Advanced bi-directional Raman pumping schemes for long span unrepeated WDM transmission systems

In this paper we describe advanced bi-directional Raman pumping schemes for extending the maximum reach achievable in long-span unrepeated WDM transmission systems operating at 10 Gb/s. We describe both first- and higher-order bi-directional Raman pumping schemes, pointing out that co-pumping, although critical in terms of noise transfer from pump to WDM signals, can be very effective in extending the maximum system reach. New co-propagating Raman pump lasers, characterized by high power levels and low relative intensity noise, are also proposed to further increase the maximum achievable span loss, avoiding transmission penalties induced by relative intensity noise transfer.