Vertical coupling

Consider this example:

Two Si waveguides on top of each other with a spacing of SiO2 and clad of SiO2. I can use FDTD to simulate coupling between the waveguides. However, Mode Solver doesn’t help. I tried following the approach outlined here: https://kb.lumerical.com/en/pic_passive_waveguide_couplers_evanescent.html

But I got stuck at the very first step as the modes found belong to either waveguide but there was no such mode where the field is in both waveguides.

Is there a limitation I am unaware of in Mode Solver, or am I doing something wrong?

Hi Mohamed,
First, make sure you search for enough number of modes.
Secondly, It seems you can’t find the odd and even modes of the coupler. Probably, this means the coupling between the 2 waveguides it not strong enough. Try decreasing the edge-to-edge gap.
Next, I would recommend adding an over-mesh in the gap region.

You are welcome to upload your file to help you find what is wrong.

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