An Interview with Georg Mohs and Mark Enright, SubCom

Editor’s Note: Perhaps the biggest topic of interest for the submarine fiber optics industry in recent years has been Spatial Division Multiplexing (SDM).  We have written many times that SDM is coming and speculated on what its impacts may be on the market, but now SDM is here, with the first SDM cable in the water and carrying traffic. 

SubCom was the supplier of that first SDM cable (Dunant) and it seems an opportune time for SubCableWorld to speak with two of their executives about the technology.  Georg Mohs has been with SubCom for almost 20 years in various technical roles and is currently running the R&D organization as Vice President of Research and Development and Chief Technical Officer.  Mark Enright, Vice President of Customer Solutions, is responsible for system engineering and product management and has been with SubCom for more than 30 years.

The following are their thoughts about SDM technology and its impact:

Georg Mohs: SDM is a departure from the way we used to think about the submarine cable system.  Previously, we thought about how we wanted to increase the capacity of a fiber pair.  We would use wavelength division multiplexing or, later on, coherent technology to try to get as much capacity as we could on each pair.  SDM is really a departure from that way of thinking.  In the SDM world, we are increasing and improving the capacity of the entire cable.  It’s no longer about the capacity of a single fiber pair.  It’s now really about optimizing the capacity of the cable. 

And that brings several benefits.  One is that it allows us to use power better.  That’s really a consequence of the Shannon Limit, which tells us that you can improve capacity only logarithmically with power.  Therefore, if you have a certain amount of power and you split that onto two fiber pairs, for example, the two fiber pairs at half the power will have more capacity than one fiber pair at full power.  So SDM allows us to use power more efficiently.  Another benefit is that by using power across multiple fiber pairs, it also improves the reliability of the system by sharing more pumps across more fiber pairs.  The pumps in the repeater are the most vulnerable parts, with the lowest reliability.  Sharing those pumps improves the reliability of the repeater as a whole. 

Mark Enright: Another benefit of SDM involves the type of optical fibers we can use.  When we take advantage of the optical power and spread it over more fiber pairs, it allows us to use less exotic fiber because we have power management.  We can use more ordinary fibers, which are more cost effective. 

Georg Mohs: SDM is somewhat of an enabler in that regard for the very high cable capacities.  We have to power our cable from the shore so there is a limitation to how much power can be provided.  The power determines how much capacity you can ultimately support on the cable.  So SDM, with its higher efficiency in the use of that power, also enables us to go to these high capacity cables. 

Mark Enright: This is a good time to mention our 18kV power products.  These are qualified to increase the output voltage of our power feed equipment and we’ve qualified all of our undersea elements to operate at a higher voltage.  Previously the maximum voltage was 15,000 volts.  So we have an extra 3,000 volts to feed these very large pipe networks. 

Georg Mohs: An extra 3,000 volts may not sound like much but the power increases quadratically, so it’s really a 44% increase in terms of the power available. 

Mark Enright: I think everybody has seen the Telegeography reports on 40% bandwidth demand compound annual growth rate (CAGR) and the ever increasing need for capacity, so there’s a drive towards larger and larger pipes.  SDM is a cost-effective way to manage that.  The increase in market demand is really showing itself in all of the recent project awards as well as future plans for networking.  I would guesstimate that 90% of the opportunities out there are SDM.  The only places that aren’t are very small regional interconnections that don’t have the need yet.  Their 40% CAGR is still below a fiber pair’s capacity, which right now as we see from press releases is about 25 Tbps.  That’s a lot of capacity for a regional network.

But it’s not a lot of capacity for the intercontinental Internet Content Providers (ICPs) that need larger and larger pipes and many, many fiber pairs.  Right now we’re up to 24 fiber pairs.  That’s basically half a Petabit of capacity per cable.  The ICPs want this capacity.  There is definitely an entrenched demand for SDM. 

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Georg Mohs


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Mark Enright


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