Transparent optical networks may suffer from power transients due to slow EDFA gain and noise dynamics. Network reconfiguration using ROADMs or equipment failure may cause abrupt changes of the signal power in individual or multiple channels. In conjunction with the nonlinear EDFA gain response acting in the millisecond range (and stabilization circuits with finite response time), this may lead to strong power fluctuations in the other channels with negative effect on the BER. The problem of transients is especially important in networks with complex topology, as power dynamics can be amplified significantly due to the numerous feedback and amplification mechanisms in such networks.
This application example illustrates the propagation effect of power transients in a ring network (higher-order transients) [1, 2]. shows the simulation setup of the four-node optical ring, a simple but realistic use case for investigating the propagation of power transients. The nodes are connected by amplified, dispersion-compensated fiber spans, as depicted in . All EDFAs have an automatic pump control on the basis of a proportional integrated controller. At each node, various numbers of channels can be added and dropped according to the ring connectivity that is displayed in .
In this example, Node 2 initiates a power transient when it drops 8 out of 16 channels (without adding 8 new ones), which 'travels' around the ring network. The control of dynamic events is performed by using the simulation interface shown in . This interface enables you to start the simulation in the steady state by reading the corresponding module (transmitters, amplifiers) parameters from a file.
shows the resulting power overshoots at Node 2 and Node 3. Note that the higher-order transient, which reaches Node 3 after a full loop through the ring, has still noticeable amplitude. Node 2 experiences power dynamics after a full round-trip delay. This demonstrates how dynamical reconfiguration of a node can affect the operation of other nodes in the networks and illustrates the need for careful consideration of transients in the network design phase.
Keywords: ROADM, Transient, Ring network, Transparent networks, Optical amplifiers, Dynamics
Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.
 D.C. Kilper et al., OFC'2006, OTuk6.
 M.D. Feuer et al., in Optical Fiber Telecommunications V B, 2008.