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Laser action summary
Step 1 : Choose a proper lasing medium
Step 2 : Establish population inversion by suitable
pumping
Step 3 : Stimulated emission takes place
Step 4 : Positive feed back (optical resonator)
Step 5 : Amplification of light
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Characteristics of laser
Directionality

The directionality of a laser beam is expressed in
terms of the full angle beam divergence which is twice
the angle that the outer edge of the beam makes with
the axis of the beam.

The outer edge of the beam is defined as a point at
which the strength of the beam has dropped to 1/e times
its value at the centre.
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At d1 and d2 distances from the laser window, if the
diameter of the spots are measured to be a1 and a2
respectively, then the angle of divergence (in degrees)
can be expressed as
(a 2  a 1 )
 
2(d 2  d 1 )
For a typical laser, the beam divergence is about 1
milli radian.
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(ii) Monochromaticity
•The degree of monochromaticity is expressed in terms
of line width (spectral width)
•The line width is the frequency spread  of a spectral
line
•The frequency spread  is related to the wavelength
spread  as
 = -(c/2)  
•The three most important mechanisms which give rise to
the spectral broadening (frequency spread) are Doppler
broadening, Collision broadening and natural broadening
.
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(1) Doppler broadening
The atoms which emit radiation are not at rest at the
time of emission and depending on their
velocities
and the direction of motion, the frequency of the
emitted radiation changes
slightly
and
this
broadening is called Doppler broadening.
(2) Collision broadening
If the atoms undergo collision at the time of emitting
radiation there will be change in the phase of the
emitted radiation resulting in frequency shift and is
known as collision broadening.
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(3) Natural broadening
In solid materials, an atomic electron emitting
energy in the form of a photons leads to an exponential
damping of the amplitude of the wave train and the
phenomenon is called natural broadening.
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Coherence
The purity of the spectral line is expressed in terms
of coherence Coherence is expressed in terms of
ordering of light field.
(1) Temporal coherence
(2) Spatial coherence
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(i) Temporal coherence
Temporal coherence refers to correlation in phase at a
given point in a space over a length of time.
i.e, if the phase difference between the two light fields
E1 (x,y,z,t1) and E2 (x,y,z,t2), is constant, the wave is said
to have temporal coherence.
The maximum length of the wave train on which any two
points can be correlated is called coherent length.
coherent lentth
Coherent time =
velocity of light
The high degree of temporal coherence arises from the
lasers monochromaticity.
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(ii) Spatial coherence
Spatial coherence refers to correlation in phase at
different points at the same time.
i.e, if the phase difference between the two light fields
E1( x1,y1,z1,t) and E2 (x2,y2, z2,t) is constant, the wave
is said to have spatial coherence.
The high degree of spatial coherence results, since the
wave fronts in a laser beam are in effect similar to those
emanating from a single point source.
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(4) Intensity or Brightness
When two photons each of amplitude ‘a’ are in phase with each other,
then by young’s principle of superposition the resultant amplitude is ‘2a’
and the intensity is proportional to (2a)2 i.e, 4a2.
In laser, many number of photons (say n) are in phase with each other,
the amplitude of the resultant wave becomes ‘na’ and hence the intensity
is proportional to n2a2.
Thus due to coherent addition of amplitude and negligible divergence,
the intensity increases enormously.
i.e., 1mw He-Ne laser can be shown to be 100 times brighter than the
sun.
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Difference between spontaneous emission and stimulated
Spontaneou
emission
s
emissionb(o
rdinary
light)
Not required
Stimulated
emission
(laser light)
Monochrom
aticity
Less
High
Directionality
Less
High
Intensity
Less
High
Coherence
Less
High
Property
Stimuli
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Required
11
Essential components of a laser system
Active
Medium
Pumping
Mechanism
Optical
resonator
Active medium or Gain medium
It is the system in which population inversion and
hence stimulated emission (laser action) is established.
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
transition.
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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
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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.
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
silvered mirror.
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Types of Lasers
Based on its pumping action
•Optically pumped laser
•Electrically pumped laser
Basis of the operation mode
•Continuous wave Lasers
•Pulsed Lasers
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According to their wavelength
•Visible Region
•Infrared Region
•Ultraviolet Region
•Microwave Region
•X-Ray Region
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According to the source
Dye Lasers
Gas Lasers
Chemical Lasers
Metal vapour Lasers
Solid state Lasers
Semi conductor Lasers
other types
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Gas lasers
Laser gain
medium and
type
Operation wavelength(s)
Pump source
Applications and notes
Helium-neon
laser
632.8 nm (543.5 nm, 593.9 nm,
611.8 nm, 1.1523 μm, 1.52 μm,
3.3913 μm)
Electrical discharge
Interferometry, holography, spectroscopy,
barcode scanning, alignment, optical
demonstrations.
