GATE 2021 PH – Question 25
A three-level laser has levels 0, 1 and 2 as shown. Atoms are pumped from 0 to 2; laser emission occurs from 1 to 0. If $n_i$ and $\tau_i$ denote level populations and lifetimes, which conditions are necessary for laser action?
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Correct answer: (A) $\tau_1>\tau_2$; (B) $n_1>n_0$
Explanation
Pumped atoms must leave level 2 rapidly and accumulate in metastable level 1. Thus the upper laser level needs a longer lifetime than the pump level, $\tau_1>\tau_2$. Net stimulated emission on the laser transition requires population inversion between levels 1 and 0, here $n_1>n_0$. Neither a long lifetime of level 2 nor a larger population in level 2 is required. Select A and B.