By Brian Lavallée, Ciena Senior Director of Portfolio Marketing

Editor’s Note: At SubOptic 2019, SCW had a chance to sit down with Ciena’s Brian Lavallée.  SDM, or Spatial Division Multiplexing, was undoubtedly the hottest topic at the conference this year and Brian had talked about it during a presentation the day before.  I asked him to talk some more about the technology and how it will impact the submarine cable industry.

Mr. Lavallée: “Let’s start with what SDM is and what it isn’t.  In the submarine networking world today, SDM means distributing transmission over an increased number of fiber pairs. This is not to be confused with multiple cores within a single fiber, which is a future technology discussion that will likely take place.

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Brian Lavallée

Today, the average modern submarine cable has 6 to 8 fiber pairs.  With SDM, you’re looking at 16 fiber pairs today, and potentially many more fiber pairs in the future.  A couple of the wet plant vendors have already announced SDM technology.  People are talking about an even a higher number of fiber pairs in the future – say, 24 and up.  Initially, we at Ciena are looking at 16 fiber pairs based on wet plant vendor commercial announcements.  The intent is to have less capacity on each fiber pair when compared to existing systems, but more capacity in total when all fiber pairs are added up.  That said, operators will still want to maximize the per fiber pair capacity, based on what is technically possible.

The reason for doing this is to use modems that have potentially “less performance”, because you don’t need the latest modem technology, but you will need more of them.  There is only so much electrical power that you can push down a cable without melting it, so the only way you can do this is to distribute the power so that it is lower per fiber pair but spread across more pairs.  In the end, you get more capacity for a similar amount of electrical power and operate in a region that is “easier” to implement.

You’ve seen announcements in the recent past of speeds like 26 Tbps per fiber pair.  With SDM you’ll see less – maybe 16-18 Tbps, and potentially higher as things become real.  But as you’ll have more fiber pairs, the total capacity of the cable will be greater.  So, we’re not looking at ultimate capacity per fiber pair anymore, we’re looking at wet plant capacity as a whole. 

All the previous development went into how much capacity you could get out of a fiber pair.  With SDM you’re backtracking a little bit, rather than trying to get as close as we can to the Shannon Limit, and we’re pretty darn close now.  So, let’s pull back a little bit and launch less light into the fibers, with less capacity and fewer non-linear effects, so your modems don’t have to be the ultimate in performance.  Now our belief is, on the SDM side, that people will still want to have as much traffic as they can on an SDM cable, so the technology we build still needs to be applicable to a 16-fiber-pair wet plant just as it is to an 8-pair wet plant.  It’s about increased capacity within the wet plant at a lower cost per bit. 

There have been announcements made that these cables are available and you’ll see them going into the water soon.  A lot of this is being pushed by the big Internet content players.  They’re investing a lot and they have several white papers on this where they talk about the SDM cables.  They’re doing the investment; they’re pushing the technology and we’ll see it soon. 

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Claude Shannon, MIT Photo

We’re so close to the Shannon Limit right now that it’s getting very hard to get appreciable returns like we had in the past.  We used to get step increases in capacity.  But now we’re so close to it that the industry is saying, “do we keep going for diminishing returns or do we look at a new wet plant technology?”  Now our view at Ciena is that we build the modems and we want to provide as much capacity as we can into any fiber, be it SDM or non-SDM.  There are a lot of cables already in the water so our technology is readily applicable to those and it will continue to be.  The older cables were not optimized to be coherent, so we did the best we could over those cables.  The newer generation cables, those in the last 5-6 years, were optimized for coherent, that’s why you see capacity jumps.  And these new SDM cables will be further optimized for coherent supporting many more fiber pairs so we’ll get even higher capacities. 

It all has to do with Shannon.  He wrote that paper in 1948, but it was theoretical.  Now the opto-electronics caught up to his theory and now we’re living it.  And people have no choice but to start looking for solutions.  In the past, the innovations have mostly been in the modems, given submarine wet plants and their inherent performances were already deployed on seabeds around our planet.  There have been some minor improvements on the wet plant – a couple more fiber pairs – but the vast majority of innovation has been in the modems.  Now it’s time to go back and look at the wet plants.  Once they optimize the wet plant, you’ll see another spike of innovation and associated performance. 

Shannon Limit formula

Formula for Shannon's Limit

Basically, with Shannon’s Limit, we have three things to play with to get more capacity: increase bandwidth (the spectral width of the amplifiers), higher launch power, and a lower noise figure.  We’ve tweaked those to the point where there’s not much left to tweak.  So why not start adding more fiber pairs in cable and more cables themselves?  It’s a pretty exciting time in this industry right now.”