Unexpected and strange 3D waveguide simulation error?

Hi all,

Firstly thanks for viewing my question.

I am building a simple fiber light propagation model by combining boundary mode analysis and frequency domain analysis.

When the fiber core is set to be small, it is ok, the result is right. However, when I increase the fiber core diameter, comsol hints:

Undefined value found:
NaN or Inf found when solving linear system using SOR

Here I am absolutely sure that the corresponding effective refractive indices entered in boundary mode analysis for two cases are right.

It is very strange here.

Appreciate your help here.

Marvin