(a) Optical Pumping: In this process the population inversion is achieved by using strong sourcen of light such as gaseous discharge, flash lamp or arc lamp. This process keeps more atoms in the upper excited (metastable) state. This process is suited to solid state (e.g. Ruby) and liquid (e.g., dye) lasers.
(b) Electrical Pumping: In this process the population inversion is achieved by using an intense electrical discharge in the medium which is in the gaseous form. This process is suited to gas lasers and semiconductors. The discharge converts the gas into plasma and the population of the upper level also
increases.