Laser Notes
Laser Notes
Laser Notes
Lasers
By
Ø What is Laser?
Ø Properties of Laser
interacts
Radiation matter
Spontaneous Stimulated
Absorption
Emission Emission
Absorpti Atom be initially in the lower state E1, If a
on photon of energy h ν is incident on the
atom in the lower state, the atom absorbs
the incident photon and gets excited to the
higher energy state E2.
E2
E=hν
E2
hν=E2/ E1
hν
Inducing hν=E2/ E1
photon)
Where, B21 is the
E1 proportionality constant,
N2 is population of the
higher energy level, R21 is
Rate of stimulated
Atom*+Photon Atom+(Photon+Photon)
emission.
Necessary condition for lasing transition
ØStimulated Emission
3- state system
ØPopulation Inversion
E3
(N2 > N1)
ØMetastable State
E2
E1
Population inversion: The redistribution of atomic energy levels
that takes place in a system so that laser action can occur. Normally, a
system of atoms is in temperature equilibrium and there are always
more atoms in low energy states than in higher ones. Although
absorption and emission of energy is a continuous process, the
statistical distribution (population) of atoms in the various energy
states is constant. When this distribution is disturbed by pumping
energy into the system, a population inversion will take place in which
more atoms will exist in the higher energy states than in the lower.
Ø Liquid Lasers
Ø Dye Lasers
Ø Semiconductor Lasers
Essential components of a laser system :
Active medium or Gain medium : It is the system in which
population inversion and hence stimulated emission (laser action) is
established.
Active Pumping Optical
Medium Mechanism resonator
Pumping Source:
-The principle behind Nd-YAG laser is optical pumping. The population
inversion is achieved by a flash light either using xenon or krypton
flash tube. As a result, Nd ions are transported into the excited levels.
Resonating Cavity:
- In the Nd-YAG laser, a rod of 5 to 10 cm length and 6 to 9 mm
diameter is used. The ends of the rod are polished and made optically
flat and parallel. The optical cavity is formed either by silvering the two
ends of the rod or by using two external reflecting mirrors. One mirror
is made hundred percent reflecting while the other mirror is left
slightly transmitting to draw the output
This laser system has two absorption bands (0.73 m and 0.8
m)
Optical pumping mechanism is employed.
Laser transition takes place between two laser levels at 1.06 mm.
Working:
When flash lamp is switched on, neodymium ions acquire energy from
the flash light. The Nd ions are excited to energy levels E3 and E4 by
absorbing energy with wavelengths of respectively 0.73 µ m and 0.80
µm.
The Nd ions are not stable in the excited state, it makes a non –
radiactive transition from E3 and E4 to a metastable state E2 . Ions can
stay for a long time in this state until population inversion is achieved.
When population inversion is achieved between E2 and E1 state, a
stimulated emission takes place from the energy levels E2 to E1 by
emitting radiation of the wavelength 1.064 µm.
Nd:YAG applications :
The important industrial uses of YAG and glass lasers have been in
materials processing such as welding, cutting, drilling.
Since 1.06 m wavelength radiation passes through optical fibre
without absorption, fibre optic endoscopes with YAG lasers are
used to treat gastrointestinal bleeding.
Remote sensing applications
Medicines for endoscopic applications, medical surgery, dental
surgery etc.
CO2 LASER
Introduction :
CO2 lasers belong to the class of molecular gas lasers.
In the case of atoms, electrons in molecules can be excited
to higher energy levels, and the distribution of electrons in the
levels define the electronic state of the molecule.
Besides, these electronic levels, the molecules have other
energy levels.
2
7
Symmetric stretching mode (m)
Optical resonators :
A pair of concave mirrors placed on either side of the
discharge tube, one completely polished and the other partially
polished.
2
9
Pumping :
Population inversion is created by electric discharge of the
mixture.
When a discharge is passed in a tube containing CO2,
electron impacts excite the molecules to higher electronic and
vibrational-rotational levels.
This level is also populated by radiationless transition from
upper excited levels.
The resonant transfer of energy from other molecules, such
as, N2, added to the gas, increases the pumping efficiency.
3
0
He
NaCl
window
Electrode
Electrode
DC
excitation
CO2 LASER
3
1
4
V ib ra tio n a l
m
e n e rg y tra n s fe r
m
3
2 n n
iot oit
ait ait
c cx
x
E E
y
gr
e
n 1
E
C O 2 G ro u n d S ta te N 2 G ro u n d S ta te
3
3
Disadvantages:
Advantages:
3
4
The Application of Lasers
Ø Science and engineering applications
precise measurements and spectroscopy
Fiber optic communication
Communication between planets is possible with Laser
used in Holography
Ø In Medicine
for the treatment of detached retinas
eye surgery and in cancers treatments
Ø In Industry
cutting, welding, melting
To test the quality of the materials
for the heat treatment of metallic and non- metallic
materials.
Military applications:
Principle: