Microstructure fibers for gas detection

The paper deals with experimental investigation of capabilities of microstructure fibers (MSFs) for evanescent-wave chemical sensing. It presents results on the sensitivity of MSFs of a “grapefruit” type to gaseous toluene in mixtures with nitrogen which flow through the fiber holes. The sensitivity is determined on the basis of changes of the output optical power from the fiber caused by refractive-index changes of the fiber cladding induced by toluene and determined at 670 nm. Specific changes of spectral absorbance due to C–H overtones of toluene in a range of 1600–1800 nm are also utilized. Results obtained both with unmodified fibers and with fibers modified by the application of methyltriethoxysilane-based xerogel layers onto the hole walls are presented. The results show that fibers modified by the xerogel layers are sensitive to gaseous toluene in the cladding holes. Changes of the output power can also be determined with fibers unmodified by the xerogel layers, namely for saturated toluene mixtures. The presence of toluene in the detection layers was proved by spectral measurements.