May 28, 2013
Noise-cancelling Light Beams Could Speed Up The Internet
redOrbit Staff & Wire Reports - Your Universe Online
A concept similar to that used with noise-cancelling headphones could lead to a faster, more reliable Internet, researchers reported this week in the journal Nature Photonics.
The idea involves transmitting two separate beams of light — mirror images of one another — down a fiber optic cable, and then recombining them on the receiving end to cancel out any noise the signal may have gathered along the way. The approach uses a well-established method of dealing with interference by inverting the noise to its phase conjugate.
In the current study, researchers used the technique to send a signal of 400 Gb/s down 12,800 km of optical fiber — a distance greater than even the longest trans-oceanic fiber cable, and a speed four times faster than the highest commercially available service.
The limiting factor in determining how far a given light beam can travel is how much power is contained in the signal. However, the greater the power, the more the light actually interacts with the material of the fiber, rather than simply passing through it. This interaction is what adds the noise that degrades the quality and reliability of the data.
The researchers wanted to explore the idea of using phase conjugation to cancel this noise.
Light waves that travel over fiber optic cables consist of a pattern of peaks and troughs just as sound waves and sea waves do. These waves are manipulated to represent information.
The "phase conjugate" of a given light beam is a separate beam in which every peak becomes a trough, and vice versa, so that the second beam is the inverse of the first and the two signals cancel each other out.
Although ideas to make use of phase conjugation to cancel out the noise added by fiber optic links are not new, they involve adding devices midway along the links to repeat the signal. This can be problematic since these devices must sometimes be installed at a location in the middle of an ocean floor.
"Sometimes you may send data from London to New York, sometimes you may send it from London to Paris. The links are changing and you cannot keep sending people to the middle of the link," said Xiang Liu of Bell Laboratories in New Jersey, the study´s lead author, in an interview with BBC News.
Liu and his team created a pair of phase-conjugate beams, each carrying the same data.
"At the receiver, if you superimpose the two waves, then all the distortions will magically cancel each other out, so you obtain the original signal back," he said.
"This concept, looking back, is quite easy to understand, but surprisingly, nobody did this before."
If the noise on the beams can be cancelled out, the power can be increased, allowing the data to travel longer distances.
Furthermore, since fidelity can be preserved, the data doesn´t need to be repeated for error correction as often, meaning the phase conjugation method is also a way to achieve higher data speeds.
The approach also conserves bandwidth since there is less of a need to resend data that had become corrupted.
"Nowadays everybody is consuming more and more bandwidth - demanding more and more communication," Liu said.
"We need to solve some of the fundamental problems to sustain the capacity growth."