Optical frequency transfer via 1840 km fiber link with superior stability

We investigated the transfer of a 194 THz optical frequency along a 1840 km fiber link. The fractional frequency instability expressed as the modified Allan deviation is 2.7 × 10
⁻¹⁵
at 1 s with 4 × 10
⁻¹⁹
after 100 s. The transferred frequency shows no systematic offset within an uncertainty of 3 × 10
⁻¹⁹
. Analyzing the underlying noise structure reveals a τ
⁻²
response in the modified Allan deviation, which is confirmed by calculations of the frequency instability from frequency counter data as well as from phase noise data.