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

In optically transparent wavelength-division-multiplexed (WDM) networks, sudden variations in the number of propagating wavelengths generate optical-power transients that can degrade the transmission performance of the surviving wavelength channels. Computer simulation is employed to study the effect 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 expressed in terms of Q-factor penalty, which reflects their optical power and optical-signal-to-noise ratio (OSNR) variations. Switching-induced wavelength power reductions are found to generate the largest penalties. It is shown that the e-factor degradation is mostly caused by the receiver decision-threshold misalignment relatively to its optimum position and that the OSNR-dependent degradation is negligible. Therefore, receiver automatic gain control (AGC) can be used to suppress the signal. The AGC response should he much faster than in a conventional optically preamplified receiver to avoid error bursts. The time within which the Q-factor penalty reaches 1 dB is identified as the parameter that dictates the AGC response speed.