Active Medium Gain Curve with lasing and without - Hole Burning

In figure 5.8, both small signal gain and saturation gain are plotted as a function of frequency.

Small signal gain Curve appears identical to the fluorescence line shape (figure 5.3), with one maximum at the frequency of the basic mode (n₀).
The value of the saturation gain drops for each lasing mode, from the small signal gain to threshold gain (G_A)_th
This process is called “hole burning” in the gain curve.

Conclusion:
Each moment, most of the energy stored inside the active medium is not used to create the radiation out of the laser.