Learning Forum

[Saha]

I am trying to use a long (>/= 10ps) cw pulse in the sim with custom source code. However for specifying pulse time longer than 7ps (particularly at 7097.8fs as shown in zoomed figure) the simulation stops and diverge error appears. The sim works for pulse (or sim time) duration less than 7ps.
For divergence error, I have tried some parts of the checklist suggested in the article- https://optics.ansys.com/hc/en-us/articles/11277217507603. It confirmed the error is not due to PML. Fot dt stablity issue I am not  exactly sure how it would affect, as I am only using a nondispersive nonlinear dieletric and field amplitude 1 at the moment. Yet I tried the sim with reduced value of 0.5, which still diverges.
I would really appreciate any suggestion regarding this issue,specially on use of a very long CW pulse like this.

[Guilin Sun]

Most likely this is due to the co-called "late-time instability" issue. FDTD is a weakly stable algorithm. any numerical errors can trigger this instability, such as single precision of the numerical data. You may search only for more information.

The late-time instabiity usually occurs for long simulation. You may check if you really need that long simulation. and if this nonlinear simulation involves gain, please significantly increase "autoshuoff max".

You may also need to add 3 profile monitors (xy, yz,and xz plane) and record data inside PML, and movie moitors. and then check where the diverge begis. It might be from PML. So you can modify the PML types and number of layers. 

I particular, the custom source should turn on gradually to not excite higher frequencies unexpected. You can use a time monitor and check its spectrum to see if there is a resonance.