A new research activity developed by the Group is the study of light-atoms interaction using optical fibres. We use a special kind of optical fibres called microstructured optical fibres, where the microstructured geometry acts as the low-index material needed for guiding light through total internal reflection. Most of the optical signal is propagating in the solid silica core, with a small evanescent fraction of the guided optical field present in the air holes (see Figure.1 below). The air holes are filled with a gas-phase material, and light-atoms interactions are studied by looking at the absorption spectrum at a given wavelength.
Optical fibres are very effective spectroscopic tools despite the small overlap between the optical field and the gas molecules (1-3%, see Figure. 1c in blue) because interactions occur over a very long distance (typically 5-10m). Moreover the possibility to hermetically seal the optical fibre cell by fusion splicing to standard fibres further extends the application area of such fibres.
There are several issues related to the practical handling of microstructured fibres. The fusion splicing to standard fibres requires localized heating at the splice point to soften the silica glass. With the fibre holes facilitating the diffusion of heat trough the fibre, the silica struts forming the low-index cladding will tend to melt and the holes to collapse, resulting in a strong deformation of the microstructure and high coupling losses. Proper splicing parameters must therefore be used to ensure high-quality splices.
Another important issue while splicing microstructured fibres arises once it is filled with a gas of interest. Usually the fibre gas cell is filled with a low-pressure gas to decrease the linewidths of the absorption lines, resulting in very precise measurements of the absorption wavelengths. Because the electrical arc requires an input of oxygen, the fusion splice is performed at atmospheric pressure, resulting in a contamination of the gas of interest by ambient air and a decrease in wavelength accuracy. One way to overcome this is to take advantage of the high permeability coefficient of helium gas in silica glass. The fibre is loaded with high-pressure helium gas before splicing, and then the helium gas will diffuse out of the fibre in a matter of hours. One can then obtain a fully hermetic fibre gas cell, which can then be used as a wavelength reference for metrological purposes or as a very compact and reliable device to study light-atoms interactions.
Such fibre gas cell was recently used by our Group to shine light on a very interesting question concerning light-atoms interaction. Recent works by the photonic community have suggested that slow light can enhance the absorption efficiency of molecules by extending the interaction length. Our Group being a pioneer and an expert in modifying and controlling the speed of a light signal, we could experimentally clarify the effect on the absorption of light by gas molecules. The transmitted light spectrum was measured in slow light and normal conditions without modifying the experimental implementation, as shown in Figure. 2. We could demonstrate that light slowed down by modifying the material properties of the optical fibre core using stimulated Brillouin scattering has no impact on the absorption efficiency of gas molecules. However, slow light based on modifications of structural properties may lead to entirely different results due to unique features of multiple reflections and artificially extended interaction lengths. This configuration is currently under investigation in our Group.
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Thevenaz, L., Chin, S., Dicaire, I., Beugnot, J. C., Mafang, S. F., & Herráez, M. G. (2009, October). Experimental verification of the effect of slow light on molecular absorption. In 20th International Conference on Optical Fibre Sensors (Vol. 7503, p. 75034W). International Society for Optics and Photonics.
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Chin, S., Dicaire, I., Beugnot, J. C., Mafang, S. F., Herraez, M. G., & Thévenaz, L. (2009, July). Material slow light does not enhance Beer-Lambert absorption. In Slow and Fast Light (p. SMA3). Optical Society of America.
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Dicaire, I., Beugnot, J. C., & Thévenaz, L. (2010). Analytical modeling of the gas-filling dynamics in photonic crystal fibers. Applied optics, 49(24), 4604-4609.