what is population inversion in laser

Population inversion is a state in which a higher-lying energy level is more strongly populated than a lower-lying level in a system, such as a laser gain medium. This state cannot occur under conditions of thermal equilibrium, where the level populations are described by a Boltzmann distribution. To achieve population inversion, energy needs to be pumped into the system to excite the atoms. When a population inversion exists, an upper-state system will eventually give off a photon of the proper wavelength and drop to the ground state. This photon, however, can stimulate the production of other photons of exactly the same wavelength because of stimulated emission of radiation. This process can lead to a massive increase in photons given off, which is what is wanted inside a laser. A population inversion is required for laser operation, but cannot be achieved under normal circumstances.