If you expect modes that have considerable radiative losses, you need to use PML boundary conditions as the metal boundaries might introduce unphysical modes and protect the radiative energy from leaving the simulation region. You can read more about the limitations(and advantages) of metal boundaries here.

Note that if you use PML BCs you need to make sure that the PML is far enough from the guided, non-radiative modal fields otherwise you might introduce unphysical loss. This should be part of the convergence testing.

**TE Polarization fraction and TE/TM fraction definition:**

Since the TE/TM polarization definition can vary in different application areas, here is a mathematical definition of the “TE polarization fraction” and “TE/TM fraction” from Knowledge Base in order to avoid any confusion:

The “TE polarization fraction Ex” for propagation along the z direction is defined by the following equation:

where |Ex|^2+|Ey|^2 corresponds to |E_parallel|^2 (since we are considering the polarization of the modes, we only consider the fields parallel to the mode cross section). For propagation along the x/y direction (ie. “TE polarization fraction Ey/Ex”), this is similarly defined:

and

“TE/TM fraction” is defined by the following equations:

and

where and refers to the field component in the direction of propagation (ie. perpendicular to the mode cross section).