I think the problem is "the index change is about 0.291" . It is too large to be considered as purturbation. It will certainly creates other frequencies. So the injection power is lost to other unwanted wavelengths. You can check the spectrum from a time monitor, and use log scale-frequency you will notice other wavelengths.
In reality, you may not get such large index change, or you want to avoid it! The pertubation is propotional to Chi3*Intensity. Therefore if Chi3 is correct you will need to reduce Intenisty, or check Chi3 to see if it is the real number for the material. Chi3 should not be arbitrary but only the real material property. Please modify them accordingly. Of course you do not need to use very weak intensity. you will need to use proper intenisty for a given Chi3.
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