AN INVESTIGATION OF COATED TAPERED OPTICAL FIBERS

In this paper we analyse the operation of an evanescent-field sensing device fabricated by coating a tapered single-mode optical fibre with a layer containing fluorescent material. Chemical reactions with the s urrounding external medium, such as changing pH levels, induce changes in the fluorescence properties, which are detected by the evanescent field present in the sensing layer. The device has the advantage of de livering the pump efficiently and collecting the subsequent fluorescen ce simultaneously. The small size of the paper waist concentrates the modal field, resulting in a high local intensity. Because the incoming fibre is single mode, a large fraction of the total power is availabl e to interact with the layer, unlike coated multi-mode fibres where on ly the highest-order modes have a significant field beyond the core. W e consider the dependence of the efficiency of fluorescence recapture on the refractive index and thickness of the coating and present resul ts which show that this can be improved by using a coating of higher r efractive index. We discuss how this leads to a constraint on the thic kness.