New technique doubles distance of fibre optic communications

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UCL researchers have demonstrated a new way to process fibre optic signals that could double the distance data travels error-free through transatlantic sub-marine cables, and cut long-distance transmission costs.

As demand for high-speed communications grows, more information is being sent through the existing fibre infrastructure using different frequencies of light to create the individual data signals. However, these signals can interact with each other and cause distortions, meaning the data is received with errors.

According to UCL professor of optical communications and networks Polina Bayvel, the team wanted to find ways to increase the amount of information being carried over existing fibre optic infrastructure.

They used a 16QAM (quadrature amplitude modulation) super-channel made of a set of frequencies that could be coded using amplitude, phase and frequency to create a high-capacity optical signal. The super-channel was then detected using a high-speed super-receiver, and an algorithm was used to undo digital distortion based on calculations of the length of the fibre optic cable and the properties of this. As a result, the signals were transmitted error-free over 5890km compared to the previous 3190km.

As the receiver is at the end of the cable, operators would not have to add new components to the mid-line, while increasing the distance the signal can be carried means communications do not have to be electronically boosted on their journey, an important factor when cables are buried underground or at the bottom of the ocean. Changing the receivers is also far cheaper and easier than re-laying cables.

’If we know how distortion is caused, we can undo it,’ said Prof Bayvel. ‘Most other work in this area has only been able to detect on channel at a time and has been unable to take into account distortion cased by neighbouring channels. We have shown it is possible to have a wide band coherent receiver where all channels can be received.’

The researchers will now test their new method on denser super-channels commonly used in digital cable TV (64QAM), cable modems (256QAM) and Ethernet connections (1024QAM).

The findings are part of the EPSRC-funded UNLOC project, a collaboration between UCL and Aston University that aims to unlock the capacity of digital communications.