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New combination of techniques could double data rates

Future systems could transmit 223 Tbit/s over transoceanic distances

The latest theoretical work by UNLOC researchers has determined an optical communication system capable of transmitting 223 Tbit/s over 2000km of standard fibre. UNLOC is an EPSRC funded research programme led by Prof Polina Bayvel at UCL.

A technique known as probabilistic shaping has been used alongside signal correction techniques for the first time and could potentially double the world record data rate over transoceanic distances.

Data rates in optical communication systems must be increased to keep pace with the global demand for data. This research proposes a combination of techniques which make the most of the optical fibre cable currently in place in our data infrastructure.

The optical signals in fibre cable - which carry 99% of our data - must be amplified to ensure data can be transmitted far enough at high enough rates, particularly over transoceanic lengths of thousands of kilometres. The dominant methods of amplifying the signal are Erbium-doped fibre amplifiers (EDFA) and Raman amplification.

Research led jointly by UNLOC Senior Research Fellow Dr Tianhua Xu and UCL PhD student Daniel Semrau, has assessed for the first time the achievable data rates using both amplification schemes, with the application of probabilistic shaping and compensation techniques to remove signal-signal interactions.

“If our proposed combination of techniques is applied, a 2000km transmission using EDFA could achieve rates of 75 Tbit/s and 223 Tbit/s using a Raman amplification scheme. For comparison, 80 Tbit/s could transmit data from everything you look at in a year in a single second.” explained Dr Tianhua Xu.

Traditionally signals are transmitted with the information bits evenly distributed, whereas probabilistic shaping unevenly distributes the bits within the signal according to a predetermined pattern. This technique increases the sensitivity and efficiency of the signal transmission, in turn increasing the maximum potential data rate in an optical communication system.

“The most exciting bit is that someone can use our research to assess the maximum achievable data rate over a set distance for the system they are using” Dr Xu added.

The team proposes that probabilistic shaping, when combined with compensation techniques, achieves the limit of possible data rates imposed by the interaction between transmitted signal and noise in optical amplifiers.

The research further clarifies which modulation format (number of information bits per transmitted symbol in the signal) allows for the highest data rate. In the systems examined, it is shown that there are no gains from using the highest modulation formats over a lower modulation format (256QAM), when the transmission distance exceeds 3200km in EDFA systems and 6000km in Raman-amplified systems. Lower modulation formats are much less complicated to implement and therefore more feasible in real-world long haul transmissions.

This research was published in Optics Letters where it can be viewed in full: 'Achievable information rates estimates in optically amplified transmission systems using nonlinearity compensation and probabilistic shaping'.