working and construction of ruby ,HE NE and CO2 LASERs

L
A
S
E
R
ight
mplification
timulated
mission
adiation
LASER
It is a device to produce a strong,
monochromatic, collimated and
highly coherent beam of light and
depends on the phenomenon of
“stimulated emission”.

2
INCANDESCENT VS. LASER
LIGHT
1. Many wavelengths
2. Multidirectional
3. Incoherent
1. Monochromatic
2. Directional
3. Coherent

06/03/2025
TCT
3
If an atom is initially in a lower energy state, it can rise
to a higher state 2 by absorbing a quantum of
radiation (Photon) of frequency n given by
Where E1 and E2 are the energies of the atom in
states 1 and 2 respectively. This process is known
as Absorption of Radiation.
Absorption of Radiation
h
E
E 1
2


ABSORPTION
OF
RADIATION

06/03/2025
TCT
4
The probable rate of occurrence of the
absorption transition 1 2 depends on the
properties of states 1 and 2 and is
proportional to the energy density m(n) of the
radiation of frequency n incident on the
atom. Thus
Where B12 is proportionality constant and is
known as Einstein’s coefficient of Radiation.
ABSORPTION
OF
RADIATION
N1

06/03/2025
TCT
5
E1
E2
hn
Absorption
of
Radiation
E2 E2
E1
E1
hn =E2-E1
Before After
Energy Level Diagram for Absorption

06/03/2025
TCT
6
SPONTANEOUS EMISSION
Consider an atom initially in the higher
(excited) state 2. Excited state with higher
energy is inherently unstable , hence the
atom in excited state does not stay for
longer time and it jumps to the lower
energy state 1 emitting a photon of
frequency n. This phenomenon is called
Spontaneous Emission of Radiation.
Spontaneous
Emission

06/03/2025
TCT
7
If there is an assembly of atoms , the radiation
emitted spontaneously by each atom has a
random direction & a random phase and is
therefore incoherent from one atom to another.
The probability of spontaneous emission 2 1
is determined only by the propeties of states 2
and 1. This is denoted by
A21which is known as Einstein coefficient of
Spontaneous Emission of Radiation . In this
case the probability of spontaneous emissions
is independent of it.
Spontaneous
Emission
𝑁𝑠𝑝=𝐴21 𝑁 2

06/03/2025
TCT
8
Before
E2
E1
hn =E2-E1
After
E2
E
1
Energy Level Diagram for Spontaneous Emission
hn
E1
E2
Spontaneous
Emission

06/03/2025
TCT
9
STIMULATED EMISSION
According to Einstein, an atom in an excited state
may, under the influence of the electromagnetic
field of a photon of frequency n incident upon it,
jumps to a lower energy state, emitting an
additional photon of same frequency (n).Hence,
two photons, one original and the other emitted,
move together. This is Stimulated Emission of
Radiation. The direction of propagation, phase,
energy and state of polarization of the emitted
photon is exactly same as that of the incident
stimulating photon, so the result is an enhance
beam of coherent light.
Stimulated
Emission

06/03/2025
TCT
10
The probability of stimulated emission transition
2 1 is proportional to the energy density m(n) of
the stimulating radiation and is given by
where B21 is the ‘Einstein’s coefficient of
Stimulated Emission of Radiation’.
Stimulated
Emission
N2

06/03/2025
TCT
11
Before
E2
E1
After
E2
E1
hn
hn
hn
Energy Level Diagram for Stimulated Emission
hn
E1
E2
06/03/2025
hn
hn
Stimulated
Emission

06/03/2025
TCT
12
In thermal equilibrium at temperature T, with
radiation frequency n and energy
density u(v). Let N1
and N2
be the number of
atoms in energy states 1and 2 respectively at any
instant. The number of atoms in state 1 absorb a
photon and give rise to absorption per unit time
For equilibrium Absorption = Emission
N1
B12
u(v) = N2
[A21
+ B21
u(v)]
1

06/03/2025
TCT
13
According to Boltzmann distribution law number of
atoms N1
and N2
in energy states E1
and E2
in thermal
equilibrium at temperature T are given by

06/03/2025
TCT
14
Substituting N1
/N2
in Eq (5.5), we get
Comparing it with Plank’s Radiation law

06/03/2025
TCT
15
We get
(i) ;B12
= B21,
The probability
of spontaneous emission is same as that of
induced absorption. This means that if these two
processes will occur at equal rates, so that no
population inversion can be attained in a two-
level system.
(ii) The ratio of spontaneous
emission and stimulated emission is proportional
to v3
. This implies that the probability of
spontaneous emission dominates over induced
emission more and more as the energy
difference between the two states increases.

