Multicolor Wavelength-Agile Lasers At Your Service

By Roland Piquepaille

Laser lights can be used for optical sensing applications, for example to identify unknown gases emitted by an engine. And as these unknown substances react differently to different wavelengths, researchers at the University of Wisconsin at Madison have developed unique wavelength-agile lasers. And I'm amazed by the beauty and the simplicity of their idea. They're using white lasers which produce all colors simultaneously -- but with a twist. The white laser light goes through a 20-kilometers long optical fiber before reaching its target. And because different colors 'travel' at different speeds, this produces independent results for the different wavelengths. The researchers are using spectral resolutions smaller than a thousandth of a nanometer and they are able to get all the results within a millionth of a second. This method could be used to design cleaner engines or data storage applications in a few years. Read More...

Let's start with some technical explanations about this technology developed by Professor Scott Sanders in his labs.

Sanders' laser builds on a phenomenon known as supercontinuum generation, in which researchers convert single-color lasers, such as a green or a red laser, into a multicolored beam using a special kind of optical fiber. Photonic crystal fibers enable them to generate this "white" laser beam, says Sanders.
While that method produces a range of laser colors-and thus, a large amount of information-the drawback is that the white laser delivers all of the colors simultaneously, says Sanders. Rather, researchers want to measure rapidly their subjects' responses to individual colors.
So by directing the laser through an additional optical fiber about 20 kilometers long, Sanders created what he calls a "color-dependent speed limit." Although all of colors leave the white laser at the same time, red travels through the fiber more quickly, while blue brings up the rear, and the rest of the colors fall somewhere in the middle. In photographs, they look like a continuous stream; in reality, each color exits the long fiber one after the other, like drops from a faucet. The entire laser scan occurs in a couple of millionths of a second.

Below is a photo showing how UW-Madison engine researchers gather useful data about the gases they study by using these wavelength agile lasers (Credit: UW-Madison College of Engineering).

Using wavelength agile lasers

Here is a link to a higher quality of this picture (3,264 x 2,448 pixels, 5.04 MB).

This research work about 'rainbow' lasers is making the cover story of Optics and Photonics News in its May 2005 issue. Full access to the paper (PDF format, 6 pages, 446 KB) is available via this page about "Wavelength-Agile Lasers."

The figure below, which shows the evolution of wavelength-agile lasers within the author's laboratory, has been extracted from this article (Credits: UW-Madison College of Engineering and Optics and Photonics News).

Evolution of wavelength agile lasers

These colorful lasers should soon be used in such applications as spectroscopy or high-speed scanning.

Sources: University of Wisconsin at Madison, April 28, 2005; and various websites

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