Rami Arieli: "The Laser Adventure" Chapter 5 Section 1, page 1
5.1 Fluorescence line shape of the laser

Laser action inside matter is possible only for those wavelengths for which this material have fluorescent emission.

Fluorescence line is described by plotting spontaneous emission radiation intensity as a function of frequency (or wavelength), for the specific lasing transition.

Figure 5.1 describes a simplified energy level diagram for Helium Neon laser (other examples of He-Ne laser were shown in examples: 3.1, 4.2, 4.3, 4.4, and will be explained more in section 6.1.1).

Figure 5.1: Energy Level Diagram for Helium Neon laser.

The main transition in the visible spectrum is from level E5 to level E2, and the emission is at red wavelength of 632.8 [nm].