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UK researchers probe limits of coherent optical transmission


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esearchers 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 on the installed base of standard singlemode fibre – a capacity almost ten times greater 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. 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. In October 2016, Nokia Bell Labs reported an experiment based on this technique, which achieved 65Tb/s transmission over a single 6,660km-long fibre. 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); but probabilistic shaping redistributes them to reduce the occurrence of high-power symbols and minimise signal-to-signal interactions. Te latest investigation from UCL, carried out


as part of the 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


Dr Tianhua Xu and Daniel Semrau in the UCL Optical Networks Lab


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. ‘Te 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. Tis would help operators optimise the configuration of their optical transport systems.


Te research clarifies which modulation format


will provide the highest data rate. In the systems examined, researchers found no advantage in using a higher-order modulation format (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. Tat’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. Te research was published inOptics Letters: https://doi.org/10.1364/OL.42.000121.


AT&T to trial 400 Gigabit Ethernet in 2017


AT&T intends to be the first in the industry to demonstrate 400 Gigabit Ethernet (GbE) services across its production network, and will launch trials for business customers in early 2017. Te US service provider reported that data traffic on its network grew more than 150,000 per cent between 2007 and 2015, and continues to increase. Introducing 400 GbE is a natural progression, the company says. ‘Although there have been efforts focused on


400 GbE viability and industry standards over the past couple of years, we are excited to be the first to implement a pilot,’ said Rick Hubbard,


18 FIBRE SYSTEMS Issue 14 • Winter 2017


senior vice president, AT&T network product management. ‘400 GbE has the potential to transform how our largest retail and wholesale customers manage their networks today.’ Te 400 Gigabit Ethernet testing will be


performed in three phases using optical equipment from vendors Coriant and Ciena, as well as open source soſtware. Phase 1 will demonstrate that AT&T’s nationwide network is 400G-ready. Optical gear from Coriant will be used to carry a true 400 GbE service across a long-distance span of AT&T’s global backbone from New York to Washington. Phase 2 will bring


400 GbE services to customers in a metro area. Using coherent optical transport equipment from Ciena, AT&T will carry the 400 GbE service on a single 400G wavelength across its OpenROADM- based metro network. In the third and final phase, AT&T will test the


first 400 GbE open router platform. Te ‘disaggregated router’ platform uses merchant silicon and open source soſtware – another industry first, it asserts. AT&T hasn’t named its partners for this phase, but it’s worth noting that Coriant has also been developing a new architecture based around disaggregated routers.


UCL


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