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The chemical sensing capabilities of an optical fibre long period grating (LPG)
coated with a functional material are explored. The LPG period and coating
thickness are chosen such that the mode transition region, characteristic of
LPGs coated with nanostructured coatings, corresponds with the phase matching
turning point, ensuring optimum sensitivity. The functional material is a
calixarene, which is shown to exhibit sensitivity to the aromatic compounds
toluene and benzene, while being relatively insensitive to the aliphatic
hydrocarbon hexane. A nanoscale cavity in the calixarene molecule entraps the
VOC molecule, thereby altering the refractive index of the coating and
influencing the transmission spectrum of the LPG. A sensitivity to toluene of
231 ppmv has been achieved, based on a spectrometer resolution of 0.3 nm. This
is a weak molecular interaction thereby ensuring the sensor gives a reversible
response when the VOC is removed. The recovery time is of the order of 15 s
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