“…In the finite difference approaches of (4) (the standard FDM) and (3), (5) (frequency domain FD-BPM), the main research interests are: developing of socalled improved FD schemes with more accurate treatment of the dielectric interfaces, [9][10][11][12], and extending the range of FD-BPM applicability to the waveguide structures changing in the direction of propagation, [5,6,13,14].…”