06/03/2025
TCT
16
METASTABLE ENERGY LEVEL
When light of suitable wavelength falls on atoms, their electrons jump to a
higher energy state. When the incoming radiations are removed, the excited
electron goes back to its original level within a duration of 10−8
seconds.
However, when an electron goes to a metastable state, it remains there for
a relatively longer duration of 10−3
seconds. This phenomenon leads to
accumulation of electrons in the metastable state, since the rate of
addition of electrons to the metastable state is higher than the rate of their
de-excitation. Metatstable state are formatted by the adding impurities in
the pure material. .it is life time is 10-6
sec to 10-3
sec. This energy state
having long time period due to the los in the energy by which its time in
increased which can be explained by the below relation of Heisenberg’s
ΔE. Δt=h
Conduction
band
Valence
Band

Condition for the
Laser Operation
If n1 > n2
• radiation is mostly absorbed absorbowane
• spontaneous radiation dominates.
• most atoms occupy level E2, weak absorption
• stimulated emission prevails
• light is amplified
if n2 >> n1 - population inversion
Necessary condition:
population inversion
E1
E2

06/03/2025
TCT
18
METASTABLE ENERGY LEVEL
When light of suitable wavelength falls on atoms, their
electrons jump to a higher energy state. When the incoming
radiations are removed, the excited electron goes back to
its original level within a duration of 10−8
seconds. However,
when an electron goes to a metastable state, it remains
there for a relatively longer duration of 10−3
seconds. This
phenomenon leads to accumulation of electrons in the
metastable state, since the rate of addition of electrons to
the metastable state is higher than the rate of their de-
excitation. This leads to the phenomenon called population
inversion, which forms the basis of lasing action of lasers.

06/03/2025
TCT
19
PUMPING
 Optical pumping
 Electric discharge
 Inelastic atom-atom collisions
 Direct conversion
 Chemical reaction
The process of achieving population inversion
is known as “pumping” of atoms. Most
commonly used methods are as follows :
E1
E2

working and construction of ruby ,HE NE and CO2 LASERs

21
MAIN COMPONENTS OF A
LASER
1. Active Medium and Active Centre
The active medium may be solid crystals such as ruby or Nd:YAG, liquid
dyes, gases like CO2 or Helium/Neon, or semiconductors such as
GaAs. Active mediums contain atoms whose electrons may be excited
to a metastable energy level by an energy source. This atom are known
as active centres.
2. Excitation Mechanism (Pump)
Excitation mechanisms pump energy into the active medium by one or
more of three basic methods; optical, electrical or chemical.
3. High Reflectance Mirror
A mirror that reflects essentially 100% of the laser light.
4. Partially Transmissive Mirror
A mirror that reflects less than 50% of the laser light and transmits the
remainder.
Optical Resonator-
1.Two Mirror
arrangement
2.
length=nλ /2
3. Optical axis of
both mirror is
colinear with active
medium
Pump
Active Medium &
Active Centre
High Reflectance
Mirror

INTRODUCTION
.
The first laser to be operated successfully was ruby laser.
A ruby laser is a solid state laser that uses synthetic ruby crystal as its
gain medium.

LASER CONSTRUCTION
 Gain medium
 Ruby is a crystal of aluminium oxide (Al2O3) in which some of the
aluminion ions (Al3
+) are replaced by chromium ions (Cr3
+). This is
done by doping small amounts of chromium oxide (Cr2O3) in the
melt of purified Al2O3.
 These chromium ions give the crystal a pink or red color depending
upon the concentration of chromium ions. Laser rods are prepared
from a single crystal of pink ruby which contains 0.05% (by weight)
chromium. Al2O3 does not participate in the laser action
 The ruby crystal is in the form of cylinder. Length of ruby crystal is
usually 2 cm to 30 cm and diameter 0.5 cm to 2 cm

 Active medium or active center: Chromium ions act as active
centers in ruby crystal. So it is the chromium ions that
produce the laser.
 Pumping source: A helical flash lamp filled with xenon is used
as a pumping source Thus, optical pumping is used to
achieve population inversion in ruby laser.
 Optical resonator system: The ends of ruby crystal are
polished, grounded and made flat. Thus the two polished
ends act as optical resonator system.

