Can Mode Decomposition be used to calculate the transmission/reflection of waveguides

[niubiworker]

I am trying to use mode decomposition to calculate the transmission and reflection of the structure shown in the figure, but I am not sure if this method is feasible. Can anyone help me?
Although I have obtained a relatively conventional calculation method in the previous discussion(Thanks again for his help), I saw in the Meep example that there is a method for mode decomposition, so I would like to try it out!