Post on 21-Dec-2015
description
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.