITCH comes into play. After all, our SWITCHlan network is based on optical fibre, and we consider it our job to give the researchers access to our expensive infrastructure and expertise. The only condition is, of course, that the stable operation and future development of our network services are not affected.

Although this condition is not disputed, implementing it is not easy. All signals that are transmitted long distances via optical fibre use infrared light with a wavelength ranging from 1530 to 1570 nanometres, and we already use the middle of this range for our network services. However, there is still plenty of room outside of this range for other applications. 

 

On the right wavelength

The first challenge presented by this project was finding a wavelength for which suitable lasers, amplifiers and filters could be obtained.

The second challenge was the high level of transmission stability required. Even the smallest vibrations or temperature fluctuations affect the signal in the optical fibres. The solution? These small deviations are continuously compensated by the transmitter, which evaluates a signal reflected by the receiver. For this, both the sent and reflected signals need to be transmitted via the same optical fibre.

Another very important consideration in this project is that the reference signal can later be used by other researchers and compared with other European time references.

This innovative application is a good example of how seriously we take our role as an academic network and how we also support projects from Switzerland’s research community that represent totally new territory for us.

About the author
Ernst   Heiri

Ernst Heiri

Ernst Heiri studied computer science at the ETH Zurich. He has been with SWITCH since 1995. Today he is responsible for the planning and commissioning of new fibre-optic connections. His interests are in the fields of engineering and the operation of optical network infrastructures.

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The ultra precise second

The primary frequency standard FoCS-2 performs the second by determining the transition frequency of cesium atoms. These are first slowed down by laser beams, i. e. they are cooled to temperatures as low as a few micro Kelvin. The cold atoms are then launched on a parabolic flight in a continuous beam to determine the transition frequency in the microwave cavity. FoCS-2 performs the second with a relative measurement uncertainty of 10-15, which corresponds to a deviation of one second in 30 million years.

Infographic FoCS-2 (in German and French)

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