New fibres carrying ‘twisted light’ reach terabit speeds

28 June 2013
By Edd Gent
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Transmitting "twisted light" down fibre optic cables can boost internet speeds to more than 1 terabit per second

A new kind of optic fibre that can carry “twisted light” could provide internet speeds of over a terabit a second.

The technology relies on donut-shaped laser light beams called optical vortices, or orbital angular momentum (OAM) beams, in which the light twists like a tornado as it moves along the beam path rather than in a straight line.

Optical vortices were previously thought to be unstable in optical fibre, but Boston University (BU) Engineering Professor Siddharth Ramachandran has now designed a fibre capable of propagate them.

In a paper appearing in today’s issue of journal Science, he and Alan Willner of University of Southern California (USC) demonstrate not only the stability of the beams in optical fibre but also their potential to boost internet bandwidth.

"For several decades since optical fibres were deployed, the conventional assumption has been that OAM-carrying beams are inherently unstable in fibres," 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 fibres."

Traditionally, bandwidth has been enhanced by increasing the number of colours, or wavelengths of data-carrying laser signals – essentially streams of 1s and 0s – sent down an optical fibre, where the signals are processed according to colour.

Increasing the number of colours has worked well since the 1990s when the method was introduced, but now that number is reaching physical limits.

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

Unlike the colours, 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 and Willner's approach combines both strategies, packing several colours into each mode, and using multiple modes.

Unlike in conventional fibres, OAM modes in these specially designed fibres can carry data streams across an optical fibre while remaining separate at the receiving end.

In experiments appearing in the Science paper, Ramachandran created an OAM fibre with four modes (an optical fibre typically has two), and he and Willner showed that for each OAM mode, they could send data through a one-kilometre fibre in 10 different colours, resulting in a transmission capacity of 1.6 terabits per second, the equivalent of transmitting eight Blu-Ray DVDs every second.

The research is the result of collaboration between optical fiber experts at BU and optical communication systems experts at USC and Ramachandran and Willner also collaborated with OFS-Fitel, a fibre optics company in Denmark, and Tel Aviv University.

"Siddharth's fibre represents a very unique and valuable innovation. It was great to work together to demonstrate a terabit-per-second capacity transmission link," says Willner, electrical engineering professor at the USC Viterbi School of Engineering.

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