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L A S ER T ECHNOLOGY longer wavelengths in what is known as the mid-infrared range. This range is also called the ‘fingerprint region’ because it is here that the signatures of a great many important chemical and biological molecules are to be found. If SuperK lasers with such long wavelengths can be developed, it will be possible to do away with tracers in many contexts and view the substances under investigation directly. This will, for example, make it possible to identify cancer at a much earlier stage than today. MOR TEN ANDERSEN ! Further information Professor Ole Bang, DTU Photonics, oban@fotonik.dtu.dk PHO TO LEICA MICROSYS TEMS The SuperK laser from NKT Photonics is used in Leica’s new confocal microscope. The SuperK is the box on the table to the left. How the new type of laser works A conventional laser generates light with a fixed wavelength. When the light maintains a specific wavelength, it is much easier to construct an optical fibre that can transmit it with very little loss. For years, fixed wavelength lasers have been used in advanced biological microscopes—known as confocal microscopes. The method applied here utilizes the fact that molecules absorb light at different wavelengths. There are a number of tracers that absorb light at wavelengths that a laser can be designed to emit, which means that if you are examining a cell sample, for example, you can add a tracer and then use a laser to illuminate the mixture. The resulting image will show how much of the substance to which the tracer attaches is present in the sample. Confocal microscopes are used in the pharmaceutical industry and in research laboratories all over the world. However, there are some tracers for which there are no lasers with a corresponding wavelength. In addition, there may be situations in which researchers want to examine a sample for multiple tracers at the same time. These considerations led to the idea of developing lasers that do not simply generate light at a fixed wavelength. This eventually led to the development of what are known as ‘SuperK’ lasers. The K stands for ‘continuum’ (spelled with a K in Danish) which refers to the fact that the laser light comprises all the wavelengths within the interval it is designed to cover—which professionals in the field call a ‘continuum’. However, this is easier said than done because light featuring multiple wavelengths normally propagates at different speeds in a given medium. This means that there is a risk that the result will simply be a muddle of signals at different speeds. In collaboration with DTU Photonics, NKT Photonics has developed a system whereby high-intensity laser light is emitted into a glass fibre, which features a microstructure of holes that allows light at different wavelengths within the given range to propagate at nearly the same speed over a long length. The combination of the high intensity and the small core area of the optical fibre produce a continuum within the fibre. For the record, most of this light is infrared light, which is invisible to the naked eye. The SuperK sources currently cover the entire range from 400 to 2,400 nanometres (nm), whereas the human eye can only see light in the 400–750 nm range. However, most customers need a narrower range and filter the light before they use it. For example, Leica uses only the 470–670 nm range. Continuing its partnership with DTU Photonics, NKT Photonics is working to expand the range of the SuperK laser even further. This image of neurons in a rat’s brain is an example of what you can see with the confocal Leica microscope. PHO TO LEICA MICROSYS TEMS >> 36 Technical University of Denmark


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