Accurate time/frequency comparison and dissemination through optical telecommunication networks

Today’s best clocks rely on transitions in the optical frequency domain. They outperform the best caesium-based atomic clocks in both accuracy and stability. However, a future SI realisation of the second, based on optical atomic clocks, will need a system capable of transferring time and frequency traceability with greater accuracy than the well-established, satellite-based time and frequency systems can achieve.

The EMRP project Accurate time/frequency comparison and dissemination through optical telecommunication networks (NEAT-FT) investigated the techniques needed to ensure accurate optical time and frequency transmissions across Europe using optical fibres. A major achievement of this project was the demonstration that fibre optic frequency transfer will offer orders-of-magnitude better stability and accuracy than existing satellite-based atomic clock comparison methods.

The project:

• Developed a versatile tool box for establishing optical, long-distance time and frequency links including performance assessment and fault detection. This opens the way to accurate and high resolution optical clock frequency comparisons over international fibre networks.
• Developed and performance tested a 2000 km dedicated fibre-optic link suitable for comparing frequencies generated by optical clocks.
• Developed optical regeneration methods and demonstrated a 1500 km stable fibre-optic link using a shared public telecommunication network, showing that dedicated links may not be required between the frequency generator and its user.
• Established and characterised bi-directional fibre-optic links between NMIs enabling the first international comparison of optical clocks without the use of satellite-based systems.
• Developed protocols and techniques for accurate time transfer accessed at the levels of precision required for the international atomic time scale.

This EMRP project has successfully transmitted optical frequency and timing signals over longer distances with greater accuracy and stability than previously possible, using both dedicated and commercially available optical fibres. With access to such fibres, it is now possible for users in research and commerce to benefit from highly accurate time and frequency standards that were previously only available at NMI’s. The linking of NMI based atomic clocks by fibre-optics will eventually offer a viable and more accurate back-up system for Co-ordinated Universal Time - the international standardised time signal for telecommunications, finance and the internet.

As a result of the project a first step towards a European scientific network has been realised that enables optical atomic clock comparisons at the highest level of accuracy. This is a significant step towards the proposed SI redefinition of the second using more accurate optical atomic clocks to replace current caesium atomic clocks.

EMPIR projects 15SIB05 OFTEN and 15SIP04 TIMEFUNC build on this work.