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LASER:
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
Introduction Basic concepts of laser Metastable state Principle pumping schemes Types of laser Characteristics oflaser beam Applications oflaser Holography 3.8.1 Introduction 3.8.2 Principles of holography 3.8.3 Recording ofhologram 3.8.4 Reconstruction of the image
3.8.5 Difference between Hologram and ordinary photograph 3 .8.6 Important properties of a hologram 3.8.7 Application of Hologram
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Engineering Physics
now assume that the atom is initially in level 2. Since E2 > E1, the atom will tend to decay on its own to level 1. The corresponding energy difference (E2- E1) must therefore be released by the atom. When this energy is in the form of an electromagnetic wave or photon, the process is called spontaneous emission. The frequency u of the radiated wave is then given by the expression.
Ez Ez Ez E2 -EI -----------------(1) 2 2 lw:Ez-Et hll
h hv Where h is the Planck's constant.]
-'V\1'+ Note that radiative emission is just one hv of the two possible ways for the atom
1 E, E, E, to decay. The decay can also occur in a (a) (b) (c) non-radiative way for some levels. In
Fig. 3.1 (a,b,c) this case the energy difference E2 - E1 is delivered in some form other than
electromagnetic radiation (e.g., it may go into kinetic energy of the surrounding molecules).
3.2.2 Stimulated Emission:
Let us again suppose that the atom is found initially in level 2 and that an electromagnetic wave of frequency v given by equation (1) is incident on the atom (Fig. 3.1a). Since this wave has the same frequency as the atomic there is a finite
to 2 � 1. In this case the energy difference E2 -E1 is delivered in the form of an electromagnetic wave which adds to the incident one. This is the phenomenon of stimulated emission.
There is, however, a fundamental distinction between the spontaneous and stimulated emission processes. In the case of spontaneous emission, the atom emits an electromagnetic wave which has no definite phase or directional relation with that emitted by another atom. In the case of stimulated emission, since the process is forced by the incident electromagnetic wave, the emitted light by the atom is in phase with that of the incident electromagnetic wave. The emitted wave is also in the same direction as that of the incident wavJThe of stimulated emission was first put forward by A. Einstein in 191 7.
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ftJ Absorption:
Laser
Let us now assume that the atom is initially lying in level I (Fig. 3 .1.( c)). If this
is the ground level, the atom will remain in this level unless some external stimuli is applied to it. We shall assume then that an electromagnetic wave of frequency v (given
by equation (1)) is incident on the material. The energy difference E2- E1 required by
the atom to undergo the transition is obtained from the energy of the incident electro
magnetic wave. This is called absorption.
�pulation Inversion:
Let us consider the number of atoms N, per unit volume, that exist in a given energy state E. This number, called population N is given by Boltzmann's equation
E
N- N eksT
- 0
Here No is the population in the ground state. (E = 0), k8 is the Boltzmann's constant and T the absolute temperature.
It is clear from the above equation that population is maximum in the ground
state and decreases exponentially as one goes to higher energy states. If N1 and N2 are the populations in two states, a lower state E1 and a higher state E2 we have
-El
N2
Nl E,
ekBT
from which it follows that
-(E2-E1)
Nz = Nl e KaT
clearly N2 < N1 since E2 > E1. Since N1 > N2, whenever an electromagnetic wave is incident, there is net absorption of the radiation.
For laser action to take place, it is absolutely necessary that stimulated emission
predominate over spontaneous emission. This is possible only if N2 > N1 (i.e., the upper levels are more populated than the lower levels). This situation in which N2 > N1 is called population inversion.
69
::'ngineering Physil:s
(b) {a)
Fig. 3.2
The concept of population inversion can be best illustrated if we consider a system that has three energy states (a-three level system). These states may be designated as E1, E2 and E3• When the is in equilibriu� the upper most state E3 is
least and the lowest state E1 is populated most (Fig. 3.2(a)). the curve shows a Boltzmann distribution. Since the population in the various states is such that N3 <
N2 < N1, the system is absorptive rather than emissive. But on excitation by outside energy, it is possible that N2 exceeds N1 (this is possible ifE2 happens to be a metastable state i.e., an energy state with a long life time and the transition probability between levels 3 and 2 is very high). Thus population inversion is achieved and is shown in Fig. 3.2b.
�Pumping:
For achieving and maintaining the condition of population inversion, we have to raise continuously the atoms in the lower energy to the upper energy level. It requires energy to be supplied to the system. Pumping is the process of supplying energy to the laser medium with a view to transfer it into the state of population inversion. Because N1 is originally very much larger than N2, a large amount of input energy is required to momentarily increase N2 to a value comparable to N1•
These are number of techniques for pumping a collection of atoms to an inverted state. Those are
1. Optical pumping 2. Electrical discharge 3. Direct conversion.
In Optical Pumping, a light source such as a flash discharge tube is used to illuminate the active medium. This method is adopted in solid state laser.