WORKING
 Ruby Laser is based on three energy level. The upper
energy level E3 short lived , E1 ground state, E2
metastable state with life time of 0.003 sec.
 When a flash light falls on ruby rod it absorbs radiations of
5500 Å are absorbed by Cr3+ which are pumped to E3.
 The ions after giving their energy to crystal lattice decay to
E2 state undergoing radiation less transition.
 The spontaneous emission of Cr3+ ion at E2 level initiates
the stimulated emission by other Cr3+ ion in metastable
state.

In metastable state, the concentration of ions in E2 increases, while that of E1
decreases.Hence, population inversion is achieved.
And laser action is produced between the E2 and E1 of wavelength 6934 Å
which having red color
Metastable
state
E3
E2
E1
5500 Å 6934 Å

APPLICATIONS
 Ruby laser have declined in use with the discovery
of better lasing media. They are still used in a
number of applications where short pulses of red
light are required.
 Many non-destructive testing labs use ruby laser
to produce holograms of many large objects such
as aircraft tires to look for weaknesses in the
lining.
 Ruby laser is extensively used in tattoo and hair
removers.

Laser beam used for LASIK procedure.
Removal of Tattoos using laser.

DRAWBACKS OF RUBY LASER
 As the terminus of laser action is the ground state,
it is difficult to maintain the population inversion.
This fact results in ruby laser’s low efficiency.
 The ruby laser requires high power pumping
source.
 The laser output is not continuous but occurs in
the form of pulses of microsecond duration.
 The defects due to crystalline imperfection are
also present in ruby laser.

ND:YAG LASER
Nd: YAG is a solid state laser four level laser.

Lasing medium :
The host medium for this laser is Yttrium Aluminium Garnet (YAG = Y3
Al5
O12
)
with 1.5% trivalent neodymium ions (Nd3+
) present as impurities.
The (Nd3+
) ions occupy the lattice sites of yttrium ions as substitutional
impurities and provide the energy levels for both pumping and lasing transitions.

 The length of the Nd: YAG laser rod various from 5cm to 10cm
depending on the power of the laser and its diameter is
generally 6 to 9 mm.
 The laser rod and a Krypton flash lamp are housed in a
elliptical reflector cavity
 Since the rod and the lamp are located at the foci of the ellipse,
The light leaving one focus of the ellipse will pass through the
other focus after reflection from the silvered surface of the
reflector. Hence the entire flash of light gets focused on the
laser rod.
 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

• When the flash tube switch on a flash is generated and after the
reflection from the cavity this focused on the Nd:YAG rod.
• The ground state (E0 ) 3+ ion go to higher energy band E3&E4
𝑁𝑑
by optical pumping through the absorption of wavelengths of
7300Å and 8000 Å light .
• As E3&E4 is unstable exited state life time of E3&E4 is very less.
So, 3+ in E3&E4 go to energy level E2 rapidly by spontaneous
𝑁𝑑
emission.
• As E2 is metastable state. It is population very soon and does
population inversion is achieve between E2 and E1.
• Now when external photon is insider 3+ ion they go to energy
𝑁𝑑
level E1 by stimulated emission.
• During the stimulated emission from E2 to E1 light amplification
also take place and so, the light produced is form of laser beam of
wavelength 1.064µm.
• The 3+ ion in energy level E1 goes to ground state E0 by
𝑁𝑑
spontaneous emission
WORKING

Nd:YAG/ Nd: Glass laser applications :
These lasers are used in many scientific
applications which involve generation of other
wavelengths of light.
The important industrial uses of YAG lasers
have been in materials processing such as
welding, cutting, drilling.
In medical use as YAG beams penetrate the
lens of the eye to perform intracular procedures.
YAG lasers are used in military as range finders
and target designators.

Historical facts
The Helium-Neon laser
was the first continuous
laser.
It was invented by Ali
Javan, W.Bennet in 1961.

