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CHAPTER-II

LASERS(Light Amplification by Stimulated Emission of Radiation

STR!CT!RE"

#$%$& I'TR)!CTI'*+E,TI,ES

#$%$%CHARACTERISTICS LASER *EA.#$%$/ATTE'!ATI' LI0HT I' PTICAL .E)I!.#$%$#PP!LATI'#$%$1STI.!LATE) A*SRPTI'#$%$2PP!LATI' I',ERSI' A') ACTI,E S3STE.#$%$4E.ISSI'#$%$5SP'TA'E!S E.ISSI'

#$%$6 STI.!LATE) E.ISSI'#$%$&7 LASI'0 ACTI'#$%$&& P!.PI'0#$%$&% PTICAL P!.PI'0#$%$&/ T3PES LASERS

A R!*3 LASER * He-'e LASER C SE.IC')!CTR LASER

#$%$ APPLICATI'S LASERS#$%$&1 S!..AR3

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#$%$& I'TR)!CTI'"

Laser is one of the outstanding inventtion of the 20thcentury . Laser is a photonic device, which isresponsible for the new birth of new field, namely photonics. In interference of light we use theterm coherence between two sources of light. The two sources are coherent when they vibrate insame phase or there is a constant phase difference. In recent years some sources are developedwhich are highly coherent, i.e.the radiation gives out by all the emitters in the source is in mutuaagreement not only in phase but also in direction of emission and in polarization. These coherentsources are called lasers. LA!" stands forLight Amplification by Stimulated Emission of Radiation. Though it is a light source of

light,lasers differ vastly from the traditional light sources. It is not used for illumination purposesand is used for radio wave and microwave transmitters and produce a highly directional coherentmonochromatic beam . laser is a tool in metal wor#ing , entertainment electronics, opticalcommunications, bloodless surgery weapon guidance in wars and in a wide verity of other fields.The first successful laser was built by T.$.%aiman in &'(0 . laser is a device to produce a powerfulmonochromatic light of beam in which the waves are coherent. The beam emerges as a narrowbeam which can travel over long distances without much loss of energy.

*+ECTI,ES"After studying this chapter you should able to) #now

About the features of lasers.

About the special characteristics of lasers. About absorption, spontaneous emission, stimulated emission and population inversion.

About different types of lasers li#e solid state laser, gas laser, and semiconductor laser.

The applications of lasers in different fields.

#$%$% SPECIAL CHARACTERISTIC EAT!RES LASER *EA.)The most outstanding characteristics of the laser beam are its high degree of directionality ,monochromaticity , coherence and brightness.

)i8ectionality" the laser beam is highly directional. e*) a laser beam &0cm in diameter when

beamed at %oon+s surface.,-,000#m away is more than /#m wide. The laser beam spreadis less than 0.0&mm for a distance of &m.

.onoch8omaticity" a laser produces light in more or single wavelength i.e. the line widthassociated with laser beams are e*tremely narrow.

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Cohe8ence" a laser beam is spatially and temporally coherent to an e*traordinary degree.

Tempo8al cohe8ence"it refers to relative phase or coherence of the two waves attwo separate locations along the propagation direction of two beams.

Spatial cohe8ence" it referred to transverse coherence. it is possible to observeinterference effect from two independent laser beams.

*8ightness" the laser beam is highly intense as compared to ordinary sources of light. It can

be used for such operations as weding which involve reaching high temperatures.

#$%$/ATTE'!ATI' LI0HT I' PTICAL .E)I!." hen light travels through a medium , a gradual reduction in its intensity occursmainly because of the process of absorption of light and scattering of light in the medium.

&. Light absorption occurs because part of incident light is transformed into the energy ofmotion of atoms in the medium.

2. Light is scattered when it encounters obstacles of sizes smaller than a wavelength. The reduction in intensity with distance in medium is called attenuation of length.

1ig)

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#$%$# PP!LATI'" A material medium is composed of identical atoms which are characterized by a specific systemof energy levels. These energy levels are common to all atoms in the medium. A certain number ofatoms occupy a certain energy level. The number of atoms per unit volume that occupy a givenenergy level is called population of that energy level.A particular medium has atoms, which are characterized by only two energy levels. Let !& be theground level and !2 be the e*cited level. Atoms are differently distributed in these two energy

levels. Let the populations in these two energy levels be & and2 respectively. The number ofatoms in the lower energy level is always larger than the number of atoms in the higher energylevel .under normal conditions higher the energy of energy levels, lesser is population. $ence&332. uch distribution of atoms among the energy level is termed normal distribution .if energydifference 4!25!&6530 again 27&. They show as long as medium is in thermal e8uilibrium thepopulation of higher energy level cannot e*ceeded the population of lower energy level.

