UNIT-III

Main componentsof a Laser-

The main conponents of a laser givenas-

1.Active Material - Active material such as the

substance have many number of free electrons.

And also have Met stable states.

Resonant Civity- it is a specially prepared

cylindrical tube in which light intensity can boil

up by multiply reflections. The ends of the tube

are silvered, one end is made perfectly

reflecting while the other end is made partially

transmitting , so that a very intense bean can

emery cut from is.

Pumping system- By this the population

inversion is achieved . in population inversion

state the Number of atoms in hither energy state

are more than that in lower energy state (i.c.N 2

>N1 ) Under ordinary conditions most of the

atoms are in the ground state or lower energy

state E1 and every small Number of a forms in

higher energy state E2 i.e.(N2<N1 ) under normal

condition so that there is a little chance of

stimulated emission as compared with

absorption therefore an incident photon is more

likely to be absorbed them to cause stimuli ted

emission. A large number of atoms are available

in higher energy state (E2) stimulated emission

is promoted (i.e N2>N1)This condition makes

probability of stimulated emission greater than

the probability of induced absorption.

Principle of laser action- The principle of laser

is based on stimulated emission. The stimulated

emission is the process in which a photon of

right energy may induce an atom in excited

state to emit a photon and makes a transition to

the ground state. The excited atom would itself

emit photon and make a transition to the ground

state. But the incident photon of right energy

induces the excited atom to emit photon earlier.

The process may be represented symbolically as

follows-

Atom + proton→ atom+2 photon

This type of transition with emission of photon

is called the stimulated emission. The special

characteristic of stimulated emission is that the

two photons that are emitted coherent and

possess exactly the energy and same direction.

in the laser action, consider a group of a toms

all in the same excited state. A passing photon

may cause stimulated emission in one of these

a tons. This results in two coherent photons of

same frequency. Each of these two excited

atoms. Thus there will be now four more

excited atoms emit four photons. Now there will

be produce 8 photons. The process in

cumulative with the result that we get an intense

beam of coherent photons moving in the same

direction. This is the profile of operation.

Types of laser- There are several ways in which

we can classify the different types of laser,

Accordingly laser are broadly classified in to

five categories-

Solide state lasers- the term solid state has

different meaning in the field of electronics and

laser. A solid state laser is one in which the

active center are fixed in a crystal or glasry

material . the solid state lasers are-

.Rady laser

Nd- yag laser

Nd –Glass laser

Tunable solid state laser etc.

2.Gas Lasers- Gas lasers are the most widely used

lasers. They have range from the law used in collage

laboratories and very high power co 2 laser used in

industrial application .there are different types of

gas lasers ion lasers natural atom lasers and

molecular lasers.

The first gas laser was (He-He)laser, which was

demonstrated in 1961 at Bell telephone

laboratories.in USA by Ali javan.the different types

of gas lasers are

He-He laser

Argon laser

He-cd laser

Carbon diocxide laser

Excimer laser.

3.Dye lasers or liquid laser- Dye laser belongs to the

family of liquid laser the active material is a dye

dissolved in a host medium of a liquid solvent, such

as ethylene glycol. The dye laser differs from others

lasers that emit light at a fixed wavelength.

4.Semiconduetor lasers- Semiconductor lasers is

fabricated p-n junction device that emits current

light ,when it is forword biased.

The first semiconductor lasers made in 1962 by

R.N.Hall and his coworkers of general electric

R&D Laboratories in U.S.A.It was made from

gallium arsenide (GaAs),which orated at low

temperature and emoted light in the near infrared.

5.Chemical Laser- Chemical Lasers depends on the

excitation of atoms in the laser systems deseribed so

far was accomplished by optical pumping of an

electric discharge . the distribution of energy an

energy level system which follows a chemical

reaction might result exclusively in excited state

leaving lower states completely depopulated. Hence

achemical reaction could be extremely efficient this

types of laser

Hu Laser

Hf Laser

Solide state laser- The laser systems using active

material in solid state are called solid state lasers.

The material used for laser active should have story

absorption bands and high degree of quam tum

efficiency for fluorescent transists. The crystals of

glases having these characteristics are doped with

small amount of depots the do pants have optical

transitions between inner incomplete atomic shells .

The dopant materials having this characteristic are-

Transition elements like er3+

Rare earth elements like Nd+3

Actinide series elements like U3+

The premed materials must have pumping transition

in the broad band. The optical pumping can be

achieve by sources of light like are lamps, flash

lamps. The host material crystalline solide and

glasses, must have the following.

