Rami Arieli: "The Laser Adventure" Chapter 5 Section 2, page 1
5.2 Loop Gain

Each time the laser radiation pass through the active medium, it is amplified, as was explained on population Inversion (section 2.6).

Contrary to amplifying the radiation, there are many losses:

  1. Scattering and absorption losses at the end mirrors.
  2. Output radiation through the output coupler.
  3. Scattering and absorption losses in the active medium, and at the side walls of the laser.
  4. Diffraction losses because of the finite size of the laser components.
These losses cause some of the radiation not to take part in the lasing process.

A necessary condition for lasing is that the total gain will be a little higher than all the losses.

Loop Gain is defined as the net gain (amplification less losses) that the radiation see in a round trip transmission through the laser. It is measured as the ratio between radiation intensity at a certain plane (perpendicular to the laser axis), and the radiation intensity at the same plane after a round trip through the laser.