Spectrum change of Gaussian pulse in waveguide by FDTD method

In this paper, an interesting, surprising phenomenon, spectrum change of Gaussian pulse propagating in waveguide, is revealed. It is found that the spectrum power of zero frequency of Gaussian pulse is changed into the spectrum power of cut-off frequency of H₁₀ mode in rectangular waveguide when FDTD simulation is implemented in the absence of nonlinear material in waveguide. The cause of the spectrum change is explained based on the mode field structure at cut-off frequency