Machine Design

Photonic chips take a step towards practical applications

Also from Cornell researchers comes a broadband light amplifier on a silicon chip, considered a significant step towards photonic microchips, devices in which photons replace electrons as the means of communication.

A four-wave mixer inputs two photons at a pump wavelength and converts them into two new photons, one at the signal wavelength and one at a wavelength equal to twice the pump wavelength minus the signal wavelength. The new signal photons combine with the originals to create an amplified signal. The idler photons are a copy of the signal at a new wavelength.

A technique called fourwave mixing "pumps" or amplifies a light signal using another light source as the two beams travel inside a narrow waveguide. Although four-wave mixing amplifiers have been made with optical fibers, such devices are tens of meters long. The current waveguide is a silicon channel just 300 550 nm, smaller than the wavelength of infrared light traveling through it. This tightly confines the pump and signal beams, allowing for energy transfer between the two. Silicon waveguides are relatively inexpensive, and photonicsonsilicon can easily combine with electronics on the same chip.

The devices were tested with infrared light near 1,555 nm, the same wavelength used in most fiber-optic communications circuits. Amplification took place over a range of wavelengths, from 1,512 to 1,535 nm. Longer waveguides gave greater amplification in a slightly narrow range, from 1,525 to 1,540 nm. Refining the process could boost bandwidth and amplification, predicts researchers.

An advantage this scheme has over previous methods of light amplification is that it works over a wide range of wavelengths. Photonic circuits may first serve as repeaters and routers for fiber-optic communications, where several different wavelengths simultaneously travel over a single fiber. The Cornell device makes it possible to amplify the multiplexed traffic all at once. The process also creates a duplicate or "idler" signal at a different wavelength, so the devices could convert one wavelength to another.

Other potential applications for the silicon waveguides include all-optical switching, optical signal regeneration, and optical sources for quantum computing. Support for the work comes from the National Science Foundation.

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