Fluorescence line-shape

Fluorescence line is described by plotting spontaneous emission radiation intensity as a function of frequency (or wavelength), for the specific lasing transition. In figure 5.1 for the He-Ne laser it was for transition between E₅ and E₂.

When the transition is between narrow levels, the fluorescence line is narrow.

The narrower the fluorescence line the easier (less energy required) it is to achieve population inversion.

The ideal fluorescence line-shape is a spike pulse with width approaching zero, as described in figure 5.2a.

In reality, each energy level have a finite width, as described in figure 5.2b. Thus, many transitions can occur between different regions in the upper lasing level to different regions in the lower laser level. All these transitions, plotted as a function of frequency, make the fluorescence line shape shown in figure 5.3.