Argon laser
454.6 nm, 488.0 nm, 514.5 nm
(351 nm,457.9 nm, 465.8 nm,
476.5 nm, 472.7 nm, 528.7 nm)
Electrical discharge
Retinal phototherapy (for diabetes),
lithography, confocal microscopy, pumping
other lasers.
Krypton
laser
416 nm, 530.9 nm, 568.2 nm,
647.1 nm, 676.4 nm, 752.5 nm,
799.3 nm
Electrical discharge
Scientific research, mixed with argon to
create "white-light" lasers, light shows.
Xenon ion
laser
Many lines throughout visible
spectrum extending into the UV
and IR.
Electrical discharge
Scientific research.
Nitrogen
laser
337.1 nm
Electrical discharge
Pumping of dye lasers, measuring air
pollution, scientific research. Nitrogen lasers
can operate superradiantly (without a
resonator cavity).
Carbon
dioxide laser
10.6 μm, (9.4 μm)
Transverse (high power) or
longitudinal (low power)
electrical discharge
Material processing (cutting, welding, etc.),
surgery.
Carbon
monoxide
laser
2.6 to 4 μm, 4.8 to 8.3 μm
Electrical discharge
Material processing (engraving, welding,
etc.), photoacoustic spectroscopy.
Excimer
laser
193 nm (ArF), 248 nm (KrF),
Excimer recombination via
Ultraviolet lithography for semiconductor
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308 nm (XeCl), 353 nm (XeF)
electrical discharge
manufacturing, laser surgery
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.
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LECTURE 4
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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.
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LECTURE 4
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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.
Nitrogen here plays the role that He plays in He-Ne laser.
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LECTURE 4
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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 coherence.
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 rotational motions.
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LECTURE 4
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Fundamental Modes of vibration of CO2
Three fundamental modes of vibration for CO2
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.
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LECTURE 4
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In the bending mode, the molecule ceases to be exactly
linear as the atoms move perpendicular to the molecular
axis.
In asymmetric stretching, all the three atoms
oscillate: but while both oxygen atoms move in one
direction, carbon atoms move in the opposite direction.
The internal vibrations of carbon dioxide molecule
can be represented approximately by linear combination
of these three normal modes.
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CO2 Laser
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Independent modes of vibration of CO2 molecule
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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 energy level.
This energy level corresponds to one of the vibrational rotational level of CO2 molecules, designated as level 4.
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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 state.
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
.
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LECTURE 4
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Energy level diagram
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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 configurations.
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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
10.6m.
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LECTURE 4
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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 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.
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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.
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Nd: YAG laser
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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 9mm.
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.
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•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
• The system is cooled by either air or water
circulation.
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Energy level diagram
Simplified energy level diagram for the neodymium ion in YAG showing
the principal laser transitions
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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 1.06mm
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LECTURE 4
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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 optical
pumping.
Continuous output powers of over 1KW are
obtained.
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Note: Nd: Glass laser
Glass acts as an excellent host material for
neodymium
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
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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
bleeding.
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LECTURE 4
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YAG beams penetrate the lens of the eye to perform
intracular procedures.
YAG lasers are used in military as range finders and
target designators.
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Semiconductor (Ga-As) lasers
Introduction
The semiconductor laser is today one of the most
important types of lasers with its very important application
in fiber optic communication.
These lasers use semiconductors as the lasing medium and
are characterized by specific advantages such as the
capability of direct modulation in the gigahertz region,
small size and low cost.
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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 electron-hole pair.
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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 junction.
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When a current is passed through a p-n junction under
forward bias, the injected electrons and holes will increase
the density of electrons in the conduction band.
 The stimulated emission rate will exceed the absorption
rate and amplification will occur at some value of current
due to holes in valence band.
 As the current is further increased, at threshold value of
the current, the amplification will overcome the losses in
the cavity and the laser will begin to emit coherent
radiation.
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Simple structure (Homojunction)
• The basic semiconductor laser structure in which the
photons generated by the injection current travel to the
edge mirrors and are reflected back into the active area.
