Question

Discuss various types of pumping that can be used in lasers.            [10]

Answer 1

Pumping in laser is the process to achieve population inversion.There are a number of ways to achieve population inversion.They are

1.Optical Pumping-In this polpulation inversion is achieved by an external light source.Here photons of right frequency raise the groud state atoms to the excited state .These atoms in the excited state decays spontaneously to the desired metastable state and metastable state has greater lifetime than ground state. This achieves the majority atoms in the higher energy states.In optical pumping as pumping source flash lamps or arc lamps are commonly used. Solid state lasers (ruby laser) uses optical pumping mechanism .

2.Electrical Pumping-In this a high voltage electric discharge acts as puping source.When this electric discharge passes through the laser medium the highly accelerated electron collides with the atoms of the medium .As a result of this atoms of the medium in the lower enegy state excite to the higher energy state.These excited atoms jump to metastable state of higher lifetime .Thus most of the atoms concentrated in the higher energy state and hence achieve population inversion.This method is used in gas lasers like Helium-Neon laser.

3.Chemical Pumping-Some chemical reactions can produce atoms or molecules in its excite state more than the normal state atoms or molecules.These kind of reactions can be used to produce population inversion .For exampl hydrogen fluride molecule produced in an excited state when hydrogen and flurine combine chemically.

4.Gas Dynamic Pumping-In this method a gas at high pressure is heated to high temperature.Then this gas is allowed to expand through a nozzle .This expansion occures at supersonic velocities.This hot gas contains many molecules in the higher energy states.Since the gas undergoes adiabatic cooling upper state molecules take much longer time to deexcite to equilibrium state than lower energy molecules.Thus majority of the gas molecule will be in higher energy state to achieve population inversion.

5.Thermal energy pumping-Here population inversion is achieved in heating the laser medium .On heating the medium lowe energy atoms get sufficient energy to excite to the higher energy state.