• ESSENTIAL COMPONENTS OF A LASER;
• TYPES OF LASER,
• CO 2 LASER,
• Nd – YAG LASER (Doped Insulator laser),
UNIT III Lecture 3 1
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 mechanism : It is the mechanism by which
population inversion is achieved.
i.e., it is the method for raising the atoms from lower
energy state to higher energy state to achieve laser
UNIT III Lecture 3 2
DIFFERENT PUMPING MECHANISMS :
i. Optical pumping : Exposure to electromagnetic radiation of
frequency = (E2-E1)/h obtained from discharge flash tube
results in pumping Suitable for solid state lasers.
ii. Electrical discharge : By inelastic atom-atom collisions,
population inversion is established.
Suitable for Gas lasers
iii. Chemical pumping : By suitable chemical reaction in the
active medium, population of excited state is made higher
compared to that of ground state Suitable for liquid lasers.
iv. Optical resonator : A pair of mirrors placed on either side
of the active medium is known as optical resonator. One
mirror is completely silvered and the other is partially
silvered. The laser beam comes out through the partially
UNIT III Lecture 3 3
Types of Lasers(Based on its pumping action) :
•Optically pumped laser
•Electrically pumped laser
• Basis of the operation mode
•Continuous wave Lasers
According to their wavelength :
•Visible Region, Infrared Region, Ultraviolet Region, Microwave
Region, X-Ray Region and etc.,
According to the source :
•Dye Lasers, Gas Lasers, Chemical Lasers, Metal vapour
Lasers, Solid state Lasers, Semi conductor Lasers and other
UNIT III Lecture 3 4
gain Operation Pump
medium wavelength(s) source
390-435 nm (stilbene), spectroscopy,
460-515 nm (coumarin birthmark removal,
102), 570-640 nm isotope separation. The
(rhodamine 6G), many tuning range of the
others laser depends on which
dye is used.
UNIT III Lecture 3 5
MEDIUM PUMP SOURCE APPLICATIONS AND NOTES
632.8 nm (543.5 nm, 593.9 Interferometry, holography,
nm, 611.8 nm, 1.1523 μm, Electrical discharge spectroscopy, barcode scanning,
1.52 μm, 3.3913 μm) alignment, optical demonstrations.
454.6 nm, 488.0 nm, 514.5
Retinal phototherapy (for diabetes),
Argon nm (351 nm,457.9 nm,
Electrical discharge lithography, confocal microscopy,
laser 465.8 nm, 476.5 nm, 472.7
pumping other lasers.
nm, 528.7 nm)
416 nm, 530.9 nm, 568.2 Scientific research, mixed with argon
nm, 647.1 nm, 676.4 nm, Electrical discharge to create "white-light" lasers, light
752.5 nm, 799.3 nm shows.
Many lines throughout
visible spectrum extending Electrical discharge Scientific research.
into the UV and IR.
UNIT III Lecture 3 6
PUMP SOURCE APPLICATIONS AND NOTES
Pumping of dye lasers, measuring air
Nitrogen pollution, scientific research. Nitrogen lasers
337.1 nm Electrical discharge
laser can operate superradiantly (without a
Transverse (high power) or
Carbon Material processing (cutting, welding, etc.),
10.6 μm, (9.4 μm) longitudinal (low power)
dioxide laser surgery.
Material processing (engraving, welding,
monoxide 2.6 to 4 μm, 4.8 to 8.3 μm Electrical discharge
etc.), photoacoustic spectroscopy.
Excimer 193 nm (ArF), 248 nm (KrF), Excimer recombination via Ultraviolet lithography for semiconductor
laser 308 nm (XeCl), 353 nm (XeF) electrical discharge manufacturing, laser surgery
UNIT III Lecture 3 7
MEDIUM AND PUMP SOURCE APPLICATIONS
Chemical reaction Used in research for laser
2.7 to 2.9 μm for
in a burning jet of weaponry by the U.S. DOD,
Hydrogen Hydrogen fluoride
ethylene and operated in continuous wave
fluoride laser (<80% Atmospheric
nitrogen trifluoride mode, can have power in the
(NF3) megawatt range.
~3800 nm (3.6 to MIRACL, Pulsed Energy
4.2 μm) (~90% chemical reaction Projectile & Tactical High Energy
Atm. transmittance) Laser
Laser weaponry, scientific and
Chemical reaction materials research, laser used in
COIL (Chemical 1.315 μm (<70%
in a jet of singlet the U.S. military's Airborne laser,
delta oxygen and operated in continuous wave
iodine mode, can have power in the
UNIT III Lecture 3 8
LASER GAIN OPERATION
MEDIUM AND WAVELENGT APPLICATIONS
Printing and typesetting applications,
441.563 nm, fluorescence excitation examination (ie. in
325 nm U.S. paper currency printing), scientific
vapor laser Electrical research.
567 nm, 615 metal vapor Rare, scientific research, amateur laser
nm mixed with construction.
up to 24 gas.
wavelengths Rare, scientific research, amateur laser
between red construction.
