System degradation due to coherent crosstalk in WDM network nodes

Summary form only given. It is well known that crosstalk is a major concern in a wavelength division multiplexing (WDM) network. The most severe case of crosstalk-induced system degradation arises when the signal and the crosstalk originate from the same laser. Because distributed feedback (DFB) lasers can have coherence length on the order of tens of meters, coherent interference between signal and crosstalk can be of concern in a WDM network node where crosstalk paths arise from components´ finite isolation. An example is an add-drop node formed by a back-to-back mux/demux pair: leakage of light through unintended paths in the node gives rise to coherent interference at the output of the node. Detailed studies have been done when the path length difference between the signal and crosstalk paths is longer than the laser´s coherence length. However, the case where the path length difference is shorter has not received much attention. This interference is referred to as coherent cross talk in this paper. We refer to other forms of crosstalk as incoherent. We show that coherent crosstalk differs from its incoherent counterpart in that it doesn´t induce a deterministic power penalty for the network, but instead gives rise to multipath interference and fading, a familiar phenomenon in mobile communication systems.