I’m not too sure if a bit of the script is missing, but I got an error when running it (the variable Lp is not defined).
Regardless of the actual structure, here’s a few elements of response.
Typically, for long propagation distances, you can consider the EME and varFDTD solvers of MODE Solutions. Note the varFDTD solver can be used for planar waveguiding structures where there’s little coupling between the vertical slab modes.
If the waveguides are tilted in the propagation plane, then varFDTD could be possible, provided the waveguides have the same thickness.
I’m not too sure the EME solver can easily be used here, first because of the slab (the modes are not confined laterally, so it can b tricky) and also because of the tilt (that said, it could be worth trying!). The EME advantage is you can easily do sweeps over the length.
In fine, a full 3D FDTD simulation is recommended to validate the results from other solvers.
EME is a frequecy domain method, so you would have to run a sweep over the wavelength to get the full results.
On the other hand, varFDTD and FDTD are time domain method, meaning you can get the full spectrum in 1 simulation.
The mode source in FDTD and varFDTD can be tilted. On the other hand, monitors can only record data from the mesh, so they will be along the axis. The mode expansion monitor can be tilted and provide the coupling efficiency to a selected mode.
Regarding EME, I believe the ports can be tilted, although I never used them that way.
It would be great if you could provide the missing part of the script, and also let us know what result you would like to extract from the simulation. That will allow to provide a better recommendation to which solver to use.