Fiber design could boost Internet bandwidth

Ashley N. Rice, ashley.rice@photonics.com

A new kind of optical fiber stable enough to transmit donut-shaped laser beams called optical vortices, or orbital angular momentum (OAM) beams, promises to increase bandwidth dramatically to meet today’s ever-increasing demand for data-intensive activities like video streaming.

“For several decades since optical fibers were deployed, the conventional assumption has been that OAM-carrying beams are inherently unstable in fibers,” said Boston University engineering professor Siddharth Ramachandran, who designed the new fiber.

Data flow inside an optical fiber with an output in an orbital angular momentum (as characterized by the twisted phase pattern of the output beam). These novel fibers developed at the University of Southern California and Boston University enable the use of orbital angular momentum as an additional degree of freedom for data transmission, helping scale bandwidth. Photo courtesy of David Steinvurzel, background image Inmagine Ltd.

“The main design philosophy was driven by the question – ‘Is there a way to separate different vector modes in a fiber?’ ” Ramachandran told Photonics Spectra. “If there is, then some combinations of vector modes, which lead to OAM beams, would not couple with other parasitic modes, and thus remain stable. We found that optical fiber waveguides that are shaped like rings does the trick of separating the aforementioned modes.”

In new work published in Science (doi: 10.1126/science.1237861), Ramachandran, University of Southern California electrical engineering professor Alan Willner and colleagues demonstrate not only the stability of the beams in optical fiber, but also their potential to boost Internet bandwidth.

Since the 1990s, bandwidth has been increased by sending multiple data streams down the cable fiber optic line by making each stream a different wavelength, or color, a process known as wavelength division multiplexing. An emerging strategy to boost bandwidth today is to send the light through the 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 from different modes mix together; determining which data stream came from which source requires computationally intensive and energy-hungry digital signal-processing algorithms.

Combining both approaches into a new strategy called OAM mode-division multiplexing, the investigators packed several colors into each mode and used 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.

Ramachandran’s OAM fiber had four modes (an optical fiber typically has two), and he and Willner showed that for each OAM mode, they could transmit 400 Gb/s in just a single wavelength of light – or 1.6 Tb/s across 10 wavelengths – over the course of 0.68 mi (1.1 km).

“This distance is good for applications in data centers, but for long-haul communications, we would need to study if this concept could work (i.e., whether OAM beam can continue to be stable) over thousands of kilometers of fiber,” Ramachandran said. The team would also need to investigate how many modes to which their concept is scalable.

“Aside from these two fundamental questions that we need to study, there are a few other practical considerations” before the technology can be practically applied, he said. “While the fiber supports/enables propagation of multiple data stream simultaneously, one would still need the technology to add and receive all these data streams into, and from, the fiber, respectively.

“Our demonstration used large-bulk optic devices for the lab demo, but the encouraging news is that compact, efficient and potentially cost-effective multiplexing technologies that can multiplex/demultiplex multiple OAM beams is being developed by several groups, in free-space optical elements, or even as integrated-optic components.”

The work was funded by DARPA, and Ramachandran and Willner collaborated with OFS-Fitel, a fiber optics company in Denmark, and with Tel Aviv University in Israel.