• Photoelectron collisions take place and produce more
photons, which continue to bounce back and forth between
the two edge mirrors.
• This process eventually increases the number of
generated photons until lasing takes place. The lasing will
take place at particular wavelengths that are related to the
length of the cavity.
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Basic semiconductor laser structure
a) Side view b) Projection Hetero structures
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Heterostructures
The hetero structure laser is a laser diode with more
than single P and N layers. GaAs/AlGaAs is a Hetero
junction laser. The notations P+ and N+ and P- and Nindicate heavy doping and light doping respectively. The
P-N structure consists of the two double layers, P+ - Pand N+ - N- .
A thin layer of GaAs is placed at the junction, the
active region. The substance is selected because the
electron-hole recombinations are highly radiative. This
increases the radiation efficiency.
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The P and N regions are lightly doped regions that have
an index of refraction n2 less than n1 of the active region.
These three layers, n2-n1-n2, form a light waveguide
much like the optical fiber, so that the light generated is
confined to the active region.
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Laser heterostructure (a) Schematic projection (b) Refractive index profile
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Excimer laser
•
An excimer laser or exciplex laser is a form of
ultraviolet chemical laser which is commonly used in eye surgery
and semiconductor manufacturing.
•
The term excimer is short for 'excited dimer', while
exciplex is short for 'excited complex'.
•
An excimer laser typically uses a combination of an inert
gas (Argon, krypton, or xenon) and a reactive gas (fluorine or
chlorine).
• Under the appropriate conditions of electrical stimulation, a
pseudo-molecule called a dimer is created, which can only exist
in an energised state and can give rise to laser light in the
ultraviolet range
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•
Laser action in an excimer molecule occurs because
it has a bound (associative) excited state, but a repulsive
(disassociative) ground state.
•
This is because noble gases such as xenon and
krypton are highly inert and do not usually form chemical
compounds.
•
When in an excited state (induced by an electrical
discharge or high-energy electron beams, which produce
high energy pulses), they can form temporarily-bound
molecules with themselves (dimers) or with halides
(complexes) such as fluorine and chlorine.
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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 picoseconds) disassociates back into two unbound
atoms. This forms a population inversion between the two states.
Most "excimer" lasers are of the noble gas halide type, for
which the term excimer is strictly speaking a misnomer (since a
dimer refers to a molecule of two identical or similar parts): The
correct but less commonly used name for such is exciplex laser.
The wavelength of an excimer laser depends on the
molecules used, and is usually in the ultraviolet region
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Excimer Wavelength
Excimer
Wavelength
ArF
193 nm
KrF
248 nm
XeBr
282 nm
XeCl
308 nm
XeF
351 nm
CaF2
193 nm
KrCl
222 nm
Cl2
259 nm
N2
337 nm

Excimer lasers are usually operated with a pulse
rate of around 100 Hz and a pulse duration of ~10 ns,
although some operate as high as 8 kHz and 30 ns.
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All commercial excimer lasers employ the
modules. Laser light is generated in the laser cabinet.
The electrical energy required by the laser to
form laser pulses is generated by the high voltage supply.
A gas supply and a vacuum pump are required to
fill the laser with the appropriate laser gas mixture.
The control computer is usually linked to the laser
cabinet and high-voltage supply by a fiber optic network.
The computer provides laser function user control.
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Typical excimer laser configuration
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Uses
The UV light from an excimer laser is well absorbed by
biological matter and organic compounds. Rather than burning
or cutting material, the excimer laser adds enough energy to
disrupt the molecular bonds of the surface tissue, which
effectively disintegrates into the air in a tightly controlled
manner through ablation rather than burning.
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.
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These properties make them useful for surgery
(particularly eye surgery), for lithography for
semiconductor manufacturing, and for dermatological
treatment.
Excimer lasers are quite large and bulky devices, which is
a disadvantage in their medical applications, although their
size is rapidly decreasing with ongoing development.
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Free electron laser
Introduction
Free electron laser, or FEL, is a laser invented and developed by
J.M.J. Madey in 1971. It is a powerful and challenging
combination of particle-accelerator and laser physics .
FEL is a relativistic electron tube that made use of the open
optical resonator, that shares the same optical properties as
conventional lasers such as emitting a beam consisting of
coherent electromagnetic radiation which can reach high power,
but which uses some very different operating principles to form
the beam.
