FEATURE OPTICAL COMPONENTS


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Digital signal processors meet Moore’s law


Silicon technology has powered rapid advances in coherent optical communication. Now silicon’s progress is set to slow, but that’s just one of many concerns the industry is preparing to overcome, finds Andy Extance


F


orty years ago, Intel co-founder Gordon Moore predicted that the number of transistors in an integrated circuit would double approximately every two years – a


prediction that proved astonishingly accurate. Optical communications has progressed at a


much slower pace. Historically electronic components have become about 70 per cent faster each year, while in contrast the capacity of optical transmission systems and their optical interfaces has only increased by about 20 per cent annually. At 100G, optical transmission hit a roadblock


that forced the industry to move in a new direction. And in doing so, the fortunes of the optical industry have become more tightly tied to the progress of silicon-based electronics than they ever were before.


Dispersion intolerant Delivering 100G performance required equipment vendors to overcome the physical limitations faced by conventional WDM transmission technologies, such as low tolerance to chromatic dispersion. Matthias Berger, senior director, optical hardware, at Nokia in Nuremberg, Germany, compares this challenge to an ice cube leſt outside the freezer. Like the ice cube melting, a bit travelling through the fibre spreads out over time. Aſter long enough – a few hundred kilometres in the case of the bit – nothing recognisable is leſt. Besides chromatic dispersion, 100G signals


suffered from other impairments, such as lower polarisation mode dispersion tolerance, and intensified nonlinear fibre effects. Te answer was coherent optical transmission, which encodes data using both the phase and amplitude of the signal. Standard photodetectors can only measure the intensity of light; therefore, to extract the


10 FIBRE SYSTEMS Issue 12 • Summer 2016


information at a coherent receiver requires powerful digital signal processing (DSP). Concurrently, networking system complexity


moved from only using carefully designed optical links to compensating for signal distortions using powerful silicon-based DSP chips in the coherent receiver. ‘Te DSP knows what the bit looked like when the signal was launched into the fibre, and it undoes all linear distortions,’ explained Berger. ‘You’re moving more complexity into transistors, which are fairly low cost.’ Even as expensive first- and second-generation


DSP technologies progressively prove themselves in practice, communications service providers are now looking at next-generation capabilities to prepare for the future. Te improvements that they can make are in large part enabled by moving to the next process node in silicon technology. However, in 2016 the news for those whose future relies on silicon processors could be considered worrying. Silicon processing power has followed the


exponential curve of Moore’s law since the 1970s – but in February Intel announced that it is slowing that pace. As circuits get smaller, scientists and engineers are beginning to see difficulties, such as heat dissipation and current leakage, which affect the ability of electronic circuits to advance at the same rate. Below 10nm transistors


near the atomic scale, beyond which it seems impossible to get any smaller. Tis clearly influences the future for DSPs – but


it’s just one of many hurdles technology providers know they must overcome to keep pace with galloping data demand. On the client side, in internet companies’


hardware racks, 100G coherent networking technology can exploit the pluggable CFP specification family of modules that integrate DSP devices within their packages. Intended to plug into switches or routers – connecting them into WDM systems – such modules are available from various vendors, who can exploit DSP devices from specialist digital electronic companies. Announcements at the Optical Fibre Communications conference in Anaheim, California, in March 2016, showed what the immediate future holds for these DSP devices. For example, the ExaSPEED 200 DSP


announced by Japan’s NTT Electronics can serve 100G QPSK long-haul and 200G 16QAM metro optical links. It is available to start sampling early in the second quarter of 2016 with a production release in the second half of 2016. Similarly, the third- generation CL20010 LightSpeed-II from Irvine, California’s ClariPhy, is a 200G coherent single-chip transceiver with a DSP engine. ClariPhy is partnered in a CFP2 analogue coherent optics


PSE-2s: Ultimate flexibility for optimising capacity and reach


Modulation formats introduced on Nokia’s PSE chip include 250G 16QAM, 200G 8QAM and a novel 100G scheme called set-partition QPSK


Nokia


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