70
Laser
In Electrical discharge method, the electric field causes ionization of the m•:
dium and raises it to the excited state.
In Semiconductor diode laser, a direct conversion of electrical energy in to
light energy takes place.
3.3 METASTABLE STATES:
An atom can be excited to a higher level by supplying energy to it. Normally,
excited atoms have short lifetimes and release their energy in a matter of
(1 o 9s) through spontaneous emission. It means that atoms do not stay long enough at
the excited state to be stimulated. As a result, even though the pumping agent continuously raises the atoms to the excited level, they undergo spontaneous transitions and
rapidly return to the lower energy level. Population inversion cannot be established
under such circumstances. In order to establish the condition of population inversion,
the excited atoms are required to 'wait' at the upper energy level till a large number of
atoms accumulate at that level. In other words, it is necessary that the excited state has
a longer lifetime. A metastable state is such a state. Because of restrictions imposed
by conservation of angular momentum, an electron excited to a metastable state can
not return to the ground state by emitting a photon, as it is generally expected to do .
. Such a state, which single-photon emission is impossible, has an unusually long
time and is called a metastable state. Atoms excited to the metastable states remain
excited for an appreciable time, which is of the order of 1 o-6 to 1 o-3s. This is 103 to 106
times the lifetimes of the ordinary energy levels.
Therefore, the metastable state allows accumulation of a large number of
excited atoms at that level. The metastable state population can exceed the population
at a lower level and establish the condition of population inversion in the lasing medium. It would be impossible to create the state of population inversion without a
metastable state. Metastable state can be readily obtained in a crystal system contain
ing impurity atoms. These levels lie in the forbidden band gap of the host crystal.
Population inversion readily takes place as the lifetimes ofthese levels are large, and
secondly, there is no competition in filling these levels, as they are localized levels.
There could be no population inversion and hence no laser action, if meta
stable states do not exists.
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Engineering Physics
3.4 PRINCIPAL PUMPING SCHEMES:
Atoms in general are characterized by a large number of energy levels. Among
them only three or four levels will be pertinent to the pumping process. Therefore,
only those levels are depicted in the pumping scheme diagrams. Two important pump
ing schemes are widely employed. They are known as three-level and four-level pump
ing schemes.
3.4.1 Three LEVEL Pumping Scheme :
A typical three-level pumping scheme is shown in Fig. 3.3. The state E1 is the
ground level; E3 is the pump level and E2 is the metastable upper lasing level. When
the medium is exposed to pump frequency radiation, a large number of atoms will be
excited to E3 level. However, they do not stay at that level but rapidly undergo down
ward transitions to the metastable level E2 through non- radiative transitions. The
E2 Metastable State
E1 Ground State
(a) Fig. 3.3
Upper Lasing
Lower Lasing Level (b)
atoms are trapped at this level as spontaneous transition from the level E2 to the level
E1 is forbidden. The pumping continues and after a short time there will be a large
accumulation of atoms at the level E2• When more than half of the ground level atoms
accumulate at E2, the population inversion condition is achieved between the two
levels E1 and E2• Now a chance photon can trigger stimulated emission.
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Laser
3.4.2 Four-Level Pumping Scheme :
A typical four-level pumping scheme is shown in Fig. 3.4. The level E1 is the
ground level, E4 the pumping level, E3 the metastable upper lasing level and E2 the
lower lasing level. E2, E3 and E4 are the excited levels. When light of pump frequency
v is incident on the lasing medium, the active centers are readily excited from the p
ground level to the pumping level E4• The atoms stay at the E4 level for only about
10-8s, and quickly drop down to the metastable level E3• As spontaneous transitions
from the level E3 to level E2 cannot take place, the atoms get trapped at the level E3•
E4
E1
Pumping Level
vp �Metastable State
E2
Ground State
(a)
E1
Upper Lasing Level
Es
E2
Fig . 3.4 (b)
The population at the level E3 grows rapidly. The level E2 is well above the ground
level such that (E2- E1) > kT. Therefore, at normal temperature atoms cannot jump to
level E2 from E1 on the strength of thermal energy. As a result, the level E2 is virtually
empty. Therefore, population inversion is attained between the levels E3 and E2• A chance photon of energy hv = (E3 - E) emitted spontaneously can start a chain of
stimulated emissions, bringing the atoms to the lower laser level E2• From the level
E2, the atoms subsequently under go non-radiative transitions to the ground level E1 and will be once again available for excitation.
