Four-wave mix

researchers have created a broadband light amplifier on a silicon chip, a major breakthrough in the quest to create photonic microchips. In such microchips, beams of light travelling through microscopic waveguides will replace electric currents travelling through microscopic wires.

A team of researchers working with Alexander Gaeta, Cornell professor of applied and engineering physics, and Michal Lipson, assistant professor of electrical and computer engineering, used the Cornell NanoScale Facility to make the devices.

The amplifier uses a phenomenon known as four-wave mixing, in which a signal to be amplified is ‘pumped’ by another light source inside a very narrow waveguide. The waveguide is a channel 300 x 550 nanometres (nm) wide, smaller than the wavelength of the infrared light travelling through it. The photons of light in the pump and signal beams are tightly confined, allowing for transfer of energy between the two beams.

The advantage this scheme offers over previous methods of light amplification is that it works over a fairly broad range of wavelengths. Photonic circuits are expected to find their first applications as repeaters and routers for fibre-optic communications, where several different wavelengths are sent over a single fibre at the same time. The new broadband device makes it possible to amplify the multiplexed traffic all at once.

The process also creates a duplicate signal at a different wavelength, so the devices could be used to convert a signal from one wavelength to another.

Although four-wave mixing amplifiers have been made with optical fibres, such devices are tens of metres long. Researchers are working to create photonic circuits on silicon because silicon devices can be manufactured cheaply, and photonics on silicon can easily be combined with electronics on the same chip.

‘A number of groups are trying to develop optical amplifiers that are silicon compatible,’ Gaeta said. ‘One of the reasons we were successful is that Michal Lipson's group has a lot of experience in making photonic devices on silicon. ‘That experience, plus the manufacturing tools available at the Cornell NanoScale Facility, made it possible to create waveguides with the precise dimensions needed. The waveguides are silicon channels surrounded by silicon dioxide.

Computer simulations by the Cornell team predicted that a waveguide with a cross section of 300 x 600nm would support four-wave mixing, while neither a slightly smaller one -- 200 x 400nm -- nor a larger one -- 1,000 x 1,500nm -- would. When Lipson's Cornell Nanophotonics Group built the devices, those numbers checked out, with best results obtained with a channel measuring 300 x 550nm.

The devices were tested with infrared light at wavelengths near 1,555nm, the light used in most fibre-optic communications. Amplification took place over a range of wavelengths 28nm wide, from 1,512 to 1,535nm. Longer waveguides gave greater amplification in a range from 1,525 to 1,540nm. The researchers predict that even better performance can be obtained by refining the process.

They also predict that other applications of four-wave mixing already demonstrated in optical fibres will now be possible in silicon, including all-optical switching, optical signal regeneration and optical sources for quantum computing.