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Gas, liquid, or solid-state lasers such as diode lasers,
which rely on bound atomic or molecular states, FELs use
a relativistic electron beam as the lasing medium, hence the
term free electron.
This gives them the widest frequency range of any laser
type, and makes many of them widely tunable, currently
ranging in wavelength from microwaves, through terahertz
radiation and infrared, to the visible spectrum, to ultraviolet,
to soft X-rays.
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Beam creation
Free Electron Laser Diagram
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A FEL can be created by a beam of electrons is accelerated to
relativistic speeds.
The beam passes through a periodic, transverse magnetic field.
This field is produced by arranging magnets with alternating poles
along the beam path.
This array of magnets is sometimes called an undulator, or a
"wiggler", because it forces the electrons in the beam to assume a
sinusoidal path.
The acceleration of the electrons along this path results in the
release of a photon .
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Viewed relativistically in the rest frame of the electron, the
magnetic field can be treated as if it were a virtual photon.
The collision of the electron with this virtual photon creates an
actual photon (Compton scattering).
Mirrors capture the released photons to generate resonant gain.
Adjusting either the beam energy (speed/energy of the electrons) or
the field strength tunes the wavelength easily and rapidly over a wide
range.
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Compton scattering is complicated in itself, it is
easier to say that the electrons are forced onto a sinus
path by the undulator and then switch in a rest frame
moving along the undulator.
where the electrons are oscillating, but not moving
otherwise, and emit dipole radiation, and than switch
back into the rest frame of the undulator to see that
this dipole radiation is transformed into a forward
emitted radiation of shorter wavelength.
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The photons emitted are related to the electron beam
and magnetic field strength, an FEL can be tuned, i.e. the
frequency or color can be controlled.
Laser is that the electron motion is in phase (coherent)
with the field of the light already emitted, so that the fields
add coherently.
The intensity of light depends on the square of the
field, this increases the light output.
Moving along the undulator any radiation will still
move with the speed of light and pass over the electrons and
lets them communicate to get in synchronization.
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Same light (that is radiation) is introduced from the
outside.
Depending on the position along the undulator the
oscillation of an electrons is in phase or not in phase with
this radiation.
The light either tries to accelerate or decelerate these
electrons.
It thereby gains or loses kinetic energy, so it moves faster
or slower along the undulator.
This causes the electrons to form bunches.
They are synchronized, and will in turn emit
synchronized (that is coherent) radiation.
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X-Ray Free-Electron Lasers
A free-electron laser that emits X-rays with a
wavelength of the size of an atom (about
1 Å) can be built because of a favorable and interesting
phenomenon of self-organization of the electrons in a
relativistic beam, known as the free-electron laser
collective instability.
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This instability takes an electron beam with a random
electron position distribution, and changes it into a
distribution with electrons regularly spaced at about the x-ray
wavelength, producing what could be called a 1-dimensional
electron crystal.
The radiation from this crystal has the new and exciting
properties
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Medical applications
It was reported that at infrared wavelengths, water in
tissue was heated by the laser, but at 915, 1210 and 1720 nm,
subsurface lipids were differentially heated more strongly
than water.
The possible applications include the selective destruction
of sebum lipids to treat acne, as well as targeting other lipids
for the treatment of cellulite and atherosclerosis.
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Military applications
FEL is also considered by as a good candidate for
anti-missile directed-energy weapon.
Significant progress is being made in increasing
FEL power levels (already at 10 kW) and it should be
possible to build compact multi-megawatt class FEL lasers.
(Airborne megawatt class free-electron laser for defense and
security).
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A specialized application that has received a
significant attention is the use of lasers in space
communications, where atmospheric interference is
not a problem, the distances are enormous, and the
data rates and system weight are more significant
than the cost of individual components.
A second application is that of a rapidly installed,
terrestrial communications link for short distances, as
between adjacent office building in a city.
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Holography
Definition
The technique of recording of the complete
information of an object (ie, its amplitude and phase)
is called Holography (Holo – whole; graphy –
recording)
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Comparison between holography and photography
Property
Photography
Holography
1. Illumination
Ordinary light
Laser
2. Recording
parameter
Amplitude
Both amplitude and phase
3. Imaging
2-Dimensional
3-Dimensional
4. Recording
medium
Ordinary Photographic
film
Very high resolution film
5. Special
requirement
Not Applicable
Vibration isolation table (it
requires long exposure)
6. Special property
When cut into pieces,
information is lost
When cut into pieces each
bit carries full information
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Recording process
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Recording Technique – Construction of a Hologram
A monochromatic laser beam from the source is made to
fall on beam splitter.
