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FAQ on Material Fitting and Dispersive Material Simulation in FDTD

General Dispersion Settings Instructions:​

  1. When the dispersion fit curve is a horizontal line, FDTD treats the fitted material as non-dispersive. When the fit curve is a straight/curve line, FDTD treats the fitted material as dispersive.

  2. If the user needs to treat the fitted material as dispersive, but the fit result is a horizontal line (non-dispersive), try to reduce the rmse (Fit Tolerance) parameter until a sloped line is obtained.

  3. Recommended oscillator number settings: For bands with shorter wavelength ranges, such as the O band or C band, it is recommended to set the number of oscillators to 1 or 2 (the criterion for small wavelength ranges is: the range is less than 20% of the central wavelength). After fitting, check the fit, if the fit is poor when set to 1, try using 2.

  4. The fitting wavelength range is recommended to be the same as the simulation wavelength range.

  5. It is recommended to update to the latest version for using dispersion features, as dispersion functionality is continuously updated and corrected.

Solutions for Non-convergence of FDTD in Version V3.2.7.0 Dispersion Calculation:​

  1. It is recommended to keep the number of oscillators less than or equal to 2 within the acceptable range of fitting errors.

  2. If the calculation diverges and the number of oscillators ≤ 2, you can appropriately increase the fitting bandwidth.

  3. Use Staircase instead of Curve Mesh.

Settings to Note for Using Dispersion Simulation in Version V3.2.7.0:​

  1. In the GUI, when using the dispersion fitting function, you must go to fitting configuration -> Fit Plot. Plot the fit curve and Save it before it truly becomes a dispersive material and uses FDTD dispersion simulation.

  2. The background material in this version can only be set to non-dispersive materials.

  3. When all materials in the simulation region set with dispersion fitting have horizontal fit curves, the materials are treated as non-dispersive. But due to the use of the dispersion fitting function and the invocation of FDTD dispersion simulation, a logical conflict error occurs. It is recommended not to use the dispersion fitting function for non-dispersive scenarios. 3.1 SDK Recommendations: First perform dispersion fitting on the material, output the fit curve to check whether the material is dispersive. If non-dispersive, comment out/delete the material fit-related code before running FDTD simulation.

    3.2GUI Recommendations: When modeling, first add the material and check the fit curve. If the fit is non-dispersive, do not use the dispersion fitting function. If the material in an already created model uses the dispersion fitting function, but the fit curve is a horizontal line, it is recommended to delete and re-add the material.