Multi-channel directional coupler as an evanescent field optical sensor

We investigate the use of multi-channel directional couplers as evanescent field optical sensors. If the surrounding analyte can infiltrate the region between the array waveguides then these sensors can exploit the exponential dependence of coupling on the analyte refractive index. Analysis based on supermodes and the principle of self-imaging is used to develop design guidelines for optimizing sensor performance. It is shown that the quasi-periodic characteristic of these sensors facilitates a trade-off between sensitivity and device length.