‘Twisted’ light could give bandwidth an upgrade

BOSTON U. (US) — A new fiber optic technology has the potential to dramatically boost bandwidth for Internet users and providers.

Described in the journal Science, the technology centers on donut-shaped laser light beams called optical vortices, in which the light twists like a tornado as it moves along the beam path, rather than in a straight line.


Widely studied in molecular biology, atomic physics, and quantum optics, optical vortices (also known as orbital angular momentum, or OAM, beams) were thought to be unstable in fiber, until Siddharth Ramachandran, an engineering professor at Boston University, recently designed an optical fiber that can propagate them.

In the paper, he and collaborators from OFS-Fitel, a fiber optics company in Denmark, University of Southern California, and Tel Aviv University demonstrate not only the stability of the beams in optical fiber but also their potential to boost Internet bandwidth.

Bandwidth roadblocks

“For several decades since optical fibers were deployed, the conventional assumption has been that OAM-carrying beams are inherently unstable in fibers,” says Ramachandran. “Our discovery, of design classes in which they are stable, has profound implications for a variety of scientific and technological fields that have exploited the unique properties of OAM-carrying light, including the use of such beams for enhancing data capacity in fibers.”

Funded by the Defense Advanced Research Projects Agency, the technology could not come at a better time, as one of the main strategies to boost Internet bandwidth is running into roadblocks just as mobile devices fuel rapidly growing demands on the Internet.

Traditionally, bandwidth has been enhanced by increasing the number of colors, or wavelengths of data-carrying laser signals—essentially streams of 1s and 0s—sent down an optical fiber, where the signals are processed according to color. Increasing the number of colors has worked well since the 1990s when the method was introduced, but now that number is reaching physical limits.

Keep the modes separated

An emerging strategy to boost bandwidth is to send the light through a fiber along distinctive paths, or modes, each carrying a cache of data from one end of the fiber to the other.

Unlike the colors, however, data streams of 1s and 0s from different modes mix together; determining which data stream came from which source requires computationally intensive and energy-hungry digital signal processing algorithms.

Ramachandran’s approach combines both strategies, packing several colors into each mode, and using multiple modes. Unlike in conventional fibers, OAM modes in these specially designed fibers can carry data streams across an optical fiber while remaining separate at the receiving end.

In experiments appearing in the Science paper, Ramachandran and coworkers created an OAM fiber with four modes (an optical fiber typically has two), and showed that for each mode, they could send data through a one-kilometer fiber in 10 different colors, resulting in a transmission capacity of 1.6 terabits per second.

“A typical cable Internet connection to a home delivers 1-10 megabits per second of data,” Ramachandran explains, “which means that the transmission capacity we demonstrated with OAM modes in our fiber represents a capacity equivalent to one million simultaneous cable-internet connections today.”

Source: Boston University