Predictions and measurements of a multimode waveguide interferometer operating in a fibre coupled, ``dual-mode regime are reported. With a 1.32 micrometer source, a complete switching cycle of the output beam is produced by a 10.0 nanometer incremental change in the 8.0 micrometer width of the hollow planar mirror waveguide. This equates to a fringe spacing of $simlambda /130$. This is an order of magnitude smaller than previously reported results for this form of interferometer.
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