510.6 nm, Dermatological uses, high speed
Copper vapor laser
578.2 nm photography, pump for dye lasers.
discharge Rare, dermatological and photodynamic
Gold vapor laser 627 nm
UNIT III Lecture 3 9
SOLID STATE LASERS
Holography, tattoo removal. The first type of
Ruby laser 694.3 nm Flashlamp
visible light laser invented; May 1960.
Material processing, rangefinding, laser target
designation, surgery, research, pumping other
(combined with frequency doubling to
1.064 μm, produce a
Nd:YAG laser laser
(1.32 μm) green 532 nm beam). One of the most
power lasers. Usually pulsed (down to
UNIT III Lecture 3 10
MEDIUM OPERATION PUMP
AND WAVELENGTH(S) SOURCE
2.94 μm p, laser Periodontal scaling, Dentistry
Mostly used for continuous
pumping of mode-locked
laser Ti:sapphire or dye lasers, in
orthovanad 1.064 μm
diode combination with frequency
doubling. Also used pulsed for
marking and micromachining.
UNIT III Lecture 3 11
LASER GAIN OPERATION
MEDIUM AND WAVELENGT APPLICATIONS
Nd:YCOB is a so called "self-frequency
doubling" or SFD laser material which is
~1.060 μm both capable of lasing and which has
(~530 nm nonlinear characteristics suitable for
oxoborate laser diode
at second second harmonic generation. Such
harmonic) materials have the potential to simplify
)3 or simply
the design of high brightness green
Used in extremely high power (terawatt
scale), high energy (megajoules)
multiple beam systems for inertial
glass glasses), Flashlamp,
confinement fusion. Nd:Glass lasers are
(Nd:Glass) ~1.054 μm laser diode
usually frequency tripled to the third
harmonic at 351 nm in laser fusion
UNIT III Lecture 3 12
LASER GAIN OPERATION
MEDIUM AND WAVELENG APPLICATIONS
Spectroscopy, LIDAR, research.
This material is often used in
sapphire 650-1100 Other
infrared lasers to produce
(Ti:sapphire) nm laser
ultrashort pulses and in amplifier
lasers to produce ultrashort and
UNIT III Lecture 3 13
strontium(or Frequency quadrupled Remote
calcium) Nd:YAG laser pumped, atmospheric
~280 to 316 nm
aluminum excimer laser pumped, sensing, LIDAR,
fluoride copper vapor laser pumped. optics research.
doped Typically tuned in Flashlamp, laser diode, Dermatological
chrysoberyl the range of 700 mercury arc (for CW mode uses, LIDAR, laser
(alexandrite) to 820 nm operation) machining.
UNIT III Lecture 3 14
SEMICONDUCTOR LASERS :
medium and Applications
Semiconductor 0.4-20 μm,
laser diode depending on
Electrical weapons, machining,
(general active region
current welding, pump sources
for other lasers.
GaN 0.4 μm Optical discs.
UNIT III Lecture 3 15
Optical discs, laser
communications. 780 nm
Compact Disc player laser
AlGaAs 0.63-0.9 μm
is the most common laser
type in the world. Solid-
state laser pumping,
Electrical machining, medical.
InGaAsP 1.0-2.1 μm solid-state laser pumping,
Vertical cavity 850 - 1500 nm,
surface emitting depending on Telecommunications
laser (VCSEL) material
Hybrid silicon laser Mid-infrared Research
UNIT III Lecture 3 16
OTHER TYPES OF LASERS :
Laser gain Operation
medium wavelength(s Pump source Applications
and type )
100 nm -
Free several mm);
relativistic electron research, material
electron one free
beam science, medical
laser electron laser
tunable over a
UNIT III Lecture 3 17
Lasing in ultra-hot of a sub–10 nm X-
samarium plasma ray laser, possible
formed by double applications in high
pulse terawatt scale resolution
"Nickel-like" X-rays at 7.3 irradiation fluences microscopy and
Samarium nm created by holography,
laser wavelength Rutherford operation is close to
Appleton the "water window"
Laboratory's at 2.2 to 4.4 nm
Nd:glass Vulcan where observation of
laser. DNA structure and
the action of viruses
and drugs on cells
can be examined.
UNIT III Lecture 3 18
Complete 1-2 μm
Raman Other laser, mostly
1-2 μm for signal amplification
scattering in Yb-glass fiber
fiber version for
a nonlinear lasers
; optical solitons
UNIT III Lecture 3 19
CO2 lasers belong to the class of molecular gas
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
Besides, these electronic levels, the molecules have
other energy levels.
C.K.N. Patel designed CO2 laser in the year 1964.
UNIT III Lecture 3 20
Active medium :
It consists of a mixture of CO2, N2 and helium or
water vapour. The active centres are CO2 molecules
lasing on the transition between the rotational levels
of vibrational bands of the electronic ground state.
Optical resonators :
A pair of concave mirrors placed on either side of
the discharge tube, one completely polished and the
other partially polished.
UNIT III Lecture 3 21
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.
UNIT III Lecture 3 22
Nitrogen here plays the role that He plays in He-
A carbon dioxide (CO2) laser can produce a
continuous laser beam with a power output of
several kilowatts while, at the same time, can
maintain high degree of spectral purity and spatial
In comparison with atoms and ions, the
energy level structure of molecules is more
complicated and originates from three sources:
electronic motions, vibrational motions and
UNIT III Lecture 3 23
Fundamental Modes of vibration of CO2 :
Three fundamental modes of vibration for
Symmetric stretching mode (frequency 1),
Bending mode (2) and
Asymmetric stretching mode (3).
In the symmetric stretching mode, the oxygen
atoms oscillate along the axis of the molecule
simultaneously departing or approaching the
carbon atom, which is stationary.
UNIT III Lecture 3 24
In the ‘bending mode’, the molecule ceases to be
exactly linear as the atoms move perpendicular to the
In ‘asymmetric stretching’, all the three atoms
oscillate: but while both oxygen atoms move in one
direction, carbon atoms move in the opposite
The ‘internal vibrations’ of carbon dioxide molecule
can be represented approximately by linear
combination of these three normal modes.
UNIT III Lecture 3 25
UNIT III Lecture 3 26
INDEPENDENT MODES OF VIBRATION OF CO2 MOLECULE
UNIT III Lecture 3 27
The energy level diagram of vibrational –
rotational energy levels with which the main physical
processes taking place in this laser.
As the electric discharge is passed through the
tube, which contains a mixture of carbon dioxide,
nitrogen and helium gases, the electrons striking
nitrogen molecules impart sufficient energy to raise
them to their first excited vibrational-rotational
This energy level corresponds to one of the
vibrational - rotational level of CO2 molecules,
designated as level 4.
UNIT III Lecture 3 28
collision with N2 molecules, the CO2 molecules
are raised to level 4.
The lifetime of CO2 molecules in level 4 is quiet
significant to serve practically as a metastable
Hence, population inversion of CO2 molecules is
established between levels 4 and 3, and between
levels 4 and 2.
The transition of CO2 molecules between levels 4
and 3 produce lasers of wavelength 10.6 microns
and that between levels 4 and 2 produce lasers of
wavelength 9.6 microns .
UNIT III Lecture 3 29
ENERGY LEVEL DIAGRAM
UNIT III Lecture 3 30
The He molecules increase the population of level
4, and also help in emptying the lower laser levels.
The molecules that arrive at the levels 3 and 2
decay to the ground state through radiative and
collision induced transitions to the lower level 1,
which in turn decays to the ground state.
The power output of a CO2 laser increases linearly
with length. Low power (upto 50W) continuous wave
CO2 lasers are available in sealed tube
UNIT III Lecture 3 31
•Some are available in sizes like torches for medical
use, with 10-30 W power.
• All high power systems use fast gas-floe designs.
• Typical power per unit length is 200-600 W/m.
• Some of these lasers are large room sized metal
working lasers with output power 10-20 kW.
• Recently CO2 lasers with continuous wave power
output exceeding 100 kW.
• The wavelength of radiation from these lasers is
UNIT III Lecture 3 32
Nd: YAG Laser (Doped insulator 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
The (Nd3+) ions occupy the lattice sites of
yttrium ions as substitutional impurities and
provide the energy levels for both pumping and
UNIT III Lecture 3 33
When an (Nd3+) ion is placed in a host crystal
lattice it is subjected to the electrostatic field of
the surrounding ions, the so called crystal field.
The crystal field modifies the transition
probabilities between the various energy levels
of the Nd3+ ion so that some transitions, which
are forbidden in the free ion, become allowed.
UNIT III Lecture 3 34
Nd: YAG laser
UNIT III Lecture 3 35
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 linear 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 emitted by the lamp is
effectively coupled to the rod.
The ends of the rod are polished and made
optically flat and parallel.
UNIT III Lecture 3 36
•The optical cavity is formed either by silvering
the two ends of the rod or by using two external
• One mirror is made hundred percent reflecting
while the other mirror is left slightly transmitting
to draw the output
• The system is cooled by either air or water
UNIT III Lecture 3 37
ENERGY LEVEL DIAGRAM
Simplified energy level diagram for the Nd-ion in YAG showing the
principal laser transitions
UNIT III Lecture 3 38
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.
UNIT III Lecture 3 39
OUTPUT CHARACTERISTICS :
The laser output is in the form of pulses with
higher repetition rate
Xenon flash lamps are used for pulsed output.
Nd: YAG laser can be operated in CW mode
also using tungsten-halide incandescent lamp for
Continuous output powers of over 1KW are
UNIT III Lecture 3 40
Note: Nd: Glass laser :
Glass acts as an excellent host material for
As in YAG, within the glass also local electric fields
modify the Nd3+ ion energy levels.
Since the line width is much broader in glass than
in YAG for Nd3+ ions, the threshold pump power
required for laser action is higher.
Nd: Glass lasers are operated in the pulsed mode
at wavelength 1.06 m III Lecture 3
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 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
UNIT III Lecture 3 42
•YAG beams penetrate the lens of the eye to
perform intracular procedures.
•YAG lasers are used in military as range finders
and target designators.
UNIT III Lecture 3 43