Characteristics-

The material must be hard and chemically inert.

The material must power excellent optical, the

small and mechanical properties.

The material should have no internal strain or

impurity

The refractive index of material should be

constant the ragout

The material should not be damaged by

radiation.

The valiancy of do pant ion must match with

the host ion. the cries tagline materials used as

hosts have clearer and sharper emission of

fluorescence as compared to glasses .the

commonly used crystalline hats are sappier

( Al2o3)yttrium aluminum garnet (YAG) and

calcium – tungsten oxide(Cawo4) the commonly

used glass hosts are silicate and phosphate

glasses. Some of common combination of do

pant and hostage –

Cr+3−Al2 o3

Nd +3-YAG

Nd+3-Cawo

Nd+3- Class

The spectral range of solid state laser with in

0.6 micro to 2.5microns.

Selection of Material – for selection of material

for selected state lasers, the following points

must be kept in mind.

1.The fluorescent line with of the material should be

quite narrow.

2.There should be broad and intense absorption

bands.

There band must match with the spectral cut put of

the pump for effective utilization of pump radiation.

3. The host crystal must be strong and tough to work

eve under of severs ,operating conditions.

4. the non- radioactive life time of the upper level

taking part in laser action must be much greater than

the spontance oust radioactive life time of the atom.

This condition is necessary for better internal

efficiency of laser.

Difficulties in laser process- 1. The like time of an

excited state is of the order of 10 -8 second. This

means that before on excited atom be stimulated to

emit photon, it is most likely to make a spontaneous

emission. The photon, it is most likely to make a

spontaneous emission are not coherent . the Number

of excited atoms next and Number of atoms (No) in

ground state are related as Mex=Noe∆ E/ KBT

Where ∆ E=( E2−E1)is the energy difference between

the ground state and the excited state, k B is the

Bolt2 Man’s constant and it is temperature in

Kelvin. For radiation by a toms ∆ E<<KBT ,Hence

in thermal equilibrium the population of higher

energy states in much smaller than that of

ground state.

The incident photon of appropriate energy is

likely to be absorbed by absorption is same as

that of stimulated emission. As that of

stimulated emission. As the population of

unexcited atoms is much larger than that of

excited atoms ans moreover that life time of

excited states is very short. Due to these two

reasons most of the photons of the incident

radiation cause induced absorption so the

probability of stimulated emission is very small.

Removal of above difficulties- the above

difficulties may be removed by-

.Taking to active material having three energy

states, out of which the intermediate energy

state is metes table state.

Choosing a suitable resonant cavity, having

perfect reflector at the other end.

Creating population inversion by pumping, so

that the number of atoms in excited met stable

state is more than the atoms in ground state.

Three level lasers- Two level laser is not

possible practically due to following two

reasons-

In a two level laser, the population inversion is

not possible.

The upon met stable level must have narrow

band to get a monochromatic transition, while

the pumping radiation has a comparatively

broader band of frequencies. this will effect the

efficiency of laser system.

Out the these two reasons, the first one is more

serious . that is why laser action employs three

or four energy states. To explain the laser

action considers a three energy level active

material as show in the figure.

In the figure E1 ground state energy ,E2 is the energy

met stable state and E3 is the energy of excited state.

By supplying energy (by optical pumping)a large

Number of atoms are raised from ground state level

E1 to any of the levels in the broad, excited Energy

band E3. In excited state, atones decal rapidly to

Energy level E2 by spontaneous emission. The

Energy difference level E2 greater then number N1

in ground state E1 so population inversion achieved

this condition necessary for laser action.

Let N1,N2 and N3 present the Number of atoms per

unit volume in energy levels E1,E2 And

E3,respectively. we assume that only these three

levels are populated and the transitions take place

only bet between levels. Then the total number of

atoms per unit volume.

N=N1+N2+N3 ……………………..…

(1)

Population of level 3 changes due to induced

abruption from level 1 to level 3 (1→3)1 stimulated

emission from level 3→2

There for the rate of change of population of level 3

is given by the equation.dn3

dt=℘ (n1−n2 )−N3 T 32…………. (2)

Where we is the probability of induced absorption

for transition 3→1and T32 is the probability for

transition 3→2

Further T32 =A32 +S32 ………………………………..

(3)

Where is Einstein’s coefficient corresponding to a

radioactive Transition rate S32 in the non-radiative

transition rate from level 3 to level 2.

