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Optical links hold key to PCB-processor gap

Posted: 27 Feb 2012 ?? ?Print Version ?Bookmark and Share

Keywords:optical link? interconnect? processor? Internet?

The International Technology Roadmap for Semiconductors has emphasized the widening gap between the growth of processor performance and the lagging ability of I/O. Aggravating this scenario is that data sent over the Internet increases by a factor of 100 every 10-12 yearsabout the pace of growth of processor performance.

"A byte-per-flop gap is opening up that's a major limit on architectures that gets worse and worse as we head to 2020 and beyond," noted David Miller, researcher at an optical lab at Stanford.

"We are getting close to the point where optical is more attractive," stated John Stonick, a Synopsys scientist and chair of the panel. "But optical interconnects have always been the next thing, and the big questions are when and how we get there."

Keishi Ohashi, an optical expert from NEC Corp., said optical may follow a path of hard disk technologies. It took nearly twenty years for powerful magneto-resistive heads to emerge from their beginnings in labor-intensive magnetic coils, he said.

Bert Offrein, an optical expert with IBM Research, noted milestones and challenges IBM has seen. In 2008, the company built Roadrunner, the first petaflop computer, using optical interconnects to each server board.

Last year IBM created its Power P775 high-end server that brought optical links to the processor. It fused 56 fiber cables to modules with 56 transceivers integrated with help from Avago on to each IBM CPU.

"It required 100 additional assembly steps to bring the optics to the chips in addition to building transceivers themselves," said Offrein. "That's justified for some high-performance systems, but for general servers we need something easier."

Board makers are experimenting with many ways to integrate optical links, prototyping 10Gb/s channels now, but none are ready for commercial production, he said. One future vision is of 3D stacks that include an optical layer, but it may require tiny mechanical mirrors or waveguides in the substrate, challenges companies such as IBM are wrestling with in research labs, he added.

Mario Paniccia, head of a silicon photonics research lab at Intel, called for flexible solutions that could address a range of applications from high-volume consumer markets to high-end exascale servers.

Paniccia's lab demonstrated last year transmitter and receiver chips made in "CMOS-like" processes that sent the equivalent of 50Gb/s of data over fiber using four 12.5G channels. "It was not a product, but it gave us confidence," he indicated.

Future versions of the technology could increase channel speeds and the number of parallel lines to hit rates of 400Gb/s and someday even a Tb/s, he added.

Big questions remain unanswered, including what sort of chip-level connectors such links should use, Paniccia said. In addition, optical links will always consume more power than electronic ones at similar data rates, he added. Ultimately, "I think silicon photonics will happen in increments as applications need more bandwidth, distance or new form factors," he said.

Stanford's Miller said Intel's work is a bright spot in the field. Most optical companies such as Finisar and Luxtera lack the financial clout to take on the heavy duty work needed in silicon photonics. Miller pointed to promising research that indicates plenty of headroom for optical technology. For example, he discussed early work on quantum well waveguides and optical antennas the size of a transistor gate.

Others are looking for ways to use germanium to embed light sources in chips without hitting the thermal limits of silicon. Researchers at Berkeley are using external light sources, integrating photonic modulators on chip, he said.

- Rick Merritt
??EE Times

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