The Sky’s the Limit

Ramachandran Team’s Light Transmission Discovery Published by Science

By A.J. Kleber

It’s undeniable that the amount of data people are generating in digital spaces is constantly, exponentially growing. We tend to think of information as ephemeral, hovering insubstantially in “the cloud,” but in reality there are physical limits to how our data is stored and transmitted, and this continual increase in content is beginning to pose a challenge to the optical fibers that form the infrastructure through which it travels.

The information is sent in the form of beams of light, which can be maintained and relayed over globe-spanning distances using a phenomenon called “total internal reflection,” which allows light to bounce off the walls of an optical fiber “light pipe” with minimal loss. However, the capacity of a given optical fiber is limited—a limit our expanding data generation threatens to exceed. Fresh solutions are needed, which is where Distinguished Professor of Engineering Siddharth Ramachandran (ECE, Physics, MSE) and ECE PhD candidate Zelin Ma come in.

Siddharth Ramachandran (ECE, Physics, MSE)

In a new paper published in Science, Ma and Ramachandran, along with industry collaborator Poul Kristensen of OFS Optics, demonstrate their groundbreaking solution—one which not only cracks the problem of the upcoming capacity crunch, but may also yield a more energy-efficient means of signal transmission than traditional methods.

One existing approach to alleviating capacity crunch involves configuring an optical fiber to support several separate data channels. Light travels down these channels in spatially distinct patterns, each of which carries as much data as a single standard fiber. Ramachandran and his team had previously played a pivotal role in the development of this concept, akin to expanding the number of lanes in a highway to allow for increased traffic. Unfortunately, this tends to lead to “crashes”—information leaking between channels. This leakage corrupts every channel, thereby rendering information transmitted in all channels irretrievable–making this method a stopgap at best, not an effective solution.

Instead of thinking in terms of cars and roads, Ramachandran suggests a more celestial framework.

“High-topological charge light beams” behave differently from the standard beams used in optical communications today; rather than moving in a straight line, they twist as they travel, generating a “centrifugal barrier” similar to those created by the rotation of binary stars. Just as centrifugal barriers keep such stars from crashing into one another, they can also operate to keep these unusual light beams contained within an optical fiber over significant distances. That is, these twisted beams do not need total internal reflection, previously thought to be necessary for transmitting light, to remain confined to the optical fiber. Unlike total internal reflection, this effect is also significantly more robust, allowing for many more data channels, without that pesky leakage problem.

Ramachandran’s team has successfully demonstrated this new method by packing as many as 50 data channels into a single kilometer-long optical fiber; 25 times the capacity of conventional fibers. They theorize that this improvement is only the beginning—and if their approach is as scalable as they suspect, it could have a truly global impact.