System performance and channel equalization of optically-amplified WDM long-distance links and ring networks

In this thesis, we analyze the transmission of many wavelength-division-multiplexed (WDM) channels through a cascade of Erbium-doped fiber amplifiers (EDFA) in both long-distance links and ring-based networks. For a megameter long-distance system, optimal operating conditions are found for achieving a high signal-to-noise ratio (SNR) per channel with as small an SNR differential as possible between 20 WDM channels spaced 0.5-nm apart. Critical issues addressed include: (a) the non-uniformity of the EDFA gain with wavelength; (b) the link loss between amplifiers; (c) the small-signal gain per amplifier; and (d) the input signal power. We then determine the optimal conditions for passively equalizing many WDM channels while maintaining a high SNR for all channels. We also analyze the system performance and required device characteristics when using AOTF's to dynamically equalize the non-uniform gain of EDFA cascades. The equalization is achieved by independently varying the multiple passbands of the AOTF using a simple feedback mechanism. In the laboratory, we experimentally demonstrate passive equalization of the non-uniform gain of cascaded EDFA's over a 7.4-nm wavelength range by periodically using notch filters along a 1000-km transmission link. As an extension of cascaded EDFA's, we have also analyzed the critical parameters in an optically-amplified WDM ring network which represents an infinite cascade of amplifiers in a closed loop. We find that a ring can accommodate 25 nodes when incorporating an EDFA and a channel-dropping filter in each node, a significant advance over non-amplified rings. We compare several node configurations to find the optimal one that provides the best performance for a wide range in system variables including input signal power, inter-node link loss and its variability, power-tapping ratio, and small-signal gain.