FDTD for 3D Waveguide

Dear Friends,

I have a great request and would be very grateful for your help.

I am looking for 3D FDTD code for the rectangular waveguide. Any configuration and physical problem for 3D rectangular waveguide is fine. I am not an expert in FDTD and just want to know how the program for 3D rectangular waveguide could be written. Thank you very much in advance.

Tato