ROY RUBENSTEIN
Large scale data centres use hundreds of thousands of optical transceivers. Image supplied by Facebook.
over 10km using PAM-4. “This platform can be scaled to perform 400 Gig using a 4-by- 100 Gig or an 8-by-50 Gig WDM approach,” says Martin Guy, CTO & strategic marketing at Teraxion. “In the long run, data centre operators are interested in WDM over single- mode fibre and we believe silicon photonics can play a significant role here.” Silicon photonics start-up Aurrion is another eyeing WDM. “We perceive that the market has a lot of use for WDM,” says Eric Hall, vice president of business development at Aurrion. “When that is enabled on silicon photonics, much will come out of that, not only better instantiations of existing products but the development of new, more complex designs.” Aurrion has demonstrated a 4x25 Gig WDM design. Mellanox, which acquired Kotura, is another using silicon photonics for WDM in the data centre. It is a founding member of the OpenOptics MSA. “It [the MSA] supports more narrow line spacing, and scales to eight and 16 channels,” says Arlon Martin, senior director, marketing at Mellanox. “We think that is an interesting play at 400 Gigabit.” Mellanox is also using silicon photonics for 100 Gig PSM4. Challenges Indium phosphide specialist Oclaro has not stated any silicon photonics product plans but says once the performance and cost gets to a level it needs, it will consider using the technology.
that while silicon photonics had no technical hurdles, it had no edge compared to DML-based designs. “That is what brought us to parallel single mode,” says Welch. Luxtera uses one laser across multiple lanes for its PSM4 designs. “It is cheaper than multi-mode while having the long reach of single mode,” he says. Luxtera is now sampling its 100G (4x25G) PSM4 and will be in general production later this year. Finisar and Lumentum have several photonic technologies in-house and both have now added silicon photonics to their design toolkit. The companies consider using silicon photonics for any new product design and assess its merits against their existing technologies. Finisar backs VCSELs for the installed base of multi-mode fibre and believes indium phosphide DML lasers are best suited for 10 Gig single-mode transceivers. “All those millions of units that are sold using DMLs; nobody needs a new technology to make those parts,” says Eng. Where silicon photonics is starting to show an advantage is for multi-channel designs and where a channel speed of greater than 25 Gig speeds is needed, says Eng. Here silicon photonics competes with indium phosphide modulators and PAM-4 modulation. Finisar has demonstrated a silicon photonics single-channel transceiver operating at 50 Gig. Teraxion has shown a 100 Gig silicon photonics transceiver
Adam Carter, chief commercial officer at Oclaro, highlight concerns about the robustness of the supply chain for silicon photonics and manufacturing challenges. “A lot of people own the technology internally but that does not mean that it is broadly available,” says Carter. “And the foundries that are available today are boutique.” There is also much more to making a part than the fab process, says Brandon Collings, Lumentum’s CTO. “The ideal view is that silicon photonics solves so many problems,” says Collings. “But even when you decide to use silicon photonics, there is a whole pile of issues - packaging, testing and electronics - that need to play into the solution that are not CMOS-centric.” “When I first got involved in silicon photonics, I perceived that it was an opportunity to drive a different cost structure,” says Carter who, before joining Oclaro, was at Cisco when it bought Lightwire.”I’m a little bit maturer now in my thinking.” “The performance is there, it is now a manufacturing question,” says Facebook’s Schmidtke. Silicon photonics benefits from volume, and the volume is here with the large data centre requirements. Moreover, she expects to see that level of manufacturing maturity proven in the coming year. If Schmidtke is right, silicon photonics will no longer need to state its case as early as ECOC 2016.
metro-regional and long-haul networking. Silicon photonics has not shown itself to be the overwhelming solution for such long distance networks but that could change, says LightCounting. Cisco developed the CPAK after the input/ output (I/O) traffic it needed for its equipment line cards could not match the packet processing capabilities of its network processor (NPU).“By the time someone could provide us with the optics that could achieve the I/O density, that silicon investment would have gone stale,” says Russ Esmacher, director, packet optical sales at Cisco Systems. Cisco acquired silicon photonics start-up Lightwire and the resulting CPAK gave it a year’s time-to-market advantage, says Esmacher: “It also helped us tremendously in terms of power consumption.” Cisco went from a 100 Gig line card to a 400 Gig one while keeping the power consumption fixed, despite moving to a 400 Gig NPU and four 100GBASE- LR4 CPAKs. “We ship product, and to ship, you need to have product differentiation against a pretty competitive landscape,” says Brian Welch, Luxtera’s director of product marketing. “There are indium phosphide directly modulated lasers (DMLs) on one side and VCSELs on the other, and you need to take both on.” Luxtera looked to WDM for product differentiation but found
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