Operation of caesium fountain frequency standards with remote hydrogen maser references

We report on the operation of a Cs fountain primary frequency standard using as a reference for its microwave source a hydrogen maser located in a remote laboratory up to hundreds of kilometres away. The 10 MHz reference signal for the microwave synthesizer is transferred via a dedicated optical fibre link with an electronically stabilized time delay. Such referencing to a remote maser was first tested with a 50 km spooled fibre on two separate fountain systems using either a quartz-based microwave source or an ultrastable optically-derived one. We subsequently ran two fountains with quartz-based local oscillators, one co-located with a referencing maser and the second linked to it by 34 km of buried optical fibre forming part of telecommunication infrastructure. Finally, the second fountain was referenced to a different maser located much farther away via a link extended to 390 km. We find that in all configurations the addition of the delay-stabilized fibre link, delivering the remote 10 MHz reference, does not degrade the short-term stability of the fountain standard-as long as the bandwidth of a servo locking the local oscillator to the maser reference is optimized.