Reply To: MODE: Strange mode result from FDE solver

[Amrita Pati]

Hi Henrick,

Thanks for sharing these details! 

When enlarging the YZ view, sometimes a small gap appears and disappears that shouldn't be there. Is that what you mean?

I believe that might just be an artifact of the visualization. I was referring to an actual physical gap between the substrate and boundaries. Or in other words, the substrate terminating before the PML boundaries. But that is not the case here.

From the screenshot, it looks like you are simulating a bent waveguide. I missed that detail from your first post. There are a few important considerations while simulating them, the most important one I would say is the simulation span. In straight waveguides, the modal energy is more or less localized in the core region. In a bent waveguide, however, there is coupling into the radiation modes that can interact with the PML creating artifical losses/gain. Typically, it is suggested that the fields at the PML should decay to about 1e-05 to 1e-04 of the max fields. I will recommend running a few simulations with increasing simulation span, especially in the direction opposite to the bend as this where most of the radiation takes place. You can monitor how the different properties like losses, neff, etc. change over these spans. They should converge if the simulation is large enough and that would be the right choice of simulation span. You can consider using a mesh override region for just the waveguide, and keep the mesh relatively coarse (default mesh) in the background.

Let me know if you also see this strange behavior for larger simulation spans.

Regards,
Amrita