Researchers see photonic chips being created from silicon

In many communication systems, data travels via light beams transmitted through optical fibres.

Once the optical signal arrives at its destination, it is converted to electronic form, processed through electronic circuits and then converted back to light using a laser. The new device could eliminate those extra electronic-conversion steps, allowing the light signal to be processed directly.

The component is a ‘diode for light’, said Caroline Ross, Toyota professor of materials science and engineering at MIT, who is co-author of a paper reporting the device that was published recently online in the journal Nature Photonics.

According to a statement from MIT, 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 explained, because without such a device, stray reflections could destabilise the lasers used to produce the optical signals and reduce the efficiency of the transmission. Currently, 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.

They ended up using a form 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 and has a different index of refraction depending on the direction of the beam.

The researchers were reportedly 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 travelling 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 said. She added that this could make it much easier to commercialise than a system based on different materials.

‘A silicon platform is what you want to use… there’s a huge infrastructure for silicon processing,’ Ross said. ‘Everyone knows how to process silicon. That means they can set about developing the chip without having to worry about new fabrication techniques.’

This technology could greatly boost the speed of data-transmission systems for two reasons. First, light travels much faster than electrons. Second, while wires can only carry a single electronic data stream, optical computing enables multiple beams of light, carrying separate streams of data, to pass through a single optical fibre or circuit without interference.