Optical frequency transfer via a 1840 km fiber link has been investigated. Twenty fiber amplifiers and two fiber Brillouin amplifiers are needed to compensate for 420 dB of loss. Active noise compensation reduces the instability to 2.7 x 10^-15 at 1 s with 4 x 10^-19 after 100 s.

Full article:

Phys. Rev. Lett. vol. 111, p. 110801 (2013)

https://doi.org/10.1364/CLEO_SI.2013.CM4N.1