Introduction
 A helium-neon laser, usually called a He-Ne laser, is a
type of small gas laser. HeNe lasers have many industrial
and scientific uses, and are often used in laboratory
demonstrations of optics.
 He-Ne laser is a four-level laser.
 Its usual operation wavelength is 6328 A, in the red
portion of the visible spectrum.
 It operates in Continuous Working (CW) mode.

Construction of He-Ne laser
 The setup consists of a discharge tube of
length 80 cm and bore diameter of 1.5cm.
 The gain medium of the laser, as suggested
by its name, is a mixture of helium and neon
gases, in a 5:1 to 20:1 ratio, contained at low
pressure (an average 50 Pa per cm of cavity
length ) in a glass envelope.
 The energy or pump source of the laser is
provided by an electrical discharge of around
1000 volts through an anode and cathode at
each end of the glass tube. A current of 5 to
100 mA is typical for CW operation.

 The optical cavity of the laser typically consists of a
plane, high-reflecting mirror at one end of the laser tube,
and a concave output coupler mirror of approximately 1%
transmission at the other end.
 HeNe lasers are normally small, with cavity lengths of
around 15 cm up to 0.5 m.

He-ne energy level diagram
 He*(23
S1) + Ne1
S0 → He (1
S0) + Ne*4s2 + ΔE
 He*(21
S) + Ne1
S0 + ΔE → He (1
S0) + Ne*5s2

Energy
Helium Neon
Excitation by
Electron collision
He-Ne
collision
Ground State
Diffusion
to walls
Fast Radiative
transition
3.39 mm
IR
632.8 mm
Visible
1.15 mm
IR
20.61eV
21
s
23
s
19.82eV
1s
2s
3s
20.66eV
19.78eV
20.30eV
18.70eV
3p
2p

Applications of He-Ne
laser
 The Narrow red beam of He-Ne laser is used in supermarkets to read
bar codes.
 The He- Ne Laser is used in Holography in producing the 3D images
of objects.
 He-Ne lasers have many industrial and scientific uses, and are often
used in laboratory demonstrations of optics.

ADVANTAGE
 He-Ne laser has a very good coherence property.
 Cost He-Ne laser is less for most of the laser.
 Construction of He-Ne laser is also not very complex.
 He-Ne laser provide inherent safety due to low power
output.
 He-Ne laser tube has very small length approximately
from 10 to 100 cm and best life time of 20,000 hours.
 He-Ne laser can produce three wavelength that are
1.152 μm , 3.391 μm and 632.8 nm in which the
632.8 nm is most common because it is visible
usually in red colour.

DIS ADVANTAGE
 He-Ne laser is low gain system/device
 It is relatively low power device means its output is low.
 To obtain single wavelength laser, the other two wavelength of laser need
suppression, which is done by many techniques and device, So it is require
extra technical skills and increase the cost also.
 High voltage requirement can be considered its disadvantage.
 Escaping of gas from laser plasma tube is also its disadvantage

06/03/2025 TCT 49
CO2 LASER
The carbon dioxide laser (CO2 laser) was one of the
earliest gas lasers to be developed (invented by Kumar
Patel of Bell Labs in 1964), and is still one of the most
useful.
Carbon dioxide lasers are the highest-power continuous
wave lasers that are currently available.
They are also quite efficient: the ratio of output power to
pump power can be as large as 20%.
The CO2 laser produces a beam of infrared light with the
principal wavelength bands centring around 9.4 and
10.6 micrometers.
CO
2
LASER

06/03/2025 TCT 50
CO2 is a four level molecular laser which uses the
transitions that occurred between different
vibrational states of the carbon dioxide molecule.
Carbon dioxide molecule consists of a central
carbon atom with two oxygen atoms attached on
either side. There are three independent modes of
vibration of the carbon dioxide molecule :
 Stretch mode
 Bending mode
 Asymmetric mode
 Rotational Mode
CO
2
LASER

06/03/2025 TCT 51
CO
2
LASER
O O
C
Stretch Mode
Bending Mode
Asymmetric Mode
Rotational Mode
VIBRATIONA
L MODES OF
CO2

52
Construction
The device consists of a discharge tube having a bore of
cross-section of about 1.5 mm2
and a length of about 260
mm.
The discharge tube is filled with a mixture of CO2 , N2 and
He gases in the ratio of 1: 2 : 3. Some water vapours are
also added .
The active centres are CO2 molecules & the lasing occurs
on the transitions between the vibrational levels of the
electronic ground state.
Since, the heat produced in CO2 laser is immense, water is
used as a coolant and water inlet & outlet ports are thus
provided in the vessel that surrounds the discharge tube.
CO
2
LASER

