Population inversion, Pumping method,He- Ne laser & Application of laser
Population Inversion
Normally the population density of electron is more in the
lower energy level than the higher energy level. i.e., Na >
Nb
The process of achieving greater population density of electrons
in the higher energy level as compared to lower energy level is called
population inversion.
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Pumping method
The process by which atoms are raised from lower energy
level to the upper energy level so that necessary population inversion is
obtained is called pumping. There are various methods of laser pumping:-
i)
Optical Pumping - To raise the electron from lower
energy level to higher energy level, if we use light energy (Photon) then this
pumping is known as optical pumping.
ii) Direct electron excitation - In a discharge tube when a
potential difference is applied, then electrons emit & these electrons
collide with electrons of active medium & raise it to higher energy level. This
produces the required population inversion. This method is called direct electron
excitation.
iii) Inelastic atom-atom collision - One type of atom raised
to their excited state; this atom Collide with another type of atoms. After
collision another atom reached to the higher energy level. This method is
called inelastic atom-atom collision. E.g. - He-Ne laser.
iv) Chemical conversion - In active liquid medium, the
energy for raising the electrons from lower level to higher level is provided
by adding chemical in liquid medium; this method of population inversion in
known as chemical conversion.
v) Direct conversion - In active medium, the energy for
raising the electrons from lower energy level to higher energy level is
provided by direct electrical energy. This method of population inversion is
known as direct conversion.
Most
of the laser based on it. It is the best way of population inversion.
He-Ne laser
Construction
A
discharge tube with completely filled with the mixture of He & Ne gases is
taken, one fully silvered plane mirror is fixed at the left side of tube &
partially silvered plane mirror is fixed on the right side. An arrangement is
made for supplying electrons in the discharge tube to He & Ne mixture by
means of potential difference.
Working
When electric potential is applied to the gas mixture of He
& Ne (Ratio - 10:1); electrons are accelerated in the discharge tube. These
accelerated electrons collide with He atoms & excited them to higher energy
level (let me say F2 & F3).
When He atoms in levels F2 & F3 collide with the Ne atoms
in the ground state E1, then energy exchange takes place & thus results in
the excitation of Ne atoms to the level E4 & E6 & de-excitation of the
He atoms to the ground level F1. Therefore the electric discharge through the
gas mixture continuously populates the Ne excited levels E4 & E6. This
helps to create a state of population inversion between the level E4 (or E6)
& lower energy level E5 & E3.
The following three transition will occur:-
- E6 to E5 with laser 3.39 micro meter.
- E6 to E3 with laser 6228 Å
- E4 to E3 with laser 1.15 micro meter.
Fully silvered mirror shows reflection for small wave length
laser (i.e., 3.39 & 1.15 micro meter). Thus photon of these wavelength is
eliminated & only 6328 Å emerges
through the partially silvered mirror.
Application of laser
a) Medicine - Eye Surgery vision correction, tattoo
removal, hair removal etc.
b) Military - In few cases laser are used as a
weapon.
c) Communication - Optical fiber communication;
used for long distance optical data transmission.
d) Industry - Cutting, drilling, welding, soldering
etc.
e) Holography - In recording and reconstruction of
image in holography.
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