>>LASER is an acronym for Light Amplification by
Stimulated Emission of Radiation.
>>The most important optical device to be
developed in the past 50 years.
>>Invented in 1958 by Charles Townes and Arthur
Schawlow of Bell Laboratories
>>Was based on Einstein’s idea of the
“particle-wave duality”of light.
Incandescent vs laser light
Principle of lasers
The principle of a laser is based on three
a) stimulated emission within an amplifying
b) population inversion
c) an optical resonator.
Einstein’s quantum theory
According to Einstein, the interaction of
radiation with matter could be explained in
terms of three basic processes:
An incoming photon excites the atomic system
from a lower energy state into a higher energy
state. This is called absorption or
sometimes stimulated absorption.
It is called stimulated absorption because of the
fact that the atoms absorb the incident energy at
certain frequencies only.
Consider an atom (or molecule) of the
material is existed initially in an excited
state 2.. Since 2> 1, the atom will tend to
spontaneously decay to the ground state
1, releasing a photon of energy h = 2- 1
in a random direction . This process is
called “spontaneous emission
When an incident photon of energy h = 2- 1 passes
by an atom in an excited state 2, it stimulates the
atom to drop or decay to the lower state 1. In this
process, the atom releases a photon of the same
energy, direction & phase as that of the photon
The net effect is two identical photons (2h ) in the
place of one, or an increase in the intensity of the
It is precisely this processes of stimulated emission
that makes possible the amplification of light in
Population Inversion and Laser
If the higher energy state has a greater
population than the lower energy state, then the
light in the system undergoes a net increase in
N2 > N1….. Population Inversion
Can be created by introducing a so called metastable
centre where electrons can piled up to achieve a situation
where more N2 than N1
The process of attaining a population inversion is
To create population inversion, a 3-state system is
The system is pumped with radiation of energy E31
then atoms in state 3 relax to state 2 non
The electrons from E2 will now jump to E1 to give
Optical cavity or resonator
As there are certain losses of the emitted photons within
the material itself one has to think about the geometry
that can overcome these losses and there is overall gain.
This requires an optical cavity or resonator.
A pair of mirrors placed on either side of the active
One mirror is completely silvered and the other is
The laser beam comes out through the partially silvered
Construction & working of laser
A representative laser system is shown in Figure
It consists of three basic parts:-
An active medium with a suitable set of energy
levels to support laser action.
A source of pumping energy in order to establish
a population inversion.
An optical cavity or resonator to introduce optical
feedback and so maintain the gain of the system
overcoming all losses.
Active laser medium or gain medium:
Laser medium is the heart of the laser system
and is responsible for producing gain and
subsequent generation of laser. It can be a
crystal, solid, liquid, semiconductor or gas
medium and can be pumped to a higher energy
it must have a metastable state to support
Excitation or pumping mechanism:
>>There are various types of excitation or pumping
mechanisms available, the most commonly used
ones are optical, electrical, thermal or chemical
techniques, which depends on the type of the
laser gain medium employed.
>>eg:Solid state lasers usually employ optical
pumping from high energy xenon flash lamps
Gas lasers use an AC or DC electrical discharge
through the gas medium
In practice, photons need to be confined in the system to
allow the number of photons created by stimulated
emission to exceed all other mechanisms. This is
achieved by bounding the laser medium between two
Then Stimulated photons can bounce back and forward
along the cavity, creating more stimulated emission as
Plays a very important role in the generation of the laser
output, in providing high directionality to the laser beam
as well as producing gain in the active medium to
overcome the losses.
means that the wavelengths of
the laser light are in phase in space and
Factors that compromise coherence:
1. thermal fluctuations
2. vibrational fluctuations
3. emission of multiple wavelengths
Temporal Coherence – How long do the
light waves remain in phase as they
Coherence Length = 2/n
Spatial Coherence – Over what area does
the light remain in phase?
electric current is discharged through a gas to
produce coherent light.
The first continuous-light laser & the first laser to
operate on the principle of converting electrical
energy to a laser light output
He-Ne Atomic gas lasers
CO2 molecular gas lasers
PROPERTIES OF GAS LAERS
Good beam quality
High frequency stability
Low energy consumption
He-Ne laser is a four-level laser.
Its usual operation wavelength is
632.8 nm, in the red portion of the
It operates in Continuous Working (CW) mode.
The Helium-Neon laser was the first continuous
simplified energy level diagram
of He Ne laser
Advantages of He-Ne Laser
He-Ne laser has very good coherence property
He-Ne laser tube has very small length approximately
from 10 to 100cm and best life time of 20.000 hours.
Cost of He-Ne laser is less from most of other lasers.
Construction of He-Ne laser is also not very complex.
He-Ne laser provide inherent safety due to low power
Disadvantages of He-Ne
It is relatively low power device means its output power
He-Ne laser is low gain system/ device.
To obtain single wavelength laser light, the other two
wavelengths of laser need suppression, which is done by
many techniques and devices. So it requires extra
technical skill and increases the cast also.
High voltage requirement
Escaping of gas from laser plasma tube.
Applications of He-Ne laser
The Narrow red beam of He-Ne laser is used in supermarkets to read bar
Red HeNe lasers have many industrial and scientific uses. They are widely
used in laboratory demonstrations of optics in view of their relatively low
cost and ease of operation .
Argon ion laser
as the name implies it uses high purity
argon gas as the lasing medium.
Based on light amplification in ionized
argon in a gas discharge
powerful gas lasers, which typically
generate multiple watts of optical power in
a green or blue output beam with
high beam quality.
Advantages of Argon Laser
Production of multiple wave lengths is the
main advantage plus characteristic of
argon as well as other ion lasers.
Argon lasers produce high power output
as compared to He-Ne laser.
Argon laser is a higher gain system.
Argon laser like He-Ne has very less
divergence, typically about 1 milli radian.
Disadvantages of Argon laser
The overall efficiency of argon laser is very less
usually lies between 0.01% and 0.1%.
Large amount of power requirement is also its
Construction is very difficult.
Cost of argon laser is not as low as He-Ne laser.
Power supply of high voltages required, because
due to solenoid there is extra burden on it.
Applications of argon ion laser
Argon ion lasers are also used extensively in
scientific, research and educational applications
Excimer lasers produce intense pulsed output in the
Here the lasing molecule is one consisting of a halogen and
an inert gas.
The term excimer is short for 'excited dimer', while
exciplex is short for 'excited complex'.
Under the appropriate conditions of electrical stimulation, a pseudo-
molecule called a dimer is created, which can give rise to laser light
in the ultraviolet range
Laser action in an excimer molecule occurs because it has a bound
(associative) excited state, but a repulsive (dissociative) ground
In an excited state they can form temporarily-bound molecules with
themselves (dimers) or with halides (complexes) . The excited
compound can give up its excess energy by undergoing spontaneous
or stimulated emission, resulting in a strongly-repulsive ground state
molecule which very quickly (on the order of a picosecond)
disassociates back into two unbound atoms. This forms a population
inversion between the two states.
The heart of the laser is the discharge tube. This is first filled with a
low-pressure mixture of an inert gas (e.g.,krypton, argon, xenon)
and a halogen or halide gas(fluorine or hydrogen chloride), and then
pressurized with an inert buffer gas of either neon or helium. The
laser tube has two parallel electrodes running almost the entire
length of the tube. The laser is pulsed by discharging across these
electrodesThis stripe-shaped discharge lasts from 20 to 50 ns
depending on laser gas, laser parameter and discharge pulser
design. The resultant plasma contains a high concentration of an
excited transient complex (e.g.,ArF, KrF, XeCl, or F2), which emits
ultraviolet laser light.Among the most commonly used wavelengths
are 248 nm and 308 nm..
ADVANTAGES OF EXCIMER
Excimer lasers are powerful.
Directly generate intense, short ultraviolet
High optical resolution: less than 1m.
Shallow absorption depth:0.1 to 0.5µm.
Small interaction volume.
Energy highly absorbed by materials.
Uniform power density over relatively large area.
High peak power: approximately 107 watts.
High performance discharge circuit is required to generate laser
Laser gas mixture is toxic and corrosive.
Reactivity of lasant mixtures result in impurities formation during
A computer control system is required to maintain stable laser light
Changes in gas chemistry affect beam shape and quality.
Advanced optical materials are required to efficiently transmit the
Optic transmissivity degrades over long-term exposure to high-
power UV beams.
Optical surfaces and coatings are damaged quickly by laser light if
not kept clean.
Uses of excimer laser
Excimer lasers have the useful property
that they can remove exceptionally fine
layers of surface material with almost no
heating or change to the remainder of the
material which is left intact. These
properties make them useful for surgery
(particularly eye surgery) for lithography
for semiconductor manufacturing, and for
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 molecule.
Besides, these electronic levels, the molecules
have other energy levels.
C.K.N. Patel designed CO2 laser in the year
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
Population inversion is created by electric discharge of
A carbon dioxide (CO2) laser can produce a
continuous laser beam with a power output of
Can maintain high degree of spectral purity.
High spatial coherence.
The CO2 laser is the most efficient
laser, capable of operating at more than 30%
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.
In surgical procedures because water (which makes up
most biological tissue) absorbs this frequency of light
Other medical uses are
"solid-state" refers to a crystal or glass.
Is usually optically pumped.
Nd:YAG (Yttrium Aluminum Garnet which is
A ruby laser is a solid-state laser that uses a
synthetic ruby crystal as its gain medium.
The first working laser was a ruby laser made
by Theodore H. Maiman at Hughes Research
Laboratories on May 16, 1960.
Ruby lasers produce pulses of visible light at
a wavelength of 694.3 nm, which is a deep red
color. Typical ruby laser pulse lengths are on the
order of a millisecond.
