Notes-Module 4

Population inversion is necessary for laser operation, where more atoms are in the excited state than the ground state.
In a two-level laser system, population inversion is not possible as both the rate of absorption and stimulated emission become equal in the saturated state.
In a three-level laser system, atoms are pumped from level 1 to level 3, and then non-radiative transitions take place from level 3 to level 2.
Level 2 is a metastable state, and atoms relax to level 2 instead of level 1, achieving population inversion between level 2 and level 1.
The short lifetime of level 3 ensures a faster relaxation rate, requiring a minimum pump power for population inversion.
In a four-level laser system, atoms are pumped from the ground level (level 1) to level 4, and then non-radiative transitions occur to level 3.
Laser light is emitted when atoms transition from level 3 to level 2.
The short lifetime of level 2 allows rapid relaxation of atoms to level 1, creating population inversion between level 3 and level 2 even with small pump power.