Ethan is in conference so I take over.This post is the same as this FDTD vs varFDTD (ansys.com).
While ideally varFDTD can be the same as 3DFDTD, in some cases, or in most cases they may not give the same result. This is because, varFDTD decomposes the original 2D mode into two 1D modes, eg, E(y,z)=E(y)*E(z), so only when it can be exactly decomposed in this way througout the whole device can the results be identical.Â
That said, usually varFDTD is used to get a quick result. 3D FDTD should be more accurate, and we strongly recommend users to have 3D FDTD for final verification.
As you can see from the two screenshots, the scattering is different: varFDTD has only 1D scattering whereas 3D FDTD has 2D scattering, eg, along the waveguide wall.Â
To improve accuracy, you may increase the mode-source/monitor size and make sure no mode field is clipped.Â
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