Toward a Petabit Trans-Atlantic Submarine Cable

Last week, I had the opportunity to moderate a Light Reading webinar on the topic of “The Race for Capacity: Space-division Multiplexing in Next-generation Submarine Cables.”  A replay of the webinar is now available.

Our guest speakers were Dr. Steve Grubb, Global Optical Architect for Facebook; Dr. Sergei Makovejs, who heads up submarine optical fiber development at Corning; and Dr. Pierre Mertz, one of Infinera’s submarine transponder gurus.

The three speakers were able to cover all four major areas of the topic.  First, why is demand in the submarine market so high?  Second, what are the capacity limitations on today’s highly advanced submarine cables like the trans-Atlantic MAREA cable?  This section introduced a new submarine cable architecture known as space-division multiplexing (SDM).  Third, what are the developments in the next generations of submarine transponder design that will allow SDM cables to maximize transmission capacity?  Then the final section looked at the evolution of SDM toward trans-Atlantic cables that could potentially carry a petabit per second of capacity in the future.

For those of you not familiar with SDM, the idea is as follows. Cables like MAREA represent the pinnacle of submarine fiber quality today – using advanced large-area, positive-dispersion fibers, and with somewhat shorter spacing between the in-line amplifiers.  These design decisions enable record-breaking deployed capacity (in contrast to field trial capacity) over MAREA using Infinera’s fourth-generation Infinite Capacity Engine (ICE4) technology.  In fact, the features that enable this level of fiber pair capacity were covered in a previous Light Reading webinar, a replay of which is also available.

However, while the capacity per fiber pair in MAREA may be at record levels because of the type of fiber and high amplifier power levels, the amplifier power budget acts a limit on the number of fiber pairs in the cable – which is eight pairs for MAREA.

A future SDM cable would deliberately operate the amp chain at a lower power level so that more fiber pairs could be deployed in the cable.  While the capacity in each fiber pair would be somewhat lower than a MAREA-style pair, this would be more than made up for by the larger number of pairs in the cable.  If I were to summarize SDM in one line, it would be:

Take a small hit on the capacity per fiber pair in order to achieve a much bigger increase in the total cable capacity

While Steve Grubb explained the sort of traffic types Facebook would expect to drive the need for this capacity increase, Sergei covered the mathematics behind SDM, and also speculated on the fact that at lower amplifier power levels, it would be possible to use smaller effective area fibers, which would reduce the cost of the cable.

In his second section, Steve also pointed out that future SDM architectures might swap a copper electrical conductor for one made from aluminium, which would offer certain technical advantages and would be significantly cheaper in terms of the cost of the metal itself.  Pierre explained that within this new lower optical power environment, really effective probabilistic constellation shaping (PCS) would be one of the major factors in delivering optimal performance, and I recently wrote a blog about the need for long codewords in a high-efficiency PCS implantation.

We had some interesting questions from the audience, and one thing I’m looking forward to as we look toward 2020 is that more interesting technologies will be in lab and field trial demonstrations to show how we can genuinely move toward the petabit era in the submarine cable systems of the future.