.dn2

dt=wi ( n1−n2 )+N 3T 32−n2T21

…………. (4)

Where first term represents the net stimulated

transition between level land 2, the second term

represents the spontaneous transition from level

3 to level 2 and the last term the spontaneous

transition from level 2 to level 1. The Quantity

w1 is proportional to Einstein Coefficient B 21

and the energy density associated with lasing

transition 2→ 1if this transition is radioactive. Then

T21 <A21 where A21 the electronic

client.similargy the rate o change of population

of level is given by

.dn1

dt=w p ( n3−n1 )+w1(N ¿¿2−N 1)+n2T 21

¿ (5)

Where the first and second terms represent the

stimulated transition between levels 1 and 3 and

the last term represents the spontaneous

transition from level 2 level.

As N is constant so from equation (1)

dn1

dt +dn2

dt+

dn3

dt =0 (6)

Equation (2),(4),(5) gives the rate of change of

populations of three levels and are called the rate

equations of a three level laser system at steady

state, the time derivative of population os each level

is zero ie

dn1

dt =0,dn2

dt=0 ,

dn3

dt =0

From εgn (2 ) puttingdn3

dt=0∧N2=N 3 for condition

Wp (N1-N2)-N3T32 =0

Wp (N1-N3)-N3T32 =0 ❑⇒ Wp N1-wpN3=N3T32

=N3T32 +Wp N1 =N3 (wp+t32)

=N3wpn1

℘+t 32………………….(7)

Similarly from εgn (4) putting 3dn2

t =0

Wi(N1-N2)+N3 T32 –N2 T21=0

❑⇒ W iN i-W i H2 +N3T32-N2T21-N2T21=0

❑⇒ W iN i-N3 T32 =W iN2+N2T21=N2

❑⇒ W i W i+T 32 N3

W i+T 21………….(8)

Using εg(7) N2=W i N i+T 32(

W p N 1

℘+t32

)

W i+T 21

N2= ¿……………(8)

The population difference between level 2 and 1 is

N2−N1

N 1+N 2+N 3

Using equations (1)(2) and (8) the rutioof

population difference of level 1 and 2 and the total

Number of a atoms is

N 2−N1

N=

N2−N1

N 1+N2+N3

……… ..(9)

Similar from we finf the value of N 3, N2, N1, and put

in εgn(9)

Then we find

N 2−N1

N =w p

(T ¿¿32−T 21)−T 32 T21

3w p wi+3℘T 21+2 T 32w i+T32 w p+T

32T

21

¿……………(10)

From this equation it is obvious that for population

inversion between levels 1ie for N2>N1 then

T32>T21 ………………………..(10)

As the relaxation times of a tons in level 3 and 2

inversely proportional to the corresponding

relaxation rates. There for in according with εgn(10)

for achieving population inversion, the life time of

level 2 must be greater than the life time of level 3

i.e. energy state to must be met stable state.

In assertion to this condition in order tracheae

population inversion a minimum pump power is

required for this the minimum value of WP from

equation (9)is giving.

(wp)min (T32 –T21 )- T32 –T21 =0

⇒(wp)min =

T32 – T 21

T32−T 21……………..(11)

Thus for population inversion the following two

conditions must be satisfied (1)the level 2 must be a

meta stable state.

Wp> =T32T 21

T32T 21

Above figure represent four level E1, E2, E3, E4,…….

Are the energies of level, E 1 being ground state , E4

Exited state and E3Mta stable state and E2other met

stable state the pumping source carries some of the

atoms from ground level to one of the higher excited

level of energy E4 the life times of excited state E 2

and E4 is avout 10 -8second while that of level E 3 is

about 10 -3

Second so that E3is matestable state .there are three

transition is this process

There is a rapid non-radioactive decay from

level E4to level E3

As E3is a met stable state there is a lasing

action between E3and E2

There is a non radioactive rapid decay from

level E2to E1 however it so happens that the

transition E4❑⇒ E3 and E2❑

⇒ E1 are much faster as

compared to E3❑⇒ E2 Hence it is easy to achieve

population inversion with a four level system

than with a three level system.

The level 4 could be a collection of large number of

levels or a broad level than an optical pump milting

radiation over a band of frequencies can efficiently

excite atom from level 1to level 4 also the can not

be the upon laser level be cause the upper laser level

is required to be mirror.

Moreover the pumping power required in four level

laser is much less than a three level system. Because

in Faure level system there is no need to excite more

than half of the atoms from the ground state to get a

population inversion. This results greater efficiency

of a four level laser system than that of three level

system. He -Ne and nd –YAG lasers comes in four

level lasers.

