Noise cancellation tech quadruples optical bandwidth

May 28, 2013 // 10:27 a.m.

Tags: #bandwidth #bell-labs #fibre-optic #internet #nature-photonics #networking #noise-cancellation #optical-fibre #performance #phase-conjugation

A system of sending twin waves through an optical fibre cable to boost signal to noise ratio at the far end could lead to long-range communications with a peak data transfer rate some four times greater than currently possible.

Developed by engineers working at Bell Labs, now a subsidiary of communications giant Alcatel-Lucent, the technology of using what the team calls 'phase-conjugated twin waves' has been successful used to transmit data at 400Gb/s over a 12,800km of optical fibre - or, to put it into relative terms, around four times the data transfer rate of current long-distance optical fibres over a length significantly greater.

The team's paper, published online this weekend following its submission to the Nature Photonics journal, is hard going for non-scientists. 'We show that the nonlinear distortions of a pair of phase-conjugated twin waves are essentially anticorrelated, so cancellation of signal-to-signal nonlinear interactions can be achieved by coherently superimposing the twin waves at the end of the transmission line,' the paper's abstract reads. 'We demonstrate that by applying this approach to fibre communication, nonlinear distortions can be reduced by >8.5 dB. In dispersive nonlinear transmission, the nonlinearity cancellation additionally requires a dispersion-symmetry condition that can be satisfied by appropriately predispersing the signals.'

Simplified, the team has achieved a way of significantly boosting the signal to noise ratio in an optical fibre communication system by sending two versions of the signal whereby one is a mirror image of the other - the 'phase conjugation' of the abstract. As the signal travels, noise picked up along the way affects both waves differently: noise that flips a 0 into a 1 on one wave will flip the 1 into a 0 on the other. When the two waves are received at the far end of the signal path, the waves are superimposed on one another again to form a single wave - and the noise picked up along the way cancels itself out.

The basic concept is similar to that of active noise-cancelling headphones, which uses microphones to generate an phase-conjugated waveform of external noise. This waveform is then played back along with the music you're trying to listen to, interfering with the external noise and cancelling much of it out. The team's work, therefore, can be thought of as noise-cancellation for optical communications systems - where the noise in question is light, rather than sound.

Previous systems attempting to achieve the same goal - the eradication or reduction of noise in optical fibre communication systems - have existed, but the Bell Labs team is the first to have created a working prototype that can operate without intermediate hardware. As a result, the team's technology can in theory be applied to existing long-distance communications links - such as the undersea cables that connect much of the world together - without the need for massive re-engineering work and a team of divers.

'Nowadays everybody is consuming more and more bandwidth - demanding more and more communication,' explained Dr Xiang Liu, lead researcher on the project, in a statement to the BBC. 'We need to solve some of the fundamental problems to sustain the capacity growth.'

The team's work is published in Nature Photonics, with the full text available for purchase on the journal website.
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