Duduli

In order to achieve population inversion, we must have an energy state which has a long lifetime i.e electron can stay there for sufficiently long time such an energy state is called as metastable state.

A laser has a unique combination of coherence,monochromaticity, intensity directionality, which distinguishes it from other light sources. The multiplication of photons through stimulated emission results a coherent, powerful, monochromatic and collimated beam of light known as laser light.