Construction of Ruby Laser
The ruby laser consists of a ruby rod . which is made of
chromium doped ruby material. At the opposite ends of
this rod there are two silver polished mirrors. Whose one
is fully polished and other is partially polished. A spring is
attached to the rod with fully polished end for adjustment
of wave length of the laser light. Around the ruby rod a
flash light is kept for the pump input. The whole
assembly is kept in the glass tube. Around the neck of
the glass tube the R.F source and switching control is
designed in order to switch on and off the flash light for
Operation of Ruby Laser:
When we switch on the circuit the R.F operates. As a result the flash of light
is obtained around the ruby rod. this flash causes the electrons within ruby
rod to move from lower energy band towards higher energy band. The
population inversion take place at high energy band and electrons starts
back to travel towards the lower energy band. During this movement the
electron emits the laser light . This emitted light travels between the two
mirrors where cross reflection takes place of this light. The stimulated lazer
light now escapes from partially polished mirror in shape of laser beam.
The spring attached with the fully polished mirror is used to adjust the wave
length equal to λ/2 of lazer light for optimum lazer beam. The switching
control of the R.F source is used to switch on and off the flash light so that
excessive heat should not be generated due to very high frequency of the
movement of the electron.
The above three level energy diagram show that in ruby lasers the absorption occurs
in a rather broad range in the green part of the spectrum. This makes raise the
electrons from ground state E1 to the band of level E3 higher than E1. At E3 these
excited levels are highly unstable and so the electrons decays rapidly to the level of
E2. This transition occurs with energy difference (E1 – E2) given up as heat (radiation
less transmission). The level E2 is very important for stimulated emission process and
is known as Meta stable state. Electrons in this level have an average life time of
about 5m.s before they fall to ground state. After this the population inversion can be
established between E2 and E1. The population inversion is obtained by optical
pumping of the ruby rod with a flash lamp. A common type of the flash lamp is a glass
tube wrapped around the ruby rod and filled with xenon gas. When the flash lamp
intensity becomes large enough to create population inversion, then stimulated
emission from the Meta stable level to the ground level occurs which result in the
laser output. Once the population inversion begins, the Meta stable level is
depopulated very quickly. Thus the laser output consists of an intense spike lasting
from a few Nano sec to µsec. after stimulated emission spike, population inversion
builds up again and a 2nd spike results. This process continues as long as the flash
lamp intensity is enough to create the population inversion.
Advantages of Ruby laser
From cost point of view, the ruby lasers are economical.
Beam diameter of the ruby laser is comparatively less
than CO2 gas lasers.
Output power of Ruby laser is not as less as in He-Ne
Since the ruby is in solid form therefore there is no
chance of wasting material of active medium.
Construction and function of ruby laser is self
Disadvantages of Ruby Laser
In ruby lasers no significant stimulated emission
occurs, until at least half of the ground state
electrons have been excited to the Meta stable
Efficiency of ruby laser is comparatively low.
Optical cavity of ruby laser is short as compared
to other lasers, which may be considered a
Applications of ruby laser
Used in retina surgery and in dermatology. It has the
ability to concentrate the energy of optical radiation into
a small area and the possibility of cutting and
It is used in performing a non contact sharp contour
tissue incision and removal of even tiny structuires
without any damage to the surrounding tissues and any
possible infection to the cut.
In medical diagnostics.
Used in cataract surgey,retina detachment surgery and
glaucoma removal surgery.
The semiconductor laser is today one of the most important types of
lasers with its very important application in fiber optic communication.
Unlike other lasers, semiconductor laser does not need mirrors to
obtain the reflectivity needed to produce feedback mechanism
Basic Mechanism :
The basic mechanism responsible for light emission from a semiconductor
is the recombination of electrons and holes at a p-n junction when a current
is passed through a diode.There can be three interaction processes.
1) An electron in the valence band can absorb the incident radiation and be
excited to the conduction band leading to the generation of eletron-hole
2) An electron can make a spontaneous transition in which it combines with a hole and
in the process it emits radiation
3) A stimulated emission may occur in which the incident radiation stimulates an
electron in the conduction band to make a transition to the valence band and in the
process emit radiation.
To convert the amplifying medium into a laser
Optical feedback should be provided
Done by cleaving or polishing the ends of the p-n junction diode at right angles to
The heterostructure laser is a laser diode with
more than single P and N layers.
GaAs/AlGaAs is a heterojunction laser. This
increases the radiation efficiency
Smaller size and appearance make them good choice
for many applications.
From cost point of view the semiconductor lasers are
Semiconductor lasers construction is very simple.
No need of mirrors is in semiconductor lasers.
Semiconductor lasers have high efficiency.
The low power consumption is also its great advantage.
Due to relatively low power production, these lasers are not suited to
many typical laser applications.
Semiconductor laser is greatly dependent on temperature. The
temperature affects greatly the output of the laser.
The lasing medium of semiconductor lasers is too short and
rectangular so the output beam profile has an unusual shape.
Beam divergence is much greater from 125 to 400 milli radians as
compared to all other lasers.
The cooling system requirement in some cases may be considered
Application / Uses of
The semiconductor laser can be pulsed at
varying rate and pulse widths. Therefore
this laser is a natural transmitter of digital
Semiconductor laser is well suited for
interface with fiber optic cables used in