FEATURE TERABIT NETWORKS


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physics perspective, there is no limit: 20-30 terabit is the practical limit, because then the signal will degrade so much it is no longer suitable for practical transmission purposes. It is more of a practical limit, in that trade-off of distance and capacity.’ To get out of that ‘trade-off trap’, he said, one


would have to apply more power into the fibre. But ‘with traditional fibres, this is not possible because they are working on the non-linear limit so then the only way out would be to use new fibres. ‘Now there are a number of fibres that have


been proposed. Te first is called hollow-core fibre; the other is called multi-mode fibre. Tis is exactly what we did with Telecom Austria, where we used space division multiplexing experiment that used this new type of fibre.’ Given the expense of putting new fibre into the


degradation of the signal when using higher modulation states like 16QAM. Fischer gave an overview of why this is necessary: ‘In the classical system, there is filtering in the optical signal. If you avoid that filtering, you need appropriate signal processing for the electrical signal. If you do not have this optical filtering, then you must shape the signal before you put it onto the fibre to avoid overlapping from neighbouring channels [producing noise], and this requires additional signal processing which is also part of the technology.’ Ciena’s solution to the problem of noise over


higher distance transmission has been to implement a new version of Raman amplification. Xenos said: ‘Historically Raman has had negative connotations because it has been very difficult to deploy, so many people have stayed away from deploying Raman solutions.’ Raman amplification is a technology that was


originally developed for the optical market in the 1970s but has generally played a secondary role to the more popular technique of EDFA amplification, even though Raman can provide improved signal to noise ratio. Te high power used by the module and the


fact that Raman uses the fibre plant as the gain medium created challenges for turn-up and troubleshooting new optical connections as well as for future fibre maintenance which historically made Raman unsuitable for some applications. For example turning up a high-power Raman amplifier across a fibre span that has a dirty connector or inadequate splice quality can result in damage to the fibre plant. However these concerns have been addressed with Ciena’s new Raman based solution.


36 FIBRE SYSTEMS Issue 5 • Autumn 2014


It will be the end- performance that really sets apart one competitor from another


Xenos continued: ‘Tere is a new type of


Raman that is required, because we need to deploy very simply and we expect Raman to become more widely deployed with these higher capacity channels.’ With competing solutions to reduce noise


across super-channel based networks, it will be the end-performance that really sets apart one competitor from another. Once all the hardware has become commercially available, time will tell which solutions can achieve the best performance in terms of distance and capacity of the network. Ciena has integrated Optical Time Domain


Reflectometer (OTDR) capabilities directly into the Raman amplifier. Te integration of the ODTR capabilities provides a controlled activation by autonomously testing the fibre plant to detect unacceptable connector and fibre conditions before the Raman amplifier is turned on. Tis controlled turn-up process prevents equipment and fibre damage which could cause additional deployment costs and delay. Coriant too is using Raman to overcome reach limitations and providing an integrated OTDR. With terabit networks already in sight, is there a


limit to what can be achieved? According to Fischer: ‘Strictly speaking, from a theoretical


ground, Fischer believes that this is still a solution for the future. Nonetheless, he believes that: ‘At some point in time, for very demanding connections, the industry will consider deploying these new fibres which would increase the non-linear limit, so you can just apply more power and then you can increase the reach again even while using the higher-state modulation formats.’ Coriant has already demonstrated record- breaking transmission rates of of 57.6Tb/s over solid-core multi-mode fibre and 57.6Tb/s over hollow-core photonic bandgap fibre. Te experiment with Telecom Austria showed


that there are such alternate paths into the future. It is not yet a current solution because these fibres are not widely deployed and there are challenges in manufacturing them in mass volumes. However, Fischer believes that, in some areas


and for some applications, new fibre technology will be used. ‘Tere are some cases where people are either doing new deployments – for instance in Australia, where we supply the national broadband network. Tere are others like in the US for instance – important interconnect routes for electronic trading like in Chicago and New York. People are already deploying new fibres in a more straightforward fashion to avoid some microseconds of delay. When you talk to Corning or other fibre manufacturers, they will tell you there is a good and constant market for new fibre deployments worldwide. When people start using these new fibres in new deployments, we will start to get these capabilities into the ground and then we can start using it from a system perspective.’ He concluded: ‘People say that internet traffic is


doubling year over year, so you can calculate when all the tricks and advances in system design will hit the ceiling of available capacity. Tat is why people start thinking, now, about beyond what is available today.’l


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