How to obtain beat length (coupling length) of a 3D silicon photonic directional coupler using COMSOL

Hiii, I am trying to simulate a directional coupler using COMSOL with Beam envelope method and boundary mode Analysis. However it is getting difficult to identify even and odd modes, as some single n_eff Are showing both even and odd modes shapes for Ey and Ez . Can Someone help me to get the coupling length in COMSOL by any method?

Thanks