For maintaining a state of population inversion atoms have to be raised continuously to excited state. It requires energy to be supplied to the system. The process of supplying energy to the medium with a view to transfer it into state of population inversion is known as pumping.

Commonly used pumping types are : —

Optical pumping: light is used to raise the atoms to higher energy states.

Chemical pumping: chemical reactions are used to raise the atoms.

Electrical pumping: A strong field is applied to the atomic system with the use of high voltage power supply. The high energy electrons collide with the atoms and transfer their kinetic energy to the later. As a result, atoms rise to the higher states

Direct conversion: In this method the electrical energy directly creates the state of population inversion and laser is produced.