Illumination on curved surface generates fringe pattern in far fields

    • Kazuma Yamashita


      When I illuminate plane waves on a curved surface, the far fields are not smooth but rather oscillatory like interference fringes.

      As an example, I attached far field results for a microlens-like structure. I am wondering if this phenomenon is simply due to interference of light, or some numerical problem in the field projection. Could anyone explain what the cause is?

      Thank you in advance.

    • Taylor Robertson
      Ansys Employee


      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,


    • Kazuma Yamashita

      Dear Robertson,

      Thank you so much for your helpful reply. I did not use any script commands; I just calculated the far fields from GUI with default settings (so a semicircle of 1 m diameter is located underneath the monitor). It was weired for me that refraction at a smooth curved surface, rather than diffraction generated such interference fringes.

      And I also noticed that when there is no gap between the sphere and the periodic boundary condition (PBC being in contact with the sphere), the fringes were significantly suppressed and I obtained a very smooth angular profile (image attached). This result agrees with my intuition as the optics does not contain diffraction, so why did I obtain the fringes when there is a gap? Possibly the small gap at PBC worked as an opening that causes diffraction? Or is it due to far field artifacts or PBC?

      Best regards,No gap

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