Full duplex coming soon to a fiber near you

March 20, 2018
CableLabs is working on a new fiber technology, Full Duplex Coherent Optics, an approach that is being incorporated into CableLabs' point-to-point coherent optics specification, which is slated to be issued ...

CableLabs is working on a new fiber technology, Full Duplex Coherent Optics, an approach that is being incorporated into CableLabs' point-to-point coherent optics specification, which is slated to be issued later this year. Changing network topology configurations will not be required.

Full Duplex Coherent Optics enables a coherent signal to be transmitted over the same fiber using the same wavelength, simultaneously in the downstream and upstream directions. This differs from traditional coherent optic links, which use the same wavelength but a separate fiber for the downstream and the upstream. Non-coherent systems, on the other hand, use a single fiber, but two wavelengths.

"The coherent technical approach CableLabs has proposed uses one wavelength and one fiber, meaning it uses only one-half of fiber wavelength resources [compared to] traditional technologies," said Steve Jia, CableLabs distinguished technologist, wired technologies.

Coherent optics could potentially increase the fiber capacity of any existing access network fiber by 200 times. The way this would play out in a traditional system is that if an operator has 10 wavelengths in the downstream direction and 10 in the upstream direction, carrying coherent signals, the total system requires 20 wavelengths. With the full duplex coherent optics mechanism, the same system can carry these signals using only 10 wavelengths.

"That is half the resources of a traditional coherent optics approach. Leveraging this approach, the optical components in the network would require only one-half the number of optical ports, thereby reducing complexity as well," Jia said.

Cable operators will be able to meet the increasing bandwidth demand without the need to lay more fiber or redesign chips for signal processing, CableLabs says.

Making this all happen involves a circulator, which is a directional three-port device. CableLabs has proposed a configuration that places one at each of the common downstream/upstream optical transport segments. The circulators can take bidirectional traffic from the portion of the network simultaneously carrying downstream and upstream traffic to fiber segments that only carry one or the other.

"The circulator plays the role of an optical roundabout to route traffic," Jia said.

In the cable access environment, there are shorter lengths and, therefore, there is negligible downstream/upstream interference and interaction. Coherent optics technology is a good fit here, where it might not be so in longer fiber segments, Jia said.

"The initial use case in cable for coherent optics and full duplex coherent optics is one of traffic aggregation. It backhauls DOCSIS capacity, but it doesn't increase the peak capacity DOCSIS systems can achieve. That would be accomplished by increasing DOCSIS spectrum or DOCSIS channel efficiency," Jia said.

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