CableLabs Gives Optical Fiber a Massive Efficiency Boost

You gotta hand it to CableLabs. The non-profit cable R&D organization is not about to rest on its laurels. Nor is it willing to forego a single shred of optimization possible in the broadband access network.

In its latest move, CableLabs is introducing a new optical innovation designed to make it more efficient for operators to deploy coherent optics in the access network. The work builds on existing CableLabs efforts to apply coherent optics -- a technology typically found in long-haul networks -- to last-mile broadband infrastructure. (See New CableLabs Optical Tech Promises Big Bandwidth Boost and Full Duplex, Coherent Optics Specs Advance .)

But the newest breakthrough is actually a step-function improvement in the technology that could be critical to making coherent optics viable for actual deployment.

The CableLabs innovation project is called Full Duplex Coherent Optics. Full duplex refers to the concept of bidirectional traffic over a single medium. In this case, it's not related to that other high-profile CableLabs effort, Full Duplex DOCSIS (FDX), except that both are optimizing network infrastructure by allowing traffic to flow simultaneously in two directions. With FDX, that means bidirectional traffic over a DOCSIS network. With Full Duplex Coherent Optics, that means bidirectional traffic over an optical fiber link.

Adding the full duplex part to coherent optics is important because of the way service providers are deploying fiber in their access networks. According to Steve Jia, distinguished technologist for wired technologies at CableLabs, about 20% of residential cable deployments today rely on a single fiber that uses different wavelengths for upstream and downstream traffic. But typically, coherent optical technology requires two fibers, with one laser using the same wavelength to direct signals upstream on one fiber and downstream on the other.

Further, while only 20% of cable deployments involve a single-fiber topology today, Jia projects that percentage will grow to 60% in the near future because cable operators want to use additional fibers for other purposes, like commercial services.

In other words, while cable operators plan to rely more heavily on the single-fiber approach going forward, coherent optical technology today requires a dual-fiber approach. Hence the importance of the new Full Duplex Coherent Optics solution.

CableLabs illustration of Full Duplex Coherent Optics
CableLabs illustration of Full Duplex Coherent Optics

So how does the new technology work? The CableLabs approach introduces two optical circulators on each end of an optical link to direct bidirectional traffic. Jia describes the circulator as being like a traffic roundabout for cars, but the important point is that it allows for traffic in both directions on a single fiber and it only requires one laser, rather than two. That's a massive cost-efficiency improvement.

As Curtis Knittle, vice president of wired technologies at CableLabs, explains it, these circulators will be integrated into optical transceivers in the future, making it possible for operators to deploy the technology as they're migrating to distributed access architectures (DAA).

"As an operator evolves toward distributed access architecture, they will be installing these digital optical links," says Knittle. "One option for this digital optical link between the headend/hub and the aggregation node is this Point-to-Point Coherent Optics link that we're specifying today. The technology in the specification will be integrated into an optical transceiver, a device that plugs into another device on each end. To incorporate this Full Duplex Coherent Optics, there's not going to be any change to that particular process. The transceiver potentially will have this circulator integrated and so you'll simply plug in the transceiver to the devices on each end and connect up a single optical fiber instead of two."

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Importantly, because the full duplex component has already been tested in the CableLabs' Optical Center of Excellence (OCE), there's also an opportunity now to bring the technology to market at the same time the existing CableLabs Point-to-Point Coherent Optics specification debuts. The research organization isn't willing to commit to that outcome officially, but in a blog post, Jia does note that full duplex technology "can be seamlessly incorporated into the ongoing CableLabs' P2P Coherent Optics specification effort, which will be issued in mid-2018."

That would seem to suggest that the new Full Duplex Coherent Optics work will merge with the existing CableLabs P2P Coherent Optics project. At that point, after a specification is officially introduced, CableLabs expects to move into initial trials in 2019.

There are a couple of other points worth mentioning. First, while Full Duplex Coherent Optics holds great promise for the broadband access market, it's not likely to be transferrable to the long-haul optical industry. The addition of amplifiers over long-haul links make the full duplex configuration impractical.

Second, while CableLabs is developing the new coherent optics technology for its cable members, there's no reason the technology couldn't be applied in other fiber networks by other network operators as well. In the future, this work could proliferate further with the help of telecom standards bodies like the ITU, IEEE and OIF.

— Mari Silbey, Senior Editor, Cable/Video, Light Reading

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