Why am I seeing a significant difference in coupling efficiency from EME to FDTD?
I am simulating the evanescent coupling between two waveguides after the waveguides overlap for some distance X (i.e. how much light is transmitted from waveguide A to waveguide B when the waveguides overlap for some distance X). I have identical simulations setup in EME and FDTD (they have identical mesh sizes over the waveguides of 10 nm , very fine non uniform conformal variant 1 background meshes, PML boundary conditions, same simulation region size in Y/Z, etc.). The structures of the two waveguide are also the same (identical dimensions between simulations with the smallest feature size being 50 nm). When simulating in FDTD, the transmission for a 5 um by 5 um output monitor located 15 um after the waveguides stop overlapping is around 67%. When the monitor is located 100 um from when the waveguides stop overlapping, this is still 67%. A 20 um by 20 um monitor located at 1um from when overlapping finishes shows 98% transmission, while the same size monitor at the 15 um position shows around 87%, and again the same monitor at the 100 um position shows around 72%. The EME simulation always shows 87% for the abs(S_21)^2 and abs(S_12)^2 matrix value for a 5um by 5um port size. When I run a propagation sweep in EME to determine if the distance between the end of the overlapping region and the final port makes a difference, I find the value is 87% regardless of how long this distance is.
This seems like a significant discrepancy between EME and FDTD for the same simulation, and I'm not sure which simulation to trust before I can continue with design development. Please see my attached screenshots of the settings I'm using below. I would like to determine why the difference is occurring and make sure EME and FDTD match one another to within 2%. Thanks so much for any assistance (is there a way to upload my simulation file?).