Phase-sensitive optical data transmission: a new frontier

Fibre-optic cables are becoming the backbone of the Internet, from the main trunk lines between cities, countries and continents to the network of cables running from telephone central offices to individual homes and mobile phone base stations. A pioneering approach to data transmission, supported by EU funding, promises to increase the capacity, range and efficiency of fibre-optic networks.

Traditionally, commercial fibre-optic communications have relied on encoding the data in the amplitude of a light beam (varying the intensity of the light to transmit information). Placing devices called 'Erbium-doped fibre amplifiers' at intervals along the fibre to periodically boost signal strength overcomes propagation loss, but provides a relatively inefficient use of fibre potential.

 a new frontier
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Using the phase, rather than amplitude, of a light beam to encode data can potentially offer exponential increase in the information carrying capacity. But the extent of the improvement possible is limited: due to noise added during optical amplification and cross-talk between the different wavelength channels caused by non-linear optical interactions.

'Fibre-optic cable has huge data carrying capability and commercial systems still have orders of magnitude excess capacity but in recent years we have started to hit the practical limits in laboratory research using existing transmission techniques and conventional optical amplifier technology,' explains Professor David Richardson, Deputy Director of the Optoelectronics Research Centre (ORC) of the University of Southampton in the United Kingdom.

'Phase-sensitive amplification' (PSA) was identified theoretically as long ago as the 1960s as a potential means to amplify optical signals without adding noise. More recently it was demonstrated that it offered a means to remove phase noise (and to a lesser extent amplitude noise) from optical signals degraded during transmission - a function known as 'optical regeneration.'

Realising that improvements in optical component technology meant that a practical PSA might now be possible, a team of researchers from eight partner organisations in seven countries launched the 'Phase-sensitive amplifier systems and optical regenerators, and their applications' (Phasors) project. Supported by EUR 2.7 million in funding from the European Commission, their work has helped phase-sensitive fibre amplifiers make the leap from theoretical curiosities to practical devices.

'We knew that PSA should allow very low noise amplification as well as the removal of phase noise within optical communication systems, but there were big technological challenges ahead of us,’ Prof. Richardson, the scientific coordinator of Phasors, says. 'We successfully showed that PSA is possible, in a practical setting, and demonstrated many of its beneficial noise reduction properties along with the associated enhancements in network performance.'

Focusing on developing the technology for 40 gigabit-per-second (Gbps) broadband core networks, the Phasors team demonstrated two main devices - a phase sensitive amplifier and an optical regenerator for phase-encoded signals - that showed dramatic reductions in transmission system noise.

Phase noise - rapid, short-term, random fluctuations in the phase of a signal - is caused by a variety of processes, including quantum noise added in the amplification process and signals on different wavelengths interacting with each other in the same transmission fibre. It degrades signal fidelity and impedes network performance.

Unlike traditional amplifiers which are phase-insensitive, the Phasors amplifier is phase sensitive and was shown to reduce the noise figure to just above 1 dB. That compares with traditional erbium-doped fibre amplifiers which have a noise figure of at least 3 dB, and typically closer to 5 dB.

'Achieving noiseless optical amplification is the ultimate dream in optical amplifier research,' Prof. Richardson notes. 'The Phasors amplifiers certainly represent a major step in that direction.'

The Phasors optical regeneration subsystem eliminates interference for high-speed binary-phase-encoded signals. Whereas previous signal regeneration devices convert the optical signal into an electronic signal, slowing data transmission rates, the Phasors device directly reduces both phase-noise and amplitude-noise build-up, all within the optical domain.

The project also demonstrated the possibility of scaling the basic regeneration approach to allow the regeneration of signals with far higher levels of phase encoding than just binary, for example showing regeneration of 'Quadrature (four-level) phase-shift keying' (QPSK) for the first time.

'When we started Phasors we set out to show what is and isn't possible with all-optical signal processing and the amplification of phase-encoded signals. These devices demonstrate that the technology does work, not just in theory but in practice, and enable some very useful and enabling functionality,' Prof. Richardson explains.

With many researchers in Europe, the United States and elsewhere following up the work of the Phasors team, the researchers' achievements, in the long term, promise to enable significant improvements in the speed, capacity, range and efficiency of fibre-optic networks. In addition, the technology has important applications in a range of other fields such as optical test and measurement, sensing, and metrology.

Members of the Phasors consortium are developing commercial devices and technologies on the back of the significant advances made within the project. Components, including specialty optical fibres and high performance lasers, are already generating significant sales. Swedish partner EXFO, a global telecom test and service assurance solutions provider, has developed and is selling a test and measurement device for characterising complex phase and amplitude encoded signals based on their work within the project.

'Although the Phasors project is now over, its impact is certain to be felt for a long time into the future,' Prof. Richardson explains. 'High performance components and measurement systems are already on the market and research interest in the area of PSA technology in telecommunications and adjacent application areas is increasing. In Europe alone there are a number of nationally funded projects continuing on from our research, and there are several other major projects starting around the world.'

'Phasors has also generated a lot of interest academically,' he continues. 'A significant number of high profile papers have been published in leading scientific journals such as "Nature Photonics", and we've presented prestigious papers at major international conferences, including most recently at the European Conference on Optical Communications (ECOC 2011) held in Geneva Switzerland and the Optical Fiber Communications Conference (OFC 2012) in the United States where a substantial number of papers were presented on phase sensitive optical signal processing.'

Phasors received research funding from the European Union’s Seventh Framework Programme (FP7).

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