Reply To: FDTD:: simulation of directional coupler

[Guilin Sun]

did you put the override mesh at the coupling region? if so, please set its size to only cover the waveguide. If it is too large it will affect the mesh close to the vertical PML.

Right now you want to overcome the diverging issue. so you can try to use the suggested methods from the post to see if it works. In your case, maybe the ripples are from higher-order modes. Please use port or mode expansion monitors to verify this.