#$%$1 STI.!LATE) A*SRPTI'"The process of particle transfer from normal state corresponding to minimum energy of the systemto a higher energy state is termed as e*citation and the particle itself is said to be e*cited. Theprocess is called stimulated or induced absorption.If an atom is in the lower energy level !& .if a photon of energy hv 7 4!25!&6 is incident on atom itimports its energy to the atom and disappears. Then the atom absorbed an incident photon. As aresult of absorption of energy, the atom 9umps to the e*cited state ! 2. The transition is calledabsorption or incident absorption.

Induced absorption involves the e*citation of atom to the fi*ed energy level only. As a result decreases and 2 increases. :ut under normal conditions 2cannot be greater than &. As lightpropagate through the mediumIt gets absorbed. 2can be made greater than &using special techni8ues.

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PP!LATI' I',ERSI' A') ACTI,E S3STE.";sually the population 2of higher energy level is less than the population &of the lower energylevel. This condition is called normal condition or thermal e8uilibrium, &332%a#ing the number of particles n2 more in higher energy level than the number of particles & inthe lower energy levels 423&6 is called as population inversion .or inverted population.Thus the condition where the no of atoms in the e*cited state e*ceeds that in lower state is calledpopulation inversion. The state of population inversion is referred to as a negative temperature

state .It is not a physical 8uantity but signifying the e8uilibrium state of system. The states ofsystem, in which the population of higher energy state is more in comparison to the population oflower energy states, are called negative temperature states. hen the system has a number ofstates .Then one of them may have a negative temperature with respect to other state or states. Asystem in which population inversion is achieved is called an active system.The state of population inversion is a non e8uilibrium state. The non e8uilibrium condition isattained by employing pumping techni8ues to transfer large number of atoms from lower energylevel to higher energy level.

LIE TI.E R .EA' LIE"the particle can remain in the e*cited state for a limited time #nown aslife time .the life time of the e*cited hydrogen atom is of the order of &0 5-sec.

.ETASTA*LE STATES" the e*cited states in which the life time is greater than &0 5-sec. Thesestates are called %eta stable states.

In the process of emission there are two types. They are

1. SPONTANEOUS E!SS!ON

". ST!ULATE# E!SS!ON

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)ISTI'CTI' *ET9EE' SP'TA'E!S E.ISSI' A') STI..!LATE) E.ISSI'"

SP'TA'E!S STI.!LATE)

&. Transitions occurs from higher energy level tolower energy level

&. Transitions occurs from higher energy level tolower energy level

2. o incident photon is re8uired 2.

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#$%$&& P!.PI'0"The method of raising of the particle from lower energy state to higher energy state is called as

pumping or process of producing population inversion is #nown as pumping. This can be done bynumber of ways the most commonly used methods are&. =AL >=LL!I= =1 AT=% etcIf the active medium is transparent and non conductivity in nature then the optical pumping isused . if the medium is conducting in nature then the electric field is used to attain populationinversion.

#$%$&%PTICAL P!.PI'0"The most common method of pumping is optical pumping. >onsider a three level system. let usallow all transitions between these levels. If the system is e*posed to a e*ternal source with afre8uency>orresponding to the transform level& to level .1ig)

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if the life time of the particles in level is sufficiently long4meta stable state6 , their number inthis level will e*ceed that in level 2. in this way level is inversely populated with respect tolevel2. thus by optical pumping , the condition for population inversion is obtained4326. 1or a

light of fre8uency 2 the system acts as laser. The stimulated radiation together with originaincident radiation both coherent can stimulate the emission of more coherent , radiation by otherparticles in the higher energy state . this process can go on ,provided enough molecules are inhigher energy state.In fig 4b6 the population inversion 23&is created. $ere the necessary condition is that the state 2must be in meta stable state. =ptical pumping raises the particles to level , from where most ofthem return to state& but some go to state2. the probability of transition from state2 to state& isvery low hence in due course of time 23&, which is population inversion. In the presence ofe*ternal photon h&%, stimulated emission ta#es place from state2 to &. hence a laser beam ofenergy h&%is obtained.

