Three level pumping in Laser


Three-level pumping in laser is suitable for attending population inversion.

When atoms of ground energy state observe the photon from incident energy. It goes from lower energy or ground energy state two to a higher energy state but the lifetime of a high energy state is very short that is $10^{-8}$ $sec$ i.e. So an atom cannot stay for a long time in high energy state i.e.$E_{3}$ and then the atom goes for non-radiative transition and reach to the metastable state. In a metastable state, Atoms cannot go to a lower energy state or ground energy state directly. Therefore, These atoms come from a metastable state to a lower energy state or ground energy state by lasing transition.

Three level pumping in Laser
Three-level pumping in Laser

This is the process of three-level pumping in a laser. For better pumping efficiency, The level $E_{3}$ should be the band of energy levels instead of being a single arrow line. It allows the use of pumping radiation of wider bandwidth to excite more atoms. However, the major disadvantage of the three-level scheme is that it requires very high pumping powers. The three-level laser system can produce light only in pulses. Once stimulated emission commences, the metastable state $E_{2}$ gets depopulated very rapidly and the population of the ground energy state increases quickly. As a result, the population inversion ends. One has to wait till population inversion is again established. Thus, the Three-level laser system operates in pulse mode.