THE CASE FOR SILICON PHOTONICS
The case for silicon photonics Silicon photonics has been a technology of promise for over a decade. So it is not surprising that industry voices have started to question the technology, writes Roy Rubenstein.
Facebook has said it will use duplex single-mode as the fibre of choice for its large data centres and has selected the 2km, 100 Gig CWDM4 interface to connect equipment. “Facebook is agnostic to technology,“ says Katharine Schmidtke, strategic sourcing manager, optical technology at Facebook. “There are multiple technologies that meet our requirements.” Yet silicon photonics’ ability to produce integrated designs - with all the required functions placed in one or two chips - is appealing, says Schmidtke. Such designs are needed to scale and that requires a high- yielding process, especially when a single large data centre uses hundred of thousands of optical transceivers. This style of manufacturing is what the chip industry excels at. Silicon ends up dominating fields, not necessarily because it is the best choice in terms of performance but because it ends up being so cheap in scale, says Hochberg: “Once you can do something in silicon and do it adequately well, it tends to displace everything else from the majority of the market.” Applications Silicon photonics may be in a quiet period but products are shipping in volume. Cisco has been using its 100 Gig CPAK transceivers for several years, with two CPAKs being silicon photonics-based. And the 40 Gig PSM4 module now ships in excess of 100,000 units. Six companies offer 40 Gig PSM4 products, says LightCounting, but silicon photonics player Luxtera has a healthy start. Acacia is another selling silicon photonics-based modules. Unlike Cisco and Luxtera, Acacia offers 100 and 400 Gig coherent modules for
phosphide and gallium arsenide can. “With III-V, all the core functionality can be integrated in a single die,” says Joris Van Campenhout, programme director for optical I/O at imec, the Belgium nano-electronics research centre. “In terms of pure performance, III-V is still a bit further.” But what attracts Verizon and Facebook to silicon photonics is its ability to produce integrated designs and its potential to ramp to volume manufacturing quickly. “Fundamentally, we believe silicon photonics is a real enabler,” says Glenn Wellbrock, director of optical transport network architecture and design at Verizon. “It is the only way to get to the densities that we want to get to, going forward.” Wellbrock says indium phosphide photonic integrated circuits (PICs) can also achieve such densities. But with silicon photonics, there are many potential suppliers, given its relatively low barrier to entry compared to indium phosphide.”To date, Infinera has been the only real [indium phosphide] PIC company and they build only for their own platform,” says Wellbrock.
players - some silicon photonics players, others like Finisar and Lumentum (formerly JDSU) adding it to their portfolio - all back the technology. “I don’t think anything has fundamentally changed [regarding silicon photonics],” says Julie Eng, vice president of transceiver engineering at Finisar. “There is a lot of excitement over it, and then in developing any new technology there are always setbacks, difficulties and challenges.” Vladimir Kozlov, CEO of LightCounting Market Research, agrees. “The excitement has gone,” he says. “Now it is the long hard work to deliver products.” Promise “The great advantage silicon photonics gives you is access to first-rate fabrication infrastructure,” says Michael Hochberg, director of R&D at Coriant’s Advanced Technology Group. Silicon photonics offers 8- and 12-inch wafers, access to high-volume foundries, and an ability to manufacture complex designs in volume and with a high yield. But silicon can’t lase whereas the III-V materials of indium
ROY RUBENSTEIN
A fter steady
technological progress and a spate of start- up acquisitions, silicon
photonics has entered a quiet period and even experienced setbacks. Silicon photonics pioneer Intel announced in February it would delay the launch of its silicon photonics products by a year. And IBM - like Intel, a long-time developer of the technology - has announced a near-complete 100 Gigabit WDM transceiver but said that while it intends to use the technology internally, wider adoption will be needed if the design is to be economical. “As a general rule, the more that reality sets in, the less impressive silicon photonics gets to be,” says Mark Lutkowitz, principal at consultancy fibeReality. Yet Verizon, Facebook, Cisco, Coriant, and Mellanox Technologies, as well as optical component and module
10,000,000
40-400G Si 40-400G InP 40-400G GaAs
8,000,000
6,000,000
4,000,000
2,000,000
2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
LightCounting’s forecast showing the volumes of 40 to 400 Gig transceivers shipping between 2010-2021
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ISSUE 5 | Q3 2015
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