**5. Laser sources and stimulated emission**

Laser is a monochromatic light source (sometimes visible) and coherent with very high orientation and brightness. A comparison of the conventional light source and laser light source is given in **Figure 7** [6, 7].

The energy levels used in the laser radiation process are generally of two categories of atomic and molecular energy levels. Under normal conditions, atoms are generally in the ground state until they are excited by an external stimulus (pumping system). Then they return to the ground state in a very short time (nano/microseconds) by

**Figure 7.** *The comparison between a plain light source and a laser source.*

emitting a photon having an energy equal to the difference between the two levels. This return is possible in two ways [6, 7, 12], spontaneous emission and stimulated emission. Stimulated emission is considered in laser. When an atom is in the excited state, if a photon with an energy equal to its excitation energy passes by that atom, the photon induces it to be sunk. The photon is a laser photon if it is produced under the influence of another photon. Otherwise, it is the spontaneous emission [6, 7, 12].
