In figure 4.4 the gain curve of the active medium, as a function of frequency is marked with the lasing threshold and possible longitudinal modes of the laser.

Figure 4.4: Gain Curve of a laser.

The height of each lasing line depends on the losses in a round trip inside the cavity, including the emitted radiation through the output coupler. The shape and properties of the gain curve is explained in chapter 5.

The marked region under the curve and above the lasing threshold include the range where lasing can occur.

The height of the gain curve depend on the length of the active medium and its excitation.

The possible longitudinal modes of the laser are marked as perpendicular lines at equal distances from each other.

In section 4.1 the condition of standing waves for longitudinal modes was determined by the length of the cavity, and its index of refraction.

In figure 4.4 only 5 frequencies from those allowed inside the cavity, are above the lasing threshold. Thus only these 5 frequencies can exist at the output of this laser.