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FEATURE OPTICAL COMPONENTS


LightCounting also forecasts that at least 100,000 100Gb/s PSM4 modules will ship this year, with more than half of them silicon-photonics based. STMicroelectronics, meanwhile, is already


sampling its 100G optical engine chip design for PSM4 modules, its first silicon photonics product. ‘Silicon photonics as a market is at a turning point this year,’ claimed Flavio Benetti, group vice president, general manager digital and mixed processes ASIC division at STMicroelectronics. At the OFC show in March, Acacia announced


that since September 2014 it had shipped 13,000 of its AC100 100Gb/s coherent CFP transceivers based on its silicon photonics PIC. Te company also announced that it has started sampling optical modules based on the CFP2-ACO Implementation Agreement defined by the Optical Internetworking Forum (OIF).


Challenges Despite the recent successes, silicon photonics technology continues to contend with a number of challenges that will take time to resolve. Te most significant one remains the laser


Mario Paniccia holds up a 50Gb/s silicon photonics based data cable, an important milestone for silicon photonics technology in 2010


Although silicon photonics has only recently


become a viable technology for optical integration, its progress over the last few years has been striking. In just over a decade, the industry has gone from not knowing whether silicon could be used to make basic optical functions, such as modulators and photodetectors, to getting them to work at speeds in excess of 40Gb/s. ‘I’d argue the performance [of silicon


photonics] is close to what you can get in III-V [materials such as indium phosphide and gallium arsenide],’ said Paniccia, who was directly involved in Intel’s key silicon photonics device breakthroughs. Chris Doerr, who was at Bell Labs for more than


17 years, developed integrated optical devices first in indium phosphide and then also using planar lightwave circuits. He became hooked on silicon photonics aſter designing his first chip, an integrated coherent receiver, in 2009. When fabricating a complex design using


indium phosphide, five or six devices would need to be tested before a working one was found, he recalled. ‘In silicon it totally changed,’ he said. ‘Te yield is so high that you could assume every device was good.’ It means you can focus more on the design than the yield aspects, says Doerr, now associate vice president of integrated photonics at


Silicon photonics as a market is at a turning point this year


Acacia Communications, having joined the company in 2011. Another benefit Doerr highlights is that the


optical performance of a silicon photonics chip closely matches its simulation results. ‘Using indium phosphide, you have to worry about all the composition effects, and the etching is not as precise. With silicon photonics, you know the dimensions and the refractive indexes and you can simulate and design very accurately.’ Te result is that companies are no longer


investing their time and effort to develop basic devices in silicon photonics, but are able to develop and sell a range of integrated subsystems. LightCounting, which tracks optical component


and module shipments, highlights several silicon photonics products now shipping in volume such as Cisco’s CPAK optical transceivers, while silicon photonics start-up Luxtera has long been shipping its 40Gb/s PSM4 modules, a market that it leads.


source. Silicon alone cannot generate light, requiring that a laser source in another material is integrated onto the chip. Companies have developed several approaches to integrate the light source but a winning approach has yet to emerge. Until a low-cost approach emerges, silicon


photonics will not be able to compete with existing low-cost light sources such as vertical-cavity surface-emitting lasers (VCSELs), says Acacia’s Christopher Doerr. VCSELs dominate short-reach links, typically up to one hundred metres, although extended-reach VCSEL designs of several hundred metres also exist. Equally, coupling the laser to a fibre or the


silicon chip’s waveguide using passive alignment is another challenge. ‘Everything about silicon photonics is about low cost,’ said Southampton University’s Graham Reed. At present, to attach a laser, it is typically turned on and aligned to the chip’s waveguide. Tis requires manual intervention and is time-consuming. ‘Te ideal scenario is to put a fibre down and it


couples to the waveguide or laser and somehow you have aligned it,’ he explained. Te challenge is the discrepancy in dimensions between the 10-micron fibre core and the waveguide, which is typically between 0.35 and 0.5 microns wide. Work is on-going to use mode converters or gratings such that the resulting optical loss is low enough to make passive alignment viable. ‘People still view it [the laser attachment] as a


traditional optics assembly,’ said Doerr. ‘Until we get out of that mind-set and get to an electronics- type packaging where we can attach the fibre


Issue 12 • Summer 2016 FIBRE SYSTEMS 15


Intel/Business Wire


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