Laser scientist illuminates research

Scientists use colour to paint clearer pictures of the things they study. And as a result of a new laser system that rapidly delivers a pulsed rainbow of colours, those pictures will contain more information than ever before. Mechanical Engineering Assistant Professor Scott Sanders developed the system, which is highlighted in the May issue of Optics and Photonics News.

Laser light can indicate a research subject's characteristics. "We'd like to illuminate our subject with as many colours as possible, because we can get a lot of information about the subject by monitoring its colour sensitivity," says Sanders, who is affiliated with the University of Wisconsin-Madison Engine Research Center (ERC).

At the ERC, combustion gases comprise one area of study. "They absorb certain colours of light and not others," he says. "And so if we put in one colour, we might see some absorption or not, but if we put in a lot of colours, we can see all of the signatures of all of the gasses in the engine, because they all have some unique colour dependence."

Sanders' laser builds on a phenomenon known as supercontinuum generation, in which researchers convert single-colour lasers, such as a green or a red laser, into a multicoloured beam using a special kind of optical fibre. Photonic crystal fibres enable them to generate this "white" laser beam, says Sanders.

While that method produces a range of laser colours-and thus, a large amount of information-the drawback is that the white laser delivers all of the colours simultaneously, says Sanders. Rather, researchers want to measure rapidly their subjects' responses to individual colours.

So by directing the laser through an additional optical fibre about 20 kilometres long, Sanders created what he calls a "colour-dependent speed limit." Although all of colours leave the white laser at the same time, red travels through the fibre more quickly, while blue brings up the rear, and the rest of the colours fall somewhere in the middle. In photographs, they look like a continuous stream; in reality, each colour exits the long fibre one after the other. The entire laser scan occurs in a couple of millionths of a second.

Scott Sanders, assistant professor of mechanical engineering, exploits the varying speed of light in his new laser system, which will enable UW-Madison engine researchers to gather more useful data about the gases they study.

For the engine researchers, who study gas properties such as temperature, constituents and pollutants, the scan through colour occurs so quickly that the gas remains virtually unchanged. "We're putting in one colour at a time, but we're doing it so fast that the gas is basically frozen," says Sanders.

In this case, the researchers' goal is to use the data to design engines that run cleaner and more efficiently. But, says Sanders, the laser system could be adapted to help detect cancerous cells, scan and evaluate multiple drug samples, or gauge objects' distance from a certain point.

In addition, it could be useful for data-storage applications, particularly those that involve writing large amounts of information. "Currently, we wait for the CD to move around past the laser, so we're waiting for some mechanical motion," he explains. "To get to a new spot on the CD, we have to wait for the CD to move. With our system, each colour can be directed to a different direction, and the colour can change much faster than anything mechanical. So now we can get to a new spot more quickly, so we could consider that we might be able to write data more quickly."