Optical Properties and Sensitivity of Xerogel Detection Layers for Fiber-Optic Hydrocarbon Sensors

Xerogel layers were prepared on fiber-optic substrates and glass slides by the dip-coating method from sols composed of tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and phenyltriethoxysilane (PTES), ethanol or isopropan, HCl and water, and by drying the gels at 80°C.Layers with a thickness ranging from 200 to 1500 nm were prepared. Optical properties of the layers applied on the fibers and their sensitivity to toluene and hexane were determined from angular and temporal changes of the output power of the fibers excited by an inclined collimated light beam. These changes were measured at a wavelength of 670 nm and in a range of 1600–1800 nm.The results obtained at 670 nm show that the layers refractive index can be increased from 1.39 to 1.5 by using the PTES sols instead of the MTES or TEOS ones. High optical losses of the layers of the order of 10³ dB/cm can be explained by their porosity. The best detection limits of 0.01 vol.% of toluene and 0.2 vol.% of hexane were obtained for layers containing methyl groups. The presence of toluene in the layers was deduced from spectral bands of toluene in a range of 1600–1800 nm.