Gnewuch, H., Hua, P., Wilkinson, J.S. and Jansen, K.
Cross-sectional transformers for the signal-enhancement of intensity-based evanescent field sensors
Sensors and Actuators B: Chemical, 22, (3), . (doi:10.1016/0925-4005(94)87017-9).
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A cross-sectional transformer (CST) that allows efficient interconnection between a conventional multimode optical fibre and a broad shallow waveguide, designed for strong interaction of guided modes with sensing materials in the evanescent field, is described. A ray approach is used to determine a simple theoretical relationship between the geometrical configuration of the transformer and the enhancement in sensitivity to absorption. Thinly clad rectangular optical fibres have been drawn from a composite preform, and a CST has been fabricated using a suitably formed bundle of these fibres. An enhancement of 3.9±0.6 dB of the power collected from a broad slab waveguide compared with that collected directly by a multimode fibre is measured. The attenuation of the rectangular fibres that comprise the CST is 0.1±10.07 dB /cm at 550 nm.
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