Ultrafast Oscilloscope

Researchers at Cornell University have discovered a new method that uses an ultrafast optical oscilloscope can plot the waveform of an ultrashort-lived optical signal with a resolution of less than a trillionth of a second.

"We can make measurements of very short optical phenomena. The signal can be very weak, and it doesn't have to be repetitive," said Alexander Gaeta, Cornell professor of applied and engineering physics.

According to Gaeta, applications include analyzing intermittent glitches in fiber-optic communications and observing such fast-moving events as chemical reactions or laser fusion.

The ultrafast optical oscilloscope. Photo courtesy of Alex Gaeta Lab, Cornell University.

Other members of the research team include, Michal Lipson, associate professor of electrical and computer engineering, and Mark Foster, a postdoctoral researcher.

The innovation converts "time to frequency" using a process called four-wave mixing, in which two beams of light, referred to as the signal and the pump, are combined in a narrow channel – in this case a silicon waveguide on a chip, 300- by 750-nm in cross section. The narrow space forces the two beams to exchange energy, and a copy of the signal at a new wavelength emerges. The wavelength of the copy depends on the wavelength of the pump, and for this application the wavelength of the pump changes linearly in time.

The pump pulse is generated by a laser that outputs a broad band of wavelengths, and sent through a 50-meter length of optical fiber. Each wavelength of light travels at a slightly different speed in the fiber, so the pump pulse stretches into a stream in which wavelength varies continuously over time. In the four-wave mixing chip the stream is combined with the waveform to be analyzed, which varies in intensity over time. What emerges is a pulse in which each tiny moment of the input waveform is represented by a different wavelength of light, and the intensity, or brightness, of the light at that wavelength corresponds to the intensity of the input wave at that moment.

The result is fed into a spectrometer, which produces a graph of the intensity of light at each wavelength, and that graph corresponds to the original temporal waveform.

Lipson's research group is developing a dispersive waveguide on a chip that will replace the 50 m of fiber, as well as a spectrometer on a chip, Gaeta said, so that the entire device eventually can be fabricated on a single chip.

The work is supported by DARPA, the National Science Foundation and the New York Office of Science, Technology and Academic Research.

For more information, visit: www.cornell.edu