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#$%$&/ T3PES LASERS"

Among the various #inds of lasers, solid state lasers, gas laser, li8uid laser, dye laser andsemiconductor laser are some important types of lasers.&$ SLI) STATE LASERS ""uby laser, d)BA? laser%$ 0AS LASER " $e5e laser , >=2 laser/$ LI:!I) LASER " !uropean chelate laser, e=>l2laser#$ )3E LASER " "hodamine (? lasers1$ SE.IC')!CTR LASER "In< laser ,?aAs laser.

A R!*3 LASER"

"uby laser is a three level solid state laser and was constructed by %aiman in &'(0. it is a pulsedlaser having very high power of hundreads of mega watt in a single pulse with about &0 nanosecond duration"uby4Al2=6 laser uses a crystalline substance as a active material.$onst%uction& it consists of three main parts.4&6 an active wor#ing material) a rod of ruby crystal in the form of cylindrical rod to/ mm in

diameter and / to 0 cm long.426 A resonant cavity) it is made of fully reflecting plate at the left of ruby crystal and a partially

reflecting plate at the right of ruby crystal . both the plates are optically plane and e*actlyparallel to each other.

46 !*citing system) a helical *enon flash tube with power supply source.

'o%(ing&ruby is a crystal of aluminum o*ide 4Al2=6 in which some aluminum atoms are replacedby chromium atoms. The active material in the ruby are chromium ions >rC. when the ruby crystacontains about0./D of chromium its color is pin#.

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In a ruby laser, a pin# rod of cm length and 0./ cm in diameter is generally used. The end facesof rod are made strictly parallel, ground and high degree. The end faces are silvered that one endbecomes fully reflecting while the other end partially reflecting. The ruby rod is surrounded by ahelical *enon flash tube which provides the pumping light to raise the chromium ions to upperenergy levels. The flash of *enon tube lasts several milliseconds and the tube consumes severalthousand 9oules of energy. =nly a part of this energy is used in pumping the >r Cions while the restheats up the apparatus. 1or this purpose a cooling arrangement is used.&, 2 E represent the energy levels of chromium ion. In normal state, chromium ion is in lowerlevels. hen the ruby crystal is irradiate with the light of *enon flash, the chromium atoms aree*cited and pass to upper level where the light absorption band is /(00 A o. The absorption bandwidth of this level is about -00 Ao. ew few e*cited chromium atoms return to ground level & andother to level 2. The transition 532 are non irradiative. The chromium atoms give part of energyto crystalline lattice in the form of heat.%ost of e*cited atoms in state returns to state & but those which go to state 2 have a littlechance to return to state & spontaneously. After few milliseconds the level 2 becomes morepopulated than level &. And hence population inversion is achieved. The state of invertedpopulation is unstable and the probability of spontaneous transitions is very high. The spontaneoustransitions may cause an induced transition which produces a photon. This photon #noc#s outsecond photon from neighboring atom. The atom from which the first photon was emitted isbrought to ground level. The photons will #noc# out two more photons and their total number wilbe four and so on.

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1ig) !nergy level of ruby

* He-'e LASER(0AS LASER"The main drawbac# ofruby laser is that the output beam is not continuous though very intense.1or the continuous laser beam, gas lasers are used. In gas lasers the vapors of metals areemployed as active medium .the main advantage of gas lasers are high monochromaticity, mostpure spectrum and high stability of fre8uency. The out put power of gas laser is moderate. Theyhave wide application in science and it engineering pertucurlly in communications. It is a fourlevel laser systemIn &'(&, A.FAGA, w.:ennett and @.$erriot reported a continuous gas laser.

C'STR!CTI'"the gas laser consists of a fused 8uartz tube with a diameter of about &./ cmand -0 cm long. The tube is filled with a mi*ture of neon under a pressure of 0.&mm of mercuryand helium under a pressure of &mm of mercury . There is a ma9ority of $e atoms and minorityof e atoms. And one end of the tube there is a perfect reflector on the other end is partiareflected . the active material is e*cited by means of high fre8uency generator with a fre8uencyof several tens of %$z and an input of about /0 watt.

