The data capacity of single-mode optical fibers, while having increased by four orders of magnitude over the last 30 years, is rapidly reaching the limits imposed by the fiber’s nonlinear effects. But a bicoastal team has devised a new fiber optic technology that promises to increase bandwidth dramatically, meeting today’s ever-increasing demand for data-intensive activities like video streaming. New research by optical fiber experts at Boston University and optical communications systems experts at the University of Southern California created a new kind of optical fiber stable enough to transmit donut-shaped laser beams called optical vortices, also known as orbital angular momentum (OAM) beams. OAM beams are generating interest not only in communications, but also atom manipulation and optical tweezers. “For several decades since optical fibers were deployed, the conventional assumption has been that OAM-carrying beams are inherently unstable in fibers,” said BU engineering professor Siddharth Ramachandran, who designed the new fiber. “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.” 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 enable the use of orbital angular momentum as an additional degree of freedom for data transmission, helping scale bandwidth. Courtesy of David Steinvurzel; background image Inmagine Ltd. In new work published in Science this week, Ramachandran, USC 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. The strategy by Ramachandran, Willner and colleagues, OAM mode-division multiplexing, combines both approaches. They 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 miles (1.1 km). “This is very impressive,” University of Rochester physicist Robert Boyd told Science. “I can imagine a huge commercial market.” In related work last year, the researchers reported in Nature Photonics that OAM encoding could be used to send 2.5 Tb/s of data (about five Blu-ray discs) through free space (See: Twisting Light Sends Data Speeds Soaring). That method didn’t work, however, when it was tried in a standard optical fiber. The work was funded by DARPA, and Ramachandran and Willner collaborated with OFS-Fitel, a fiber optics company in Denmark, and Tel Aviv University in Israel. For more information, visit: www.usc.edu or view the paper, “Terabit-Scale Orbital Angular Momentum Mode Division Multiplexing in Fibers,” in Science (doi: 10.1126/science.1237861).