06/03/2025 TCT 53
Apparatus
CO
2
LASER

54
Working
When current passes through the mixture of gases then N2
molecule get excited to the metastable state .
These excited N2 molecule cannot spontaneously lose energy
so at that level number of N2 molecule increases.
These N2 molecule return to the ground state through inelastic
collisions with the ground state CO2 molecules. Due to this
CO2 molecules are excited to E5 level.
The CO2 molecules are also excited to E5 level through
collision with electrons. The (020) and (100) states marked
as E3 and E4 levels act as lower levels.
CO
2
LASER

The population inversion is achieved between E5 level and
the levels E4 and E3.
The transition between E5 level to E4 level produces for
infrared radiation at the wavelength 10.6 m and between E5
level to E3 at wavelength 9.6 m.
The levels E3 and E4 are also metastable states and CO2
molecules fall to lower level E2 through inelastic collision
with unexcited CO2 molecules at the lower laser level
inhibits the laser action.
The presence of He with CO2 decreases the population at
level E3 by colliding and the lasing action continues. This
laser also works in continuous mode (CW) and has
efficiency upto 45%.

06/03/2025 TCT 56
The Population Inversion in the laser is achieved by the following
sequence:
 Electron impact excites vibrational motion of the nitrogen. Because
nitrogen is a homonuclear molecule, it cannot lose this energy by
photon emission, and its excited vibrational levels are therefore it’s
a metastable and live for a long time.
 Collisional energy transfer between the nitrogen and the carbon
dioxide molecule causes vibrational excitation of the carbon
dioxide, with sufficient efficiency to lead to the desired population
inversion necessary for laser operation.
 The nitrogen molecules are left in a lower excited state. Their
transition to ground state takes place by collision with cold helium
atoms. The resulting hot helium atoms must be cooled in order to
sustain the ability to produce a population inversion in the carbon
dioxide molecules. In sealed lasers, this takes place as the helium
atoms strike the walls of the container. In flow-through lasers, a
continuous stream of CO2 and nitrogen is excited by the plasma
discharge and the hot gas mixture is exhausted from the resonator
by pumps.
CO
2
LASER

06/03/2025 TCT 57
0.3--
0.2--
0.1--
0
Nitrogen Carbon dioxide
E5
E4
E3
E2
E1
Energy transfer
through
Collisions
10.6 mm
9.6 mm
(001)
(100)
(020)
(010)
CO
2
LASER
Energy
(eV)

06/03/2025 TCT 58
CO
2
LASER
0.3--
0.2--
0.1--
0
Nitrogen Carbon dioxide
E5
E4
E3
E2
E1
Energy transfer
through
Collisions
10.6 mm
9.6 mm
(001)
(100)
(020)
(010)
Energy
(eV)
On the impact
of
He atoms

06/03/2025 TCT 59
Applications
 Because of the high power levels available
(combined with reasonable cost for the laser),
CO2 lasers are frequently used in industrial
applications for cutting and welding, while
lower power level lasers are used for engraving.
CO
2
LASER

06/03/2025 TCT 60
 Because the atmosphere is quite
transparent to infrared light, CO2 lasers
are also used for military range
finding using LIDAR techniques.
CO
2
LASER

06/03/2025 TCT 61
 They are also very useful in surgical procedures
because water (which makes up most biological
tissue) absorbs this frequency of light very well.
Some examples of medical uses are laser
surgery, skin resurfacing ("laser facelifts")
(which essentially consist of burning the skin to
promote collagen formation),
and dermabrasion.
CO
2
LASER

06/03/2025 TCT 62
 They are profitably used in almost every field including
fundamental research.
 It is used during the manufacture of electronc circuits on silicon
chips.
 They are used in CD players, laser printers, fascimile
machines,etc.
 High Power CO2 Lasers are used to bring about thermonuclear
reactions which would become the ultimate inexhaustable
power source for human civilization.
 They are used for separating various isotopes of an element.
 They are used in Holography.
&
Much More
CO
2
LASER

working and construction of ruby ,HE NE and CO2 LASERs

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