Device prevents long-distance signal degradation

Southampton University is developing a way to stop complex data signals sent by optical fibre from degrading over long distances.

Scientists working on the EU-funded international PHASORS project have created a device to amplify optical signals carrying large amounts of data by reducing the interference that gradually builds up.

The device could allow the optical signals to be sent over thousands of kilometres, something that is currently only possible with simpler transmissions carrying less data or with radio and microwave communication.

The amplifier is designed to work with signals that encode data in the phase of an optical wave (the relative position of the wave’s peaks and troughs) rather than its amplitude.

This method is increasingly important thanks to the growth of the internet, and in particular online video, as phase-encoded signals can carry more data and require less energy.

When sent over a long distance, phase-encoded signals start to degrade due to the interaction with others in the network (cross-talk). This results in random fluctuations (phase noise), which make it difficult to decode the information with high fidelity.

Southampton’s phase-sensitive amplifier restores the quality of the signal by eliminating this interference without the need for conversion to an electronic signal, which would slow its speed.

PHASORS director Prof David Richardson told The Engineer: ‘We mix the incoming signal with a signal of well-defined optical frequency and phase and we can use the outputs from that process to both amplify the signal and restore the precise phase levels of the original data stream.

‘The physics of these devices had been demonstrated before. But what we did was develop a practical means to make the device operate on signals of uncertain phase and carrier frequency.’

Existing systems use a previous Southampton invention known as an erbium-doped fibre amplifier to boost the power of flagging signals. These are fitted every 50km to 100km in optical transmission networks to boost the energy of the signals. However, these are phase-insensitive and so cannot restore the phase fidelity of signals.

Southampton’s prototype device works for bi-polar systems that have two levels of coding. ‘These are still relatively simple signals that we’re working with,’ said Richardson.

‘The next step is to work with more complex forms of encoded signal. We want to push it to systems that have four or even eight levels because by doing that we can squeeze even more information through an optical fibre.’

The €3.7m (£3.1m) PHASORS project is partly funded by the European Commission under the Seventh Framework Programme (FP7). It is led by Southampton’s Optoelectronics Research Centre (ORC) but also involves research teams from universities in Sweden, Ireland and Greece, and industrial partners in Switzerland, Ireland and Denmark.