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UK researchers probe limits of coherent optical transmission

Researchers from University College London in the UK have calculated that optical systems should be capable of providing transmission capacities of up to 223Tb/s over transoceanic distances using the installed base of standard singlemode fibre – almost ten times greater capacity than systems commercially available today.

Keeping pace with the global data traffic growth is increasingly challenging for optical systems developers as the capacity of standard singlemode fibre is close to theoretical limits. How close it is possible to get is a hot topic of research (see our feature article, Breaking the glass ceiling).

Recently, a technique known as probabilistic constellation shaping has been introduced that increases the resilience of the signal to noise, enabling signals to go faster or further. Last year, Nokia Bell Labs used this technique to enable 65Tb/s transmission over a single 6,660km-long fibre (see Nokia Bell Labs touts 65 terabit transoceanic transmission trial.)

Traditionally coherent optical signals are transmitted with the information units (symbols) evenly distributed across the constellation (a representation of the amplitude and phase of each symbol), whereas probabilistic shaping redistributes them to reduce the occurrence of high-power symbols and minimise signal-to-signal interactions.

The latest investigation from UCL, carried out as part of the £4.8 million five-year EPSRC-funded UNLOC programme, analyses the effect of combining probabilistic shaping with nonlinear compensation techniques (such as digital back propagation) using two common optical amplification methods – erbium-doped fibre amplifiers (EDFAs) and Raman amplification.

Senior research fellow Dr Tianhua Xu and PhD student Daniel Semrau found that, with the combined techniques, the achievable data rate over 2000km of standard singlemode fibre was 75Tb/s using EDFAs, and 223Tb/s using a Raman amplification scheme. “For comparison, 80Tb/s could transmit data from everything you look at in a year in a single second,” they commented.

“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 Tianhua Xu added. This would help operators optimise the configuration of their optical transport systems.

The research also clarifies which modulation format will provide the highest data rate. In the systems examined, the researchers found no advantage in using the highest-order modulation formats (1024QAM) over a lower-order modulation format (256QAM), once the transmission distance exceeded 3,200km in EDFA systems and 6,000km in Raman-amplified systems.

That’s important because lower-order modulation formats are much less complicated, and therefore more likely to be cost-effective to implement in real-world optical transmission systems, the researchers point out.

The 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'.

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