Optical-Frequency Transfer over a Single-Span 1840 km Fiber Link

To compare the increasing number of optical frequency standards, highly stable optical signals have to be transferred over continental distances. We demonstrate optical-frequency transfer over a 1840-km underground optical fiber link using a single-span stabilization. The low inherent noise introduced by the fiber allows us to reach short term instabilities expressed as the modified Allan deviation of 2×10^-15 for a gate time τ of 1 s reaching 4×10^-19 in just 100 s. We find no systematic offset between the sent and transferred frequencies within the statistical uncertainty of about 3×10^-19. The spectral noise distribution of our fiber link at low Fourier frequencies leads to a τ^-2 slope in the modified Allan deviation, which is also derived theoretically.