Theory of wave propagation in periodic structures

A study is made of the propagation of electromagnetic waves in a one-dimensional medium with a periodic variation of the refractive index. An asymptotic method is used to reduce the problem to the solution of well-known equations for coupled waves in which the expression for the coupling coefficient differs from the traditional form. The difference is analyzed and it is found that the new coupling coefficient makes it possible to extend the range of validity of the coupled-wave equations to the case of high Bragg resonances.