How to identify the fundamental TE mode and TM mode in the multilayer structure

- July 6, 2022 at 9:15 am
  
  liyaqian
  Subscriber
  
  I am trying to simulate an edge coupler structure with FDE. The cross section of the edge coupler includes two nitride layers in oxide as shown in the following picture. Now I want to get the fundamental TE/TM mode of the facet of the structure. Considering the high index Si substrate, "modal analysis" in FDE solver cannot use max index. And I am confused what should I set the search index?
  If I ignore the Si substrate and use the max index. Then how to identify the TE/TM mode? Thank you very much.
- July 6, 2022 at 9:04 pm
  
  Guilin Sun
  Ansys Employee
  
  When high index material appears in FDE, usually it begins from the largest refractive index. If you know the effective index roughly, you can deselect the "use max index", and select "near n" close to the effective index. If more silicon is exposed to FDE, this might not work well. disabling the Si is an option, or only expose a small part of it. You may also need to use PML BCs instead of Metal BCs. However you may need to increase the number of trial modes. This is because the solver will give both the propagation modes and the PML modes and the latter are disposed, eg, not shown in the mode list.
  As for " how to identify the TE/TM mode? " if you use Lumerical definition of TE/TM, please check the "TE polarization fraction", if it is more than 50%, it would mainly the TE polarization. However in case of symmetry and degeneracy, TE and TM may be mixed up. You can use symmetry BCs to isolate them.https://optics.ansys.com/hc/en-us/articles/360034382694-Symmetric-and-anti-symmetric-BCs-in-FDTD-and-MODE
  Please try.