BRIAN SMITH ROADM NETWORKS
more power per channel. So as channel bandwidths scale from 50GHz to higher bandwidth, EDFA arrays associated with the multicast switch must support significantly more total power (Figure 4). For a typical 8x16 MCS, the EDFA array output power requirement becomes impractical even for a 150GHz channel width. This inherent ‘bandwidth bottleneck’ means that there is a limit to the maximum channel bandwidth and port count that an MCS can practically accommodate. If an MCS based network is deployed with an EDFA array that cannot support higher channel powers and thus wider bandwidths, that network may not support future coherent transceivers. A FUTURE-PROOF ROADMNETWORK Recent advances in liquid crystal on silicon (LCoS) and optical design technology have allowed the development of a commercial contentionless MxN WSS which meets the needs of next generation CDC ROADM networks. Functionally, the high-loss power splitters of the MCS are replaced by low-loss wavelength selective switches, tightly integrated in such a way as to provide both add and drop capability within a single module. Flexible grid filtering from the WSS eliminates the accumulation of out-of-band noise generated by transmitters in the add direction, while removing multichannel noise at coherent receivers in the drop path. Consequently, coherent Tx and Rx requirements can be relaxed and overall system performance is improved as a result (Figure 5).
Growing the channel bandwidth significantly reduces the number of CDC modules required and consequently, minimizes the number of line WSS ports required within each degree. This not only lowers the cost for the same target capacity, but consumes significantly less power in a smaller equipment footprint. THE BANDWIDTH BOTTLENECK Current generation CDC mux/demux solutions leverage PLC-based multicast switches (MCS) constructed using a cascade of 1xN power splitters and Mx1 selector switches. Multicast switches possess high loss, driving the requirement for erbium-doped fibre amplifier (EDFA) arrays to be located between the degree WSS and the MCS. The maximum power required by each EDFA is driven by the loss of the MCS and the required power per port. Increasing MCS port count, increases loss and consequently, the maximum power required by the EDFA array. Modern coherent signals have nominally constant power across their spectrum so as the overall bandwidth of the channel increases, the total power of the channel increases proportionally. For example a 150GHz width channel has 5dB (3x) higher power than a 50GHz width channel, while a 300GHz channel possesses 8dB (6x)
Figure 2. Number of active channels/node for 50GHz channels vs growing channel bandwidth
IMPACT ON ROADM INFRASTRUCTURE The impact of employing increasing channel bandwidth on ROADM infrastructure is best described using a simple example that assumes 25% add/drop traffic at a 200Tbps node. Figure 3 shows the resulting CDC node architecture based on the channel growth options described earlier. The 50GHz grid node must support many more active channels, and as a result needs almost four times the number of CDC modules to achieve the same target add/drop capacity compared to an approach where channel bandwidths are progressively increased.
Figure 3. Comparison of 200Tbps node architecture with fixed 50GHz grid vs increasing channel bandwidth
Figure 5. Low loss contentionless MxN removes the need for EDFA array
With no EDFA arrays in the add/drop path, there are no limitations on channel bandwidth or power as would be the case with multicast switch implementations. The result is that contentionless MxN WSS can seamlessly support wider bandwidth channels of any total optical power. Future CDC ROADM networks must be compatible with evolving coherent interfaces without incurring constraints on channel bandwidth and/or power. While today’s MCS-based solution with EDFA arrays cannot easily provide this flexibility, contentionless MxN WSS will provide a path to higher capacity CDC ROADM networks with improved performance and lower total cost of ownership.
Figure 4. Power scaling of MCS EDFA arrays with increasing bandwidth
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| ISSUE 14 | Q3 2018
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