Welcome to the Journal of Optical Communications and Networking (JOCN) OFC 2020 Special Issue. Published over two months (January and February 2021, Vol. 13, Nos. 1 and 2), the special issue includes extensions of invited and top-scored optical networking papers that were presented at OFC 2020. The OFC Technical Committee worked with the JOCN Editor-in-Chief to determine which authors were eligible to participate in this issue. The authors were required to include significant new material as compared to their OFC paper. All papers underwent the usual JOCN peer-review process. All of the authors, reviewers, editors, and JOCN staff are acknowledged for their diligence in producing this high-quality special issue.

For the first time, the JOCN Special Issue includes papers from OFC authors who presented tutorials on optical networking topics. The issue includes two excellent invited tutorials. One covers the history and future of passive optical network (PON) transceivers. In doing so, it traces the history of PONs and provides perspective on where PONs are heading with respect to capabilities and requirements. The second tutorial also addresses PONs but from a softwarization angle, a topic of high interest to carriers. In particular, it details the main components of the Software Defined Networking (SDN)-Enabled Broadband Access (SEBA) architecture.

There are a number of other papers addressing access and metro networks, ranging from 100G PONs (coherent or direct-detection), to forward error correction (FEC) evaluation using Markov models, to programmability via vertical-cavity surface-emitting laser (VCSEL)-based sliceable bandwidth-variable transponders, to the disaggregation of the PON dynamic bandwidth allocation (DBA) functionality.

Disaggregation and open systems are major themes in the special issue, including open line systems, open transponder interfaces, and evaluation of an open-source tool for quality-of-transmission estimation. Several of the papers include experimental results from either a proof-of-concept testbed or an actual production network.

Machine learning (ML) is another noteworthy theme, including a good practical overview of the topic that addresses when ML makes sense in a production environment. ML techniques such as domain abstraction for transfer learning are also explored in two papers. Another paper uses ML to assist in the abstraction of the physical layer in multi-domain environments. In a paper with a completely different direction of application, ML is proposed for use in attack detection. Yet another paper proposes using reinforcement learning to address user mobility in a PON-based 5G backhaul network.

A relatively new multi-layer architecture, i.e., a wavelength-division multiplexed layer over a space-division multiplexed layer (WDM/SDM) is explored in an invited paper. The paper proposes scalable algorithms and hierarchical switch architectures for a spatial channel network, which can significantly reduce the cost and complexity of future networks as traffic demands continue to grow. We expect this approach to spur further architectural research in the near future.

Finally, we would like to highlight a very well written paper that analyzes the impact of using co-packaged optics for high-performance computing and data center networks. It clearly states the benefits of the technology, most notably increased escape bandwidth; proposes architectures enabled by this advantage; and performs comparisons with a baseline architecture over an array of traffic patterns.

Overall, the special issue provides a good mix of covering current areas of interest in optical networking while still exploring new directions. We look forward to what OFC 2021 will bring.