Spinning with Light’s Possibilities: Prof. Ramachandran Receives Phase II Grant from DARPA


By Rachel Harrington

Pictured above is the spiral phase structure of an optical vortex, extracted by interfering it with a conventional light beam.
Pictured above is the spiral phase structure of an optical vortex, extracted by interfering it with a conventional light beam.

When we think about light, what often comes to mind are the beams that keep the world from being a dark and dreary place or the electricity and bulbs that keep our homes bright.

In all these cases, light’s energy is coming from the center of its beams, but sometimes light can possess orbital angular momentum. When this is the case, those beams, called optical vortices, allow no energy in the center and essentially look like donuts. The energy, rather than moving in a straight line along the beam path, spins around the donut.

In May 2011, a team headed by Associate Professor Siddharth Ramachandran (ECE, MSE) that includes Steve Golowich of MIT Lincoln Laboratory and Poul Kristensen of OFS-Fitel received a grant from the Defense Advanced Research Project Agency (DARPA) to study optical vortices and whether optical fibers can be used to generate and transmit them.

Previously it was thought that optical vortices, while exotic and interesting, have little use because of their instability in fibers, but recent work by Ramachandran and Kristensen suggests that novel optical fiber structures can help transmit these beams, which opens up the prospect of using them for telecommunications. Pleased with their initial results, DARPA has awarded a Phase II grant worth approximately $800,000 to Ramachandran, Golowich, and Kristensen to continue this project.

“We are delighted to have received this highly competitive award, for it provides the necessary resources to investigate much more complex optical fiber structures which we expect to use to generate very high order orbital angular momentum states,” said Ramachandran. “This has never been done, but if the investigations are successful, we expect the results to help increase the data carrying capacity of optical fibers significantly.”

Prof. Siddharth Ramachandran (ECE, MSE)
Prof. Siddharth Ramachandran (ECE, MSE)

He noted that learning more about optical vortices could lead to advances in fields outside telecommunications, such as DNA sorting, high-resolution imaging and nonlinear and quantum optics.

Already, Ramachandran and his team have published several reports showing the use of these beams to perform quantum encryption for secure networks and for studying spin-orbit interactions in quantum systems.

At the European Conference on Optical Communication in September, the team demonstrated a significant breakthrough that led to a fourfold data capacity enhancement over a single wavelength channel with fibers that use these exotic beams. With their new funding, the researchers hope to continue making progress on this work.

Ramachandran will present a talk on optical vortices at the Future of Light Symposium on November 29.