1ig)

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9R;I'0"when a discharge is passed through the gas , the electrons are accelerated towards thepositive electrode. @uring their passage the collide with $e atoms and e*cite them into upperstates labeled $e2and $e.These are meta stable states . The atoms remain in these levels for a sufficiently long time. owthese atoms interact with neon atoms which are in ground state. The interaction e*cite the neonatoms to their %eta stable state labeled eand e(while $e atoms returns to their ground state.As the energy e*change continuous, the population of neon atoms in e*cited states increases moreand more. hen an e*cited ne atom passes spontaneously from meta stable states e and e(tostates eand e/ it emits a photon. This photon travels through gas mi*ture. If this photon ismoving parallel to the a*is of tube, it is reflected bac# and forth by the reflectors until itstimulates an e*cited neon atom and causes it to emit a fresh photon in phase with stimulating

photon. This process is continuous and laser beam builds up in tube. hen the beam becomesintense a portion of it escapes. Through partially reflected end. The neon atom now passes downspontaneously to a lower state e2by emitting incoherent light. 1inally the e atom comes toground state through collision with tube walls. This is radiation less transition.?as laser emit light which is more directional and monochromatic. ?as lasers are capable of

operating continuously with out need of cooling.

C SE.IC')!CTR LASER (0a-As LASER"emiconductor laser is also called as diode laser. These have important applications in fiber optic

communication.

PRI'CIPLE"when a current is passed through a p5n 9unction, p region positively biased. $oles arein9ected from p region into n region and electrons from n region to p region. the electrons andholes recombine and release of energy ta#es place near the 9unction region .this energy is calledactive energy depends on type of semi conductor . In semiconductors li#e germanium and silicon ,most of the energy released in the form of heat because the recombination of carriers of oppositesign ta#es place through interaction with atoms of crystal. :ut in case of ?allium Arsenide theenergy is released as light because the atoms of the crystals are not involved in the release ofenergy. The wavelength of emitted light depends on activation energy of the crystal.

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hen e*citing currents are small, a small part of carriers undergo recombination. The process ofspontaneous. The laser radiation is random and incoherent. hen the current density is increasedthe emission becomes more and more coherent and radiation intensity increases. ow efficiencyof ?aAs lasers reach 0D.hen cooled to 20#, semiconductor lasers have delivered output of more than 2 watts ofcontinuous power. The semiconductor lasers may reach &00D efficiency. They have a high stabilityof output fre8uency.

#$%$6 APPLICATI'S LASERS":ecause of high directionality, high coherence, high monochromaticity and high intensity lasers

are widely used in the fields of4&6 communication 426computers 46industry 46scientific research 4/6military operation

4(6medicine

LASERS I' C..!'ICATI'"In case of optical fiber communication, semiconductor lasers diodes are used as optical sourcesand its band width is used very high compared to radio and microwave communication.4&6 as it has large band width , more amount of data can be sent.426 ignals cannot be tapped.

46 @ue to its high directionality and less divergence, it has greater potential used in spacecrafts and submarines.

46 ;sed in high speed photo copiers and printers.

LASERS I' C.P!TERS"4&6 In LA , data can be transferred or transmitted from the memory storage of one

computer to another computer using laser for short time426 >@5"=% stores large amount of data used in >@5"=%s during recording and reading the

data.

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#$%$&1S!..AR3"

LA!" means light amplification by stimulated emission of radiation. !*citation of atoms from lower energy state to higher energy state due to interaction of

radiation with matter is #nown as stimulated absorption.

hen electron in the e*cited level !2falls spontaneously to lower energy level !&after itslife time a photon is emitted. This process is called spontaneous emission.

hen electron in the e*cited level !2 is induced by a photon of energy !25!&the electronmoves to lower energy level !& emitting another photon of energy !25!&. This process iscalled stimulated emission.

timulated emission of radiation results in amplification of light.

To get the LA!" beam the population of e*cited state must be greater than the population

of lower energy state. This condition is called population inversion. There are different pumping mechanisms to e*cite the atoms from lower energy state tohigher energy state.

There are different methods to get laser beam for e*ample ruby laser, $e5e laser, and solidstate laser.

@ue to high energy laser beam possess so many applications in many fields li#e engineering,medicine, military etc.