Hollow optical fibre guides UV laser light
German researchers have tested a new type of optical fibre with a hollow core which is able to guide UV laser light without being damaged and with acceptable loss. Such light, which is required for applications such as spectroscopic investigations on ions or atoms, would normally damage conventional optical fibres.
Optical fibres usually have a solid glass core, which is coated with an optically thinner material. The laws of physics ensure that an infrared light beam is kept inside the fibre thanks to total reflection and that it can be transported over long distances, eg, through telecommunications networks, without significant loss. UV light, however, has a short wavelength and is therefore strongly absorbed by the glass used in most types of optical fibres, damaging them in the process.
Researchers at the Max Planck Institute for the Science of Light (MPL) found a certain type of optical fibre which is particularly well suited for UV light: a microstructured photonic crystal fibre (PCF) with a ‘Kagome structure’ - a special pattern consisting of triangles and of hexagons in a regular arrangement - and a hollow core of 20 µm in diameter. This core ensures a singlemode guiding of the light, ie, with a spatial intensity distribution similar to the shape of a Gaussian bell-shaped curve.
Metrological experts from the QUEST Institute, based at the Physikalisch-Technische Bundesanstalt (PTB), were tasked with investigating whether this transport was really singlemode and damage-free. They found that in the case of the UV beam used, with a wavelength of 280 nm, singlemode transmission was possible; even after more than 100 h in operation at a power of 15 mW, no UV-induced damage could be detected. In spectroscopic investigations on trapped ions, the UV laser beam was stabilised by the new fibre, allowing an improved interrogation of the ions’ internal state.
Besides use in spectroscopic methods (eg, astronomy, chemistry or fundamental research in physics), the method could also be useful for researchers who are developing quantum computers; in that field, the internal states of a particle are the new digital 0s and 1s. Additional fields of application could include fluorescence microscopy in biology, the investigation of process plasmas, combustion studies on soot or the spectroscopy of greenhouse gases.
The research has been published in the Optical Society of America’s journal Optics Express.
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