Let N1, N2, N3, N4, =constant ……………(1)

The rate of change of population pr unit volume of

level 4 is giving by

Dn4

dt=℘ (n1−n4 )−T4 N 4 …………….(2)

Where wp (n¿¿1−n4)¿represents the net rate of

stimulated transited occurring between levels 1and 4

caused by pumping, T4 is the net relaxation rates

from level 4 to any lower level (1,2,3…....)then T 4

=T43+ T42+ T41……………….(3)where T4 represent

total relaxation rates (i.e. both radioactive and non

radioactive)T43 is much greater than T24 and t41so

most of the atoms pumed to level 4 dropdown to

level 3 the second term in equation(2) represent the

rate at which atoms deejay from level 4

The rate of change of population perunit volum of

level 3 will be given by-

Dn4

dt=¿ T32 –T4 +w i (N2 –N3)- =0 T3 –N3 ………………..

(4)

Where T3 =T32+T31

In equation (4) the first termterpresentsthe rate at

which the items jump from level 4 to level 3 the

second term represent the rate of stimulated

transition from level 2 to level 3 due to presence of

laser radiation and the third term represents threat of

loss of atoms from level 3 to levels 2 and 1 through

spontaneous transitions. Similarly the population

rate of level N2 and would be

Dn4

dt=¿ T32 –T4 +w i (N2 –N3)- =0 T3 –N3 ………………..

(5)

AndDn4

dt=¿ T41 N4 +wp (N4 –N1)+T31 N3+T21N2………..

(6)

The inter predation of different term in above

equations are similar as in equation(4)

As N is constant for all Rates then

Dn1

dt+

Dn2

dt+

Dn3

dt+

Dn4

dt+

Dn5

dt =0……………..(7)

For steady state-

Dn1

dt=

Dn2

dt=

Dn3

dt=

Dn4

dt=¿0…………..………(8)

In most four level lasers the atoms from level 4 go

primarily to level 3 and hence T42<< T43 and T41 <<

T43 Also the atoms from level 3 go to level 2 i.e.

T3 =T32

Under these circumstances we see that in order to

obtain population inversion N3> N2 we must have

T21>T32 this means that the rate of decay of a toms

from level 2 to 1 must be greater than the rate of

decay from level 3to2 under this condition the

creation of population inversion between levels 3

and 2 is independent of the pumping power (wp) but

the magnitude of population inversion depends on

wp.

For a good laser action T3 << T43 and T21>> T32 and

T4 = T43 under such an approximation the population

difference

.n3−¿n2

n=

℘/T 3

1−¿¿¿………….(9)

Equation(9) shoes that the population inversion can

be achieved much more easier in four level system

that the three level system.

It consist of pink ruby cylindrical rod whole ends

are optically flat and parallel .one end is fully

silvered and other is only partially silvered. The rod

is surrounded by a glass tube upon which is would a

coiled flesh lamp filled with xenon gas as shown

above figure.(a)

The ruby rod is a crystal of aluminum oxide (Al 2 o2 )

Doped with 0.05% chromium oxide (CR 2 o3 ),so that

some of the aluminum atoms in the crystal lattice

are replaced by chromium ions (Cr+++)it is the small

amount of chromium ions which given pink color to

the Ruby.

As seen from the energy level diagram the

chromium ion has two energy band E 1 and E2 above

its ground level G and met stable level in slightly

below the energy land E1 the level M has a life time

of about 3×10-3 second)

Which is about 105 t imes greater than the life time of

other exited levels E1 and E2 (which is ≈10-8 second)

When a flash of light falls upon the ruby rod, the

green and yellow light photons are absorbed by the

chromium ion which are excited to energy states E 1

and E2 there absorption transitions 1 and 2 are the

pumping transition he excited ions give up by

collision a part of their energy to the crystal lattice

and pass to the mates table state. In the

corresponding transition 3and 4 are thus non

radioactive since the state in has Avery long life, the

number of ions in this state goes on increasing and

exceeds the number in ground state G thus

population inversion is established between

detestable state M and ground state G. Shown above

figure. (2)

When an (exited) ion passes spontaneously from the

detestable state to the gaunt state (transitions) it

emit a red photon of wave long in 6943A this photon

travels through the ruby red and it is moving

parallel to the axis of the crystal is reflected back

and forth the causes it to emit a fresh photon in

phase with the stimulation photon (stimulated

transitionsition 6) the process is repeated again and

again because the photons repeatedly move along

the crystal, being reflected from its ends. The

photon thus multiplies, when photon beam

sufficiently intense part of it emerges through the

partially silvered end of the crystal.

