Rami Arieli: "The Laser Adventure" Chapter 5 Section 1, page 6
Broadening the Fluorescence line
Certain mechanisms are responsible for broadening the linewidth of a laser: 1. Natural broadening.

2. Doppler Broadening.

3. Pressure broadening.

For many applications, especially when temporal coherence is required (as explained in chapter 10), a small linewidth of the emitted laser wavelength is required.