3.4.3 COMPARISON OF FOUR-LEVEL LASER WITH THE THREE-LEVEL
LASER:
1. In the three-level pumping scheme, the terminal level of laser transition is
simultaneously the ground level. Therefore, in order to achieve population inversion
more than half of the ground level atoms have to be pumped up to the upper lasing
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Engineering Physics , ____ ___
level, such that N2 > N/2. As the number of atoms in the ground level is very large, high pump power is required in order to promote N/2 atoms and establish the required population inversion.
On the other hand, in the four-level pumping scheme, the terinal level of laser transition is virtually empty and population inversion condition is readily established even if a smaller number of atoms arrive at the upper lasing level. Therefore, relatively small pumping power is required to establish population inversion in four level pumping schemes.
2. In case of three level pumping scheme, once stimulated emission commences, the population inversion condition reverts to normal population condition. ceases as soon as the excited atoms drop to the ground level. Lasing occurs again only when the population inversion is reestablished. The light output therefore is a pulsed output.
In case of four level scheme, the condition of population inversion can be held without interruption and light output is obtained continuously. Thus, the laser operates in continuous wave ( cw) mode.
rPES OF LASERS:
There are several ways in which we can classify lasers to different types. We prefer here to classify the lasers on the basis of the material used as active medium. Accordingly, they are broadly divided into four and few subcategories as mentioned below
1. Solid State Laser -Ruby laser - Nd : YAG laser
2. Gas Laser - He-Ne laser - C02 laser
3. Liquid Dye Lasers
4. Solid State Diode Laser or Semiconductor Diode Laser.
Among all the various types of lasers, few out of them are discussed below of
our interest.
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Laser
�UBY LASER: \a , 3.5.1.1 Introduction:
Ruby laser belongs to the class of solid state lasers. The term solid state has solid state laser is one in
Flash lamp
Schematic of a ruby laser.
Fig. 3.5
m-acrystal or glassy material. state electrically
Laser car \ beam
the first laser. It was invented in
Maiman, U.S.A. The ruby laser rod is in fact a synthetic ruby crystal, Al203 crystal, doped with chromium ions
at a concentration of about 0.05% by weight. C2" ions are the actual active centers and have a set of three energy levels suitable for lasing action whereas aluminum and oxygen atoms are inert.
3.5.1.2 Construction: The schematic of a laser is 3 0 50 Ruby rod is taken in the form
of a cYlindrical rod and 0.5 e�actly parallel and a�eaTsoperpen
n�flection while the 5 ' • ��- "" �..4l.,.-- -�- --
other is silvered to give 10% and 90% reflection. The silvered faces .. - ' , '
the Fabry-Perot resonator. is a helical photo- filled with xenon. by the power !he
lamp flashes of light. '" • ' ,.,
·-
3.5.1.3 Working: laser uses a three-level pumping scheme. The energy levels ofCr3+ ions in
the __ _9:r.e !here are t�o levels at E2. When the flash lamE
• < � -,.� ·-="" , c
discharge generates an intense burst of white light lasting for a few The · - _ • -,. •• .._.
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Engineering Physics
bands E3 and blue comp�nents light. The energy levels in these bands have a very small lifetime(� l0-9s) .
�<:lergo t�����_?2• 'Ole_p��_2f levels at E2 times 1Il_9!e
" """-�" -� . , th�1ifetime of l�yel.
PumpIng
E' 3
�-Photon "\f'ij'i 6943 A
Stimulated Ernission
Ground State
Fig. 3.6
ions at more ion .. ,,. · _.,._,, ""' " . ' · .v " , - tion accumulates atE level, the state of l»""''·'"l. _ 'v# • , •. ;· �--l-<'•r-"> ;> - '"" "-� -· ' 2 - '·,£\� � '.. , .,, Eopulat(()l1 inversi2B .,i§..��tablished pe-
and E1levels. A chance photon emitted spontaneously by a c�+ ion initiates a chain of stimulated emissions by other c�+ ions in the metastable state. Red photons of wavelength 6943 A traveling along the axis of the ruby rod are repeatedly reflected at the end mirrors and light amplification takes place. A strong intense beam of red light emerges out of the front-end mirror.
Note that the green and blue components of light play the role of pumping agents and are responsibJt': for population inversion.
" The <;>f =colo�r: a�i i��-of the which
· actually gets amplified. The xenon flash lasts for a few milliseconds. However, the laser does not operate throughout this period. Its output occurs in the form of irregular pulses of microsecond duration. It is because the stimulated transitions occur faster than the rate at which population inversion is maintained in the crystal. Once stimulated transitions commence, the metastable state E2 gets depopulated very rapidly and at the end of each small pulse, the population at E2 has fallen below the threshold value required for sustained emission of light. As a result the lasing ceases and laser becomes inactive. The next pulse appears after the population inversion is once again restored. The process repeats.