He-Ne (Helium Neon) LASER

He-Ne (Helium Neon) laser consist of a narrow

discharge tube filled with a mixture of Helium and

Mean in the ratio of 10 % the pressure inside tube is

maintained at 1 mm of tlg.M1andM2 are two mirrors

(plane or concave) kept at the ends of the take.

Mirror M1is highly silvered and M2 is partially

transparent as Shawn above figure (a)

M1andM2 to gather constitute a resonating system.

When electrodes are connected to a source of high

potential electric discharge passes. Helium atoms

excited first due to collision of helium atoms with

neon atoms they are excited. These oscillations

result in population inversion is reached any of the

spontaneously emitted photons will trigger laser

action in tube. A will collimated monochromatic

intense beam emergs out of the tube .Gas laser are

preferred compared to ruby laser because the laser

beam is more directional and monochromatic intence

beam emergs out ot the tube .Gas laser are preferred

compared to ruby laser be cause the laser beam is

more directional and monochromatic. Moreover gas

laser can be operated continuously and there is no

need for frequent cooling as in case of solid state

laser.

The laser action takes place in neon atoms he is the

mixture serves the only purpose to enhance the

excitation process the strongest emission line occur

between 25 and 2 p level with a wake ling in 11573

A0

IONICLASERS ❑⇒ I. Argon ion laser ❑

⇒

There is a special class of gas lasers called noble –

ion lasers. These lasers operates in ionized species

of the gases like Argon neon, krypton and xenon,

therefore these are called .Ionic lasers . the argon

and krypton ion lasers provide out put in the form of

continuous wave, through some of them can operate

in pulse mode too.

Argon ion laser consist of Argon ion as laser

medium and operates in visible and ultraviolet

spectral region. About 25 different visible

wavelengths can be evolved with argon ion laser

between 408.9 and 686.1nm. they can also operates

in tho ultraviolet wavelengths between 275 and 363

nm. A schematic diagram of Argon ion laser is given

blow-

Like He-Ne laser, Argon ion laser consists of a

resonant cavity (or tube) made of graphite or

beryllium oxide having He-Ne laser, an additional

tube called the Return Tube is provided in Argon

ion laser.

Due to high current density, Argon ion (Ar t) move

towards negative electrode and electrons ( e) moves

towards positive electrode, the return longer than

the laser tube itself to arrest the electric discharge

and stop it from passing through the laser tube. At

the high

current density the tube may get damaged due to

collision by not ions with the tube walls.

The temperature of ions in about 3000 k. that is

why the tube is made of graphite or beryllium oxide.

Asiatic magnetic field is applied parallel the tube In

the discharge region to reduces to diffusion rate of

electrons towards the walls of the tube .the most

intense wavelength .the most intense wavelength are

λ1=488 nm (in blue region) and λ2=514.5 nm (in gradation

region) the two wave long the are quite important

because the photo emissive cathode photographs and

human eye have maximum response. At this wave

long the oscillation response at this wavelength may

be selected by placing prism inside the resonant

cavity.

2. Krypton ion Laser (An ionic Laser)

Krypton ion laser is similar to Argon ion laser in

energy levels and characteristics. It is note that the

krypton ion laser can be produced by Argon-ion

laser, just by replacing Argon by krypton in the

resonant tube. Due to difference in wake lengths the

laser reflectors are required to match the laser wove

length. The wave long range 406.7 n m to 676.4 n m

.the dominant wave length are 406.7

nm,413.1nm,530.5nm,568.2 n m,647.1nm and

676.4nm. The strongest transition is at 676.4nm is

red region of visible spectrum. The emption ion

lasers provide brooder spectrum of wave lengths and

high power output than give by Argon ion lasers.

Ionic lasers find their applications in pumping dye

lasers, call sytometry, stereo lithography, laser

printing sand photo the eye. Argon ion lasers are

very efficient pumping derive for titanium- sapphire

lasers and krypton ion lasers are used in call

cytometry this means these laser beams can count

different categories of living calls.

Molecular gas laser -(Carbon Dioxide Lasers)

In molecular gas laser the active material is in

molecular state .the molecular lasers are carbon

dioxide and nitrogen lasers .Carbon dioxide lasers is

the most deficient laser amongst the different of

presentable available lasers. These lasers used a

mixture of carbon dioxide and nitrogen.

