This video is taken from the FDTD Learning Track on Ansys Innovation Courses.
We saw the different types of boundary conditions that can be set in the “Boundary conditions”
tab of the solver region.
In this video, we will go over some additional tips for setting up boundary condition.
When using the PML boundaries, the “extend structure through pml” option is selected
When this option is enabled, the solver will automatically extend any structure touching
the inner edge of the boundary through the full thickness of the PML boundary.
This prevents reflections that can occur if a material interface exists within the boundary
In this example, the waveguide is terminated at the inner edge of the simulation volume.
When the ‘extend’ option is not used, a large reflection occurs because the waveguide
does not extend through the PML absorbing region.
Typically extending structures through the PML is desired, however, the following cases
are examples when you don’t want to automatically extend the structure through the PML.
If you have an object which is angled, the simulated object will be extended in the direction
perpendicular to the PML so it changes the structure that is simulated from the actual
In this case, you should turn off the “extend structure through pml” option and make sure
that the structure that is set up extends all the way through the PML region.
If you have to pattern, you want to manually set up the same patterning in the PML boundary
When using Periodic or Bloch boundary conditions, a similar issue exists.
These boundaries are one mesh cell thick, as shown with the shaded blue region.
To obtain the most accurate results, the structure should be defined within the boundary condition
This is most easily accomplished by drawing an extra unit cell of the structure on each
side of the simulation region.
To check that the appropriate symmetry conditions are chosen for the given source polarization,
you can simply check the CAD view.
If the color of the shaded half of the simulation region matches the color of the source polarization
arrow, the shading should be parallel to the source arrow direction.
If the color of the shaded region does not match the source arrow, the shading cuts through
the arrow perpendicularly.
Symmetry conditions can also be used as a tool to restrict the allowed symmetry of the
For example, you could use symmetry to restrict the polarization of modes or a waveguide or
to isolate modes of resonant cavities so that only the modes with the desired symmetry can