This video is taken from the EME Learning Track on Ansys Innovation Courses.

## Transcript

In general, increasing the number of modes should make the results more accurate, and

this makes sense intuitively, however, there are rare cases where this is not true in practice.

Consider this periodic waveguide Bragg grating.

Using the mode convergence sweep tool and sweeping up to 150 modes in each cell, we

can see that the reflection, absolute value squared of S11, seemingly converges

by 30 modes, but the results start to show sharp peaks and dips as the number of modes

increases, and then above 118 modes the results are smooth again.

What can happen is that as you include more modes, at some point a high order mode is

found in one cell which has very little overlap with any modes in neighboring cells.

This can lead to large perturbations of the S matrix of the cell interface, which also

affects the internal and user s matrix results when energy conservation is applied.

The S matrix can be perturbed when the energy conservation setting is set to either "make

passive" or "conserve energy", or if the CVCS subcell method is used.

If energy conservation is set to "none" and the CVCS subcell method is not used, then

this problem will not occur.

The perturbations in the S-matrix cause sharp peaks or dips that can bee seen in the S parameter

plot from the mode convergence sweep, like we see here.

As you further increase the number of modes, a matching mode may be found in the neighboring

cell and the plot becomes smooth again.

Note that energy conservation will perturb the full S-matrix, so even if no power gets

coupled into the mode which has no matching mode in neighboring cells, the S parameter

results can still be affected.

Although this type of effect where you see large perturbations in the S-matrix doesn't

commonly occur for most devices, we almost always want to use the "make passive" energy

conservation setting, so the best practice is to always perform a mode convergence

sweep for any device that you are simulating just to make sure that you are getting S parameter

results over a range where the results are smooth.

For this particular example, the results in the range between using 19-30 modes is almost

the same as the results in the range 118 modes and above, but the simulation is faster using

fewer modes so you may choose to operate in the range using about 25 modes for further

simulations of this device.