There are two ways to go about increasing a data center's capacity: (a) increase the number of parallel fibers through which the data travels or (b) increase how much data gets transmitted through existing fibers.
Option (a) is a pretty straightforward approach, particularly for data centers that rent fibers. Yet it’s expensive, not only in price but also in power consumption — hence the interest in finding ways to increase the transmission capacity of fibers already in use. As transponders and multiplexers — devices that combine multiple signals into one — become more sophisticated, so too do the available signal encoding/decoding processes.
Now, a cutting-edge signal transmission mechanism that was experimentally demonstrated a few years ago is online as a real-time bi-directional transmission system. Capitalizing on a very high spectral efficiency, the system has proved capable of supporting 78 interleaved, 400 gigabit-per-second (Gb/s) channels with a 31.2 terabit-per-second (Tb/s) fiber capacity. This sort of transmission capacity and rate — twice the standard 200 Gb/s found in most applications — could serve as a significant upgrade for current data center interconnections, in which nearby data centers are coupled together to form a single, larger center. Moreover, the setup used commercially available transponders and components compliant with current network standards.
Developing the Technology
After testing a unidirectional transmission configuration, a team led by Thierry Zami of Nokia wanted to improve the signal-to-noise power ratio. "It was important for us to maintain simple amplification, only based on erbium doped fiber amplifiers, and to use standard fibers," Zami says. "To increase the system margins observed with the unidirectional set up, we could have decided to make the same unidirectional experiment with slightly larger channel spacing, for instance. But we said 'no,' because we wanted to remain compliant as much as possible with the standard grid."
The team instead developed a bidirectional transmission setup in which even and odd 400 Gb/s channels transmit in opposite directions over a single 90-kilometer fiber. Signal-to-noise ratio was measured to be at least twice as large, compared to the unidirectional version. In addition, while the unidirectional setup used an individual 50-gigahertz multiplexer, the bidirectional setup utilized two 100-gigahertz-spaced multiplexers to create the same 50-gigahertz channel spacing. As a result, the system benefits from wider filtering that exhibits better tolerance to frequency detuning.
A research team from Nokia will report on the technology at OFC 2018, an annual optical communications event, which runs March 11-15 in San Diego.