When an optical gain medium is pumped optically or electronically, an electron is pumped (excited) from a lower energy level to an upper energy level. The excited medium is eventually relaxed to some lower energy level by radiating energy – as upper energy levels are generally less stable than lower energy levels – and sometimes the energy radiation takes a form of photon. This is how spontaneous emission and stimulated emission take place, and is schematically illustrated in Figure 1.
Figure 1: Spontaneous and stimulated emission.
Properties of photons
Properties of photons generated by spontaneous and stimulated emission are quite different. Spontaneous emission takes place without interaction with other photons, and the direction and phase are random. Stimulated emission takes place when the excited electron interacts with another photon. Both the direction and phase are “copied” from the other photon when stimulated emission takes place, and it is the most important phenomenon for creating a highly directional and highly coherent light source (e.g. laser diode, fiber laser, and optical amplifier). The importance is quite evident from the fact that LASER is an abbreviation for Light Amplification by “Stimulated Emission” of Radiation.
Stimulated emission takes place only when the gain medium is pumped strongly enough and population inversion is created, while spontaneous emission takes place no matter population inversion exists or not.