Peculiarities of the Doppler effect in a multimode waveguide

The paper considers peculiar features of the Doppler effect that occurs when fundamental and higher-order modes reflect at a movable dielectric layer in multimode waveguides. Relationships for calculating frequencies of different modes of waves that reflect at and pass through the dielectric layer are derived. It is established that with increasing order of the mode, its frequency gets less dependent on the layer motion rate for the reflected waves, at the same time showing a stronger dependence for the transmitted waves. The interaction of waves with the inhomogeneous movable layer leads to the excitation of higher-order modes of the reflected and passed waves, so that each mode corresponds to a different frequency shift (multi-frequency Doppler effect).