Mode calculations with FDTD for a waveguide array - mesh parameters

Hello everyone,

I have a problem with FDTD Solutions concerning mode calculations with a mode source. My structure is a rib waveguide array with 7 guides. Depending on the mesh structure, the field distribution for the fundamental mode may be incorrect.

I obtain the following field distribution for the fundamental mode with an automatic non -uniform mesh with accuracy 8 :

The maximum amplitude should be at the center of the structure. It is indeed the case for a mesh with lower accuracy (7 or 6) or with a uniform mesh:

Here are the corresponding files :
WG7_rib_wSi=450nm_dRib=110nm_S=950nm_mesh_auto_non_uniforme.fsp (247.0 KB)
WG7_rib_wSi=450nm_dRib=110nm_S=950nm_mesh_uniforme.fsp (285.3 KB)

I have noticed the same problem with a different geometry and with a uniform 7 nm x 7 nm mesh :
WG7_dRib=150nm_S=850nm_Periode=1300.fsp (285.3 KB)

In this case by choosing a 6 nm x 7 nm we obtain the expected field distribution.

If you have any ideas how to fix the problem, I would be glad to hear them.

Hi @flore.h,

I think the issue might come from the mesh. As you define a fixed mesh size, the material interfaces that define the waveguide can fall within a mesh cell:

Although the conformal mesh helps to get a better accuracy, you can end with waveguides seen as of different widths and different spacing.

I did a quick try using a non uniform mesh and adding mesh override regions on each waveguides to get this plot:

Each waveguide is aligned with the mesh: