Simulating Step index fiber in 3D

I am trying to simulate step-index fiber in 3D.
But which physics i should add?


In 2D model of comsol model library they added "Modal Analysis".
But in 3D should i have to use "Boundary mode analysis"??
If so, how can I use it?


I am a novice in comsol. So, please help.

Thanks in advance.