Excited by a field that is localized at the center of the strip, the multimode propagation in this raised strip waveguide leads to a periodic focusing and defocusing pattern of the optical power density, an almost periodic process (not as strictly as suggested by the figure). The frames of this animation show the local intensity on cross section planes (the black lines indicate the permittivity boundaries) at different positions on the propagation axis. Note that this concerns stationary fields; the time coordinate of the animation replaces the propagation distance. The gif-image was created via MATLAB by the direct m-file output of the WMM programs. See the WMM-Manual for a detailed description.