A carbon dioxide molecule is made up of three

atom (one carbon and two oxygen atoms) thus in

addition to electronic motions, atoms in the

molecule may vibrate in different modes or rotate

about different axes. Three modes of vibration of

molecule of carbondioxid shown as-

Thus the molecule is characterized not only by

electronic levels but also by rotational and vibration

energy levels.

Each electronic level is split in to various

vibration sub level (due to vibration motion) and

each vibration level is further subdivided in to

various rotational sublevels. The energy difference

between various electronic levels corresponding to

visible region, while the energy difference between

various visible region, while the energy difference

between various vibration level corresponding to

infrared region and the energy difference between

vicious rotational levels corresponding to far in

fared region of the spectrum.

Carbon dioxide lasers have been prepared in

different structure

1. Wave guide carbon dioxide laser

2. Transverse excitation atmospheric laser.

3. Longitudinal Excited laser

4. Gas dynamic laser.

Diagram of longitudinally excited Co 2 laser is

shown.

It is conventional gas discharge lager having long

cylindrical narrow glass enclosure with electrodes

at both ends. The windows W1 andW2 at the ends

of enclosure are fitted at an Angle the electrodes

current to a radio frequency power supply which

provides current for the discharge. The enclosure

is filled with a mixture of Co 2 in the Ratio

00.8:1.the additional N2gas to Co2 increasing the

efficiency

of this laser.

Carbon- dioxide lasers are much more efficient as

compared to other gas laser. The reason is that in

other gas lasers, the de-excitation from the lower

laser level to the ground level involves a

sufficient amount of energy, which is last and is

not contributed to output laser beam. On the other

hand in carbon dioxide lesser. The level taking

part in laser action is vibration rotational levels

of the lowest electronic level . these levels are

very close to the ground level and a large portion

of the input energy is converted in to the output

laser energy there by resulting in very high

efficiency.

Exciter laser ❑⇒ Exciter laser is a molecular laser

in which the active molecule is always in excited

state the lasing action used vibration energy

levels of the molecule.

The rare gases like helium, neon argon and

krypton are considered as inert gates and in

general the rare gas atoms do not form molecule

with any other atom. The reason is that rare gas

atoms form closed shells of electrums. However,

if the rare gas atoms are in excited state, the

criterion is different An evicted atom of

neon(Me) is similar to sodium (Na) atom and

hence it is viable of taking part in a chemical

reaction similarly an exited argon atom (Ar)

behave like potassium (K)atoms.

The following chemical reaction takes place

K+F2→ (K+F)+F

In analogy one aNy expect the reaction of the type-

Ar+F2=(Ar+F-)+F

The exciter lasers are formed by combining arraigns

atom and a halogen atom (chlorine, fluorine,

bromine or iodine)the rare gas halide, so formed is

always in exited state. Whose energy is related to

the excited state of the component atom of rare gas.

If the rare gas is not excited no reaction with the

halogen donor (F2 ) will take place. On account of

repulsion between positive charge .the atoms will

repel each other at small nuclear spacing. Thus a

bound active molecule is in excited state hence the

name Drummer laser. The excited molecular state is

formed by one atom in ground state and the other

not exist .In around state the molecule dissociated in

to atoms. The lasing action takes place due to

transition between upper bound state and the lower

unbound level.

Above figure shows the energy versus inter nuclear

separation in ground state and excited state. The

ground state is repelling so the male clue does not

exist in this state, which means that molecule is in

dissociated (atomic) stated for excited state the

curve touches a minimum, which means the

existence of molecule in excited state. Exciter lasers

may be prepared in several forms A typing structure

is shown in the figure and is called wave guide

structure.

In this structure the discharge tube is made of glass.

The tube has region, of a narrow lateral diminution

(less than 1 mm) the gain medium has length o.5 m

to 1m .the metallic transverse electrodes E1and E2

are fixed externally to the bore the electrodes

provide apre-ionising pulse thigh voltage and a

radio frequency main pulse to the tube .in wave

guide laser the excitation by radio frequency source

in an excel laser is quite effective and efficient.

The exciter laser have the following unique char

acterisics ❑⇒

I. After undergoing laser transition from excited

state to ground state, the molecule gets

dissociated. This indicates that the lower laser

level is always empty.

II. There is no well defined rotational vibration

transition us existence .the transition has broad

energy. As a result a tunable laser radiation over

the broad band can be obtained.

III. The population inversion is examiner laser is

achieved automatically, because the upper level

in excited state is populated, while lower

molecular level is always empty due to

dissociation of molecule in constituent atoms.

IV. The exciters lasers lasers operate inultravoolet

region of the speetrum,how over some of them do

operate in usable region also.

