Rayleigh scattering limitations in distributed Raman pre-amplifiers

Recent results demonstrated that the capacity of installed transmission systems can be upgraded by means of Raman gain with minor changes at the receiver terminal and, most importantly, no changes to the cable. Improvements in the power budget of 7.4 dB were achieved in transmission experiments showing a four-fold capacity increase by time-division multiplexing (TDM) or wavelength-division multiplexing (WDM) up to an aggregate bit rate of 40 Gbit/s. As previously reported, the performance improvements that can be obtained depend on fiber characteristics such as the Raman gain coefficient, and the fiber loss at the pump and signal wavelengths. Here, we report on the limitations imposed by Rayleigh scattering in distributed Raman pre-amplifiers. Experimental as well as theoretical estimates of the maximum increase in power budget as a function of fiber parameters are presented.