Estimation of the higher order modes in transforming time dependent data to frequency dependent data in the time domain simulation

One of the advantages of a time domain numerical analysis method such as the FD-TD method is that the broad band characteristics are obtained by the Fourier transform of the pulse responses. However, the generation of higher order modes makes it difficult to estimate the frequency dependent characteristics by the above technique. This problem also occurs in analyzing guided structures. It seems to be important to predict the s-parameters accurately even if there are higher order modes. Chu and Chaudhuri (1990) have proposed a technique to extract the modal parameters from the FD-TD results in the analysis of dielectric waveguide problems. However, the technique seems to be able to be used in the steady state. Dehler et al. (1993) also has proposed the technique to obtain the scattering matrix considering multiple modes. However, concrete results have not been shown. In the present paper, a new technique for overcoming the above problem is described. The technique consists of common Fourier transforms and Prony´s method. In this technique, first, the Fourier transforms are carried out to transform the time domain data to frequency domain data at each spatial observing point, and next, Prony´s method is used to separate modes by using the previously obtained Fourier transformed values at the several observation points spatially distributed. In the technique, whole fields values over a waveguide cross section are not needed, but only values at one point have to be considered. The technique can be applied to systems including an arbitrary number of modes.<>