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 increment
al 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.
We show that the time frequency analysis of the autocorrelation function is, in many ways, a more appropriate tool to resolve fractional revivals of a wave packet than the usual time domain analysis. This advantage is crucial in reconstructing the in
itial state of the wave packet when its coherent structure is short-lived and decays before it is fully revived. Our calculations are based on the model example of fractional revivals in a Rydberg wave packet of circular states. We end by providing an analytical investigation which fully agrees with our numerical observations on the utility of time-frequency analysis in the study of wave packet fractional revivals.
