Reply To: Query related to Rough waveguide 3D FDTD, Simulation time, Negative Transmission

[Amrita Pati]

Hi Naveen,

The PML boundaries/number of PML layers can affect the results of the simulation. But it is very hard to tell if 48 layers are going to be enough for a given simulation as it is very specific to the simulation and depends upon a lot of parameters including the simulation size, the injected fields, the field profile. To reduce the amount of reflection from PML, as a rule of thumb we recommend that the monitors must be lambda/2 distance away from the PML boundaries. As an example, if the source wavelength is 1 um, make sure that you have 0.5 um between the PML and the monitor. I will recommend performing convergence testing with regard to the PML layers if you think they are affecting the results. In covergence testing, we run a simulation with increasing number of PML layers to make sure that the results converge but I understand that it can be difficult here, given that the simulation is taking so long. You could run another simulation with let's say 64 layers and see if the results are changing. That way we will know that the PML is affecting the results. As a workaround instead of simulating the whole device, reduce the size along the propagation length and then run the simulation for different PML layers and then see if that is changing the results.

Ports are a combinaion of mode source, DFT monitor, and mode expansion monitors. So, you can obtain the total transmission as well as the amount of transmission into specific modes of interest (You can select which modes you are interested in, not just the fundamental mode). The "T" result is the total transmission result. But the S-matrix gives you the coupling into the modes you selected.

It is possible to accurately capture the scattered power/transmission from the surface roughness perpendicular (X-Y plane) to the propogation direction (Z axis)? How?

You can calculate the transmission/reflection from the ports, but I am not sure how you'd capture the scattered fields accurately. Let me think about this a little bit and get back to you.

We shouldn't be using varFDTD for surface roughness. How varFDTD works is that it calculates an effective mode of the device based on the vertical permittivity profile, and then propagates this effective mode in 2D. As a result, it is generally used in flat optics where there is little to no vertical coupling between modes. The vertical profile will change with the consideration of surface surface roughness, this change won't be taken into account in varFDTD, since the effective mode is only calculated at one point in the waveguide structure. So, varFDTD will consider the geometry to be flat rather than having any roughness.

Regards,
Amrita