Modulation Scheme Dramatically Increases Capacity, Range of Next-Generation Fibers

A new phase-modulation (PM) scheme may have found the “sweet spot” for long-distance communications over next-generation optical fibers.

Infinera Corp. of California and BICS SA/NV of Belgium recently completed an extended laboratory trial of three PM schemes for transmission over OFS Terawave large-area/low-loss (LA/LL) optical fiber, which is optimized for terrestrial cables.

The results show the dramatic improvement in optical reach and capacity that can be expected as these new fibers are deployed, Infinera said.

One submarine trial showed that 8 quadrature amplitude modulation (8QAM) offered optical reach up to 7400 km with a 50 percent increase in capacity for the new LA/LL fiber. In existing fiber, the reach was more than 2200 km and capacity was also increase by 50 percent.

The trials also explored quadrature phase-shift keying (QPSK) and 16QAM.

“With the optical reach we achieved using this new type of LA/LL fiber, we could envisage a future in which PM-QPSK could be used to close trans-Pacific links, PM-8QAM could cross the Atlantic, and PM-16QAM could span a European backbone,” said Steve Grubb, an Infinera fellow and trial architect. “This would enable between 50 percent and 100 percent increase in new submarine or terrestrial cable capacity.”

BICS Senior Product Manager Eric Loos said PM-8QAM could be useful for service providers seeking to boost capacity in response to growing demand from cloud-based services.

“PM-8QAM seems to offer a sweet spot for longer metro, regional and even long-haul distances, and could give us enough reach to include optical protection,” Loos said.

LA/LL fibers are being deployed in a handful of submarine cables now, and will likely be the technology of choice for future submarine cable builds, Infinera said.

The deployment of new LA/LL fiber in terrestrial applications could be delayed because of the large base of existing fiber. Existing fiber types typically have a much lower optical performance compared to LA/LL fiber, the company said.

In existing fibers, high-order modulations like 16QAM could double the capacity of QPSK (the de facto 100-Gb/s modulation) would reduce reach by 80 percent. This makes 8QAM a promising option over existing fibers for both submarine and terrestrial applications, Infinera said.

BICS owns a 100-Gb/s-capable optical network covering all major data centers and Internet protocol (IP) exchanges in Europe, including direct connections to more than 100 countries and incorporating 70 submarine cables.

Infinera develops large-scale photonic integrated circuits for intelligent transport networks, which enable carriers, cloud network operators, governments and enterprises to automate, converge and scale their data center, metro, long-haul and submarine optical networks.