Controlled side coupling of light to cladding mode of ZnO nanorod coated optical fibers and its implications for chemical vapor sensing

• We have demonstrated controlled side coupling of light into the cladding mode of ZnO nanorods coated multimode optical fibers.
• The coupling is mainly governed by the light scattering from the ZnO nanorods, which was tuned by controlling the length, diameter and surface coverage of the nanorods.
• At optimum conditions, avg. coupling efficiency of 2.65% was obtained with ZnO nanorods with length ∼2.2 μm.
• The nanorod coated fibers are used to demonstrate chemical vapor sensing with simple and cost effective sensor setup.

Controlled light coupling from surrounding to the cladding mode of zinc oxide (ZnO) nanorod coated multimode optical fiber induced by the light scattering properties of the nanorod coating and their applications of sensing are reported here. A dense and highly ordered array of ZnO nanorods is grown on the cladding of silica fibers by using low temperature hydrothermal process and the effect of the hydrothermal growth conditions of the nanorods on the light scattering and coupling to the optical fibers is experimentally investigated. The nanorod length and its number per unit area are found to be most crucial parameters for the optimum side coupling of light into the fibers. Maximum excitation of the cladding mode by side coupling of light is obtained with ZnO nanorods of length ∼2.2 μm, demonstrating average coupling efficiency of ∼2.65%. Upon exposure to different concentrations of various chemical vapors, the nanorod coated fibers demonstrated significant enhancement in the side coupled light intensity, indicating the potential use of these ZnO nanorod coated fibers as simple, low cost and efficient optical sensors. The sensor responses to methanol, ethanol, toluene and benzene vapor were investigated and compared, while the effect of humidity in the sensing environment on the sensor performance was explored as well.

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