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We investigate optical frequency transfer over a 1400~km loop of undergroundfibre connecting Braunschweig and Strasbourg. Largely autonomous fibreBrillouin amplifiers (FBA) are the only means of intermediate amplification,allowing phase-continuous measurements over periods up to several days. Over ameasurement period of about three weeks we find a weighted mean of thetransferred frequency's fractional offset of $(1.1\pm0.4)\times10^{-20}$. Inthe best case we find an instability of $6.9\times10^{-21}$ and a fractionalfrequency offset of $4.4\times10^{-21}$ at an averaging time of around30~000~s. These results represent an upper limit for the achievable uncertaintyover 1400 km when using a chain of remote Brillouin amplifiers, and allow us toinvestigate systematic effects at the $10^{-20}$-level.

Brillouin amplification supports $1\times10^{-20}$ accuracy in optical frequency transfer over 1400~km of underground fibre