76
�!CONDUCTOR LASER:
3.5.2.1 Introduction:
Laser
A semiconductor diode laser is a specially fabricated pn junction device, which
· emits coherent light when it is forward biased. R.N. Hall and his coworkers made the
first semiconductor laser in 1962. It is made from Gallium arsenide (GaAs) which
operated at low temperatures and emitted light in the near IR region. Semiconductor
lasers working at room temperature and in continuous wave mode are produced by
1970. Now, p-n junction lasers are made to emit light almost anywhere in the spec
trum from UV to IR. Diode lasers are remarkably small in size (0.1 mm long). They
have high efficiency ofthe order of 40%. Modulating the biasing current easily modu
lates the laser output. They operate at low powers. In spite of their small size and low
power requirement, they produce power outputs equivalent to that ofHe-Ne lasers.
The chief advantage of a diode laser is that it is portable. Because of the rapid ad
vances in semiconductor technology, diode lasers are mass produced for use in opti-
communications, in CD players, CD-ROM drives, optical reading, high speed
laser printing etc wide variety of applications.
3.5.2.2 Principal and Theory:
A semiconductor is a material with electrical properties intermediate to those
of a conductor and an insulator. The allowed energy values of the valence electrons in
semiconductors occur within two well defined energy bands separated by an energy
gap known as band gap. A pure semiconductor crystal has exactly enough electrons
to fill all the states in the lower band, namely valence band. However, when a cova
lent bond is just broken, an electron is just set free. Then w·e say that the electron
jumped into the upper band, namely conduction band. The electron jumping to the
conduction band leaves behind a vacancy in the valence band. The vacancy is called a
hole and is assigned a positive charge and a mass equivalent to that of an electron. In
a pure semiconductor, for each covalent bond broken an electron and hole are gener
ated. Therefore, the number of electrons in the conduction band and the number of
holes in the valence band are equal. When a conduction electron falls into the valence
band, it recombines with a hole there. The electron rejoins the broken covalent bond
and therefore both the electron and hole disappear. The recombination energy is re
leased in the form of heat in silicon and germanium crystals. In some crystals it is
released in the form of light.
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Engineering Physics
Doping with small amounts of impurities can drastically increase the electrical
conductivity of a pure semiconductor. When the dopant is a pentavalent element, each
dopant atom contributes an electron to the conduction band without creating a hole
simultaneously in the valence band. Hence the addition of the pentavalent element
mcreases the number of conduction electrons which become the majority carriers in
the silicon crystal. As negatively charged electrons are current carriers in this crystal,
it is called a n-type semiconductor. On the other hand, a trivalent dopant atom pro
duces a hole in the valence band without the simultaneous generation of electron in
the conduction hand. Hence the addition of he trivalent element increases the number
of holes which become the majority carriers in he silicon crystal. As positively charged
holes are curent carriers in this crystal, it is called a p-type semiconductor.
There is a reference level in the energy band diagram of each type of semicon
ductor. The reference level is called the Fermi level. The Fermi level EFP is nearer to
the top of the valence band the p-type material and the Fermi level EFN nearer to the
bottom of the band in the n-type material. When the p-type and n-type
materials are joined at the atomic level to form a pn-junction device, equilibrium is
attained only when equalization of Fermi levels takes place. The energy levels in p
region move up and those in n-region move down till the Fermi levels ( EFP and EFN)
in both the regions come to the same level. The mutual displacement of the energy
levels on both sides of the junction causes a bending of the energy bands around the
junction.
3.5.2.3 Construction:
Current flow Metal
Contact Active region
Meta Contact
Optically flat and parallel faces
[cleaved (11 O) planes]
Schematic of a semiconductor diode laser
Fig. 3.7
78
Fig. 3.7 shows the schematic of
a semiconductor laser. A simple
diode makes use of the same
semiconductor material, say,
GaAs on both sides of the junc
tion. Starting with a heavily
doped n-type GaAs material, a
p-region is formed on its top by
diffusing zinc atoms into it. A
heavily zinc doped layer consti
tutes the heavily doped p-region.
The diode is extremely small in
Laser
size. Typical diode chips are 500 m long and about 100 m wide and thi,:·l . The top
and bottom faces are metallized and metal contacts are provided to pass ',:1rough
the diode. The front and rear faces are polished parallel to each other and perpendicu
lar to the plane of the junction. The polished faces constitute the Fabry-Perot resona
tor. In practice there is no necessity to polish the faces. A pair of parallel planes cleaved
at the two ends of the pn-junction provides the required reflection to form the cavity.
The two remaining sides of the diode are roughened to eliminate lasing action in that
direction. The entire structure is packaged in small case which looks like the metal
case used for discrete transistors.
