TE and TM polarization

In a planar or rectangular waveguide propagating along $z$ with a substrate normal along $y$:

• TE (transverse electric) mode: the electric field vector lies in the substrate plane ($E_x$, perpendicular to $y$). Equivalently, $E$ is perpendicular to the plane of incidence at the substrate interface.
• TM (transverse magnetic) mode: the magnetic field lies in the substrate plane; the electric field has a component perpendicular to the substrate ($E_y$).

Note the terminology depends on convention. In planar slab waveguide literature, TE/TM are defined relative to the substrate normal. In fiber optics, the same labels can refer to the principal polarization axes of the fiber. In free-space optics, s-polarization (German "senkrecht") corresponds to TE and p-polarization corresponds to TM relative to a reflection plane.

In rectangular waveguides (SOI strip, SiN strip), the fundamental modes are typically labeled TE-like and TM-like because they are not purely transverse to the propagation direction — they carry small longitudinal field components from the rectangular cross-section. Common shorthand omits the "-like" qualifier.

Why the distinction matters:

Many PIC devices are designed to operate in TE only. Coupling unpolarized or randomly-polarized fiber input requires either polarization control (a manual paddle or active controller) or polarization diversity — two parallel paths that separately process TE and TM and recombine the outputs.

For an SOI 220 nm × 500 nm strip waveguide at 1550 nm:

The wider waveguide dimension breaks the cross-section symmetry that would otherwise produce degenerate TE/TM modes.