Following the launch of a Coherent Optics specification project about a year ago, CableLabs followed up this week with Full Duplex Coherent Optics, an effort that aims to beef up fiber capacity and enable the technology to work on fibers in cable access networks.

RELATED: CableLabs Eyes Coherent Tech go Give HFC a Future-Proofing Capacity Boost 

Per this blog post by Steve Jia, distinguished technologist, wired technologies at CableLabs (with contributions from CableLabs Fellow Dr. Alberto Campos), the Colorado-based cable R&D organization claims that Full Duplex Coherent Optics will enable the following set of capabilities:

-Double the bi-directional capacity of each fiber;

-Multiply the capacity of each existing access network fiber by over 200 times; and,

-Make Coherent Optics technology well suited for deployment in many more cable access network fibers.

Under the current plan, the new scheme will be incorporated into the ongoing CableLabs P2P Coherent Optics spec effort, which is targeting a release in mid-2018. The Full Duplex Coherent Optics initiative is moving ahead even as work continues on Full Duplex DOCSIS, an annex to DOCSIS 3.1 that targeting multi-gigabit symmetrical speeds over HFC networks.

RELATED: Full Duplex DOCSIS Plows Ahead 

Last year, CableLabs said it had figured out a way to adapt point-to-point coherent technology, in use for decades for long-haul networks, to work with short-haul access networks, holding that the move would enable MSOs to mine more capacity out of their HFC networks by “more than 1,000 times.”

In this week’s post, Jia reiterated that cable’s interest in coherent optics stemmed from the “somewhat limited fiber count between the headend and the fiber node” and the need to maximize the capacity of this scarce resource.

He noted that there are two fundamental topologies to achieve bidirectional P2P coherent transport -- dual-fiber and single-fiber – adding that a survey of MSOs found that 20% of existing cable access networks use a single-fiber topology whereby downstream and upstream transmission to nodes takes place on a single strand of fiber. CableLabs, Jia added, sees this single-fiber topology rising to 60% over the next five years among MSOs.

While dual-fiber requires a second fiber (one for the downstream and another for the upstream), the single-fiber approach transmits the up and down at different wavelengths using two lasers. The first option is difficult because of the fiber scarcity and the second one – the addition of another laser – is expensive.

CableLabs is proposing an alternative that uses two optical circulators (characterized as low-cost, passive, but directional devices) on each end in a special configuration.

“Instead of using two fibers, a single fiber is connected for bidirectional transmission,” Jia said. “Most importantly, instead of using two lasers, a single laser is employed for single-fiber coherent systems.”

Regarding its application to cable and the use of direction-division multiplexing in the optical domain, the claim is that it can double the whole fiber system capacity, and can work with 100G, 200G and future 400G systems.

CableLabs also said this works for short and long wavelengths, as its Optical Center of Excellence has tested it at distances of up to 100 kilometers.

Keeping tabs on future demands of the access network, CableLabs said it has adapted coherent technology, in use for decades for long-haul networks, to work with short-haul access networks.

As a potential future-proofing option, this application of coherent technology could put MSOs in position to mine more capacity out of their HFC networks, by “more than 1,000 times,” Alberto Campos, distinguished technologist at CableLabs, said in this blog post about the work, noting that the R&D house has already demonstrated a 50x capacity bump over what analog optics can achieve today on 80 kilometers of fiber.

He said the approach, paired with a move to new distributed architectures, could remove limitation on analog optical transport that requires higher power levels countered by the reduction of the number of optical analog carriers the fiber can support due to fiber “non-linearity effects.”

“HFC Networks have been typically designed with 6 to 8 fibers connecting the hub to the fiber node,” he explained. “Two of these fibers are used for primary downstream and upstream connection and in some cases two additional fibers are used for redundancy purposes. The rest of the fibers were left for future use. Unfortunately, a large amount of these ‘future use’ fibers, because of an ever-increasing demand for bandwidth, have since been repurposed for business services, cell backhaul, node splits and fiber deep architectures. In some cases, only the two primary fibers that are feeding the fiber node remain available for access transport.”

He said this  fiber shortage problem will only grow as fiber demand for business services and wireless backhaul increases, which is where coherent technology offers an opportunity.

CableLabs, he said, has “re-engineered the coherent link to meet the special conditions of the access network,” that improved performance at lower cost than long-haul or metro environments.

Citing labs results, CableLabs achieved 256 Gbps over 80 km on a single wavelength “with minimal dispersion compensation,” resulting in about 26 times the capacity of what can be achieved over an analog optical carrier fully loaded with 1.2 GHz worth of DOCSIS 3.1 signals, Campos said.

CableLabs plans to develop specifications that apply the benefits of coherent optics to access networks, and will welcome vendor involvement in their creation.

CableLabs announced the new approach in tandem with its Winter Conference taking place this week in Orlando, Fla. The event if off-limits to the press, but here’s a look at what’s being discussed there, according to a draft of the event agenda obtained by Multichannel News.