Combining fiber Brillouin amplification with a repeater laser station for fiber-based optical frequency dissemination over 1400 km

We investigate optical frequency dissemination over a 1400 km long fiber link in looped configuration over a pair of underground fibers between Braunschweig and Strasbourg. This fiber link is the first to combine fiber Brillouin amplifiers with a repeater laser station. Phase-coherent operation over more than five days is demonstrated. We analyze the repeatability of the performance over four campaigns and present results of 65 days in total. The weighted mean of the fractional frequency offset of the transferred optical frequency over the complete data set is $(-1.1 pm 0.4) times 10^{-20}$. By analyzing the stabilization signals of the two individual fibers, the correlation of the phase noise on the two fibers is shown to be >98%.