Shafiq,
Your issue is accuracy due to mesh resolution. The 1/10 lambda rule is only a rule of thumb for stability of the simulation in general. There are a number of problems with this guideline particularly with your design.
- The rule is actually 1/10 lambda in the highest index material you are using. This means you should be using 0.03897 as a mesh resolution not 0.155. This is where the value is obtained when using the auto setting.
- You also need to look at your structure dimensions. Your waveguides have cross sections of only 0.3 um x 0.3x, at 0.155um mesh you have anywhere between 1 and 4 points across your waveguide which is not nearly enough. Further take a look at your structure in the 2D Refr_Idx_Re(y) tab and see how stepped your ring is, you will not achieve accurate results with this configuration.