IBM Gives ‘Cheap’ Fiber Cables New Lease on Life
As computers get faster and data gets bigger, it is essential that data transfer speeds keep up so networks don't become the bottleneck. This is true whether the transfer is over long-haul or short-haul links. For the latter use case, multimode optical fiber is a popular option. Cheaper than its single-mode cousin, multimode fiber can connect computers within a single building or campus. But this type of fiber was struggling to keep pace with capacity demands.
Researchers at IBM have set a new speed record on multi-mode fiber, indicating that these cables might still have a few good years left. The research team achieved 64 Gb/s speeds over a cable 57 meters long using a vertical-cavity surface-emitting laser (VCSEL). The rate was 14 percent faster than the previous record and 2.5 times faster than typical commercial solutions.
According to the researchers, this breakthrough will extend the viability of the technology, satisfying the growing needs of server, datacenter, and supercomputer deployments through at least the end of the decade.
Many in the industry thought that boosting transmission rates would require more complex types of modulation, such as pulse-amplitude modulation-4 (PAM-4), but the IBM record was achieved using standard NRZ (non-return-to-zero) modulation. Researcher Dan Kuchta of IBM's TJ Watson Research Center takes this to mean that multimode technology "has at least one or two more generations of product life in it."
The researchers say their device is ready for commercialization.
The experiment employed VCSEL lasers from 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," 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.
As a general rule of thumb, the usable data transfer rate is about 1.7 times the bandwidth. For the VCSEL laser, with a bandwidth of about 26 GHz, this "rule" would limit the rate to 44 Gb/s. The equalization technique enabled the team to exceed this guideline by more than 45 percent.
The technology was only viable at lengths up to 57 meters, but that is long enough for many short-haul applications. Approximately 80 percent of the cables in enterprise datacenters and nearly all of the cables used in supercomputing are within the 50 meter range, according to the researchers.
The results of the research will be presented at the 2014 OFC Conference and Exposition, which takes place March 9 through 13 in San Francisco.
