Margo Anderson is senior associate editor and telecommunications editor at IEEE Spectrum.
A team of researchers in Japan and the United Kingdom have smashed the world record for fiber optic communications through commercial-grade fiber. By broadening fiber’s communication bandwidth, the team has produced data rates four times as fast as existing commercial systems—and 33 percent better than the previous world record.
The researchers’ success derives in part from their innovative use of optical amplifiers to boost signals across communications bands that conventional fiber optics technology today less-frequently uses. “It’s just more spectrum, more or less,” says Ben Puttnam, chief senior researcher at the National Institute of Information and Communications Technology (NICT) in Koganei, Japan.
Puttnam says the researchers have built their communications hardware stack from optical amplifiers and other equipment developed, in part, by Nokia Bell Labs and the Hong Kong-based company Amonics. The assembled tech comprises six separate optical amplifiers that can squeeze optical signals through C-band wavelengths—the standard, workhorse communications band today—plus the less-popular L-, S-, and E-bands. (E-band is in the near-infrared; while S-band, C-band, and L-band are in what’s called short-wavelength infrared.)
All together, the combination of E, S, C, and L bands enables the new technology to push a staggering 402 terabits per second (Tbps) through the kinds of fiber optic cables that are already in the ground and underneath the oceans. Which is impressive when compared to the competition.
“The world’s best commercial systems are 100 terabits per second,” Puttnam says. “So we’re already doing about four times better.” Then, earlier this year, a team of researchers at Aston University in the Birmingham, England boasted what at the time was a record-setting 301 Tbps using much the same tech as the joint Japanese-British work—plus sharing a number of researchers between the two groups.
Puttnam adds that if one wanted to push everything to its utmost limits, more bandwidth still could be squeezed out of existing cables—even just using current E-band, S-band, C-band, and L-band technology (ESCL for short).
“If you really push everything, if you filled in all the gaps, and you had every channel the highest quality you can arrange, then probably 600 [Tbps] is the absolute limit,” Puttnam says.
Getting to 402 Tbps—or 600
The “C” in C-band stands for “conventional”—and C-band is the conventional communications band in fiber optics in part because signals in this region of spectrum experience low signal loss from the fiber. “Fiber loss is higher as you move away from C-band in both directions,” Puttnam says.
For instance, in much of the E-band, the same phenomenon that causes the sky to be blue and sunsets to be pink and red—Rayleigh scattering—makes the fiber less transparent for these regions of the infrared spectrum. And just as a foggy night sometimes requires fog lights, strong amplification of signals in the E-, S-, and L-bands are crucial components of the ESCL stack.
“The world’s best commercial systems are 100 terabits per second. We’re already doing about four times better.” —Ben Puttnam, NICT
Previous efforts to increase fiber optic bandwidths have often relied on what are called doped-fiber amplifiers (DFA)—in which an optical signal enters a modified stretch of fiber that’s been doped with a rare-earth ion like erbium. When a pump laser is shined into the fiber, the dopant elements in the fiber are pushed into higher energy states. That allows photons from the optical signal passing through the fiber to trigger a stimulated emission from the dopant elements. The result is a stronger (i.e. amplified) signal exiting the DFA fiber stretch than the one that entered it.
Bismuth is the dopant of choice for the E band. But even bismuth DFAs are still just the least-bad option for boosting E-band signals.They can sometimes be inefficient, with higher noise rates, and more limited bandwidths.
So Puttnam says the team developed a DFA that is co-doped with both bismuth and germanium. Then they added to the mix a kind of filter developed by Nokia that optimizes the amplifier performance and improves the signal quality.
“So you can control the spectrum to compensate for the variations of the amplifier,” Puttnam says.
Ultimately, he says, the amplifier can still do its job without overwhelming the original signal.
Chigo Okonkwo, associate professor of electrical engineering at the Eindhoven Hendrik Casimir Institute at TU Eindhoven in the Netherlands, added that new optical amplifiers certainly need to be developed for E-, S- and L-bands as well as the standard C-band. But too much amplification or amplification at the wrong place along a given cable line can also be like too much of a good thing. “If more photons… are injected into the fiber,” he says, “It changes the conditions in the fiber—a bit like the weather—affecting photons that come afterward, hence distorting the signals they carry.”
Pushing Data Rates Into the World
Puttnam stresses that the research team didn’t send one signal down through a commercial-grade fiber optic line that in itself contained 402 trillion bits per second of data. Rather, the team separately tested each individual region of spectrum and all the various amplifiers and filters on the line that would need to be implemented as part of the overall ESCL package.
But what matters most, he says, is the inherent utility of this tech for existing commercial-grade fiber.
“Adding more wavelength bands is something that you can do without digging up fibers,” Puttnam says. “You might ideally just change the ends, the transceiver—the transmitter and the receiver. Or maybe halfway, you’d want to change the amplifiers. And that’s the most you would [need to] do.”
“Optical fiber networks must be intelligent as well as secure and resilient.” —Polina Bayvel, University College London
According to Polina Bayvel, professor of optical communications and networks at University College London, those same transceivers that Puttnam referenced are a next-stage challenge for the field.
“Transceivers need to be intelligent—akin to self-driving cars, able to sense and adapt to their environment, delivering capacity when and where it’s needed,” says Bayvel, who has collaborated with members of the team before but was unaffiliated with the present research.
To that end, AI and machine learning (ML) techniques can help next-generation efforts to squeeze still more bits through fiber optic lines, she says.
“AI/ML techniques may help detect and undo distortions and need to be developed in combination with high-capacity capabilities,” Bayvel adds. “We need to understand that optical fiber systems and networks are not just high-capacity plumbing. Optical fiber networks must be intelligent as well as secure and resilient.”
The researchers detailed their findings earlier this year at the Optical Fiber Communication Conference 2024 in San Diego.
UPDATE: 8 July 2024: This story was updated to include the perspectives of Chigo Okonkwo at TU Eindhoven.