Monday, August 2, 2010

Chip the Light Fantastic..

Closeup of a silicon surface pockmarked with holes, designed to slow light passing through it. This "slow light" waveguide, designed by a team led by Yuri A. Vlasov of IBM's Thomas J. Watson Research Center, could be used as a buffer for optical signals and thus would be a crucial component of an optical (or "photonic") computer, or an all-optical network router.

Super-fast optical computers are a step closer thanks to research breakthroughs that may lead to silicon chips that can process information as electronic bits or flashes of light.

Two discoveries announced in the past week have sped the path to the fabrication of hybrid silicon chips with both electronic and photonic components.

The first discovery, published in this week's issue of the journal Nature, foreshadows a future in which computers may run at terahertz speeds and, paradoxically, light will move much more slowly than it does today.

The other discovery, published in last week's issue of the same journal, presents a new silicon-based microtransmitter that can send optical data at 100 Gbps -- one-tenth of a terahertz.

Both teams are hoping their discoveries will fit within the present manufacturing framework -- and can be built using the same techniques as silicon semiconductor chips (technically, "complementary metal-oxide semiconductor," or CMOS).

Both must also work around what is both the inherent strength and weakness of optical computing and communications: The bits are always moving at the speed of light.

Here is where something called "slow light" comes into play. Having been studied in elaborate laboratory settings for years, light propagating in optically dense media -- media that slow light's propagation speed down considerably -- has been an area of increasing interest in photonics.

Slowing an optical bit down enables a computer to better buffer and route information traffic in much the same way that stoplights and speed limits are essential to controlling the flow of physical traffic.

The challenge has been that the only substantially light-slowing media were laser-illuminated gas clouds or specially prepared ruby crystals, neither of which are well suited for a CMOS chip.

However, a team of researchers led by Yurii A. Vlasov of IBM's Thomas J. Watson Research Center announced this week that a grid of specially perforated silicon can slow the speed of light moving through its channels by a factor of 300.

"Instead of using atomic vapors and sophisticated equipment, we wanted to build a small (optical) circuit that doesn't require any lasers and is built on the same production lines where computer chips are built," said Vlasov.

David Lackner, senior analyst for the technology research and advisory firm Lux Research, said that Vlasov's light-slowing silicon could also enable a near-term application: an all-optical network router.

In internet traffic today, Lackner said, "It doesn't matter how quickly you deliver data across the Atlantic, because on either side of the Atlantic, you have to go through routers. And that's what slows things down."

It is the translation of a network signal from optical to electronic bits, he said, that is often the bottleneck.

Said Fred Zieber, analyst and president of Pathfinder Research, "Slow light can allow you to store, briefly, an IP packet of information without converting it to electrical signals."

Of course, computerized communications take place not only across thousands of miles but also across millimeter and centimeter scales.

James Harris, the James & Ellenor Chesebrough Professor of Electrical Engineering at Stanford University, said that as a chip's clock speed increases, electronics become better suited for computation and photonics better for communication.

"As electronics keeps scaling and becoming faster, the communications bandwidth demands become ever greater at lower levels, from LANs ... to chip-to-chip to finally on-chip, there will be a push to use photons and optical interconnects for the communications function," he said. "But I think we will have electronic computational engines for a long time into the future."

Harris was one of an eight-member Stanford team that announced the fabrication of a silicon-based optical transmitter in last week's issue of Nature. The CMOS-ready transmitter, one-thousandth the size of a human hair, encodes data as light pulses ("1") or empty space ("0") at the rate of 100 Gbps.

The Stanford team's device, built around a microelectronic shutter that opens and closes rapidly, was built for communicating across a motherboard or across the length of a computer chip. This, says Harris, is where the future of electronic-photonic hybrid chips lies.

"I believe that optical communications will eventually be integrated into and used at the chip level, and this will be part of the essential elements to continue to increase the speed and functionality of electronics," he said.


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