ABSTRACT
This chapter presents the important concept of gain saturation, perhaps the second most important basic laser concept behind the development of the threshold gain equation. A key parameter of a laser amplification medium is the saturation intensity. This is the intensity that will cause the rate of stimulated emission to equal the rate of spontaneous emission. In more practical terms, it is the intensity required to cause the gain to reduce to a value of one-half of the small-signal gain. In a standing-wave laser—a configuration with two or more mirrors in which intra-cavity radiation takes the form of a standing wave with antinodes at each cavity mirror—power flows in both directions, and it is the total power which contributes to this saturation despite the fact that only power flowing in one direction, towards the output coupler, contributes to the output of the laser.
