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A flexible optical node architecture employing software-defined modulation-flexible universal transceivers is presented for beyond 100G flexible grid optical networks. Hitless spectrum defragmentation using synchronous bandwidth-variable WSS control is supported for inservice resource optimization. 1. Flexible optical node architecture employing modulation-flexible shared universal transceivers In flexible grid optical networks [1], the optical frequency spectrum is flexibly allocated in integer multiples of the minimum spectrum granularity (slot size) to satisfy heterogeneous spectrum requirement due to mixed-rate (e.g. 100G, 400G and beyond), multi-level modulation (e.g. QPSK, 16QAM, 64QAM etc.) signals. In these networks, the optical nodes can make flexible software-controlled adjustments to the transmission schemes based on both client bandwidth requirements and network conditions. An illustrative example is shown in Fig. 1: to accommodate longhaul signals, the transceiver can be configured to a relatively wide-band transmission scheme employing a highly noise-tolerant modulation scheme that is suitable for long-haul transmissions (e.g. QPSK). For short-haul signals, the transceiver can be configured to a transmission scheme that is suitable for short-haul transmissions and features high frequency utilization efficiency, but is not very noise tolerant (e.g. 16QAM). A flexible optical node architecture supporting such flexible-rate, distance-adaptive transport is shown in Fig. 2. Universal transceivers are consolidated into a transceiver pool for efficient resource utilization. A crossconnect switch fabric is located between client cards and the transceiver pool. Using this architecture, a client card can flexibly select a software-defined universal transceivers configuration that is most suitable for the client signal’s rate and reach. The optical signals from/to universal transceivers are multiplexed/demultiplexed and switched from/to proper input/output optical fibers through a color-less, direction-less, contention-less and grid-less (CDCG) ROADM. One major advantage of this node architecture is the efficient utilization of universal transceivers through sharing. Under dynamic traffic, it can achieve about 20% improvement in transceiver utilization efficiency [2] compared to a conventional node architecture without a transceiver pool. Flexible grid WDM optical network Flexible optical switch node Short-haul A few 100 km Long-haul Greater than 1000 km Universal optical transceiver Universal optical transceiver Universal optical transceiver Software-side adjustment of transmission routes and wavelength slot on an optical switch node 16QAM QPSK Reconfiguration by Software