Next Generation Coherent PONs: Technical Challenges and Outlook

After the standardization of 50 Gbit/s PONs (ITU-T recommendation G.9804.3), the next generation of passive optical networks (PONs) is expected to achieve a data rate of 100 Gbit/s per $\lambda$ and above, with a preference towards 200 Gbit/s per $\lambda$ to maintain the usual x4 bit rate increase from one generation to the next. To support such high data rates, there is ongoing discussion about introducing coherent technologies into PONs for the first time. This decision poses several technical challenges that must be addressed. For instance, will future coherent PONs be based on single-carrier solutions (i.e., pure time division multiplexing) or multi-carrier solutions (wavelength division multiplexing, frequency division multiplexing)? And how will the burst-mode coherent DSP needed for the upstream channel be implemented?