Long-Distance Frequency Dissemination with a Resolution of ${10}^{-17}$

We use a new technique to disseminate microwave reference signals along ordinary optical fiber. The fractional frequency resolution of a link of 86 km in length is ${10}^{-17}$ for a one day integration time, a resolution higher than the stability of the best microwave or optical clocks. We use the link to compare the microwave reference and a ${\mathrm{CO}}_2/{\mathrm{OsO}}_4$ frequency standard that stabilizes a femtosecond laser frequency comb. This demonstrates a resolution of $3\times {10}^{-14}$ at 1 s. An upper value of the instability introduced by the femtosecond laser-based synthesizer is estimated as $1\times {10}^{-14}$ at 1 s.

Figure 1

Allan deviation for the round-trip optical link, 86 km, in open loop (▲) and closed loop (▼), as calculated from phase measurement with 1 s sample time. Dashed line indicates the fractional frequency instability of the 100 MHz reference signal from the CSO, and dotted line the fractional frequency instability of the Cs fountain FO2.

Figure 2

Schematic of the optical link compensation.

Figure 3

Schematic of the whole measurement setup.

Figure 4

Fractional ${\mathrm{CO}}_2/{\mathrm{OsO}}_4$ frequency stability as given by the Allan deviation (●) of the repetition rate $f_r$ of the fs comb calculated from a series of 1 s gate measurement. The noise due to the optical link is negligible (▼).