Hello,
This could certainly be a real coherent phenomenon, and you can definitely observe these types of effects in optical systems. I would recomend looking the Talbot carpet for an idea. One question would be where are you projecting this light, and what script commands or methods are you using to do so?
FDTD is a always using coherent light and these are likely either monochromatic coherence fringes or artifacts of the farfield projection. In reality this may be beyond the coherence length you could use some spectral averaging to remove the coherence.
Coherence effects
Understanding coherence in FDTD simulations
Spectral averaging simple example
Farfield artifacts
Simple far field projection example
Using spatial filtering to avoid truncating fields in far field projections
Best Regards,
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