If you use the default automesh with mesh accuracy 2 or higher, it should be ok. However, from what you said, it might be because the simulation time is too short. The default 1000fs is for optic frequency. at the THz range, the simulation time is much longer. Please check the source signal and see if it is a complete pulse without cutting off.
In addition, if you can read those posts it would be helpful:
One suggestion is to signficantly increase the simulation time, and let the autoshutoff min to terminate it.
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