Hello everyone! One important concept in optics that can be a bit confusing the first time learning it is transverse modes. First, a mode in free space (or isotropic dielectric medium) is defined as a superposition of plane waves, perpendicular to the direction of propagation. In any waveguide (optical fiber, laser cavity) where a beam is restricted to finite dimensions, however, there are “allowed” modes that depend on the boundary conditions of the waveguide. Each mode corresponds to a single frequency.
Some of the most common modes are discussed below:
Transverse Electro-Magnetic (TEM) Mode – In this case, both the electric and magnetic fields are completely transverse to the direction of propagation of the beam i.e. no dimensionality of the electric or magnetic field in the direction of propagation. In laser cavities, the most common and desired mode is TEM00 mode, which means that the beam is a perfect Gaussian.
Transverse Magnetic (TM) Mode – If a TEM mode meant that there was no dimensionality of electric or magnetic field in the direction of propagation, then a TM mode means that there is no magnetic field in the direction of the beam, but there is an electric field.
Transverse Electric (TE) Mode – As you may have guessed, TE mode means that there is no electric field in the direction of propagation, but there is a magnetic field.
An image of TM and TE modes is shown below (By Courtesy Spinningspark at Wikipedia, CC BY-SA 3.0, https://en.wikipedia.org/w/index.php?curid=39217796)