Photonics

Photonics

Ansys Insight: Key FDTD Simulation Settings

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    • Khashayar Ghaffari
      Ansys Employee
      Here we detail some important settings for FDTD simulation settings that are commonly overlooked or improperly set up. First an overview:
      • Mesh size: Use mesh override regions and index monitors to make sure small features are resolved.
      • Mesh order: Use index monitors to make sure geometry object mesh ordering is correct.
      • Material fit: Use the Material Explorer to verify a good material fit to the data.
      • Simulation time: If your simulation is ending before the autoshutoff threshold is reached, increase simulation time.
      • Simulation span: Increase the simulation span until there is a half wavelength of space between the geometry and the PML boundaries (unless the geometry passes through the boundary).
      • Boundary conditions: Make sure the choice of boundary condition and boundary condition settings are correct.
      • Source span: Check that beam/mode source spans are large enough such that the input field is not cut off. DFT or movie monitors can be used to verify sources are functioning properly.


      Below is the more detailed information:

      Mesh size

      A key parameter for the accuracy of the FDTD algorithm is the number of mesh cells per wavelength. The default auto-nonuniform mesh in FDTD Solutions will attempt to automatically create a mesh that has a certain number of mesh cells per wavelength, with the number of cells determined by the “mesh accuracy” setting.

      For your initial simulations, the mesh does not need to be very refined, so a mesh accuracy of 2 or 3 is sufficient. However, even for initial simulations, it is often necessary to refine the mesh in certain areas of the simulation region with the use of mesh override objects. Areas that may require a mesh override include sections of the device geometry with small features not sufficiently resolved with the default mesh (for example, thin layers) or metal-dielectric interfaces , where the fields vary quickly. For rectilinear geometries it is best to use mesh overrides to ensure the mesh cells overlap with the geometry.

      For example, below are images of a thin layer, first with the default mesh, then with a mesh override region with a “dy” setting that ensures there are four mesh cells in the Y direction that exactly overlap with the thin layer.

      Default mesh:
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