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.
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.
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