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Optical interconnects offer sol'n to high-speed switching

Posted: 20 May 2014     Print Version  Bookmark and Share

Keywords:Oracle  optical interconnect  high-speed switching  ASIC  Oracle 

Another industry hurdle is predicted to be overcome sooner than expected. Optical interconnects from chip to board and chip to chip are starting to look like a solution to the I/O roadblock for high-speed switching within the next few years. However, the industry is still divided on whether to implement this disruptive optical/electronics integration on the wafer or in the package.

"High-speed signalling at lower power requires that optics get closer to the ASIC than ever before. There's been a long history of optics displacing electrical links, from connecting networks to connecting server racks and chassis, and the next step will be connecting the chip to other chips on the board," stated John Cunningham, a hardware architect at Oracle.

Oracle's approach is to put the I/O engine next to the logic chips in multichip modules, so the electronic drivers and the photonic components can each be built separately with the best-in-class technology. Optical components are then attached in the same package as the processer by hybrid integration.

Recent progress in advanced packaging with micro bumps and module substrates eases much of this modular integration. Issues still remain with laser efficiency, with precisely aligning the optical fibres with the lasers and photonics, and with developing the industrial volume processes to do so.

"The next hurdle is to show that the technology is scalable, and that the cost is competitive with electrical solutions," Cunningham noted. "This progression is on the industry roadmap to be developed over the next five years, but challenges still remain in developing all the building blocks and volume manufacturing processes."

Using a similar multichip module approach, Israeli startup Compass-EOS has started shipping to some service providers routers with optical connections between the chips in the backplane. Shuki Benjamin, process engineering manager, said the company is now looking for a packaging specialist to ramp up production.

Compass-EOS' approach mounts conventional optical lasers and photo diodes directly on to the processor chip with a kind of flip-chip bump, and cuts a hole in the package substrate to let out the light. Analog circuitry in the ASIC processor converts light to electrical signals and vice versa to get high bandwidth data in and out of the chip.

The arrays of lasers and photo diodes are aligned to a matching array of fibres in a fibre optic bundle to move the signals to and from the circuit board. The approach allows for direct optical links between multiple processers in the backplane of the company's routers, reducing system size and energy consumption.

Compass-EOS's technology also allows it to increase the chip I/O capacity by a factor of three, bypassing limits of BGA packages. The result is a network architecture that increases system bandwidth in orders of magnitude, indicated Benjamin.

"Large cloud data centres are driving a real need for unprecedented volumes of transceivers, up to 400Gb/s, and the silicon world is well positioned to meet those challenges," stated Eric Hall, VP of business development at Aurrion.

"We expect to see a lot of products come to market in the next year or so. When things shake out, there will be multiple vendors using the silicon infrastructure to make optical transceivers, though with different approaches to integrating the optics into the silicon."

Aurrion's approach bonds films of high performance but costly optical materials such as indium phosphide directly to the silicon wafer for wafer-level processing of large numbers of lasers and transceivers at once. "By attaching the materials to silicon and forming the lasers with the silicon it's easier to scale, you can get 16 channels for the same cost as one," added Hall.

Scaling up to large numbers of lasers and transceivers will allow uncooled wavelength-division multiplexing at 100Gb/s, he predicted. The company is developing its process at silicon foundries that would be able to scale to volume production.

CEA-Leti also takes this approach of bonding the laser material directly to the silicon for further processing at the wafer level, and reports 10 per cent wall-plug efficiency for this hybrid laser on silicon. Leti has ported its photonics library from its 200mm R&D line to its joint 300mm facility with STMicroelectronics, so a foundry service is available for volume production.

"Telecom applications will be first, since they can afford active alignment," said program head Sylvie Menezo, one of several speakers on silicon photonics at Semicon West. He expects to see commercial transceivers start coming out late this year, and silicon photonics replacing electronics in short-reach applications over the next five to eight years.

- Paula Doe
  EE Times

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