Microwave Frequencies Metrology

LOUS - Ultra-Stable ground frequency

Ultra-stable frequency and time sources play an important role in many applications of Time and Frequency metrology and of fundamental physics (clock evaluation, relativity tests, fundamental constants test ...). Clock comparisons are currently performed by satellite, GPS or TWSTFT, where the fractional frequency stability is limited by the transmission system to a few 10^-15 at one day averaging time. These methods are thus insufficient for measuring the ultimate performances of a microwave or optical standard. Access to an ultrastable frequency reference for a large number of laboratories opens the way to perform new experiments in fundamental physics. The implementation and operation of a state-of-the-art frequency standard remain a strong limitation and can be overcome by a fibre distribution system connecting reference Time and Frequency laboratories to users.

We have thus developed a new method to distribute reference signals at a frequency of 1 GHz by optical fibres of a few hundreds kilometres, in collaboration with Laboratoire de Physique des Lasers (LPL Institut Galilée / Université Paris 13). The microwave reference signal is transmitted by amplitude modulation of an optical carrier at 1.55 mm. An electronic device allows cancellation of the additional phase fluctuations (mechanical vibrations, temperature fluctuations or non-linear effects) in the fibre. Fractional frequency stabilities of 2-3x10^-15 at one second and 10^-18 at one day of averaging time have been demonstrated over 86-km transfer. A few 10^-14 τ^-1 are measured on optical link of about 200 km. The distribution system will be upgraded to fulfil the stringent distribution requirements for the future optical clocks. By this way, a new method is under-study and will consist of the direct transfer of an optical reference signal, delivered by a fibre laser which is phase locked on a frequency standard, over distances up to 1000 km to constitute a European distribution network. .

Left: Direct amplitude modulation laser diode and phase compensation system at 1 GHZ.
Right: Frequency stability of the 1-GHz dissemination system over 186 km.