On-chip optical isolator from MIT could speed all-optical computing

Cambridge, MA--Massachusetts Institute of Technology researchers have made a breakthrough towards realizing all-optical computing: an integrated photonics or chip-based optical isolator. In fiber-optic communication, data-carrying laser light is converted to electronic form, processed through electronic circuits, and then converted back to light using a laser. The new isolator device could eliminate those extra electronic-conversion steps, allowing the light signal to be processed directly.

The new component is a "diode for light," says Caroline Ross, the Toyota Professor of Materials Science and Engineering at MIT, who is co-author of a Nature Photonics paper reporting the new device. It is analogous to an electronic diode, a device that allows an electric current to flow in one direction but blocks it from going the other way; in this case, it creates a one-way street for light, rather than electricity.

This is essential, Ross explains, 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 is used to perform this function, but the new system would allow this function to be part of the same chip that carries out other signal-processing tasks.

To develop the device, the researchers had to find a material that is both transparent and magnetic--two characteristics that rarely occur together. They ended up using a form of a material called garnet, which is normally difficult to grow on the silicon wafers used for microchips. Garnet is desirable because it inherently transmits light differently in one direction than in another: It has a different index of refraction depending on the direction of the beam.

The researchers were able to deposit a thin film of garnet to cover 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 passes freely, while a beam going the other way gets diverted into the loop. The whole system could be made using standard microchip manufacturing machinery, Ross says. "It simplifies making an all-optical chip," she says. 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."

The technology could greatly boost the speed of data-transmission systems by enabling all-optical signal processing.

SOURCE: MIT; http://web.mit.edu/press/2011/optical-computing-diode.html

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