Learning Forum

[Erik M]

Dear Lumerical support team,

I am using the grating command in my research and would like to know what it does exactly. Could you please point me to the relevant literature (publications and pages)? The following webpage is unfortunately not detailed in the explanation of the method: https://optics.ansys.com/hc/en-us/articles/360034394354-Grating-projections-in-FDTD-overview.

I noticed that the results from the grating command are different than when I propagate the near-field to the far-field and integrate the diffraction peaks. Here, I use the farfield3d command and integrate the diffraction peaks normalized to the unperturbed beam (so using a flat surface instead of a periodic grating). The intensity diffracted to the diffraction orders measured with this method differs with respect to the results obtained with grating command directly. What is the reason of this difference?

Best regards,
Erik

[Guilin Sun]

Here is the grating analysis: Grating projections in FDTD overview

Please note: grating analysis is used for periodic structure with plane wave illumination, with periodic-tpye boundary conditions, Whereas the farfieldexact or farfield3D is only for non-periodic structures with PML BCs. It requires that the fields at the monitor edge is zero, and does not count the periodicity. That said, for periodic simulation you should not use farfield3D. In visualizer, it has approximate farfields from periodic simulation with assuming periodic: Understanding the Assume Periodic option in far field projections  this is an approximate method to identify the higher order diffraction.