Silicon photonics enters the broadband age

17 Jul 2006

US researchers have built a broadband light amplifier on a silicon chip that can process large numbers of wavelengths at the same time.

Photonic microchips are one step closer to emulating their electronic counterparts now that researchers at Cornell University, New York, have produced a broadband light amplifier on a silicon chip. The device, which relies on microscopic waveguides rather than wires, will help pave the way to super-fast transmission on silicon.

The on-chip amplifier was developed by a research team working with Alexander Gaeta, Cornell professor of applied and engineering physics, and Michal Lipson, assistant professor of electrical and computer engineering (Nature 441 960).

"Electronics is always going to be a bit smaller, but for those applications where you want to transmit data at high speed, photonics has advantages," Gaeta told fibers.org. "At higher and higher data frequencies - 10 or 100 GHz - if you need to go from a chip to another chip that's a few centimeters away, photonics wins out."

While silicon-on-insulator photonic amplifiers have been demonstrated previously, they were all limited by narrow gain bandwidths. Cornell's amplifier, on the other hand, can process whole arrays of wavelength channels. This makes it ideal for amplifying multiplexed traffic within optical comms repeaters and routers.

The researchers used a phenomenon known as four-wave mixing (FWM), in which a light signal confined to a microscopic waveguide draws energy from an external "pump" source situated outside the silicon, and is thus amplified. The device demonstrated amplification over a 28 nm wavelength range (between 1512 and 1535 nm). Longer waveguides exhibited greater amplification between 1525 and 1540 nm.

To make the waveguides, the team had to fabricate silicon channels measuring 300 x 550 nm with a silicon dioxide surround. A hard task, but something at which Cornell's NanoScale Facility is a dab-hand.

"Due to the large refractive index of silicon and the relatively low refractive index of the silicon dioxide, you get very strong confinement," explained Gaeta. "In that respect the waveguides can be made quite a lot smaller than the wavelength of the light. So if you're trying to make a photonic circuit, you can fit a lot more on a chip."

A convenient off-shoot of this design is that it creates a duplicate signal with identical phase information at a different wavelength, which could come in handy for wavelength-conversion applications for WDM. In addition, the researchers' progress with FWM in silicon could enable wider applications such as all-optical switching, optical signal regeneration and optical sources for quantum computing.

Many of Gaeta's contemporaries are trying to fabricate the pump source out of silicon too, but this does not concern him. "There are a lot of people trying to do that. But our design could allow for a single pump source, which in principle could [amplify] 30 or more wavelength channels. So although it doesn't completely solve the problem of transmitting on silicon, it does take a big step."