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Laser action summaryStep 1 : Choose a proper lasing mediumStep 2 : Establish population inversion by suitable

pumpingStep 3 : Stimulated emission takes placeStep 4 : Positive feed back (optical resonator)Step 5 : Amplification of light

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Characteristics of laserDirectionality      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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)dd(2

)aa(

12

12

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

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 broadeningThe 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 broadeningIf 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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CoherenceThe 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 coherenceTemporal 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 time =

The high degree of temporal coherence arises from the lasers monochromaticity.

coherent lentth

velocity of light

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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 BrightnessWhen 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 emission

Property

Spontaneous

emissionb(ordinary light)

Stimulated emission

(laser light)

Stimuli Not required Required

Monochromaticity

Less High

Directionality Less High

Intensity Less High

Coherence Less High

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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 pumpingExposure to electromagnetic radiation of frequency = (E2-E1)/h obtained from discharge flash tube results in pumping Suitable for solid state lasersii. Electrical dischargeBy inelastic atom-atom collisions, population inversion is establishedSuitable for Gas lasers

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iii. Chemical pumpingBy suitable chemical reaction in the active medium, population of excited state is made higher compared to that of ground stateSuitable for liquid lasers.

Optical resonatorA 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 sourceDye 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

typeOperation 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 dischargeInterferometry, holography, spectroscopy, barcode scanning, alignment, optical demonstrations.

Argon laser454.6 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 dischargeRetinal phototherapy (for diabetes), lithography, confocal microscopy, pumping other lasers.

Krypton laser416 nm, 530.9 nm, 568.2 nm, 647.1 nm, 676.4 nm, 752.5 nm, 799.3 nm

Electrical dischargeScientific 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 dischargeMaterial processing (engraving, welding, etc.), photoacoustic spectroscopy.

Excimer laser

193 nm (ArF), 248 nm (KrF), 308 nm (XeCl), 353 nm (XeF)

Excimer recombination via electrical discharge

Ultraviolet lithography for semiconductor manufacturing, laser surgery

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CO2 Laser

IntroductionCO2 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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Active mediumIt 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 resonatorsA pair of concave mirrors placed on either side of

the discharge tube, one completely polished and the other partially polished.

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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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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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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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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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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

10.6m.

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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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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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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) lasersIntroduction

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 processes1)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 radiation3) 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 laserOptical feedback should be providedDone 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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HeterostructuresThe 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 N- indicate heavy doping and light doping respectively. The P-N structure consists of the two double layers, P+ - P- and 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

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 Wavelength

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 laserIntroductionFree 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 HologramA 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 plateThe 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 memoriesHigh density optical storage

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High Density Optical StorageAn 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.