CableLabs investigates coherent optical access networks

CableLabs, which investigates technologies and develops specifications for cable network operators, says it has begun to apply coherent communications technology to optical access networks. The group has achieved single-wavelength 256-Gbps transmission over 80 km in a lab setting using dual-polarized 16QAM, according to a blog post on the organization's website.

In the blog, Alberto Campos, Distinguished Technologist at CableLabs, notes that coherent transmission has gained wide acceptance in long-haul and metro fiber-optic networks. He also notes that long-haul implementations can be quite expensive, in part because of the signal processing necessary to overcome various dispersion obstacles as well as nonlinear effects.

However, the shorter runs in the access network mean that these impairments aren't as strongly felt in such applications, Campos notes. Therefore, the application of coherent technology should be less expensive than it is in today's more common deployments.

CableLabs appears to have developed a coherent transmission approach designed for the fiber runs that help enable hybrid fiber/coax (HFC) connections, particularly those that will support DOCSIS 3.1 and, eventually, Full Duplex DOCSIS 3.1. Campos notes that, as cable operators re-architect their networks to support DOCSIS 3.1, they will push fiber deeper closer to their customers. Such fiber deployments will enable a "Node+0" approach, in which the nodes via which fiber transmission gives way to coax will be close enough to the customer that no amplifiers will be required for the signals to reach their destinations.

However, cable operators will wish to deploy as little new fiber as possible, which means that already deployed fibers between the headend and existing nodes will need to carry more traffic than they do now. Hence the potential demand for capacity levels that only coherent transmission currently can support.

Campos reports that the 256 Gbps shown in the lab required minimal compensation. Technicians also multiplexed eight such wavelengths onto a single fiber to produce a total transmission capacity of almost 2.5 Tbps.

Campos writes that CableLabs plans to create specifications for the use of coherent transmission in HFC networks. Announcements regarding formation of a working group toward creation of such specifications should come soon, he added.

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