Module 8 : Laser- II
Lecture   : Rate equations for Lasers
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The system, in addition to the state $ u$, has an excited state $ m$which is a metastable state, i.e. a state in which the atom has a long life time. Atoms from the upperlevel $ u$decays spontaneously to this metastable state $ m$. Life time in the level $ m$is such that the rate of spontaneous decay from level $ m$to the ground level $ g$is slower than the rate at which atoms decay from $ u$to $ m$. This results in a population inversion between the metastable level and the ground state.
 
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