“…The finite element method (FEM) [6][7][8][9][10][11], the finite difference time domain (FDTD) method [12,13], or the beam propagation method (BPM) [14][15][16] has been utilized as a numerical simulation method in these topology optimization method. Among them, BPM [17][18][19] is a quite useful technique in the analysis of long-length waveguide devices in which backward reflection hardly occurs. BPM can treat the large-scale problem which is difficult to be analyzed by FEM or FDTD due to the lack of computational resources.…”