7.6 Mitigation of spectral power excursions 215 7.6 MITIGATION OF SPECTRAL POWER EXCURSIONS Excursions of DWDM channel power levels will impact the transmission perfor- mance by changing the RX power and OSNR levels and by introducing nonlinear signal distortions (in the case of positive excursions). Whether these fluctuations will cause channel outages depends on the available system margins. Especially for large networks with long links between adjacent transparent nodes, it will be beneficial to mitigate or even compensate the spectral power excursions. Note that it is not sufficient to use gain-controlled EDFAs only as these do not control the spectral shape of the amplifier gain. Potential solutions to mitigate the impact of spectral power excursions can be divided into three categories: 7.6.1 Fast optical control of channel power levels To compensate large spectral power fluctuations, a mid-stage attenuator in the EDFAs can be used to control the tilt of the spectrum. Dynamic optical filters (or tilt filters) every few amplification spans may reshape the spectrum and block the