When a new undersea communications cable becomes operational late this year, it will break the record for a key metric: data rate times distance. In a single second, its six fiber-optic pairs, stretching roughly 13,000 kilometers (8,000 miles) between Hong Kong and Los Angeles, will be able to send some 144 terabits in both directions. That’s as much data as you’d find in several hundred Blu-ray discs. The cable’s main purpose is to connect Facebook and Google data centers in East Asia with those in the United States.
The new cable is part of an ongoing transformation of the submarine fiber-optic cable network. Originally, that network carried telephone calls and faxes. Later those subsea conduits served primarily to shuttle data between Internet users and a myriad of service providers. Now, it’s mostly transferring content and cloud-computing offerings between the data centers of a handful of tech giants.
Last year, such flows accounted for 77 percent of the traffic coursing beneath the Atlantic and 60 percent of that under the Pacific, says Alan Mauldin, research director at TeleGeography, a market-research unit of California-based PriMetrica. No wonder Facebook, Google, and Microsoft all now buy large positions in submarine cable companies and operate cable landing points. Google, for one, needs to double its transmission capacity every year to sustain the seamless appearance of its “Cloud 3.0” computing, Urs Hölzle, Google’s senior vice president for technical infrastructure, told the Optical Fiber Communication Conference and Exposition (OFC) last March. And fiber-optic cable technology has to run to keep up.
So far, the technology has been able to satisfy the exploding demand. For more than three decades, the growth of fiber-optic data rates has outpaced Moore’s Law. New types of fibers introduced in the early 1980s boosted the capacity of an individual fiber from 90 megabits per second to more than a gigabit. Better optical transmitters pushed rates to 10 gigabits per second in the 1990s. And by 2000, all-optical amplifiers combined with new optics could pack dozens of 10-Gb data streams at closely spaced wavelengths into a single fiber and carry that information hundreds or thousands of kilometers. By 2010, a more sophisticated modulation scheme increased the data rate per wavelength used so that the same fibers that had carried 10 Gb/s on a single wavelength could convey 10 times that amount. But demand has outstripped even these impressive improvements, and now the industry needs a new generation of technology to feed the bandwidth-hungry beast.
The upcoming Los Angeles–to–Hong Kong cable, called the Pacific Light Cable Network, is spearheading that new generation. “Subsea cables represent the pinnacle of optical transmission expertise, not in terms of capacity but in terms of capacity-reach product,” says Geoff Bennett, director of solutions and technology at Sunnyvale, Calif.–based Infinera Corp., which makes terminal equipment for cables. Transoceanic cables run thousands of kilometers between landing points, so what really counts for them is data rate times distance. And judged in those terms, the Pacific Light Cable—reaching a third of the way around the world—will set a record.
Such great distances are challenging in a submarine cable because optical amplifiers are required every 50 km or so to boost signal strength. Those amplifiers add noise, which then builds up along the length of the cable. Sophisticated signal processing can extract the signal from the accumulated noise, but the process isn’t perfect, which is why the achievable data rate drops with the length of the cable.
The current transpacific record is held by the Faster Cable, made by NEC and owned by a consortium including Google and five Asian telecommunication carriers (China Mobile International, China Telecom Global, Global Transit Communications, KDDI, and Singtel). That cable stretches 9,000 km, between Oregon and Japan, with an extension to Taiwan. Its six fiber pairs each carry 100-Gb signals at 100 different wavelengths, making for a total two-way carrying capacity of 60 terabits per second.
As is standard in the industry, Faster went into operation in 2016 with only some of its 12 fibers carrying live traffic. But demand was high, so “Faster filled up real fast,” Bennett says. No wonder planners at Pacific Light Data Communication of Hong Kong had already decided to provide more bandwidth for the Pacific Light Cable. The question they faced was how to do that.
One approach is to multiply the number of paths carrying the optical signals. A cutting-edge technique to do that, still confined to the lab, is to use fibers that contain many light-guiding cores so multiple optical signals could literally run in parallel. Another avenue to high bandwidth is to make fiber cores large enough for light signals to follow several different paths through the same fiber. If the core is the right size and composition, the light carrying the different signals crisscrosses but doesn’t interact. But this tactic requires optical transmitters and receivers able to get light into and out of the core at just the right angles to keep the different signals in separate modes. And like the multicore approach, this technique is still being developed.
In principle, you can combine both strategies. Fibers that contain separate cores that can each transmit using several modes have been tested in the lab, but the process requires sophisticated equipment, and this approach is expected to be costly if and when it’s ultimately deployed in the field.
