simulation of directional coupler via varFTDT -> field-profile cut off ?!



iam trying to simulate a directional coupler via varFTDT.
the problem is my field profile is always cut off after a specific propagation distance (about 100µm), no matter how i set the parameters (design of coupler, mesh settings, boundary conditions, etc).

anybody encoutered a similar problem? is it maybe due low memory?
iam running the simulations on a intel core i7-3770 processor (8M Cache, 3.4 GHz) and 8 GB RAM.

thanks for all your help!

directional_coupler_varFTDT.lms (1.3 MB)


Hi @re-fuse,

I had a quick look at your simulation, one of the problem is the simulation time: your structure is pretty long, so you need to give enough time for the light to propagate. The default simulation time is 1000fs, you may want to increase this value. It can be changed in the varFDTD properties:

There’s a couple of other settings I would change:

  • Boundary conditions: you are using metal BC, any specific reason? Usually, you would use PML.
  • The waveguides end within the simulation area. In adds a wg/air interface that creates reflections. Unless it is on purpose, I would reduce the simulation region size so the waveguides are going through the PML.
  • As a start, I would use a coarse mesh so the calculations are faster. I would reduce the size of the mesh once I’m sure the simulation is correctly set.

I hope this helps.



hi gbaethge,
thank you so much!
the reason was indeed the simulation time. i’ve never thought of that.
greets, re-fuse