ABSTRACT
To understand the operation of the laser requires a knowledge of the energy levels associated with atoms, ions and molecules. In thermal equilibrium, the energy levels are populated according to the Boltzmann distribution, which forbids the conditions in which an upper level might have a greater population than a lower level. Because, for lasing, an upper level must be more highly populated than the lower level, lasing will not take place. Lasing can take place only when a material is not in thermal equilibrium. This non-equilibrium is created by an excitation source sometimes called a “pump” source. Just as thousands of atoms, ions or molecules can be laser materials, numerous pump sources can excite the materials.
