Optical waveguide modelling and design for evanescent field chemical sensors
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Analytical and numerical techniques are presented for determining the sensitivity of optical waveguides as evanescent wave absorption sensors. Analytical expressions based on a ray optics approach are derived for planar and multimode fibre sensors. The matrix method is used for multilayer structures to determine both the sensitivity and the effect of intrinsic waveguide losses. It is shown how more complex waveguide geometries such as fibres with a D-shaped cross-section may be assessed by the finite-difference method combined with perturbation analysis. Some examples are given demonstrating how the results enable the design of sensors with optimum sensitivity.