Figure 14.31a shows the case when an incident photon is absorbed and an electron
is elevated from an energy state £ 1 to an energy state E2• This process is known as
induced absorption. lf the electron spontaneously makes the transition back to the
lower energy level with a photon being emitted, we have a spontaneous emission
process as indicated in Figure 14.31 b. On the other hand, if there is an incident photon
at a time when an electron is in the higher energy state as shown in Figure 14.31 c,
the incident photon can interact with the electron, causing the electron to make a
transition downward. The downward transition produces a photon. Since this process
was initiated by the incident photon, the process is called stimulated or induced emission.
Note that this stimulated emission process has produced two photons; thus, we
can have optical gain or amplification. The two emitted photons are in phase so that
the spectral output will be coherent.