Power transients in fiber Raman amplifiers with coherent pumping or incoherent pumping

We investigate power transient effect caused by channel addition/removal in fiber Raman amplifiers (FRAs). A comprehensive large-signal numerical model of FRAs, which incorporates time variation effects, and downstream and upstream propagations of multiple signals, pumps, and amplified spontaneous emission components, is developed and used for the theoretical analysis. The effect of pumping schemes, input signal power, pump power, the length and type of Raman fiber, and the number of added or dropped channels on the dynamics of surviving channel power fluctuations is studied. Three different drop/add cases are considered: (a) L band drop/add, (b) C band drop/add, (c) every other channel drop/add. For all three cases, power transient behaviors in fiber Raman amplifiers counter-pumped by multiple coherent pumps or multiple incoherent pumps are analyzed at the same gain. As an example, we consider a FRA pumped by six coherent pumps or two incoherent pumps. Comparison of power transients in those two FRAs is given. It is shown that power transient behaviors of surviving signal channels are almost the same for a FRA pumped by six coherent pumps or two incoherent pumps. However, a FRA with incoherent pumping suffers from slightly severer power transients than that with multiple coherent pumps.