We propose a cost-effective implementation of reconfigurable add&drop functionality in Optical Cross Connects based on the concept of common add&drop in the node (OXC with perNode Add&Drop, OXC-NAD). The proposed architecture constrains usable transponders wavelengths, but the analysis on significant mesh topologies demonstrates that this does not affect the behavior of the transparent networks. The OXC-NAD is shown to also significantly reduce the node cost irrespective of the nodal degree. IntroductionThe continuous increase in traffic demand, the requirement for cost reduction, and the need of flexibility in network topology are driving the development of transparent and reconfigurable optical mesh networks in core and metro segments. In these networks Optical Cross Connect (OXC) nodes having the ability to optically switch the traffic to/from any direction with the granularity of the single WDM channel, are preferred. The recent development of tunable switches with wavelength granularity, namely the Wavelength Selective Switches (WSSs) [1],[2], integrating different functions in a single low-loss device, is allowing the realization of reconfigurable nodes with high nodal degree, thus permitting the design of the required OXCs for mesh networks.Besides transparently switching the optical traffic, the OXCs are also needed to connect the local subnetworks to the mesh network for adding and dropping channels to/from the WDM line streams crossing the node. In the OXC architectures proposed to-date [3]-[5], the Add&Drop (A&D) functionality is implemented at every bidirectional port of the node (Optical Cross Connect with per-Port Add&Drop, OXC-PAD). This flexibility also requires each bidirectional port to be able to add or drop all the channels assigned at the subnetworks. Therefore in the nodes based on the OXC-PAD architecture the cost of the A&D functionality increases with the nodal degree.In this paper we propose a cost effective approach for the implementation of the A&D functionality in the WSS-based OXCs. This approach is based on the concept of common Add&Drop in the node (OXC with per-Node Add&Drop, OXC-NAD), and considers the A&D channels as "local traffic" exchanged between the OXC and the "adjacent local node" constituted by the subnetworks. In this way, the A&D functionality is performed by an additional bidirectional port only, therefore its cost is independent on the nodal degree. After describing the ordinary OXC-PAD architecture and the proposed OXC-NAD scheme in detail, we compare their costs as a function of nodal degree, demonstrating significant cost benefit of our proposed solution. We also show that the proposed OXC-NAD architecture introduces a constraint on the A&D channels: a specific wavelength cannot be used by more than one subnetwork of the same OXC to add and drop traffic in the transparent network. Finally, we demonstrate through numerical analysis that this constraint does not affect the behavior of the transparent network.
In this paper, we theoretically and experimentally investigate the performance of erbium-doped fiber amplifier (EDFA)-based WDM ring networks with free amplified spontaneous emission (ASE) light recirculation. We show that, with proper network and amplifier design, the lasing light generated by free ASE recirculation within the looped network provides an effective gain clamping technique, ensuring limited signal power excursions under WDM channels add-drop operations. Considering a ring network composed of eight fiber sections and eight EDFAs, maximum signal power overshoots below 2.5 dB have been measured under 23 24 WDM channels drop. Optical signal-to-noise ratio (OSNR) analysis and bit-error rate (BER) measurement at 10 Gb/s confirm acceptable performances and negligible penalties due to polarization effects and relative intensity noise transfer from laser light to WDM signals. We also propose and demonstrate a new link control technique which overcomes the main limiting factors of such networks, respectively, related to OSNR degradation, stability and survivability to fiber and EDFA breakages.
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