Module 8 : Laser- II
Lecture   : Rate equations for Lasers
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2.10 Four Level Laser :
 


One of the most popular and low cost lasers is helium-neon laser, which works on the basis of a four level system with two levels intermediate between the ground state $ g$and the pumpng level $ p$.

The ground state atoms are electrically pumped to a short lived state $ p$. Atoms from this state undergo fast decay to a metastable state $ m$. Between $ m$and $ g$, yet another short lived excited state $ i$exists. A population inversion takes place between this intermediate state $ i$and the metastable state $ m$, between which the lasing transition occurs.

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