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HFC Network

Coherent Optics 101: Coming at You at 0.69c

Nov 23, 2020

Welcome back to the CableLabs 101 series! In our previous post, we discussed the basic components of a typical hybrid fiber-coax (HFC) cable network infrastructure and the role of DOCSIS® technology in data transmission over the coaxial portion of the network. Today, we’ll focus on the fiber portion of the HFC network, as well as the coherent optics technology that’s widely considered to be the hyper-capacity future of internet connectivity.

What Is Coherent Optics Technology?

Cable’s HFC networks are “fiber-rich,” which means they’re composed mostly of fiber—a bundle of very thin, hair-like strands of glass or plastic wire. Fiber is light, durable, and most importantly, capable of transmitting a lot of data over very long distances incredibly quickly. Light travels through a vacuum at 186,282 miles per second, a universal constant that scientists denote as “c.” Although light traveling through fiber optic cable moves a little slower than that (69 percent of the speed of light in a vacuum, or 0.69c), it’s still incredibly fast at over 128,000 miles per second. That’s fast enough for a single burst of light to circle the earth more than five times in a single second.

Until recently, signals in a typical HFC network were transmitted over fiber using analog technologies: an electrical radio frequency signal would be converted to an analog optical signal, transmitted over fiber optic cables, and then converted back to an electrical signal at the fiber node. With the advent of Distributed Access Architecture technologies, which will help cable operators cost-effectively add more capacity to their networks, that same fiber is being re-used to carry digital signals rather than analog ones.

The digital fiber technology being deployed today in access networks uses an “on-off keying” approach, in which a transmitter rapidly turns the laser on and off to send a signal; each pulse can signal a single bit of digital information (a 1 or a 0). Coherent optics adds further dimensions to the optical signal to carry more information simultaneously: rather than just pulsing the light on and off, it uses other properties of light (e.g., amplitude, phase and polarization) to carry multiple bits with each burst of information rather than just one bit. That can increase the data-carrying capacity of a single fiber by as much as 70 times, compared with non-coherent technology.

How Has This Technology Evolved?

Coherent optics technology is not new. It’s been used for over 10 years in long-haul fiber networks that span thousands of miles between cities and countries. More recently, as the cost of coherent optics technology has come down and speeds have gone up (from forty to now hundreds of gigabits per second) it has seen growing deployment in metropolitan or regional networks. The one remaining frontier has been the access network—such as in a cable HFC network, which has a large number of relatively short links, requiring a very low-cost solution.

It was for this reason that CableLabs embarked on an effort to define the use of coherent optics for cable access networks: to define requirements specific to access networks, thereby promoting interoperability, scale and competition. All this reduces the cost of this technology to the point at which it could be used widely to grow the capacity of cable operator fiber networks.

This vision was realized with the publication of our initial Point-to-Point (P2P) Coherent Optics specifications (released in June 2018), which defined how to send 100 Gigabits per second (Gbps) on a single wavelength, and how to send up to 48 wavelengths on a single fiber. That was followed by our version 2 specifications (released in March 2019), which defined interoperable operations at 200 Gbps per wavelength, doubling the capacity of the network. And both specifications included support for another key technology called Full Duplex Coherent Optics, which doubles the capacity of each fiber yet again while enabling the cost-effective use of a single fiber rather than the normal fiber pair.

How Does This Technology Affect Me and My Future?

When you think about current technology trends and predictions for the future, you’ll notice a common thread. Future innovations—like holograms, 360° virtual reality (VR), artificial intelligence and so on—will all require super high-capacity, low-latency networks that can transmit a ton of data very, very quickly. We’re not talking about just long-haul networks between cities and countries, but everywhere.

This is why cable companies started investing in the expansion of their fiber infrastructure and fiber optic technology decades ago. By focusing on “fiber deep” architectures—a fancy term for bringing fiber closer to subscribers’ homes—and using technologies such as coherent optics to mine even more bandwidth out of the fiber that we already have in the ground today, we can ensure that our cable networks continue meeting the requirements of current and future innovations. Thanks to those efforts, you’ll be able to one day enjoy your VR chats in “Paris,” work in a “holo-room” and much, much more.

Learn More About Coherent Optics

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