Error results when optical fiber in 2D axisymmetric model

Hi,

Since the 2D axisymmetric model is in Cylindrical System and the optical fiber I am designing is symmetric by z-axis, so I would like to use 2D axisymmetric model to implement my fiber.

Does anyone who know why the simulation can not get right results (mode solver) when the 2D axisymmetric model is using? If the optical fiber is built in 2D model then it works well.

I guess the problem is maybe a wrong boundary condition in 2D axisymmetric model.

Thx.