Lecture 8: Laser amplifiers

Population inversion in three-level systems
Over a period the population in the metastable state N 2
increases above those in the ground state N 1 .
=>The population inversion is obtained between levels |2>
and |1>.
Drawback: the three-level system generally requires very
high pump powers because the terminal state of the
stimulated transition is the ground state. More than half the
ground state atoms must be pumped into the metastable state
to attain population inversion.
59