Everything You Want to Know About Coherent Optics for Access Networks (But Were Afraid to Ask)

Steve Jia
Distinguished Technologist, Wired Technologies

Nov 19, 2019

The cable industry has been well served by its extensive fiber deployment that took place during the initial hybrid fiber-coax (HFC) buildout. Even though cable operators have answered capacity demand through fiber node-splits in specific high demand scenarios, only recently have operators embarked on deeper-fiber roll-out strategies as part of a comprehensive long-term evolution plan.

The exponential growth in demand for capacity prompted CableLabs to explore how to best use cable’s optical infrastructure resources. This exploration led to research activities for the introduction of coherent optics in the access environment. We’re delighted to announce the publication of the book “Coherent Optics for Access Networks” by CRC Press (Taylor & Francis Group), highlighting many of CableLabs’ research activities.

The book discusses how coherent optics in the access network is re-engineered to simultaneously achieve lower complexity and higher performance afforded by the generous link margins characteristic in shorter links. This instantiation of coherent optics is not only suitable for cable access but also for telco and cellular fiber access networks.

Recent developments in the field of coherent optics for access network applications that will support point-to-point (P2P) aggregation use cases and point-to-multipoint (P2MP) fiber to the user’s passive optical network are examined. Optical industry trends as well as the conventional intensity modulation and direct detection (IM-DD) systems and newly developed advanced direct-detection architectures leveraging four-level pulse amplitude modulation format, Stokes receivers and Kramers–Krönig receivers are also presented.

This book focuses on how to adapt coherent optics technology to the access environment in ways that address major cost challenges, such as simplified transceiver design and photonic integration. An example, is the introduction of full-duplex coherent optics, which enables simultaneous bidirectional transmission on the same wavelength thereby doubling fiber’s capacity. Full-duplex coherent optics is an approach that is feasible to implement in the shorter-link-length access environment.

The book provides economical modeling for aggregation uses cases in comparison with traditional 10G IM-DD DWDM based solutions. Implementation requirements, unique to the access environment, are also provided when introducing coherent optics into access scenarios, including coexistence with existing services and security challenges. Progress on recent-specification development activities is reviewed for many industry organizations that focus on short-distance coherent optics interoperability.

In writing this book, the authors have benefitted from the numerous interactions with experts within the optical telecommunication components and systems community, in particular with the vendor and operator members that contributed to CableLabs’ point-to-point coherent optics specification. This book represents a first look of technological advances in coherent optics, in the interest of future proofing of our access networks.

Favorable coherent component cost-reduction trends are expected to continue, technological advancements will enable higher performance and simpler implementations will make coherent technology more pervasive in the access network so that exponential growth in capacity is achieved. Given the headway gained in specification generation bodies and the development progress of optical component and transceiver manufacturers focusing on shorter link distances, a future with coherent optics in the access network is upon us.