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CableLabs: ‘Full Duplex Coherent Optics’ Gives Fiber a Capacity Jolt

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.