Telecommunication fiber link for the remote characterization of a magnesium optical frequency standard

We have characterized the 24Mg optical frequency standard at the Institute of Quantum Optics (IQ), Hanover, using a clock laser at the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, via a noise compensated 73 km fiber link and present preliminary results for the stability of the Mg standard. The stability of the clock laser (λ = 657 nm) is transferred with a femtosecond frequency comb to a telecommunication laser at λ = 1542 nm. The signal is then transmitted from PTB through the fiber link to IQ. A second comb at IQ (the remote end) is used to compare the transmitted laser frequency with that of the Mg clock laser λ = 914 nm. The frequency ratio of the clock lasers νMg/νCa shows a relative instability < 10^-14 at 1 s. The upper limit for the contribution of the fiber link to the frequency instability is measured by connecting another optical fiber following the same 73 km route at Hanover computer center. The comparison performed at PTB between the local and the transmitted signal after a round trip length of 146 km showed a relative uncertainty below 1 x 10^-19 and a short term instability σy(τ)= 3.3 x 10^-15 / (τ/s).