Tilted mode expansion monitor


I would like to use the tilted mode expansion monitor to understand the mode coupling into higher order mode of a bend-straight directional coupler. However, the simulation always seems to diverge and the mode expansion calculation will always fail. Note that without the mode expansion monitor, the script runs fine and I would be able to calculated the power coupling ratio.

Could someone help me figure out what is the solution to this? Script attached.ring_gap_v4.lsf (7.4 KB)


Hi @z_zhang,

There can be many reason for the divergence. You can have a look at the page about diverging simulations. It shows a few things you can check to identify what causes the divergence.

That said, I would have a few comments on the setup:

  • The simulation region boundaries might be too close to the waveguide. The PML may affect the propagation of the mode.
  • Similarly, the mode source (and monitors), seems too small. This can affect the mode calculation. I would recommend to start with an analysis with MODE Solutions’ FDE solver, to determine the dimensions of the mode source and monitors (including the mode expansion monitor)
  • I would define the mode expansion monitor as Y-normal, To get the correct angle, you will have to set Theta to -15 and Phi to 90. As suggested in the KB page about it, the mode expansion monitor and the monitors you define for expansion should have the same position and dimensions.
  • The mesh will be challenging: you have many very thin layers (5nm), that are not resolved with the current mesh (dz is about 25nm).

I hope this will help!