Transmission higher than 1


I am trying to model a silicon nitride ring resonator using the FDTD solutions, but have been getting transmission (T) in Port 3 larger than 1. What could be causing this? I am truly confused by this and don’t know how to correct the issue. Any help would be greatly appreciated! I am new to lumerical and have not been able to get this working.

The file is below if needed:

I am also getting transmission greater than 1 for some of the simulations. This is annoying me since I try changing mesh, port positions, FDTD region sizes, etc. It is still showing problems.

In my case the transmission (abs(S)^2) is going till 100 at some frequency. I know I did something wrong.

Please help

Hi I am attaching the files -

  1. one project is working S-parameter sweep, I am able to export a dat file for Interconnect

  2. The project for which transmission is getting above 1 is attached below.
    For this project I have posted results earlier.

Both files are pretty much similar.



Dear @grodal1

Thanks for your patience.

I ran your simulation and I noticed that it stops before triggering auto shutoff. As a result of early shutoff, transmission in port3 has ripples and sometime goes above 1. Since your structure was very large, it takes a long time to finish and I was unable to run it for longer times.

Can you please increase the simulation time and run it until it trigger auto shutoff, and update me with the results.

I was also concerned why you were performing 2D simulations? You can use varFDTD to run 2.5D simulation with a better accuracy compared to 2D and faster than 3D. For a good discussion in this regard please see the links below:

Overall, for large ring resonators, you can simulate half-ring or the coupling sections and sent it INTERCONNECT. This should save you a lot of time. Also, it might be very useful to check how you can calculate coupling coefficient in a ring resonator.

Sorry for bombarding you with the links, I just wanted to give you an overview of the available options :slight_smile:
Please let me know of the approach that you are interested in, and I am happy to discuss it further with you.


Thank you very much for getting back to me! For the past hour or so I’ve been running a simulation with double the previous simulation time. Do you think this will be sufficient?

I have attempted to use the Mode simulations but that also gives me odd results–is the set up of my resonator reasonable in terms of coupling and distances of ports? The new simulation I am running has a larger FDTD region and ports that are further from the actual resonator (longer straight waveguides to accommodate this). Do you believe this could have a positive impact in the results or should I return to the previous design?

Dear @grodal1

It will be a good practice to increase simulation time enough that it triggers auto shutoff level. This is very important to study whether transmission above 1 is due to early shutoff, poor PML absorption, or some other unwanted effects.

I think your simulation was set properly. I checked the FDTD region and it was far enough from mode source. One thing that you can try to improve the results is to use override mesh on the coupling region (gap between ring and waveguide). While conformal mesh accounts for permittivity changes inside a mesh cell, it is a good idea to have a few (3-5) mesh cell in the gap region.

Other than that I think your simulation was set properly. Please keep me updated with the your findings.
I hope this was helpful.