Rami Arieli: "The Laser Adventure" Chapter 4 Section 3, page 1

4.3 Transverse Modes of a Laser

In previous sections the distribution of intensity was examined along the optical axis of the laser.

Longitudinal modes were described as standing waves between the laser mirrors.

This section examine the transverse distribution of intensity, in cross section of the beam, perpendicular to the optical axis of the laser.

These transverse modes are created by the width of the cavity, which enables a few “diagonal” modes to develop inside the laser cavity.

A little misalignment of the laser mirrors causes different path length for different “rays” inside the cavity. Thus, the distribution of intensity is not the perfect gaussian distribution to be explained shortly.