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FEATURE OPTICAL COMPONENTS


Tanks to PSE-2’s powerful chromatic dispersion


compensation, the WDM products support transatlantic or transpacific transmission. ‘Tere is also a signal processor on the transmit side, so it can do spectral shaping,’ Berger added. ‘Tat means that we can move carriers in a WDM system away from each other, in case we need very long reach but less capacity, or bring them closer together, such that the overall capacity can be much higher.’ Similar versatility is also enabled by PSE-2’s seven


modulation formats, including 8QAM, with which Nokia has pushed 400G services into long-haul applications for the first time. ‘Before PSE2, 400G applications were mainly possible in metro-type applications, around 600-800km, and now we can push beyond 1,500–2,000km,’ Berger underlined. ‘We also have two flavours of 16QAM, one supporting 400G, and another uniquely supporting 500G. PSE-2 also has implemented single carrier 400G, based on 64QAM, the most complicated modulation format in the industry right now in an available product.’ Set-partitioning quadrature phase shiſt keying (SPQPSK) has been added to specifically address very long haul terrestrial and submarine transmission, he adds. Such increased data rates are already lowering


expenses for communications companies, Berger claimed. ‘Te first PSE-2 products are out helping our customers utilise the investment in their fibres more efficiently,’ he stressed. ‘Where you had one laser for 10G before, you can now use one laser for 400G. Tat translates into higher capacity utilisation and cost savings.’


Doubling channel count Sunnyvale, California, headquartered Infinera has also exploited an in-house developed coherent DSP in its multi-terabit ‘Infinite Capacity Engine’ optoelectronic subsystem. Tis builds on the first generation 40nm DSP Infinera has been shipping for four years alongside the photonic integrated circuits (PICs) in its existing flagship platform. ‘It used to be 100G for each of five DSPs, hence


500G bandwidth,’ explained Pravin Mahajan, Infinera’s director of corporate marketing. ‘With the Infinite Capacity Engine we now have six DSPs, each with two channels. Te new, next generation FlexCoherent DSP is fabricated using a 28nm silicon process. Tat improves symbol rates from 16GBd to 33GBd, which means customers can drive more raw capacity and increase the amount of bandwidth that’s processed in the chip. Te PIC itself is capable of 1.2Tb/s, or 2.4Tb/s, depending on which modulation scheme we use. Tat has to be matched with the DSP. If we use 16-QAM modulation, it’s 200G on each of 12 carriers. We’re the first ones to reach multi-terabit capacity transmission.’


12 FIBRE SYSTEMS Issue 12 • Summer 2016 As well as doubling the number of channels,


Infinera has added capabilities like ‘in-flight integrated encryption’ to the new DSPs that Mahajan said ‘a lot of our customers have been asking for’. Other bonus features include compensating for non-linear effects whose unpredictable impact on signals is otherwise hard to fix. ‘Our advanced coherent techniques help mitigate


those non-linear penalties using soſt-decision forward error correction (SD-FEC) and gain sharing,’ he said. SD-FEC gain sharing, as far as he knows, is unique to Infinera, although ‘we would expect others to follow at some point’. Mahajan explains the concept of gain sharing


thus: ‘Two DSP channels can be in different parts of the fibre core. One channel could be traversing a good part and the other a bad part. Te weak part may not have enough strength to pass over the commissioning limit. Tat would mean you’re not able to use that part of the spectrum and hence there’s a loss of capacity and reach. Gain sharing


There will be more flexibility and intelligence in future digital signal processors


combines the strength of the strong and weak channel. Teir error codes are combined, and therefore now exceed the commissioning limit. Tis is only possible if we have a two-channel DSP. Te dual-channel DSPs also enable multi-terabit


transmission in mature and therefore relatively economical 28nm silicon. ‘Te more the capacity per single carrier, the faster the DSP has to process the signal,’ stressed Mahajan. ‘For a 400G single carrier channel in 64QAM the baud rate has to the really high. In competitor technologies that have announced 400G single-carrier technologies, they’re at the bleeding edge of electronics, at 45GBd. If we carry the same 400G capacity in multiple subcarriers instead of one single carrier in a single laser, the DSP processing goes down. Trough these techniques carriers can realise a 40-60 per cent improvement in data carried, depending on the fibre.’ Tough the Infinite Capacity Engine was


announced at OFC, products and deployments exploiting the technology won’t be announced until later in 2016. However, Infinera tested the system on Telstra’s production 9000km subsea cable link in


Nokia unveiled the PSE2 digital signal processor in March 2016


the Pacific Ocean, between Sydney and Hawaii, in late November 2015. ‘On a subsea cable, the window for a production trial is extremely limited, so we were fortunate to have Telstra allow us to test new technologies,’ Mahajan said. ‘Te best place to test their mettle is on a challenging subsea environment, because that’s the harshest.’ Like Swenson, Mahajan emphasised that the end


of Moore’s law has been heralded several times. He refers to coverage inTe Economist suggesting that it could continue for another decade. ‘As we get down to the atomic level, below 7nm, we’ll be paying very close attention to alternate approaches, including quantum computing,’ he said. However, even if an alternative isn’t forthcoming


for a while, that may not matter if companies continue to deliver innovations like combining multiple channels. ‘I believe we can continue extending the life of Moore’s law,’ Mahajan emphasised. ‘However, it’s the combination of photonics and electronics that will carry us forward beyond multi-terabits – and we’ve shown a path beyond 5Tb/s.’ Nokia’s PSE-2 is also currently fabricated using


28nm lithography. Te company is designing in 16nm processes now, and ‘first silicon’ will be available in 2016, Berger revealed, with 7nm chips likely to appear in 2017/2018. Yet he agreed with Mahajan, accentuating that photonic technology is at least as important as, if not more than, silicon electronics. ‘We’re typically increasing capacity by a factor of four while transistors just double,’ he said. ‘We cannot just use previous DSP schemes and scale it with smaller transistors. We have to invest in new and innovative architectures.’ Nevertheless, doing so successfully looks set to


bring exciting new possibilities, if what Berger predicted is right. ‘Te next generation DSP will be characterising the whole link and calculating physical limitations,’ he said. ‘It will work out the maximum possible capacity these particular links offer. Tere will be more flexibility and intelligence in future digital signal processors.’l


Andy Extance is a science writer based in Exeter, UK


@fibresystemsmag | www.fibre-systems.com


Nokia


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