Abstract: The finite-difference time-domain (FDTD) method is extensively applied in dealing with time-domain microwave imaging(MWI) problems since it is robust, fast, simple to implement. However, the FDTD method is an explicit time-stepping technique, due to the constraint of the Courant-Friedrich-Levy (CFL) stability condition, the time step needs to be as small as the size of the fine cells, which brings a major increase in computational costs. A fast nonlinear electromagnetic reconstruction algorithm for layered loss-y media by using the alternating-direction implicit finite-difference time-domain (ADI-FDTD) method is proposed. This algorithm is based on an adjoint method, and the nonlinear iterations apply the ADI-FDTD method to calculate the forward and adjoint field, and adopt the Polak, Ribiere, Polyar conjugate-gradient (PRP-CG) optimization scheme. By comparing the simulation results based on ADI-FDTD method and the FDTD method, the validity and efficiency of the proposed algorithm have been proved. Furthermore, the relative residual errors (RRE) are introduced as the iterative computation termination conditions, which further prove the accuracy of this algorithm.

Key words: reconstruction algorithm, ADI-FDTD, MWI