Condition For Laser Action
Major condition of laser action:
The two conditions to achieve lasing actions are
1. Population inversion
2. Availability of metastable states
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
emission of a photon. Whereas, the electrons can stay in the metastable state as long as 10−3s. If the excited state happens to be a metastable state, the atoms can stay excited for longer duration resulting in steady increase in the population of the excited or metastable state and one stage we can achieve the population inversion. Once this happens, the number of stimulated emissions out number the spontaneous emissions. The photons from stimulated emission will have the same wavelength, phase and direction. Once the intensity of the photons is sufficient to pass through the partially silvered
mirror, we get the laser light.