Photonics

Photonics

Strong Interference in FDTD Simulation

    • michael_zhu
      Subscriber

      Hi,

      I am simulating the scattered-field by a Si3N4 waveguide on SiO2 membrane. The strongly focused light is created by a Gaussian source with thin lens model and the focus point of the Gaussian beam is 7micron above the waveguide.

    • Guilin Sun
      Ansys Employee
      It seems that the waveguide core is very small so you used override mesh to resolved it. When a Gaussian beam meets non-uniform material/structure, it creates diffraction. Your result confirms this. For a test, if you use larger waveguide core than the beam width, the diffraction will be less.
      You can use farfiled projection and roughly check if there are larger angles than the beam divergence, which would be the side lobes due to diffraction.
    • michael_zhu
      Subscriber
      Thanks for you quick response about diffraction field.
      However, I am confused about the interference in the diffraction field, which I think should not appear. As shown in the following four figures, there is an interference pattern in the diffraction field above the waveguide, which can be alleviated by Fourier image filter. For a single-frequency Gaussian beam, I consider there is be no side lobes interference at transmission side. I wonder what result in the interference.
      raw electric field
      Fourier transform of the raw electric field
      filtered Fourier transform of the raw electric field
      the electric field without interference using Fourier image filter




    • Guilin Sun
      Ansys Employee
      You could simulate without the waveguide core and then compare the result, which in this case it should be no side lobes. Your Fourier transform shows the beam has larger spatial frequencies and not Gaussian-like distribution. As you know, the Fourier transform of the Gaussian function is also a Gaussian. so another possibility is that the source span is smaller: the geometric area should be much larger than the beam waist.
    • michael_zhu
      Subscriber
      Hi, gsun
      Actually I am curious about the cause of the fringes in electric field magnitude figure. It turns out that the change of PML layer type leads to the fringes. With the other parameters to be the same, the type PML layer is changed from stabilized to standard. Then the fringes disappear.
      However, according the the PML boundary support webpage, the stabilized PML has more layers and is more stable but in my case standard PML seems to work much better. Would you explain the phenomenon? Thank you.

      Below are the electric field magnitude figures and model screenshot under two circumstance.
      stabilized PML layer:


      standard PML layer


    • Guilin Sun
      Ansys Employee
      Sorry, I misunderstood your original question regarding to "fringe". When comparing the two images, I got to know what do you mean about the fringe.

      I believe they are due to PML reflection: Stabilized PML is mainly used to make the simulation stable but its absorption is not as good as the standard, Please refer Ansys Insight: õ┐«µö╣PMLÕô¬õ║øÕÅéµò░ÕÅ»õ╗ѵ£ëµòêµèæÕêÂFDTDÕÅæµòú
      so you are right that in your current case, the Standard PML should work fine.
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