Merit of Raman Pumping in Uniform and Uncompensated Links Supporting NyWDM Transmission

We propose a method to quantitatively evaluate the improvement of the optical signal-to-nose ratio (OSNR) enabled by Raman pumping in hybrid Raman/erbium-doped fiber amplifiers (HFA) for Nyquist-wavelength-division multiplexing (NyWDM) transmission of multilevel modulation formats on the C-band, over uniform, uncompensated and periodically amplified links. It is based on the well accepted thesis that in the considered system scenario propagation effects can be approximately and conservatively modeled as an additive Gaussian disturbance called nonlinear interference (NLI), and it exploits the GN-model for analytical calculations. As an example, we apply the developed method to three typical fiber types with fiber-span loss Af from 20 to 30 dB, and show that the pure counter-propagating pumping scheme is always close to be optimal. Using this pumping scheme, the merit of Raman pumping is very similar for all the considered fibers and its maximum is enabled by full-Raman amplification with an OSNR improvement that increases with Af , and goes from 4 to 5 dB in the explored range. We show that adding some copropagating pump, we obtain a further OSNR improvement of only up to 0.9 dB, so, for the considered scenarios, it does not emerge any indications for the use of some copropagating pump in HFA. Moreover, for counter-propagating pumping schemes, we can obtain about 80% of the maximum Raman merit setting the Raman gain in the HFA only to 60% of the overall gain. We define as moderate pumping regime this HFA operating-mode, that, besides being convenient from an energy-efficiency point of view, it is also irrelevantly affected by pump depletion and so it is very stable in case some input channels are added/dropped, as required by reconfigurable optical networks.