Coupling of periodic modes

The author examines the effect of a periodic perturbation on a periodic structure, taking linear twin-core couplers with an axially periodic refractive index as examples. The focus is on the coupling effect of the normal periodic modes of a periodic structure under a sinusoidal perturbation, since any periodic function can be represented by a Fourier series. A perturbation to an ideal waveguide structure, is shown to lead to coupling of the normal modes of the structure. For an ideal periodic structure (such as a twin-core nonlinear coupler at a fixed excitation and power), a periodic perturbation to the structure gives rise to coupling of the normal periodic modes of the structure