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POPULATION INVERSION AND PUMPING PROCESS

 population inversion

It is defined as the condition which is obtained in stimulated emission of radiation , the number of atoms In the metastable state is greater than the number of atoms in the ground state.This condition is necessary for producing a powerful laser , because maximum number of atoms in the metastable state must be stimulated to jump to the ground level.


Lasing action:

It is a chain process during stimulated emission in which every atom emits two identical photons and finally a powerful narrow beam of laser is released.
To start the lasing process the condition of population inversion must be obtained. That is the number of atoms in meta stable state must be greater than that of ground level.
In this process an external photon of selected wavelength incidents on an atom in meta stable state. Then the atom is stimulated to jump down to lower energy level by emitting two identical photons. These two photons are exactly equal to the incident photon.The two photons again incident on two more atoms and stimulates. The two atoms jump to lower energy level by emitting two photons each. Then the process is continued as a chain reaction until all the atoms in meta stable state reach the ground level. A powerful beam of laser is released between meta stable state and ground state.Each atom releasing two photons of energy hc/λ

Optical pumping:

The method of exciting the atoms of substance from ground energy state to a higher energy state is known as pumping. In optical pumping method, an optical system like a flash or discharge through a gas is made to incident on the atoms to get excited to a higher energy level.
Then some of the excited atoms jump to a meta stable state which is selected between ground and excited levels. The remaining atoms reach ground state. These atoms will be pumped back to excited state using optical system.
The transition from excited state E2 to meta stable state E3 is non radiative. The life time of atoms in the meta stable state is of 10^-6 to 10^-3 seconds which is nearly 1000 times greater than that of excited state.
After some time the number of atoms in the meta stable state is greater than that of ground state and the condition of population inversion is created between the ground and meta stable states. Then an external photon of energy equal to hc/λ is made to incident on an atom in the metastable state. The photon stimulates the atom to jump to ground state releasing two identical photons , each of energy hc/λ. Then the lasing action is started and finally a laser of energy equal to hc/λ is released between meta stable and ground state.

 


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