The articles in this special section address advancements in optical communications. The field of optical communications and networking technologies is gaining new momentum from artificial intelligence (AI) and quantum communications, while there has been continuous attention on emerging topics such as TWDM/ WDM passive optical networks (PONs), software-defined networking (SDN), and smart cities. In this second Optical Communications Series (OCS) issue of 2018, we have selected six contributions that address CMOS time-to-digital converter for optical time-of-flight sensing systems, light sources for TWDM/ WDM PONs, resource allocation in optical networks secured by quantum key distribution (QKD), networks for future services in a smart city, software-defined wireline-wireless cross-networks, and knowledge-based autonomous service provisioning in multi-domain elastic optical networks (EONs).