WHY THIS MATTERS IN BRIEF
As the world demands more data our networks need to be able to handle it.
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Researchers in Japan and Australia have developed a new multicore optic fiber able to transmit a record-breaking 1.7 petabits per second, while maintaining compatibility with existing fiber infrastructure. The team–from Japan’s National Institute of Information and Communications Technology (NICT) and Sumitomo Electric Industries, and Macquarie University in Sydney, Australia – achieved the feat using a fiber with 19 cores. That’s the largest number of cores packed into a cable with a standard cladding diameter of 0.125 millimeters.
“We believe 19 cores is the highest practical number of cores or spatial channels you can have in standard cladding diameter fiber and still maintain good quality transmission,” says Georg Rademacher, who previously headed the project for NICT but who has recently returned to Germany to take up a directorship in optical communications at the University of Stuttgart.
Most fiber cables for long-distance transmission in use today are single core, single-mode glass fibers (SMF). But SMF is approaching its practical limit as network traffic rapidly increases because of AI, cloud computing, and IoT applications. Many researchers are therefore taking an interest in multicore fiber in conjunction with Space Division Multiplexing (SDM), a transmission technique for using multiple spatial channels in a cable.
There are two common types of multicore fiber (MCF). In weakly coupled MCF, cores are precisely separated from one another to suppress crosstalk. But this typically limits the number of cores that fit into a cable.
Instead, Sumitomo Electric has designed and fabricated randomly coupled MCFs in which the cores are intentionally arranged randomly. With no need for precise spacing, the cores can be packed closer together. This increases the spatial density of the cable and the number of cores that can be used. The random arrangement also broadens the interaction between cores, enabling light from one core to couple with the light from others nearby. As Rademacher explains, a signal transmitted in any one core of Sumitomo Electric’s MCF simultaneously utilizes all 19 cores, so the fiber achieves greater data capacity by utilizing the higher spatial channel density available. Multiple-input and multiple-output (MIMO) digital signal processing is then used to separate and demodulate the individual signals at the receiving end.
Nineteen cores is “the sweet spot because the channels all behave similarly, aided by the random coupling that helps average out fluctuations in the fiber properties,” says Rademacher. And compared to weakly coupled MCFs, which require individual signal processing for each core, “only the minimum amount of digital signal processing is necessary, and so significantly reduces power consumption.”
However, independent industry observers note that other researchers have developed non-standard fiber with as many as 32 cores and realized 1 petabit per second over 200 kilometers.
“The capacity of the new randomly coupled fiber is not that remarkable. What is remarkable is that it uses standard cladding,” says Govind Agrawal, an optics expert at the University of Rochester, in New York.
In addition, Agrawal says that weakly coupled cores supporting multiple modes have achieved capacities greater than 10 Pb/s. Again, this was with nonstandard cladding diameter fiber, and the distance was limited to 11.3 km.
“This approach also requires intensive offline digital signal processing,” he adds.
Using non-standard fiber would require a reengineering of the existing optical fiber infrastructure. MFC with standard cladding, on the other hand, remains compatible with commonly used optical components, equipment, and systems, and can take advantage of existing cable mass-production methods.
Along with the new cable, another important element in the setup are the optical chips that both direct light into the individual cores of the MFC and collect the signals from the cores at the receiving end. Most of today’s optical chips are fabricated using methods similar to standard integrated circuit processing on wafers. But this limits the circuits to a two-dimensional planar structure, which doesn’t suit the geometry of new MCF, says Simon Gross, a researcher at Macquarie University’s Photonics Research Center.
To interface the MCF with the standard SMF equipment now in use—including the transmission receiver used to collect the NICT test data—Gross and his colleagues have developed a laser-inscribed compact glass chip that incorporates a three-dimensional waveguide pattern to match the geometry of the individual cores in the MCF.
“We use a laser to etch a waveguide pattern in a block of glass the size of a fingernail,” explains Gross. “The waveguides enable the simultaneous feeding of signals into the 19 individual cores of the fiber with uniform low losses.” The etching process can be automated, and it is quick, he adds. “To inscribe the waveguides for the 19-core MCF takes less than 30 seconds with just one push of a button—though back-end processing and packaging will take much longer.”
To demonstrate transmission performance of the new MCF, NICT constructed an optical transmission system at its headquarters in Koganei, Tokyo. Employing commonly used C- and L-wavelength bands, and signal-coding techniques like polarization-multiplexed 64-quadrature-amplitude modulated (QAM) signals, the researchers achieved a transmission rate of 1.7 Pb/s over a distance of 63.5 km, a record for both data capacity and for distance using standard cladding fiber. To separate the signals at the receiving end, MIMO digital signal processing was performed offline.
The biggest challenge the researchers still face is more economic than technological, says Rademacher. “For the technology to see commercialization, we need a company to invest in a few key parts.” The examples he gives are a dedicated chip for the digital signal processing, which is currently done offline, and the need for suitable amplifiers to boost the signal over long distances.
Agrawal agrees. “Even for the short distance of 63.5 kilometers, the present digital signal processing takes too long to be practical. But with further development, such fibers are likely to find use in telecom systems.”