Quaternary modulation formats for 100-Gbps optical links

The demand for 100 Gb/s optical links is rapidly spreading across all levels of the optical networking infrastructure. Many of the first deployments will be in the local area network (LAN) and metro-core and regional network environments. To address needs in LAN, the upcoming IEEE standard (IEEE P802.3ba) seeks 100 Gb/s over distances up to 40km. Furthermore metro-core/regional dense wavelength division multiplexing (DWDM) architectures require reach of several hundred km and the ability to pass through ten or more ROADMs. However, a number of fundamental challenges remain including the selection of appropriate modulation formats that are robust to a variety of nonlinearities, are sufficiently spectrally efficient, and able to withstand the strong optical filtering of cascaded ROADMs. Here we compare a variety of single-carrier quaternary modulation formats, each providing 2 bits/symbol/polarization and each likely to provide some advantages at 100Gb/s. Each format is presented with an appropriate MZM-based transmitter, and constrained by practical signal fidelity limitations that also enable comparison to experimental results from our 100G testbed. We primarily examine direct detection for cost-sensitive metro networks; however we also quantify the performance of coherent receivers, where applicable. Simulation results demonstrate the relative OSNR penalty (at a pre-FEC BER of 10-3) for a range of launch powers and adjacent channel formats.