The source of the pump or energy source is part of a laser system that energizes the laser medium. To get the laser emissions, we first need to generate population inversion. Population inversion is the process of reaching larger amounts of electrons in higher energy states compared to lower energy states. Energy sources supply enough energy to the laser medium where electrons in low energy states are particularly interested in higher energy states. Consequently, we get population inversion in the active medium or laser medium. Examples of energy sources include electrical discharges, light from other lasers, chemical reactions, and flash lights. The type of energy source used largely depends on the laser medium. Laser LAsermåler uses chemical reactions as an energy source, helium lasers use electrical discharges as energy sources and lasers Nd: YAG uses focused light from diode lasers as an energy source.
Laser media is a medium where spontaneous and stimulated radiation emission occurs. Generally, populations of states with lower energy are greater than higher energy states. However, after reaching the population inversion, the population of the state with higher energy becomes greater than the lower energy state. After receiving sufficient energy from the source, the electron in a low energy state or ground state is particularly attracted to a higher energy state (in the laser medium). Electrons in excited states do not persist for long periods because the life of electrons in excited states is very small. Therefore, after a long time, the electrons in the excited state will return to their ground state by releasing energy in the form of light or photon. This is called spontaneous emission. In spontaneous emission, each electron emits a single photon as it falls to its ground state. When this emitted photon collides with an electron in an excited state or a stable meta state, it forces the meta stable electron to return to its ground state. As a result, the electron re-releases energy in the form of photons. This is called stimulated emission. In the stimulated emission, each electron emits two photons as it falls to its ground state.
If we use electrical energy as an energy source than a single photon or a few photons (spontaneously produced) will produce a large number of photons with a stimulated emission process. Thus, the amplification of light is achieved by the laser medium. The laser medium is also known as the active medium or the gain medium. The laser medium will determine the characteristics of the emitted laser beam. The laser medium can be solid, liquid, or gas.