Amplifier Technologies for Unrepeatered Systems

This paper presents amplifier technologies that are used today in unrepeatered systems. These are based on Raman amplification or remote amplification with powerful pumps launched from the terminals. We describe their operating principle and discuss the physical effects that limit their application, particularly in the case of counter-propagating and co-propagating Raman amplification. We analyze the contributions of OSNR saturation and double Rayleigh scattering in counter-propagating amplifiers and gain saturation in co propagating amplifiers. We show that the benefits of co propagating Raman pumping decrease as the number of channels increases. We describe examples of wide bandwidth amplification and discuss the benefits of using dedicated fibers to carry only the pumps to remote amplifiers. Finally, we present Raman gain measurements over multi-core fibers with either coupled or uncoupled cores and show how the crosstalk between cores can affect the optimization of the Raman gain.