Cumulative channel gain saturation in cascaded backward-pumped all-Raman transmission spans

Raman amplification provides wideband low-noise amplification that is beneficial for high capacity, long haul transmission systems of particular interest for advanced multi-level modulation formats. In transparent mesh add-drop based systems the channel loading on the amplifiers will change during system operation, which primarily due to amplifier gain saturation, requires amplifier gain control. Gain saturation measurements used to design the gain control is traditionally measured using the total power gain as a function of total input power [1]. However, this quantity does not capture wavelength dependence important in broadband systems. Changes in the gain spectrum have been measured by uniformly varying the power on a set of input channels covering the spectrum [2] or by looking at special cases of channel loading [3]. Ideally one would like to know how the net gain of each channel, including non-linear effects, varies throughout a transmission system for arbitrary channel loading. The channel gain saturation (CGS) spectrum can be used to characterize broadband-amplifier gain saturation under variable loading conditions [4]. CGS is measured by referencing the gain experienced by a channel in the configuration of interest to the gain experienced by the same channel when no other channels are present (or any appropriate reference condition for the system). The CGS spectrum is constructed using each individual CGS measurement. In these experiments we measure the CGS spectrum through a cascade of Raman pumped transmission spans and dispersion compensating fibers (DCF) in order to examine for the first time the evolution of the saturated gain variations through a transmission system.