3.5.2.4 Working:
The energy band diagram of a heavily doped pn-junction is shown in Fig. 3 .8( a).
Because of very high doping on n-side, the donor levels are broadened and extend
into the conduction band. The Fermi
n-Type Junction (a)
p-Type
Level also is pushed into the conduc
tion band. Electrons occupy the portion
of the conduction band lying below the
Fermi level. Similarly, on the heavily
doped p-side the Fenni level lies within
the valence band and holes occupy the
portion of the valence band that lies
above the Fermi level. At thermal equi
librium, the Fermi level is uniform
across the junction.
When the junction is forward-
Ev ?iased,e�ectro_ns and �oles are
_injected
mto the JUnctiOn regwn to h1gh con-
EFP centrations. In other words, carriers are pumped by the de voltage source. At
(b)
Fig. 3.8
low forward current level, the electron
hole recombination causes spontaneous
emission of photons and the junction acts as an LED. As the forward current
through the junction is increased the intensity of the light increases linearly. However,
when the current reaches a threshold value (see Fig. 3. 9 ), the carrier concentrations in
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Engineering Physics
the junction region will rise to a very high value. As a result, the junction region (Fig. 3.8(b)) contains a large concentration of electrons within the conduction band and simultaneously a large number of holes within the valence band. Holes represent ab-
sence of electrons. Thus, the upper
t
energy levels in the narow region are having a high electron population _while the lower energy levels in the
S same region are vacant. Therefore,
0� (SPONTANEOUS the condition of population inversion EMISSION)
1- LED LASER OPERATION is attained in the narow junction re-
� OPERATION (STIMULATED EMISSION) gion. This narrow zone in which ::J
· ·
population inversion occurs s called
Laser CURREN T -+ Threshold
Light output-curnt characterstic of an ideal diode laser.
. Fig. 3.9
an inversion region or active region. Chance recombination acts of electron and hole pairs lead to emission of spontaneous photons. The spontaneous photons propagating in the junction plane stimulate the con-
duction electrons to jump into the vacant states of valence band. This stimulated electron-hole recombination produces coherent radiation. GaAs laser emits light at a wavelength of 9000 A in IR region.
�HARACTERISTICS OF LASER BEAM:
The important characteristics of laser beam are as follows -
3.6.1. Directionality:
The conventional light sources emit light uniformly in all directions. When we need narrow beam in specific direction, we obtain it by placing a slit in front of source
� of light.
� � In case of laser, the active material is in a cylindrical resonant cavity. Any light
<: <: that is traveling in a direction other than parallel to the cavity axis is eliminated and � only the light that is travelling parallel to the axis is selected and reinforced. Light
'-\. propagating along the axial direction emerges from the cavity and becomes the laser beam. Thus, a laser emits light only in one direction.
80
Laser
3.6.2 Divergence:
Light from conventional sources spreads out in the form of spherical wave fronts
and hence it is highly divergent. On the other hand light from laser propagates in the
form of plane waves. The light beam remains essentially a bundle of parallel rays. The
small divergence that exists is due to the diffraction of the beam at the exit mirror. A
typical value of divergence of a He-Ne laser is 1 o 3 radians. It means that the diameter
of the laser beam increases by about 1 mm for every meter it travels.
3.6.3 Intensity:
The intensity of light from a conventional source decreases rapidly with dis
tance, as it spreads in the form of spherical waves. One can look at the source without
any harm to his eyes. In contrast, a laser emits light in the form of a narrow beam
which propagates in the form of plane waves. As the energy is concentrated in a very
narrow region, its intensity would be tremendously high. It is estimated that light
from a typical 1 m W laser is 10,000 times brighter than the light from the sun at the
surface. The of the laser beam stays nearly constants with distance as the
light travels in the form of plane waves.
3.6.4 Coherence:
A conventional light source such as an incandescent lamp or a natural source
such as the sun produces incoherent light since they emit random wavelength light
waves with no common phase relationships. On the other hand, the waves emitted by
a laser source will be in phase and are of the same frequency. Therefore, light gener
ated by a laser is highly coherent.
The coherence length of light radiation by conventional monochromatic source
is of the order of a few millimeters. On the other hand the coherence length of light
emitted by laser beam is of the order of one km.
3.6.5. Monochromaticity:
I f light coming from a source has only frequency (single wavelength) of oscilla
tion, the light is said to be monochromatic source. Light from traditional monochro
matic source spreads over a wavelength range of 100 A to 1 OOOA. On the other hand,
the light from lasers is highly monochromatic and contains a very narrow range of a
few angstroms(< 10 A).
