Laser Gain Curve

There is a lot of similarity between the shape of the gain curve (figure 4.4) and the fluorescence line (figure 5.3). The reason is that the active medium gain curve is directly proportional to the width of the fluorescence line of the spontaneous emission.

When discussing linewidth, it is important to distinguish between the linewidth of the laser, and the linewidth of specific longitudinal mode, which can contain many longitudinal modes.

Figure 5.4 describes both the gain curve of the laser, and the longitudinal modes of the cavity

Each of the longitudinal modes has its own linewidth, and emit certain intensity.