Fig. 2

Dielectric slab waveguide with distorted core-cladding interface. f(z)-d is an arbitrary function that describes the random variations of the upper core-cladding boundary of the waveguide. The frequency spectrum of this function includes the frequency corresponding to \(\:{\beta\:}_{0}-{\beta\:}_{1}\), which provides a phase-matching condition. This frequency causes coupling between the TE0 and TE1 modes. This waveguide is equivalent to a waveguide with a sinusoidal wall with a period of 2π/(\(\:{\beta\:}_{0}-{\beta\:}_{1}\)) frequency. The 3D figure was created by Blender ver. 2.93 (https://download.blender.org/release/).