Learning Forum

[Fakhriyya Mammadova]

Hello. I optimized two different inverse-design structures in Lumerical. I am trying to connect them and see how they work together. The first structure has 2 inputs, and one output. The output of the first structure is also the input of the second structure. When I introduce the sources on the inputs of the first structure (so max 2 sources), everything works well and the results of the simulation I obtain are as expected. Nevertheless, if I add any more sources just somewhere in the space inside mesh and FDTD, the results change severely. I am not sure why. Is there any software limitation to this or is there any other explanation for how just a sole introduction of a source changes everything.

[Taylor Robertson]

Hello Fakhriyya,

I don't think this is fundamentally impossible. Another source shouldn't change the fields - Soft sources

Multiple sources could however change how the normalization works. I imagine that you are comparing Transmission results? Perhaps you could experiement with the different normlization states?

Also it could be possible that the simulation settings changed with this new source. Maybe it is using global source settings, and the global settings are different in this simulation?

If you have a bigger simulation you should check the the simulation time is sufficient, or the results can be different. This is 3D FDTD correct?

[Fakhriyya Mammadova]

Dear Taylor Robertson, thank you for your reply. I am using 2D FDTD simulation and I am not sure what you mean by normalization. Can you explain a little further, please? How exactly should I play with the states? Also, yes, I am looking at the transmission results of the monitors

[Taylor Robertson]

I would be careful with 2D simulations is this varFDTD or 2D FDTD? There are separate approximations for each polarization in both cases. Even if the sources are the same polarization, other difference like the normalization could changing this. See Understanding Frequency Domain Normalization.

Usually you would validate and reoptimize any topology optimized devices in 3D to avoid breaking the 2D assumptions. It is likely the performance will change in 3D, but the built in 2D assumptions mean that the collective results will be more consistent.

[Fakhriyya Mammadova]

I have two structures at the same time and I measure their transmissions at the output. Both of these structures have 3 inputs, and the structures are already optimized in a way, that if I set one source at the input of one structure, the output transmission is 1. So I have set 2 sources: one per each structure. The problem is that I know that the transmission at my foms must be about 1, but I receive 0.5. So I was thinking if there is something with the setup of the program that makes the source's transmission drop. Or since I use 2 sources, do I have to assume that this 0.5 is actually 1 if I have to multiply it with the number of sources?