81
Engineering Physics
�LICATIONS OF LASER:
3.7.1 Engineering:
1. The lasers have wide industrial applications. They can blast holes in diamond and hard steels.
2. Laser beam is highly intense. Hence it can be utilized in applications such as welding cutting and ablation of material. One advantage of laser is that the beam can be focused into the fine spot. Small spot size implies that high energy densities are possible.
3. The laser beam is particularly suited for welding of fine wires, contacts in miniature assemblies etc. lasers are also found suitable for machining and drilling holes.
3.7.2 Medical:
1. Lasers have wide medical applications. They have been used successfully in the treatment of detached retinas.
2. Preliminary success had also been obtained to treat the human and animal cancers.
3. Microsurgery is also possible because laser beams can be focused on very small areas (due to very narrow angular spread) and hence one harmful com ponent can be destroyed without seriously damaging neighboring regions.
4. Lasers are used for treatment of dental decay, the destruction of malignant tumors and the treatment of skin diseases.
5. In recent years genetic research has wide scope with the help of fine highly penetrating laser beam.
3.7.3 Communication:
1. Due to narrow band width, lasers are used in microwave We that is microwave communication the signal is mounted on carrier waves
by the process of modulation. If band width of carrier wave is limited, the no. of channels of message which can be carried simultaneously is limited. By use of lasers, more channels of message can be accommodated as the band width is very small.
82
Laser
2. By the use oflasers, the storage capacity for information in computers is enerally due to the narrowness o t e an w1 . he IBM Corporation is trying to transmit an entire memory bank from one computer to another by the use of laser beam.
3. Low semiconductor lasers are used in CD players, laser printers, laser office automation equipments, optical floppy disc, optical
processing devices, range finders, strain gauges, optical micrometers, velocity meters, measuring instruments, information transmission in optical communication, radar signaling etc.
3.7.4 Science:
1. Laser has applications is the field of pure science. Laser offers a wonderful opportunity to investigate the basic laws of interaction of atoms and molecules with electromagnetic waves of high intensity. Many a new optical phenomenon has been observed with lasers, which otherwise would not have been possible.
2. Due to narrow angular spread, the laser beams have become means of commu-nication between earth, moon and other satellites. The distances between various satellites can be very accurately determined by using the laser. The distance be-
.. tween earth and moon can be determined to an accuracy of± 15 m.
3. The velocities of moving objects can be very easily calculated by comparing frequency of the laser signal reflected from the object with the emitted frequency.
4. They can initiate or hasten certain chemical reactions which could not be pos-sible in the absence of suitable photons. They can be used for investigating the structure of molecules.
5. Raman spectroscopy is one in which have made so much impact that a separate branch named as Laser Raman Spectroscopy has grown rapidly.
6. By use of lasers, the Raman spectrum can be obtained for much smaller samples and faster too. Not only that but some interactions also arise due to high intensity excitation which provides additional information.
7. It is well known that enormous energy released from the sun and the stars is due to thermonuclear fusion reactions. Attempts are being made to generate fusion energy in laboratory. One of the difficulties associated with the fusion reactions
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Engineering Physics
is requirement of very high temp of the order of 108 K for the fusion reaction to
occur. This can be obtained by heating the thermonuclear material with the help
of lasers
8. Lasers are also employed for separating various isotopes of an elements. This
has enormous use for the large scale enrichment of uranium for use in nuclear
power reactors. There are many applications for the pure isotopes in medicine. In
isotopes separation using lasers beam, one makes the use of the slight differences
in the energy levels of the atoms of the isotopes due to difference in nuclear
mass. This light of certain wavelength may be absorbed by one isotope, while the
other isotope of the element may not be absorbed it. This is one of the important
simple and very useful methods to separate various isotopes.
3.7.5 Military:
Their study also oriented for military purpose. They can be served as war weapon.
It has been predicted that they become legendary 'death ray' which could ann�hilate
anything near or far.
-�OLOGRAPHY: 3.8.1 Introduction:
Image of the object are generally obtained using photographic method. In this
method a lense focuses the light reflected from three-dimensional object in to photo
graphic firm. Where, two dimensional image of the object is formed. A negative is
first obtained by developing the film and then positive is obtained through printing.
The positive print is two dimensional record of light intensity received from the ob
ject. It contains information about the square of the amplitude of the light wave that
produced the image but the information about the phase of the wave is not recorded
and is lost.
