A novel FDTD (finite-difference time-domain) method is presented for the analysis of electromagnetic wave propagation in periodic structures.The boundary conditions are set according to Floquet's theorem for periodic structures.Floquet's theorem is used twice on electric field boundary and magnetic field boundary.Thus

the computational domain is restricted to a single period

and the absorbing boundary condition can be used on the other boundaries.Maxwell's system is then solved as an initial value problem on a FDTD grid for the prediction of the (eigen) frequencies of the propagating modes for a given value of the propagation constant.