V. The excivmer lasers operate in ultraviolet region

of the spectrum, how-ever some of them do

operate in visible region also.

LIQUID (ORDYE) LASERS ❑⇒

Dye used in lasers is organic substance which

absorb in visible and near infrared and near

ultraviolet regions of the electromagnetic

spectrum. These substances are most commonly

solid which are dissolved in various solvents like

ethyl and methyl alcohol, ethylene glycol and

water etc. in these solvents the concentration of

dye molecules is of the order of 1 part in the ten

thousand (i.e.l.104 ) therefore the dye molecules

are Quite apart from one another and so each dye

molecule is surrounded only by the molecules of

the solvent.

Dye lasers may give pulsed and continuous waves

accordingly they are of three types.

1. Pulsed dye laser

2. Continuous wave (cw)lasers

3. Mode – locked dye lasers.

Working-: the energy level taking part in the

laser be on emission are different irrational

sublevels of different electronic states of the dye

molecule. A typical energy level diagrams of a

dye molecule in which so represents the

electronic ground level which suntans a large

number of rotational-irrational levels of the

molecule. The rotational sub level are so closed

that they form nearly continuum.

[Typical energy level diagram of a dye molecule

–so is the lower single electronic level

S1,S2……..Represent excited singlet level,T 1,T2

…….Represent the triplet levels]

Each electronic state is characterized by a

similar broad continuum of energy states.

Characteristic of dye molecule leads to the broad

absorption and emission spectrum of a dye

molecule .Because of the absorption of photons,

dye molecules get excited from ground level so

to higher irrational-rotational to next electronic

stat S1 now thermal redistribution in energy level

S2 takes place in about10 -115 due to which most

of the dye molecules dropdown to the lowest

irrational level B of S1 the radiation is at once

emitted when the molecules jump from level B

to Andy higher vibration level of so this is

called fluorescence . the life time τlevel B is

about 10 -95

As energy of emitted photon is lower than that

of emitted photon, the peak we velength of the

emitted florescence spectrum is higher than that

of the absorption spectrum the molecules from

state S1can also make a non radiative transition

to the triplet level T1this is called intersystem

crossing.

The schematic diagram of flash lamp pumped

dye is shown above figure this laser system

consists of a dye cavity (or cell) to flash lamp

and end mirrors. The mirrors reflect the light

back to dye solution to cause stimulation

obviously the design of laser system is similar to

solid state lasers. A Number of flash lamps have

used in dye laser xenon flash lamps

commercially available, are commonly used. for

better excitation of dye molecules, the length of

cavity generally taken equator that of the flash

lamps the dye solution is allowed to flow

continuously this is essential because even small

the mal gradient can deviously affect the

resonator characteristic the seriously affect the

resonator characteristic the spectral range of

flash lamp pumped dye laser extends- from near

ultraviolet to near infrared (340nm-850nm)

Applications of Dye lasers ❑⇒

1. Dye lasers are preferred for research. Work

where either tunable laser of ultrafast pulses

or tunable narrowband are required. They find

wide applications in spectroscopy, especially

in absorption spectroscopy of solid materials

and in photochemistry try in spectroscopy

they are used for is o toke separation.

2. Dye lasers are used in medical science for

removal of birth Marks and treatment of

cancer.

SHORT PULSE GENERATION ❑⇒

The method by which ultra short pulses are

generated in a laser is called mode. Locking

the output of the switched ruby and nod YAG

Laser system for examples Consist of a pulse

of duration is very short and results In out

bursts of very high power. If the average

energy is one joule and the pulse time is 20

nanosecond the average out put power is 50

×106waltz (so megawatts) it is interesting to

know that these pulses of Niño second

duration are made upon overlapping pulses of

even shorter duration of the order of 1 to 10

picot second (10 -12Second) the technique of

made locking enables these pulses to be

unraveled and a train of pulses of picoseconds

duration to be produced and other techniques

enable single Pico second pulse too be is

elated.

Even with small total energies the power in

such externally short pulses is very high. The

very short duration of such pulses makes them

important as props of short lived phenomenon.

There are important fields of applications the

high energy short pulses have been used for

the compression of matter to very high

densities and temperatures for thermonuclear

fusion and there are large laboratories being

assembled for this in the U.S.A and Russia

these short pulses have also been used in

photochemistry and photobiology.

One of the most important feature of organic

dye laser is their ability to produce these ultra

short optical pulses organic dyes play a

significant role in picoseconds pulse

generation as well as being efficient laser

media.

