Photonic nanostructure yields tiny optical switch

07 Jan 2008

What could be the world's smallest optical switch has been made by a team of researchers at the University of St Andrews in Scotland, and it's made out of silicon.

There are all sorts of weird and wonderful technologies for making all-optical switches that can route signals in an optical network without turning them back into electricity. Now there's another one to add to the list, and it looks particularly interesting because it is ultrasmall and has the potential to be integrated with CMOS electronics.

The new component, which was reported in the journal Optics Letters, is a 2x2 switch based on a passive optical device called a directional coupler. A directional coupler comprises two waveguides placed close enough together to allow optical energy to be transferred from one waveguide to the other. In its off state, light travelling through the top waveguide continues unperturbed to the output. But when the device is activated – in this case by heating it up – light is switched to the other waveguide, and comes out of the lower port.

The clever part of the switch is that a photonic crystal nanostructure is etched right through it. This nanostructure slows down the light to enable a strong interaction in a short distance, and so leads to the small size.

The patterned area on the device measures just 9 µm along each side, which is significantly smaller than existing devices. "At the moment optical switches tend to be millimetres in size. It is difficult to state which is the smallest optical switch ever made, but this is definitely one of them," said Thomas Krauss from the University of St Andrews, who led the research.

Krauss sees the device as a basic building block for creating optical circuits. Larger matrix switches, such as 16x16, can be made by cascading multiple 2x2 switches. "The whole point of going smaller is so that you can make the switch matrix small," he said.

A device like this could also operate as an optical modulator that encodes information on a beam of light, making it a competing technology to the tiny modulator that IBM announced last month. It is approximately ten times smaller than IBM's modulator, which means it could potentially be switched even faster, due to lower capacitance – Krauss speculates that it could run at data rates up to 100 Gbit/s. But the trade-off is bandwidth. The 2x2 switch has an operating bandwidth of a few nanometres, whereas IBM's modulator operates over the entire C-band (1530–1565 nm).

The tiny switch in the research paper was fabricated using electron-beam (e-beam) lithography, but Krauss is keen to point out that it could also be made using deep UV (DUV) lithography – which uses a tightly focussed spot of ultraviolet light from an excimer laser rather than a beam of electrons to create the photonic crystal nanostructure.

Krauss and his team have been working with Intel and IMEC, an independent research centre in Belgium, to demonstrate that nanostructures can be made using the lower cost method. "Photonic crystals have now been made successfully by DUV lithography, so the 'e-beam only, thus pricey' argument no longer applies," he contends.

Krauss had been working under a three-year contract with Intel, now finished, to investigate slow-light effects in photonic crystal waveguides. "They wanted to know what these things can do, and how easy it is to make them. We spent half the time teaching these guys how to make photonic crystal devices on their CMOS line. It was a very interesting time for both of us."

The work is also funded by the EPSRC (Engineering and Physical Sciences Research Council) as part of the UK Silicon Photonics project, a consortium lead by Surrey University. Krauss' research group recently won a further grant of £1.2 million (about €1.6 million) to pursue their studies.

But despite the involvement of companies like Intel, so far there hasn't been any commercial interest in the device. "If we can drive it electronically, make it really fast, and show integration, then people will really wake up to it," Krauss predicts.