Guilin Sun
Ansys Employee

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When simulation seems to diverge, you will need to find the root cause: is it due to material fitting, dt factor or PML. You can modify the material fitting, use metal BCs and see if it still diverges, or increase the number of PML layers. The later is is important particularly if higher mesh accuracy is used.


Please refer this post: 

Ansys Insight: FDTD varFDTD 常见的发散原因以及处理

" shorter simulation time to terminate the simulation after the light time when the light should have left the simulation domain." is not recommended, since it may need longer simulation to reach autoshutoff min in order to get correct frequency-domain result.

BTW: what quantity you use to check the result?  The sinusoidal result can only be observed through a time domain monitor, by choosing proper field component. The point time monitor should be located at the waveguide center, or where the amplitude is the maximum.