Conditions for Laser Action - Civil Engineering (CE) PDF Download

Conditions for Laser Action 
Let an atom in the excited state be stimulated by a photon of right energy so that atom makes stimulated emission. Two coherent photons are obtained. These two coherent photons

if stimulate two atoms in the exited state to make emission then four coherent photons are produced. These four coherent photons then stimulates 4 atoms in the excited state to produce 8 coherent photons and so on. As the process continues number of coherent photons increases. These coherent photons constitute an intense beam of laser. This phenomenon of building up of number of coherent photons so as to get an intense laser beam is called lasing action.

Population inversion and optical pumping:
In an order to produce laser beam there should be more number of stimulated emissions when compared to spontaneous emission. It is possible only if number of atoms in the exited states is greater than that is in the ground state. When system is in thermal equilibrium, then number of atoms in the higher energy level is always less than the number of atoms in the lower energy level. If by some means number of atoms in the exited slate is made to exceed number of atoms in the ground state then population inversion is said to have established between excited state and ground state. The method of achieving the population inversion is called pumping. If light is used to pump electrons to the higher energy level then the method is called Optical Pumping.

The essential Conditions for population inversion are
(1) Higher energy state should posses a longer life time
(2) The number of atoms in the lower energy state must be greater than the number of atoms in the higher energy state.

Metastable State: 
Population inversion can be created with the help of three energy levels as follows.
Let E₁ is the ground state of an atom. Let E₂ and E₃ are the two excited states. If an atom is excited into the energy state, within a short inter of time of 10⁻⁸ sec, atom makes a transition into the energy state E₂. Let lifetime of the atom in the energy level E₂ is of  the order of 10⁻² to 10⁻³ sec. Then atoms stay in the excited state E₂ for sufficiently long time without making any spontaneous emission.

As more and more atoms are excited from the ground state to E₃ more and more atoms are transferred from E₃ to E₂. As a result, within a short interval of time population inversion is established between energy level E₂ and E_1. The energy level E₂ in which atoms remain for unusually longer time is called Metastable state. When transition from E₃ to E₂ takes place excited atom looses energy in the form of heat without emitting any radiation. Such transitions are called radiation less transition (Non-radiative transitions).

Requisites of a Laser System 
The Three requisites of a Laser system are
(1) Energy Source or Excitation Source for Pumping action
(2) Medium Supporting population inversion called Active Medium
(3) The Laser Cavity

Appropriate amount of energy is to be supplied for the atoms in order excite them to higher energy levels. If the input energy is in the form of light energy then pumping is called optical pumping. If it is in the form of electrical energy then pumping is called electrical pumping.

Population inversion occurs at certain stage in the Active medium due to the absorption of energy. After this stage the Active medium is capable of Emitting laser light.

The Laser Cavity consists of an active medium bound between two mirrors. The Mirrors reflect the light to and fro through the active medium. This also helps to tap certain permissible part of laser energy from the active medium