Spacing between simulation edge, mode source/monitor, and mode expansion/monitor

In simulating some structures with varFDTD, I have several general questions which I hope can be addressed regarding simulation setup.

Is there a suggested minimum distance between the simulation boundary of the varFDTD region and the location of the varFDTDModeSource/varFDTDModeExpansionMonitor/varFDTDFrequencyMonitor?
How will violating the minimum distance (if there is one) affect the results of the simulation?

Similarly, if the goal of the simulation is to measure the S parameters of a structure (say a 2 port structure, so 1 input port and 1 output port, with propagation in the x direction):
- How should the varFDTDModeSource be positioned with respect to the varFDTDFrequencyMonitor for the “input”? (ie should they be separated in the x direction?)
- If the input and output waveguides have the same cross section, can/should a single mode expansion monitor be used, with both input and output frequency monitors added? Or should separate expansion monitors be used, one for input and one for output. In both cases, when should “align to frequency monitor center” be used?

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Hi @buchbinder,

There is a few things to be careful with when setting up a varFDTD (or a FDTD) simulation, regarding the mode source, the simulation region boundaries and eventually, the monitors:

Mode Source:

  • its area should be big enough so the tail of the mode is not interacting with the boundaries. Note that, if the source is larger than the simulation region, it will be truncated by the simulation region.
  • There is an area around the source represented by a grey rectangle where you shouldn’t place any monitor are the recorded fields wouldn’t have any physical meaning.

Simulation region

  • In the directions parallel to the direction of propagation, the PML should be far enough to not interact with the evanescent fields. A distance of half the smallest wavelength is usually a good start
  • In the direction of propagation, if the structure (for instance a waveguide) doesn’t go through the PML, this would add a reflection at the interface. Unless it is something you want, you should have the structure extending through the PML.

Mode expansion monitor

  • You can use 1 mode expansion monitor with several monitors, provided the the expansion is for the same waveguide cross section.
  • If the “align to frequency monitor center” box is checked, the center of the mode expansion monitor will be laterally shifted to align with the center of the frequency monitor when the expansion is calculated. This makes it possible to use the same mode expansion monitor for the expansion of modes on different waveguides that are laterally offset. This is used in the Ring Resonator example.

I hope this will help!


Thanks. This all makes sense, thanks for confirming!

You’re welcome! Something I forgot to mention, that is specific to the varFDTD solver. Since the varFDTD uses the slab mode of the waveguide to calculate the equivalent effective index, the thickness of the “varFDTD” object can affect the simulation results as well.