Thanks for your patience! I looked at you simulation in more detail, and here are a few things that I would ask you to consider:
- The position of the input port. Make sure that you are injecting the fields into the bottom waveguide and not the free space just before the start of the waveguide. Before you run the simulation, visualize the fields of Port 1. Also make sure that the structure is resolved properly by looking at the index. For ex. if you look at a structure like yours, the index and fields at Port 1 look like this:
But if you inject the fields right at the edge of the structure (where it starts, which looks like you might be doing in your simulation), it would look something like this (which is incorrect):
In case you are doing this, this could be the reason behind the large expansion error. As these are free space modes that EME is not very good at simulating and might require a large number of modes fir expansion.
- The position of the Output Ports. You should position the 2nd set of ports at the end of the taper rather than the start of the taper. What this is doing is not even considering the taper. So, even though there is a large expansion error you don't see any change in the Transmission because it is not even considering the taper structure, so the Transmission is basically 1, no matter how many modes you use for the expansion. So, what you can do is extend the simulation domain in the x-direction. With this you can also characterize the mode interaction happening at the abrupt transition in the geometry as you go from one waveguide to the other.
- In terms of the cells. I would ask you to use one cell for the bottom waveguide, and then a series of cells with CVCS subcell method for the taper. You do not need to have multiple cells at the bottom waveguide side because the cross-section of the waveguide is constant. So, even if there is an abrupt transition, you only need to consider the cross-sections of the waeguide. As long as the cross-section is constant you only need 1 cell. So, in this case the cells should look something like this:
1 cell for the bottom waveguide and multiple cells for the top waveguide as it is a taper.
If you need more insights into the setting up of the simulation, you can refer to the edge coupler example, which is also vertical coupling. Although different from your geometry it can still provide crucial insights: Edge Coupler.
After you make all these changes, if you see unexpected behavior/results, let me know and we can discuss convergence testing and error diagnostics in more detail. I believe you have already looked at the error diagnostic page. But here is another article that can give you an intuitive picture of the convergence testing process in EME: EME Convergence Testing: An Intuitive Approach.
Please let me know if you have any questions.