IBM has set a new speed record for data transmission over multimode optical fiber (which is typically used to connect nearby computers within a single building or on a campus). The researchers reckon that the achievement demonstrated that the standard, existing technology for sending data over short distances should be able to meet the growing needs of servers, data centers and supercomputers through to the end of this decade.
By sending data at a rate of 64Gb/s over a cable 57m long using a vertical-cavity surface-emitting laser (VCSEL), the researchers achieved a rate that was about 14% faster than the previous record and about 2.5 times faster than the capabilities of existing typical commercial technology, it is reckoned.
To send the data, standard non-return-to-zero (NRZ) modulation was used. “Others have thought that this modulation would not allow for transfer rates much faster than 32Gb/s,” says Dan Kuchta of the IBM T.J. Watson Research Center in New York. Many thought that achieving higher transmission rates would require turning to more complex types of modulation, such as pulse-amplitude modulation-4 (PAM-4).
“What we are showing is that that’s not the case at all,” Kuchta says. Because he and his colleagues achieved fast speeds even with NRZ modulation, "this technology has at least one or two more generations of product life in it,” he adds.
Foreground: two Chalmers VCSELs with (left) a 6 micron aperture that could operate error free up to 62Gb/s and (right) a 5 micron aperture, which set the equipment-limited record of 64Gb/s. Behind is IBM’s BiCMOS8HP VCSEL driver IC. On either side are the decoupling capacitors and connecting wirebonds. Credit: IBM.
To achieve such high speeds, the researchers used VCSELs developed at Sweden’s Chalmers University of Technology together with custom silicon germanium (SiGe) chips developed at IBM Research. “The receiver chip is a unique design that simultaneously achieves speeds and sensitivities well beyond today’s commercial offerings,” Kuchta explains. “The driver chip incorporates transmit equalization, which widens the bandwidth of the optical link. While this method has been widely used in electrical communication, it hasn’t yet caught on in optical communication,” he adds.
“Researchers typically rely on a rule of thumb that says the usable data-transfer rate is about 1.7 times the bandwidth,” Kuchta says. “That means that with the VCSEL laser, which has a bandwidth of about 26GHz, the rate would be only about 44Gb/s,” he adds. “What we are doing with equalization is we’re breaking the historical rule of thumb.”
The fast speeds only worked for a distance of 57m, so the technology is not designed for sending data across continents. Instead, it is most suitable for transmitting data within a building, says Kuchta. About 80% of the cables at data centers and most, if not all, of the cables used for typical supercomputers are less than 50m long. The new technology is ready for commercialization now, says IBM.
