Download FREE ELECTRON LASER

PH 0101
UNIT-3
LECT - 5
• Free Electron Laser,
• X-ray Free Electron Laser,
• Applications of Laser
• Holography
UNIT III
Lecture 5
1
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 .
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).
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Contd.
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.
Because 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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Contd.
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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Contd.
Often 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.
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.
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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IN INDUSTRY
Drilling :
Laser drilling using either single or multiple pulses
from a stationary laser beam produces Holes. This
technique is often called as laser percussion hole drilling.
In percussion drilling the work piece is placed at or near
the focal point of the laser beam.
Short pulses from the laser causes a small volume of
work piece material to be first illuminated, then partially
melted and partially vaporized.
The explosive escape of the work piece material causes
most of the volume of molten material to be removed as a
spray of droplets.
Any material can be drilled with hole size ranging from
0.1 mm to 1.2 mm with length to diameter ratio 100:1.
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CUTTING :
Holes with diameters larger than 1.2 mm
(approx.) cannot be produced by laser
percussion drilling because of lack of power
density in defocused beam.
Hole sizes larger than 0.5 mm are usually laser
cut rather than drilled because the percussiondrilled holes have the characteristic rough shape
that lacks the high degree of roundness.
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Countd.
• The process of laser cutting combines the
concentrated light from the laser with a high
velocity gas jet to either vapourise (non-metals)
or to melt (metals) and thus rapidly remove
material.
• Cutting is accomplished without generating any
cutting forces at high speeds.
• Laser cutting of a workpiece can be started at
any location and can be continued in any
direction.
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MARKING :
A pulsed laser with high peak power density in
conjunction
with
computer-controlled
beam
scanning system is generally used in laser marking.
As the beam is scanned across the work piece, it
vaporizes a series of overlapping blind holes to
produce smooth bottomed grooves that make up
the identification letters and symbols. T
hese grooves are 0.25 mm wide and can be of any
depth up to approximately 0.25 mm.
Uniformity of groove depth is maintained up to an
accuracy of usually  0.005 mm.
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To ensure that material is removed completely and
to minimize thermal damage to the work piece, a
special method of pulsing a laser beam is often
used with laser markers.
With such pulsed lasers (lasting only a few
nanoseconds) power densities adequate to cause
sublimation of work materials are easily attained.
It is this combination of high power density and
short pulse duration that account for the laser’s
unique ability to smoothly machine blind features in
metals.
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WELDING :
For low distortion, high speed and autogenous
welds lasers are used.
Both spot and seam welding can be performed
either through pulsed lasers or continuous wave
lasers. For seam welding pulsed lasers capable of
high repetition rates or CO2 lasers are suitable.
Parts to be welded must have a tight fit with gap of
less than 5% the thickness of the material.
Argon and helium gas is used to locally protect the
weld puddle from oxidation.
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Countd.
• One important application of laser welding involves
sealing of electronic packages in special atmospheres.
• The operation is performed inside a chamber
containing the special atmosphere, thus the packaging
contains the special atmosphere.
HEAT TREATMENT :
• Lasers are generally used for surface hardening of
steels (carbon percentage greater than 0.3) upto 2 mm
depth.
• Laser beam is defocused to produce a power density of
only 150 to 1500 W/cm2 causing heating of the part
surface.
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Contd.
• The beam is traversed across the work surface at a rate
fast enough to avoid surface melting, heating rate up to
2,00,000C/s are obtained on the surface.
• The heat is conducted from the surface into a thin finite
volume of the metal beneath the beam.
• This volume of metal is rapidly heated beyond its upper
critical temperature, transforming it to Austenite.
• As the beam passes, a steady state condition between
heat input and heat conduction will be reached lasting
from 0.01 to 0.6 second.
• When the beam has moved on, self-quenching of the
heated layer occur because of the rapid flow of heat into
the cool substrate.
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Countd.
• Carbon dioxide lasers are most often used for
surface hardening because of their cost
effectiveness.
