LASER

Module - 5

 Laser characteristics, Spatial and temporal coherence

 Einstein coefficient and their significance
 Population inversion, 2, 3, and 4 level systems
 Pumping schemes, Threshold gain coefficient
 Components of laser, He – Ne laser
 Nd:YAG and CO2 lasers and their engineering applications
Recap:

1. LASER stands for?

2. Basic properties of laser?

3. What are three quantum processes when light interacts with matter?

4. What is spontaneous emission?

5. What is stimulated emission?

3
Introduction
• LASER: Light Amplification through Stimulated Emission of Radiation

• Laser is a source of highly coherent monochromatic radiation in the optical

region (UV to IR)
• Einstein gave the theoretical basis for the development of laser in 1916
• In 1954, C.H. Townes developed a microwave amplifier based on stimulated
emission of radiation, called as a MASER
• In 1958, A. Schawlow and C.H. Townes extended the principle of mass to light -
Noble prize (1964)
• In 1960, T.H. Maiman build the first laser device (Ruby Laser)
• Laser is high technology device with variety of applications in different fields
4
Characteristics of Laser
Laser exhibits some peculiar properties compered to convectional light

1. Highly directionality

2. Highly monochromatic

3. Highly intense

4. Highly coherence

5
Highly Directionality

• The directionality of laser beam is expressed in terms of divergence

• Where 2 and 1 are the diameters of laser spots at distances of 2 and 1 respectively
from laser source
• For laser light divergence = 10−3
• The divergence of light is very low and therefore laser light is highly directional 6
Highly Monochromatic
• In laser radiation, all the photons emitted
between discrete energy levels will have
same wavelength
• As a result the radiation is monochromatic in
nature
• Due to the stimulated characteristic of laser,
it is more monochromatic than that of a
convectional light
• laser radiation -the wavelength spread =
0.001 nm
Typical Applications of narrow spectral width

Communication Spectroscopy
Holography Interferometry
Sensors 7
Highly Intense
• Laser light is highly intense and can give very high power (>1015 W)
• 1 mill watt He-Ne laser is highly intense than the sun intensity
• when two photons each of amplitude are in phase with other; the resultant amplitude of
two photons is 2a and the intensity is 4a2

• in laser much number of photons are in phase with each other, the amplitude of the
resulting wave becomes na and hence the intensity of laser is proportional to n2a2

Typical application of high power laser:

• Material processing
• Fusion
• Military
• Nonlinear optics
8
Highly Coherence
• Coherence is the property of the wave being in phase with itself and also with another
wave over a period of time and space
• This is because of orderly electronic transitions that take place in laser

• There are two types of coherence

1. Temporal coherence
2. Spatial coherence
• For laser radiation all the emitted photons are in phase, the resultant radiation obeys
spatial and temporal coherence
9
Highly Coherence (cont..)
Temporal coherence (or longitudinal coherence)
The predictable correlation of amplitude and phase at one point on the wave train w .r. t
another point on the same wave train, then the wave is said to be temporal coherence

Spatially coherence (or transverse coherence)

The predictable correlation of amplitude and phase at one point on the wave train w. r .t
another point on a second wave, then the waves are said to be spatially coherence

10
Typical applications of high colimitation
• Alignment
• Bar code readers
• Communication
• Radar

Typical applications of small focused spot

• Compact discs
• Laser printers
• Material processing
• Medical surgery
11
 Interaction of Light with Matter (Three Quantum Process)
• Consider a material medium of identical atoms characterized by many energy levels, E1, E2,
E3,….
• We consider only two energy levels E1 (ground state) and E2 (Excited state) for simplicity to
understand
• The number of atoms per unit volume at an energy level is called as population density N
• Let the population at energy levels E1 and E2 be N1 and N2 respectively
• Under normal condition, N1 > N2
• Let light radiation (which is stream of photons) be incident on the material at thermal
equilibrium
• Let ρ is a photon density
• When photons travel through medium, they likely to cause three different processes
1. Absorption
2. Spontaneous Emission
3. Stimulated Emission
12
 1. Absorption

• If a photon of energy 𝟐 𝟏 is incident on an atom, it imparts its energy to the atom and
disappears
• The absorbed energy make the atom to excite at energy E2, called as absorption transition
• The probability of absorption transition is proportional to photon density ρ
P12 ρ
P12 = B12 ρ
Where B12 is a Einstein coefficient for induced absorption
• The number of atoms excited during time Δt is
Nab = B12 N1 ρ Δt 13
2. Spontaneous Emission

