Transverse Mode - Types of Modes

Types of Modes

Transverse modes are classified into different types:

• TE modes (Transverse Electric) no electric field in the direction of propagation.
• TM modes (Transverse Magnetic) no magnetic field in the direction of propagation.
• TEM modes (Transverse ElectroMagnetic) neither electric nor magnetic field in the direction of propagation.
• Hybrid modes nonzero electric and magnetic fields in the direction of propagation.

Some authors use an alternate notation;

• H modes have a magnetic field component in the direction of propagation. H modes are equivalent to TE modes.
• E modes have an electric field component in the direction of propagations. E modes are equivalent to TM modes.

In rectangular waveguides, rectangular mode numbers are designated by two suffix numbers attached to the mode type, such as TE_mn, where m is the number of half-wavelengths across the width of the waveguide and n is the number of half-wavelengths across the height of the waveguide. In circular waveguides, circular modes exist and here m is the number of half-wavelengths along a half-circumference and n is the number of half-wavelengths along a radius.

Modes of hollow metallic waveguides filled with a homogeneous, isotropic material fall into the first two categories. Otherwise, except in cases of special symmetry, modes are generally of hybrid type. For example, light travelling in an optical fiber or other dielectric waveguide forms hybrid-type modes. The fiber modes are usually referred to as LP (linear polarization) modes, which refers to a scalar approximation for the field solution, treating it as if it contains only one transverse field component (this is accurate because of the low refractive index contrast in typical fibers, the transverse electromagnetic (TEM) type. A planar Fabry-Perot resonator or etalon can also exhibit linearly polarized TEM modes. A resonator employing curved mirrors cannot support a TEM mode. Such modes are almost always hybrid except for the special cases of the electric field polarized either radially or azimuthally. The former case corresponds to a TM mode, the latter to a TE mode. Equivalently, linear polarization and TEM modes are mutually exclusive for a Gaussian beam or any other beam with curved wavefronts.