Microdensitometer trace of the streak

recorded of ultra short pulses ❑⇒

The production of ultra

short pulses by laser mode locking have also

created some sort of challenge to measure

them. Measurement using ordinary photomulti

pliers and fast photo diodes are inadequate for

observing picoseconds pulses. This is simply

because the time solution of fast electronic

eireaut is -100ps and is much slow. The

ultrafast streak camera has been applied to the

measure mint of back ground intensity of

picoseconds pulses from a passively mode

locked dye laser. The streak camera has also

been form the initial notice in a dye laser

cavity. Ad indicated by regime, it is necessary

to employ the approach of the electron –

optical chronograph or simply chronoscope

the principal of operation is based upon

changing the time variable in to a spatial

variable as shown in figure (a) A slit image of

the light sauce is favored on the photo cathode

of a streak image tube. By applying a-high

voltage ramp to the deflection plates the

electron optical image is moved across the

tube phosphor at a velocity which approaches

the velocity of light if the slit is illuminated

by a vontimu oust light source then a

continuous track would be record with ultra

short pulses, slit images are record ashen in

figure

(b) to measure the pulse duration at the

helfwiding of pulse is measured by its streak

velocity there is a fundamental problem in

using streak cameras for picoseconds

measured by its streak velocity there a

fundamental problem in using streak cameras

for picoseconds measure emends. The problem

lies in the statistics of toot electron emission

from a photocathode as for Ex at the

neodymium laser wavelength of 1.6 um, there

is a spread of about a third of an electron volt

with a is photo cathode, at shorter wavelength

it is much greater for that reason the accuracy

of measure mint of ultra short pulses was

limited to about 60 ps which was adequate

before the advent of picoseconds laser pulses

Basically what happens is that some of the

photo electrons are going faster than the other

and after awhile the initial pulse spreads out .

the time dispersion caused by the spread of

electron velocities is over come by genera a

static high electric field close to eh photo

cathode. A method of chronoscope has been

perfected by Bradley and using this method

optical pulse down to 1 picoseconds was

easily me assured.

Semiconductor Laser ❑⇒ first Semiconductor

laser was fabricated in 1962 using Gab As in

the form of diffused p-n nomojunction. The

resonant cavity is formed by just clearing the

junction ends. When an external voltage is

applied across the junction such that p-regio

is positive with respect to nreguib, both the

depletion region and the potential barer are

reduced and the diode is said to be for word

biased. Under theism biasing the current flow

through the junction due to majority charge

barriers take place more readily these charge

carriers become minority vaddriers on

crossing the junction. These are attracted by

butlery terminals to complete the credit and

hence current flaw place.

Spontaneous Emission ❑⇒ when p-n junction is

for ward biased, the concentration of minority

carriers in opposite type regions leads to

recombination of carriers across the band

jape. This process in shown figure (b) for a

defeat band jump semiconductor material

where the normally empty electron states in

conduction band of p-type material and

normally empty hole states in the valence

band of n-type material are populated by

injected carriers, which recon bane across the

band gap. The energy released due to this

electron hole recombination is really equal to

band gap energy Eg that the excess of carrier

population decreases by recombine nation

which may be radioactive or non radioactive.

In non radiate me recombination process, the

energy releases is dissipated in the form of

lattice vibration and hence heat however ins

radioactive recombination the energy is

released with the creation of a photon the

frequency(v) or wavelength () of photon

produced in given

by

Eg = hv =hcλ this gives

λ=hceg

=¿ 1.24

eg(inev )

This process of spontaneous light emission

occurring in diode structure is called electro

lupine sconce . The amount of radiation

recombination and the emission area of

structure depend upon semiconductor

materials and the fabrication of device.

P.N.junction Laser ❑⇒ the population inversion

in semiconductor is achieved by using app.-n

junction diode of semiconductor. Heavily

doped with donors and acceptors when no

potential difference is applied a potential

barriers exists across the depletion lager of

junction diode and no vagrant flaws when a

forward bias is applied, a current flaws and

the minority carriers are inked\cited in both

hand p-regions these recombine eighth

majority carrier and light is emitted at law ear

rent density the emission is spontaneous

giving a broad maxima the junction acts as a

light emitting diode (L.E.D)when bias is

applied, the current increases and maxima

tense to get narrow .when the threshold

current is reached , a population inversion is

created close to the junction the filled levels

neat the top of the valence band. Hence light

amplification is obtained is this region and

monochromatic and highly directional beam

of light is emitted from the junction.