Population inversion is a state of a system in which the population of an excited state is more than that of a ground state. Naturally, we find the number density to be more in the ground state than in the excited one, hence the name. We can achieve population inversion only in those systems which posses a special kind of excited state called metastable state. The electrons will remain only for about 10−8s in the excited state and after that they make transitions to the ground state by