Calling attention to data they will deliver at the OFC Conference and Exhibition in early March, IBM researchers announced they successfully sent data at a rate of 64 Gbits/sec over 57 meters of multimode cable using vertical-cavity surface-emitting lasers (VCSELs). “The achievement demonstrated that the standard, existing technology for sending data over short distances [multimode fiber] should be able to meet the growing needs of servers, data centers and supercomputers through the end of this decade,” the researchers said. They characterized the speed-and-distance accomplishment as “about 14 percent faster than the previous record and about 2.5 times faster than the capabilities of today’s typical commercial technology.”
To achieve these speeds, the researchers used VCSELs developed at Chalmers University of Technology in Sweden and custom silicon-germanium chips developed at IBM Research. “The receiver chip is a unique design that simultaneously achieves speeds and sensitivities well beyond today’s commercial offerings,” said Dan Kuchta of the IBM T.J. Watson Research Center in New York. “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.”
The researchers used standard non-return-to-zero (NRZ) modulation. Kuchta noted, “Others have thought that this modulation wouldn’t allow for transfer rates much faster than 32 Gbits/sec. What we’re showing is that’s not the case at all. This technology has at least one or two more generations of product life in it.” IBM pointed out that some researchers believed achieving higher-than-32-Gbit/sec transmission rates would require turning to more-complex types of modulation.
Kuchta will describe the results of his research at the OFC Conference and Exhibition, which is being held March 9 through 13 in San Francisco. He further explained, “Researchers typically rely on a rule of thumb that says the usable data-transfer rate is about 1.7 times the bandwidth. That means that with the VCSEL laser, which has a bandwidth of about 26 GHz, the rate would be only about 44 Gbits/sec. What we’re doing with equalization is we’re breaking the historical rule of thumb.”
IBM explained that the fast speeds achieved only worked for a distance of 57 meters, so the technology is most suitable for transmitting data within a building. About 80 percent of the cables within data centers and most, if not all, cables used for typical supercomputers are less than 50 meters long, the company further stated.
For those planning to attend OFC, Kuchta’s presentation, titled “64Gb/s Transmission over 57m MMF using an NRZ Modulated 850nm VCSEL,” will take place on Thursday, March 14 at 1:30pm in Room 121 of the Moscone Center.