A much simpler option is to use many separate fibers, either bundled in a single cable or split among a number of them. But time-tested designs for transoceanic cables can handle only a limited number of fiber pairs with their long chains of power-hungry amplifiers.
The Pacific Light Cable Network adopted yet another strategy to increase carrying capacity: It ventured into a new optical band. That’s because the Faster Cable had gone as far as was practically possible in transmitting signals in the conventional, or C, band, which ranges in wavelength from 1,530 to 1,565 nanometers. But engineers at Pacific Light Data Communications’ cable supplier, TE SubCom, in Eatontown, N.J., opened up an additional transmission band at wavelengths between 1,570 and 1,610 nm, called the L (for long) band. Using both the C and L bands, along with other improvements, doubled the cable’s total capacity.
Previously, it had been easier in most situations to refine C-band technology than to combine the C and L bands, says Neal Bergano, vice president and chief technology officer at TE SubCom. But with systems coming within a factor of two of the theoretical capacity limit, he and his colleagues decided it was time to open a new band. “There is about 5 terahertz of usable bandwidth in the C band, and you can double that by adding the L band, to get a total bandwidth of about 10 THz,” says Bergano.
The optical amplifiers used for these transmissions have limited bandwidth, so a second amplifier has to be added in parallel for operation in the L band. Fortunately, the required L-band amplifiers are essentially variations on C-band amplifiers and use the same raw material, erbium, to amplify different wavelengths. So good lasers and optical amplifiers were available for L-band transmitters. Still, it was no small matter to do the rigorous engineering to make this C+L scheme work.
In April 2016, at the SubOptic conference in Dubai, TE SubCom reported that a single fiber transmitting both the C and L bands could carry 49.3 Tb/s through 9,100 km of cable, at least under laboratory conditions. This approach needed separate optical amplifiers for the two bands but could use essentially the same fibers and cable designs as were being deployed in C-band systems. The developers said they could squeeze 20 extra wavelength channels into each band in a practical system that could carry 24 Tb/s per fiber through 12,500 km of cable—an impressive accomplishment. Six months later, TE SubCom announced that it had received a contract to build the Pacific Light Cable.
In addition to the C+L approach it was pioneering, TE SubCom also made improvements in how the data gets encoded, further boosting throughput. At the OFC last March, it reported sending 70.4 Tb/s per fiber in both the C and L bands through 7,600 km of cable. Just six months later, at the European Conference on Optical Communications, it reported using different coding to send 51.5 Tb/s through 17,107 km of cable, setting a laboratory record for the bit rate–distance product.
Adding the L band was clearly a big win, so it’s natural to wonder whether it will be possible to add still other optical bands to submarine cables. Alas, developers hold out little hope for that in the near term. “Murphy wasn’t looking when the C band came along,” jokes Bergano, because everything worked remarkably well. Erbium-based optical amplifiers are powerful and almost perfectly match the wavelength near 1,550 nm where optical fibers experience the least loss. The L band is almost as good, but other fiber transmission bands are poorly suited for transoceanic cables because of limitations in the available lasers, amplifiers, or the fiber material itself.
Why not just make the cable thicker so that you can stuff in more fibers? The problem is power. “Modern submarine cables are limited by the electrical supply power you can launch at the two ends of the cable,” says Peter Winzer of Nokia Bell Labs. Terrestrial cables can carry hundreds of fiber strands because the optical amplifiers they contain can tap local power sources dotted along the way, but transoceanic submarine cables can draw power only from their ends. And every fiber in a 10,000-km transpacific cable needs as many as 200 optical amplifiers per band spaced out along the way, each of which requires energy to operate. That, and the amount of power you can send over intercontinental distances, limits undersea cables typically to eight fiber pairs at most.
How then will future subsea cables meet the ever-increasing demands for bandwidth without people having to lay more of them in parallel? One tactic is to divide long cables into shorter, island-hopping segments, which could offer more bandwidth by virtue of the power that could be injected at the junction points. But that’s not attractive to Internet giants, which want direct, low-latency routes between their data centers. Another technique is to stretch the spacing between amplifiers, sacrificing bandwidth somewhat in each fiber to reduce power consumption, which then allows more fibers to be included in the cable. Such cutting-edge schemes and other fresh approaches should help to satisfy the voracious data appetites of Facebook, Google, and the other tech giants—at least for a while.
The original version of this article first appeared in IEEE Spectrum.
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