Optical fibers play a critical role inside the atomic clock PHARAO

The PHARAO (Projet d’Horloge Atomique par Refroidissement d’Atomes en Orbite) caesium atomic clock, combined with a hydrogen atomic clock, the Space Hydrogen Maser (SHM), form the ESA’s Atomic Clock Ensemble in Space (ACES). The ESA program ACES/PHARAO is the result of a collaboration between several European organizations and laboratories.

To achieve its accurate timekeeping, the PHARAO atomic clock has been attached to the outside of the European Columbus laboratory on board the International Space Station (ISS) in 2016. Indeed, in space, operating in micro-gravity provides a much more stable and accurate time base than in the presence of gravity like on Earth.

PHARAO is not the only atomic clock in space. Actually, it is accompanied by SHM (the Space Hydrogen Maser), which uses hydrogen atoms as a frequency reference and offers better stability but for a shorter time. By coupling the two clocks, ACES provides the scientists with a unique, highly stable time reference in space.

The cold caesium atom clock PHARAO was developed by France CNES space agency and the laboratory SYRTE.

The main purpose of this atomic clock is to reach a time resolution of about 10 ps, with a relative frequency resolution of 10^-16, which corresponds to a time error of about one second over 300 million years!

To manipulate caesium atoms, the caesium tube must be linked to the laser source by means of optical fibers. In addition, time and frequency transfer is also done using optical fibers.

The interest for researchers is that they are now able to compare many different atomic clocks based on atoms other than caesium, under different gravity levels since they now have access to the data from PHARAO and SHM at 400 km high, and from high-performance ground clocks developed by laboratories in various countries worldwide.

Comparing clocks will allow scientists to test Albert Einstein’s theories on space and time, but also other theories in fundamental physics, with a precision that is impossible in laboratories on Earth.

It is in this context, and under the aegis of EADS Sodern, that SEDI-ATI has been involved in the PHARAO project. The company was chosen for its expertise in the choice of optical fibers and materials with the properties required for the space environment, as well as for its know-how in the assembly of fiber components: collimators, cables, polarization-maintaining fibers, and NASA-type FC connectors with very low outgassing materials.

Various areas of applications are concerned: International Atomic Time (TAI), Coordinated Universal Time (UTC), geodesy, Very Long Baseline Interferometry (VLBI), Atmospheric propagation of microwave signals, and support applications involving remote sensing via the GNSS network, etc.