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Super-receiver unscrambles super-channel data

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Researchers at University College London (UCL) have developed a new way to process coherent optical signals, which could double the distance that data can travel error-free through fibre-optic cables. Their work was published in the journal Scientific Reports.

In the experiment, the researchers increased the distance that their super-channel signals can be transmitted error-free, from 3190km to 5890km.  Dr Robert Maher, research associate at UCL’s Department of Electronic and Electrical Engineering and lead author of the paper, said this is “the largest increase ever reported for this system architecture”.

This technique could reduce the costs of long-haul optical communications by eliminating the need for optical amplifiers to boost the signal strength, which is important when the cables are buried underground or at the bottom of the ocean. Alternatively, it could be used to increase the bit rate of optical signals transmitted over shorter distances to achieve much higher speeds.

The technique, called multi-channel digital back-propagation (MC-DBP), makes it possible to unravel the interactions that occur between different optical channels as they travel side-by-side over an optical cable.

“Once we figure out how the channels interact with each other, we can then devise new processing techniques that replicate this journey, but in the digital domain. The virtual digital journey is then carried out on a computer using some exciting digital processing,” Maher explained.

The technique is interesting because it can compensate for nonlinear effects, such as self-phase modulation due to the Kerr effect or cross-phase modulation (XPM), which introduce phase errors into the optical signal. Coherent optical systems are particularly sensitive to phase noise because they use phase as well as amplitude to encode the optical signal.

“The challenge is to devise a technique to simultaneously capture a group of optical channels, known as a super-channel, with a single receiver. This allows us to undo the distortion by sending the data channels back on a virtual digital journey at the same time,” Maher added.

In the experiment, the researchers generated a seven-carrier super-channel, with each 10-GBaud sub-carrier modulated using DP-16QAM (dual polarisation 16-point quadrature amplitude modulation). A single super-receiver was then employed to receive and demodulate the entire super-channel simultaneously.

The researchers are confident that their technique can be commercialised, but say further work is required to improve the efficiency of the scheme and to develop electronic components to implement it. The experimental set-up used a typical desktop computer to perform the digital processing. Ultimately, the technology would need to be condensed onto a single silicon chip.

“To implement this technique in real life will require a lot of power. We will need to come up with a more efficient technique before we can use it in a real system,” Maher said. One of the issues is the trade-off between performance and power - the more channels that are processed, the more processing power is required.

The work was carried out as part of UNLOC, a collaborate research project funded by the UK Engineering and Physical Sciences Research Council (EPSRC) that aims to find new approaches to ‘unlock’ the capacity of future optical communications system.

Professor Polina Bayvel, head of UCL’s Optical Networks Group and director of UNLOC, said: “We’re excited to report such an important finding that will improve fibre-optic communications. Our method greatly improves the efficiency of transmission of data – almost doubling the transmission distances that can be achieved, with the potential to make significant cost savings over current state-of-the art commercial systems.”

The researchers now plan to test their new method on denser advanced coherent modulation schemes commonly used in digital cable TV (64QAM), cable modems (256QAM) and Ethernet connections (1024QAM).

Maher R, et al. Spectrally Shaped DP-16QAM Super-Channel Transmission with Multi-Channel Digital Back-Propagation. Scientific Reports 2015; 5 (article 8214). DOI: 10.1038/srep08214

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