Beam splitter splits the incident beam into two.
One beam is made to fall on silver coated mirror M1 and
after reflection, it is directed towards the photographic
plate – reference wave.
Another beam is made to scatter by the object – object
wave.
The reference wave and object wave interfere and the
interference pattern is recorded on a high resolution
photographic plate
The developed photographic plate is known as hologram
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Reconstruction Process
The hologram is illuminated by the reference wave
Holography is a phenomenon of wave front
reconstruction
To the observer the reconstructed wave front appears
to be coming from the object itself and a virtual
image is seen.
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Reconstruction process
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Applications of holography
1. Holographic interferometry
Double
exposure
holographic
interferometry:
Measurement of small displacements or distortions of
an object.
Real time holographic interferometry: Measurement of
strains of object as they actually deform.
Time-average holographic interferometry: Examination
of spatial characteristics of low amplitude vibrations of
an object.
2. Holographic computer memories
High density optical storage
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High Density Optical Storage
An intriguing approach for next generation data–storage uses
optical holography to store information throughout the three–
dimensional volume of a material.
By superimposing many holograms within the same volume of
the recording medium, holograms can potentially store data at a
volumetric density of one bit per cubic wavelength.
Given a typical laser wavelength of 500 nm or so, this density
corresponds to 1012 bits (1 Terabit) per cubic centimeter or
more.
In holographic storage, data are transferred to and from the
storage material as 2D images composed of thousands of
pixels, each of which represents a single bit of information.
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Since an entire “page of data” can be retrieved by a
photo detector at the same time, rather than bit–by–bit,
the holographic scheme promises fast readout rates as
well as high density.
If a thousand holograms, each containing a million
pixels, could be retrieved every second, for instance,
then the output data rate would reach 1 Gigabit per
second. (For comparison, a DVD optical–disk player
reads data 100 times slower.)
To use volume holography as a storage technology,
digital data must be imprinted onto the object beam for
recording and then retrieved from the reconstructed
object beam during readout.
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Holograghic data storage
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Data are imprinted onto the object beam by shining
the light through a pixilated device called a spatial
light modulator.
The reference beam overlaps with the object beam on
the storage material, where the interference pattern is
stored as a change in absorption, refractive index or
thickness of the medium.
A pair of lenses image the data through the storage
material onto a pixilated detector array, such as a
charge coupled device (CCD).
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To maximize the storage density, the hologram is
usually recorded where the object beam is tightly
focused
Correct reference beam must first be directed to the
appropriate spot within the storage media.
The hologram is then reconstructed by the reference
beam, and a weak copy of the original object beam
continues along the imaging path to the camera,
where the optical output is detected and converted to
digital data
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The speed of a storage device can be jointly described
by two parameters: the readout rate (in bits per second)
and the latency, or time delay between asking for and
receiving a particular bit of data.
The latency tends to be dominated by mechanical
movement, especially if the storage media has to be
moved.
The readout rate is often dictated by the camera
integration time: the reference beam reconstructs a
hologram until a sufficient number of photons
accumulate to differentiate bright and dark pixels.
A frequently mentioned goal is an integration time of
about 1 millisecond, which implies that 1000 pages of
data can be retrieved per second.
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Materials for writing permanent volume holograms
generally involve irreversible photochemical reactions
that are triggered by the bright regions of the optical
interference pattern.
A photopolymer material, for example, polymerizes in
response to optical illumination: material diffuses from
darker to brighter regions so that short monomer chains
can bind together to form long molecular chains.
And in a so-called direct-write or photo chromic material,
the illuminated molecules undergo a local change in their
absorption or index of refraction, which is driven by
photochemistry
or
photo-induced
molecular
reconfiguration.
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Most erasable holographic materials are inorganic
photorefractive crystals doped with transition metals or
rare-earth ions.
These crystals are often available in centimeter-thick
samples and include lithium niobate, strontium barium
niobate and barium titanate doped with iron, cerium,
praseodymium or manganese.
These materials react to the light and dark regions of
an interference pattern by transporting and trapping
electrons, which subsequently leads to a local change
in the index of refraction.
The trapped charge can be rearranged by later
illumination, so it is possible to erase recorded
holograms and replace them with new ones.
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