As we mention at the beginning of chapter 6.1, it is convenient to excite a laser whose active medium is solid with "Optical Pumping".
The atoms in a solid are close to each other, and the interaction between neighbors is strong. Thus, the absorption and emission spectrum ranges in solids are much wider than those of gasses. Wide absorption spectrum allows pumping of the active medium with a "conventional" light source, which has a wide emission spectrum.

In Optical Pumping the active medium is excited by illuminating it with external electromagnetic source. The photons from the external source are absorbed by the material of the active medium, thus transferring energy to its molecules.

Two types of electromagnetic sources are used in optical pumping:

• Source of wide band electromagnetic spectrum- such as Flash lamps, incandescent lamps, arc lamps, etc.
• Source of narrow band electromagnetic spectrum - another laser.