POWER SATURATION EQUILIBRIUM

CONGRATULATIONS !

Quantum mechanics states that the probability of stimulated emission from an excited state is fundamentally equal to the probability of absorption from the lower level (except for electromagnetically induced transparency EIT). Therefore when the population in the upper state equals the population in the lower state, stimulated emission and absorption occur with equal probability.

Thus when a pumping beam of radiation is resonant with a particular transition, the populations of the two levels tend to become equal if the incoming radiation is very intense.

It is difficult to construct an optically pumped two level laser because the steady-state equilibrium distribution under intense optical pumping is equivalent to thermal contact with a very high temperature heat bath, the levels have an energy difference much smaller than the thermal energy of this fictitious heat bath and become almost equally populated.

However if we add a third level slightly below the upper level, it remains relatively unpopulated because the pump radiation is very selective and is not absorbed by it. Then a population inversion can occur between the upper level and this level. If the upper state decays rapidly to this third level then we can obtain a population inversion between the third level and the ground level (This is how the first optical ruby laser functions)

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