Learning Forum

[drewski]

I'm running a simulation to determine the coupling efficiency when evanescent coupling between a double taper (on the bottom) and a single taper (on the top) in the Z direction. The coupler transmission is as expected when performing a taper length sweep, and the modes in the Y-Z plane in each cell looks as expected. However, when I put a mode monitor to look at the X-Z mode profile, it has these random spots where the Re(E) is extremely high. I've included screenshots of Re(E) in a linear and log scale (on the log scale, you can see the "lines" of high E field, which makes me think its a cell boundary or something). I also included a screenshot of the data showing the line of high E field. This does not allow me to attach my simulation .lms file (says its not an allowed file type) but I'd like to share that also.

[Guilin Sun]

Are you using EME solver? what is the smallest geometric size of the taper? I would suggest to have a FDE solver in place with the same solver region and mesh size and check the mode profile.
FDE (EME also uses FDE) solves the eigen values of the transformed/discretized Maxwell's Equations. If a local refractive index has error such as extreme value due to finer mesh (conformal variant 2), it may cause such issue.

I suggest to begin with coarser mesh and see if it improves, and contact us if the problem still exists.

[drewski]

Hi gsun Yes, I'm using the EME solver. I tried a coarser mesh for the upper Si waveguide/taper, the lower waveguide/taper, and the background simulation. The problem did not resolve itself, and the situation remained the same (this time, the electric field was even higher).
I've tried looking at FDE cross sections along the taper and the system is able to find the fundamental modes at both ends and the middle perfectly fine. The FDE modes and cross sectional modes in EME look good, its just the side view that looks no good. The smallest geometric size is 50 nm.

Thanks for your continued help.

[Guilin Sun]

Please use a refractive index monitor to get the index profile and see if there is any place the value is not normal .
If FDE can give correct result, there is no reason EME does not, since EME use FDE as its mode solver. Please make sure both of them use the same mesh size, and the same simulation cross section.
In EME, x axis is the propagation direction. If in XZ it has strong E field, you may need to check if there is any field enhancement. Does the S parameter look normal? eg, power conserved abs(S)^2<1?
If there are non-propagating modes and occasionally there is power leakage to those modes. Much higher order modes do not contribute to propagation other than field continuity. You may also need to change the number of modes and see how it improves. Try fewer modes first.

[drewski]

Hi gsun Changing the mesh size of the lower waveguide to be finer (10 nm mesh as opposed to 50 nm) solved the problem, thank you for your input!

[Guilin Sun]

Thank you for letting me know this. Have a great holiday!