Optical-power transients in long-haul WDM trunk-and-branch networks

In optically transparent wavelength-division-multiplexed (WDM) networks, su dden variations in the number of propagating wavelengths generate optical-p ower transients that can degrade the transmission performance of the surviv ing wavelength channels. Computer simulation is employed to study the effec t of wavelength-number variations, caused by wavelength switching and cable cuts, on the transmitted wavelength channels in a long-haul WDM trunk-and- branch network. The transmission degradation of the affected channels is ex pressed in terms of Q-factor penalty, which reflects their optical power an d optical-signal-to-noise ratio (OSNR) variations. Switching-induced wavele ngth power reductions are found to generate the largest penalties. It is sh own that the e-factor degradation is mostly caused by the receiver decision -threshold misalignment relatively to its optimum position and that the OSN R-dependent degradation is negligible. Therefore, receiver automatic gain c ontrol (AGC) can be used to suppress the signal. The AGC response should he much faster than in a conventional optically preamplified receiver to avoi d error bursts. The time within which the Q-factor penalty reaches 1 dB is identified as the parameter that dictates the AGC response speed.