Question

why the population inversion is not possible for two level laser ??

Answer 1

The energy being used to pump the atoms into the upper laser state has an equal probability of stimulating them back down.Therefore it is not possiblein general to pump more than half of the atoms into the excited state. ... And since population inversion is a necessity for lasing action, we don't have two levellasers.

Answer 2

Answer:

It is not possible to achieve population inversion with optical pumping in a simple two-level system

Explanation:

Because the system can absorb pump light (i.e., gain energy) only as long as population inversion, and thus light amplification, is not achieved.

No matter how much we pump the atoms in the ground state, we cannot achieve population inversion in a two-level LASER. We don't have two level lasers because population inversion is required for laser action.

Because a population inversion cannot be achieved in a two-level scheme of operation, lasing activity cannot be made to occur with only two energy levels. Because of the Boltzmann distribution, the population of the upper of the two levels is less than that of the lower in a state of thermal equilibrium.