Rami Arieli: "The Laser Adventure" Chapter 4 Section 4, page 1
4.4 Optical Cavity

In every laser cavity there are (at least) two mirrors at the end of the laser.

These mirrors are facing each other, and their centers are on the optical axis of the laser.

The distance between the mirrors determines the length of the optical cavity of the laser (L).

There are different shapes of mirrors, with different lengths between them.

A specific optical cavity is determined by the active medium used, the optical power in it, and the specific application.

The explanation here will summarize the design principles of an optical cavity: