Optical Pumping

Optical pumping is a process in which light is used to "pump" electrons in an atom or molecule from a lower energy level to a higher one. It is commonly used in the construction of lasers to pump the active medium in order to achieve population inversion.  Laser pumping is the act of transferring energy from an external source to the active medium of a laser, typically a crystal, semiconductor, suitably doped optical fiber, or gas. 

The energy is absorbed into the active medium, producing excited states in its atoms. When the number of particles in an excited state exceeds the number of particles in the fundamental state or in a less excited state, population inversion is achieved. Under this condition, the stimulated emission mechanism can take place, and the medium can act as a laser or optical amplifier.