移动坐标系FDTD方法在脉冲波传播中的应用

Abstract

The moving coordinate frame flnite-difierence time-domain (FDTD) method is developed to simulate the propagation of electromagnetic pulses over long distances. In this paper, this method is extended to three dimensional cases. The discretized form of moving coordinate frame FDTD in three dimensions is given, and is applied to calculate propagation of difierent pulses in rectangular waveguides. The widths of pulses in propagation direction increase when pluses propagate in waveguide. Dispersion phenomena appear, and slow parts of the pulses move to the tail of pulses. By comparing the results of both FDTD method and moving coordinate frame FDTD method, this method is numerically validated. Relative difierence between two methods is less than 0.1%, the simulation results are shown to be accurate. With the increasing applications on communication and detection, there is an ever-increasing in- terest in the propagation of pulses over long distance. The flnite-difierence time-domain (FDTD) method (1) is widely used in electromagnetic simulation. Because the FDTD method is a full- wave method, when simulating propagation of electromagnetic pulses over very long distance, the computational burden quickly becomes too large to afiord.This is the major di-culty why FDTD method can not be applied to simulate long distance propagation of pulses directly. In order to overcome this di-culty, moving coordinate is combined with FDTD method. Models of one di- mension as electromagnetic pulse propagation over large distance in medium (2) and acoustic wave propagation in waveguide has been studied (3) approach this method. Coordinate of the direction in which pulse's propagation is transformed into moving coordinate, which is 'frozen' with pulses, and the pulses are in small calculation zones when simulating (as shown in Fig. 1). Moving grid whose size is limited to the order of the pulse length is created, and moves along the approach of the center-of-mass of the wavepacket. This method takes advantage of the fact that when an electromagnetic pulse propagates over long distance, the signiflcant pulse energy exists only over a small part of the propagation path at any time. The FDTD meshes (in the direction of the propagation path) need only to be large enough to contain the pulses while using this method to simulate the propagation of pulses. This method is useful in particular for tracking well-collimated short pulse beam, referred to as pulsed beams or localized waves. In this paper, the models are extended to simulate electromagnetic pulses propagating in waveg- uide over large distance and results are compared with the results using FDTD approach in order to testify the validity and feasibility of the moving coordinate FDTD method.