Optical circuits on chips pave way for computing with light

Currently, in most communication systems data travels via light beams transmitted through optical fibres and is converted into electronic form once the optical signal reaches its destination. It is then processed through electronic circuits and then converted back to light using a laser. According to Caroline Ross, Toyota Professor of Materials Science and Engineering at Massachusetts Institute of Technology and co author of the paper published in Nature Phontonics, the new device could eliminate the extra electronic conversion steps, allowing the light signal to be processed directly. Described as a 'diode for light' the new component is similar to an electronic diode that 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. This, says Ross, pictured, is essential, 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, 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 used a material called garnet, which is transparent and magnetic and 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. A thin film of garnet was deposited over one half of a loop connected to a light transmitting channel on a chip. The result was that light travelling through the chip in one direction passed freely, while a beam going the other way got diverted into the loop. Ross 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. A silicon platform is what you want to use. 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." The researchers believe the technology could boost the speed of data transmission systems as light travels much faster than electrons and, 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. For communications systems Ross said: "This may be the next generation in terms of speed."