In 1948 Dennis Gabor outlined a two step lenseless imaging process. It is radi
cally a new technique of photographing the objects and is known as wave front recon-
struction. The technique is called holography. ·
The word 'holography' is formed by combining parts of two Greek words: 'holos',
meaning 'whole', and 'graphein' meaning 'to write'. Thus holography means writing
the complete image. Holography is actually a recording of interference pattern formed
between two beams of coherent light coming from the same source. In this process
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Laser
both the amplitude and phase components of light wave are recorded on a light sensitive medium such as a photographic plate. The recording is known as a hologram. Holography required an intense coherent light source. Laser was not available when Gabor formulated the idea of holography. Holographic technique became a practical proposition only after the invention of lasers. Leith and Upatnicks prepared laser holograms for the first time. In this chapter we discuss the fundamental concept of holography.
3.8.2 Principle of Holography :
a First step is the recording where p_���St4ic: is t]le recon
into th� COJLVen !.�qui�ed i? either o�t?_e step s. the r��ult
!?.fi!lt��fere!lc� an object beam which is the light scattered off the object and a coherent background, the reference beam, which is the light reaching the photographic plate directly. In Gabor's original experiments, the refemce beam and object beams were coaxial. Further, advance was made by Leith and Upatnieks, who used the reference beam at offset angle. That made possible the recording of holograms of three-dimensional objects. ·
The off-axis arrangement for generating and viewing holograms is described here.
3.8.3 Recording of The Hologram :
Laser beam
Fig. 3.10
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In the off-axis arrangement a broad laser beam is divided into two beams, namely a reference beam and an object beam by a beam splitter. The reference beam goes directly to the photographic plate. The second beam of light is directed onto the object to be photo-
Engineering Physics
graphed. Each point of the object scatters the incident light and acts as the source of
spherical waves. Part of the light, scattered by the object, travels towards the photo
graphic plate. At the photographic plate the innumerable spherical waves from the
1ject combine with the plane light wave from the reference beam. The sets of light
waves are coherent because they are from same laser. They interfere and form inter
ference fringes on the plane of the photographic plate. These interference fringes are
a series of zone- plate like rings, but these rings are also superimposed, making a
complex pattern of lines and swirls. The developed negative of these interference
fringe patterns is a hologram. Thus, the hologram does not contain a distinct image of
the object but carries a record of both the intensity and the relative phase of the light
waves at each point.
3.8.4 Reconstruction of the image:
Whenever required, the object can be viewed. For reconstruction of the image,
the hologram is illuminated by a parallel beam of light from the laser. 'Most of the
light passes straight through, but the complex of fine fringe,s acts as an elaborate
diffraction grating. Light is diffracted at a fairly wide angle. The diffracted rays form
Laser beam
,."'"' ,"' .,."" .. .... "
Virtual image
Fig. 3.11
Eye
First order
First order
Real image
two images: a virtual
image and a real im
age. The virtual image
appears at the location
formerly occupied by
the object and is some
times called as the true
image. The real image
is formed in front of
the hologram. Since
the light rays pass
through the point
where the real image
is, it can be photo
graphed. The virtual
image of the hologram is only for viewing. Observer can move to different positions
q_nd look around the image to the same extent that he would be able to, were he look
i'ng directly at the real object. This type of hologram is known as a transmission holo
gram since the image is seen by looking through it.
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Laser
The original configuration adopted by Gabor for recording hologram was a coaxial arrangement. In this arrangement the real image is located in front of the virtual image and is inconvenient for viewing or photographing. The advantage of the off
axis configuration is that the two images are separated.
3.8.5 Difference Between Hologram and Ordinary Photograph
The fundamental difference between a hologram and an ordinary photograph is like this. In a photograph the information is stored in an orderly fashion, each point in
the object relates to a conjugate point in the image. In a hologram there is no such
relationship: light from every object point goes to the entire hologram, this has two main advantages:
1. As the observer moves sideways in viewing the hologram, the image is seen in three dimensions.
2. If the hologram were shattered or cut in to small pieces, each fragment would
still reconstruct the whole object, not just part of the object.
3.8.6 Important Properties and advantages of a Hologram :
1. In an ordinary photograph each region contains separate and individual part of
the origin object. Therefore, destruction of a portion of a photographic image leads to an irreparable loss of information corresponding to the destroyed part. On the other hand, in a hologram each part contains information about the entire object. From even a small part of the hologram the entire image can be reconstructed if only with a reduced clarity and definition of the image. Therefore, a hologram is a reliable medium for data storage.
2. It is not useful to record several images on a single photographic film. Such a
record cannot give information about any of the individual images. On the other
hand, several images can be recorded on a hologram. Therefore the information holding capacity of a hologram to is extremely high. While a 6 x 9 mm photo
graph can hold one printed page, a hologram of the same size can store up to 300
such pages.
3. On a hologram information is recorded in the form of interference pattern. The
type of the pattern obtained depends on the reference beam used to record the
hologram. The information can he decoded only by a coherent wave identical to
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Engineering Physics
that of the reference wave. The reference waves can he chosen appropriately.
