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Component Simplifies Making All-Optical Chips
Nov 2011
CAMBRIDGE, Mass., Nov. 30, 2011 — A recently discovered "diode for light" could enable the creation of photonic chips, paving the way for computing with light.

Currently, in most communication systems, data travels via light beams transmitted through optical fibers. Once the optical signal arrives at its destination, it is converted to electronic form, processed through electronic circuits and converted back to light using a laser.

Caroline Ross, the Toyota Professor of Materials Science and Engineering at MIT, said the researchers found that the diode for light component could eliminate those extra electron-conversion steps, allowing the light signal to be processed directly. She noted that it is similar to an electronic diode, which allows an electric current to flow in one direction but blocks it from going the other way, creating a one-way street for light rather than electricity.

Caroline Ross, the Toyota Professor of Materials Science and Engineering at MIT. (Image: Allegra Boverman)

This is essential, she added, because without such a device, stray reflections could destabilize the lasers used to produce the optical signals and reduce the efficiency of the transmission. Currently, a discrete device called an isolator performs this function, but the new system would allow this function to be part of the same chip that carries out other signal-processing tasks.

The researchers used a thin film of garnet, which is both transparent and magnetic, to develop the device. Garnet also is desirable because it inherently transmits light differently in one direction than in another. It has a different index of refraction — the bending of light as it enters the material — depending on the direction of the beam.

The investigators deposited the garnet over one-half of a loop connected to a light-transmitting channel on the chip. The result was that light traveling through the chip in one direction passed freely, while a beam going the other way was diverted into the loop.

Ross said she believes the whole system could be made using standard microchip manufacturing machinery. “It simplifies making an all-optical chip. [The design of the circuit can be produced] just like an integrated-circuit person can design a whole microprocessor. Now, you can do an integrated optical circuit,” she said. “A silicon platform is what you want to use [because] there’s a huge infrastructure for silicon processing. Everyone knows how to process silicon. That means they can set about developing the chip without having to worry about new fabrication techniques.”

She said the technology could boost the speed of data-transmission systems because light travels faster than electrons and, although wires can carry only a single electronic data stream, optical computing enables multiple beams of light, carrying separate streams of data, to pass through a single optical fiber or circuit without interference.

The work was published in Nature Photonics; Ross was the co-author.

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A device intended to prevent return reflections along a transmission path.
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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