References: [1].

One of the most usable in the contemporary submarine and long-haul terrestrial networks are the distributed Raman amplifiers (DRA), which is due to many advantages: stimulated Raman amplification can occur in any fiber at any signal wavelength by proper choice of the pump wavelength; the Raman gain process is very fast and the effective noise figure of the DRA is smaller than the noise figure of erbium-doped fiber amplifier and the semiconductor optical amplifier.
In contrast with the EDFA which is a discrete device with an input and an output, DRA can be described as a system which consists of two pumping sources placed at the beginning and at the end of the transmission span which length is more than 100 kilometers. The optical fiber is used as an active medium. The projecting of a DRA is related with the choice of a pump power value in accordance with the transmission span length; the needed net gain coefficient and the magnitude of the added noises.
The high level of the pump power and the long actual transmission span of the distributed Raman amplifier are the reason for adding the noises due to spontaneous emission, double Rayleigh backscattering of the signal and RIN noise transfer.