Consequently without the knowledge of the shape of the reference wave front the
information encoded in the form of interference pattern on the hologram cannot
be deciphered.
4. The reconstruction of the image of the hologram can be done with reference
beam of any wavelength if it is coherent and identical to the original reference
beam. If the wavelength A of the reconstructing beam is greater than that A o of
the reference beam, the reconstructed image will be a magnified image. The mag
nification will be proportional to the two wavelengths.
5. The most significant improvement in the field of holography is that now holo-
grams can he viewed with white light. Thus the necessity for a reconstruction of
laser beam is dispensed away.
6. Yu. N. Denisyuk a Russian scientist developed a method for making colour
holograms based on Lippman's photographic process.
7. Acoustical holography is another area which is in a developmental stage. It is
easy to produce coherent sound waves. Sound waves readily propagate in in sol
ids. Therefore, a three dimensional acoustical hologram of an opaque object can
be made. By viewing such hologram in visible light the internal structure of the
object can be observed. Such techniques will be highly useful in the fields of
medicine and technology.
3.8.7 APPLICATIONS OF HOLOGRAPHY:
The principle of holography finds applications in many diverse fields. Few of
them are explained below:
1) Holographic Interferometry:
One of the most important applications of holography is in the field of interfer
ometry. Using a special technique known as "double exposure technique", the surfaces
that undergo deformations due to stress can be studied. According to this technique, the
photographic plate is first of all partially exposed to the reference beam and object beam
as usual. Afterwards, the object is allowed to undergo the deformation due to stress.
Further, the photographic plate is exposed to the new object beam along with the same
original reference beam. In this way, the photographic plate is allowed to two different
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Laser
exposures. Now the photographic plate after development forms the hologram. The ho
logram is once again exposed to reconstruction beam. From the hologram, we get two
object waves: one corresponds to the unstressed object and the other to the stressed
object. The two waves interfere and produce interference fringes. These fringes are char
acteristic of the strain suffered by the object. Furthermore, a quantitative study of the
fringe pattern produced in the object gives the distribution of strain in that object.
2) Acoustic Holography :
The principle of holography can be used to study the image formed by sound
waves. Let us apply the principle to view the image of an object in water. For this purpose
we use ultrasound waves in place of laser beam to generate reference beam as well as
object beam. An ultrasonic generator is kept inside the water. It generates reference beam
and a beam that is directed toWards underwater object. The ultrasonic waves reflected
from the object form the object beam. The reference beam and object beam interfere and
produces ripples on the still surface of water. The ripple pattern forms the hologram and
can be photographed. On the other hand, the ripple pattern may be illuminated by laser
beam and can be viewed through a telescope. The acoustic holography can also be used
to get a three dimensional image of our internal organs. Here a coherent ultrasonic beam
is splitted into two parts. The first part serves as the reference beam while the second part
is allowed to scattered by the internal organ to be studied. The scattered beam forms the
object beam. The two beams interfere and form a hologram. From the reconstructed
three dimensional image we can study in depth, the details of the concerned organ.
3) Holographic Microscopy:
With holographic technique, we can get a three dimensional picture out of the
hologram. Moreover, we can also get information about depth. In holographic micros
copy, one beam after passing through the specimen and microscope combine with refer
ence beam and produce the hologram. The reconstructed image can be seen in any cross
section desired. Thus this method provides a high depth of field compared to conventional
high power microscope. The holographic microscopy also find's its applications in the
study of time varying phenomena that occur in a certain region. This is not possible with
ordinary microscopic techniques. Here a hologram is recorded of the scene as and when
it occurs. Thus the event gets preserved in the hologram. Now, at our convenience, we
can focus through the depths of the reconstructed image and study the phenomena in
detail. Thus we can study the transient microscopic events with the help of a hologram.
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Engineering Physics
4) Other Application :
The Other application include holographic cinema, spatial filtration and charac
ter recognition, long distance holography using microwaves, rainbow holograms, fo
cused image holograms, holographies optical elements etc.
******
Question Bank
Objectives Type Questions Marks
1. Explain in brief spontaneous emission, stimulated emission, absorption,
population inversion, pumping and metastable state etc. 3 each
2 List out various characteristics of laser beam.
3 Write note on: applications oflasers, recording ofhologram,
reconstruction ofhologram, properties ofhologram, applications of
4
holography, etc. 4 each
Descriptive Type Questions or Notes:
1 Explain any two principle schemes.
2 Write note on: ruby laser, semiconductor laser, applications of lasers,
8
holography and applications ofholography etc. 8 each
Numericals:
Numericals will be asked based on articles such as: reverberation, time of
reverberation, Sabine's formula, absorption coefficient, determination of wave
length and velocity of ultrasonic waves etc.
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