Rami Arieli: "The Laser Adventure" Chapter 5 Section 0, page 1
Chapter 5: Laser Gain

The output power of the laser at specific moment is determined by two conflicting factors:

1. Active medium gain - which depends on:

a) Population Inversion (see section 2.6).
b) Fluorescence line-shape of the spontaneous emission that is related to the lasing transition (which will be explained in section 5.1).
2. Losses in the laser, which include: a) Reflections from end mirrors.

b) Radiation losses inside the active medium - due to absorption and scattering.

c) Diffraction losses - Due to the finite size of the laser components.

It is clear that a required condition for lasing is:

In a round trip path of the radiation between the laser mirrors, the gain must exceed ( or at least be equal to) the losses.