• These lasers emit the far infrared portion of the
spectrum at a wavelength of 10.6 microns which is
highly reflective on a bare metal surface.
• To maximise laser beam absorption, surfaces must
be coated with an absorptive material. Typical
coatings include flat black paint, colloidal graphite
or black oxide.
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CLADDING :
• In this process alloys are melt and selectively
deposited onto part surface.
• Defocused laser beam and a local shield gas are
used to melt the alloy.
• By using the laser to rapidly fuse powder, wire or
inplant inserts into a surface, distortion of the part
and dilution of the cladding material can be made a
minimum.
• The application of laser cladding include deposition
of wear resisting layers on valve seaks piston,
rings, rock drills, and turbine blades.
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IN MEDICINE :
Photodynamic therapy (PDT) :
• Certain chemical agents exist which have a particularly
specific affinity for malignant cells. When administered
systematically, they preferentially concentrate on the
cancer cells.
• Here they act as sensitizing agents, i.e., they make the
cancer cells more susceptible to destruction from
external energy stimulus than unsensitized cells.
• Current models of PDT laser units employ a 20 W argon
tuneable dye laser, producing a red light of 630 nm,
capable of penetrating tissues to a depth of 1.5 to 2 cm.
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Antibacterial uses :
Russian workers have used special lowenergy near infrared lasers with a wavelength of
890 nm for eradication of infections, all laser
actions on the organisms occurred through the
skin only, providing a non-invasive method of
treating such infections.
More significant work has been done in
infective conditions of the teeth, specifically the
root canal dentin and periodontal pockets, where
infections are particularly difficult to eradicate.
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Dentistry “Millenium” laser device :
Biolase Inc., California, USA has developed a laser
which employs jet of laser-powered water to cut teeth
and repair cavities painlessly.
Gastric tumours :
• Cancer of the gastric cardia producing dysphagia is
amenable to laser therapy.
• Small gastric tumors and early gastric cancers can be
treated by endoscopic Nd:YAG laser irradiation.
• Oesophageal and gastric outlet strictures can be
opened up by incision with laser.
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Ophthalmology :
• The argon laser is used in diabetic retinopathy, retinal
tears and miscellaneous retinopathesis.
Gastroenterology :
• Laser treatment of gastrointestinal disorders is still
fairly limited, even though considerable advances
have been made in gastrointestinal endoscopes.
• Endoscope is the only way by which a laser energy
may be delivered to an internal target lesion.
• Argon laser, Nd:YAG laser and diode lasers can be
passed along flexible quartz fibers.
• These are known as fiber optic lasers.
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Countd.
• The quartz fiber with the laser beam running
through it is passed through the endoscope’s
biopsy channel, a tube is placed around the fiber
through which CO2 gas is blown, both to clear
blood from the end of the fiber and off the target
site.
•
The CO2 is usually vented by a narrow-bore
nasogastric tube.
• Laser management of certain lesions, e.g.,
bleeding ulcer, is facilitated by the no-touch
property of the laser, whereby it can exert its effect
without coming in direct contact with the target.
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IN COMMUNICATION :
• Communication by laser is attractive for several
reasons.
• First is the extreme directionality of laser beams
compared to the beams produced by typical
microwave antennas.
• Another reason that optical communication
attractive is the information carrying potential of
laser beams.
• The amount of information that can be sent over an
electromagnetic wave is proportional to the
bandwidth of the wave.
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Countd.
• 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
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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
wave
reference
• Holography is a phenomenon of wave front
reconstruction
• To the observer the reconstructed wavefront
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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Holograohic mass 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 2–D images composed of
thousands of pixels, each of which represents a single bit
of information.
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Contd.
• Since an entire “page of data” can be retrieved by a
photodetector 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 pixelated 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 pixelated detector array, such as a
charge coupled device (CCD).
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Contd.
• 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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Contd.
• 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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Contd.
• 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 photochromic 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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Contd.
• Most erasable holographic materials are inorganic
photorefractive crystals doped with transition metals or
rare-earth ions.
• These crystals are often available in centimetre-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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