Depending upon the type of laser, the most commonly used pumping methods are listed below:
a) Optical pumping: In this, the population inversion is achieved by means of light energy delivered from appropriate pumping source such as gaseous discharge or flash tubes. For example, in ruby laser, xenon flash tube is used.
b) Electric discharge: pumping: this type of pumping accomplished by means of intense electrical discharge in the medium and is particularly suited to gas media like He-Ne laser and CO2 laser. The electric discharge coverts the gas into a plasma where active centers collide inelastically with free electrons and population inversion is achieved.
c) Chemical pumping: It raises active centers into the higher levels by means of suitable exothermal chemical reactions in the active medium.
d) Heat pumping: In this type of pumping, the active material is first brought to a high temperature then rapidly cooled down.
Note: This topic is referred from my book entitled “Optics and Lasers”.