• An atom can not stay in the excited state for a longer time (10-8 sec) and reverts to the lower energy
state by releasing a photon of energy 2 1
• The emission of photon by an atom without any external impetus is called as spontaneous emission
• The probability that a spontaneous transition occurs depends only on properties of energy levels
and not on the photon density.
(P21)Spont. = A21
• Where A21 is constant known as Einstein coefficient for spontaneous emission
• 1/A21 is a measure of the lifetime of the upper state
• The number of spontaneous transitions taking place during time Δt is
Nsp = A21 N2 Δt 14
Characteristics of spontaneous emitted radiation

• poly-monochromatic
• Incoherent
• less intense
• less directionality
• Example: light from sodium or mercury lamp

15
3. Stimulated Emission

• Stimulated emission was postulated by Einstein

• The phenomenon of forced photon emission by an excited atom due to the action of an external
agency is called stimulated emission
• Stimulated emission rate depends upon the number of atoms available in the excited state as well as
the energy density of photons
• The probability that a stimulated transition occur is
(P21)stimulated = B21 ρ
• Where B21 is Einstein coefficient for stimulated emission
• The number of atoms that undergo downward transition during time Δt is
Nst = B21 N2 ρ Δt 16
 Multiplication of Stimulated Photons
• The photon induced in this process propagates in the same direction as that of stimulating photon
having same frequency, phase and plane of polarization
• Thus, waves are coherent and interfere constructively
• The net intensity of light is 𝒕𝒐𝒕𝒂𝒍
𝟐

• Since atoms in the medium is very large, coherent emission leads to an enormously high intense
light and we say that incident light is amplified

17
Spontaneous vs. Stimulated Emission
Spontaneous Emission Stimulated Emission
The spontaneous emission was postulated by The stimulated emission was postulated by
Bohr Einstein
Additional photons are not required in Additional photons are required in stimulated
spontaneous emission emission
One photon is emitted in spontaneous emission Two photons are emitted in stimulated
emission
The emitted radiation is poly-monochromatic The emitted radiation is monochromatic
The emitted radiation is Incoherent The emitted radiation is Coherent
No amplification of light: less intense light Amplification of light: high intense light
𝑰𝒕𝒐𝒕𝒂𝒍 = 𝑵 𝑰 𝑰𝒕𝒐𝒕𝒂𝒍 = 𝑵𝟐 𝑰
Emitted light is non-directional Emitted light is highly directional
Unpolarized light Polarized light
Eg. Sodium and Mercury lamp Eg. Laser light
18
Steady State
Under steady state condition the absorption and emission balance each other

Nabsorption = Nspont.emission + Nstim.emission

B12 N1 ρ Δt = A21 N2 Δt + B21 N2 ρ Δt
B12 N1 ρ = A21 N2 + B21 N2 ρ

At thermal equilibrium, N1>> N2

Therefore, the process of absorption dominates the process of stimulated emission
19
Relation between the Einstein Coefficients
• The Einstein coefficients A21, B12 and B21 are interrelated. To find out the relation we
assume that
(i) The atoms and the radiation are in thermal equilibrium
(ii) The radiation is identical with black body radiation
(iii) Population densities N1 and N2 at E1 and E2 are constant in time and are distributed
according to Boltzmann law of energy levels

• Since the system is in equilibrium the upward transitions is equal to downward

transitions
B12 N1 ρ = A21 N2 + B21 N2 ρ 1

ρ [ B12 N1 - B21 N2 ] = A21 N2

A21 N2
2
[ B12 N1 − B21 N2 ] 20
Dividing both the numerator and denominator on the RHS of the eq (2) with B12N2
𝐴 /𝐵
𝜌 𝜈 = 2
𝑁 𝐵
−
𝑁 𝐵

But = 𝑒 ( )/

or = 𝑒 /

𝐴21 1
∴ 𝜌 𝜈 = /
3
𝐵12 𝑒 − 𝐵21/𝐵12
According to Plank’s law
8𝜋ℎ𝜈 𝜇 1
𝜌 𝜈 = / 4
𝑐 𝑒 −1

Energy density 𝜌 𝜈 given by eq (3) will be consistent with Plank’s law eq (4), only if

𝑨𝟐𝟏 𝟖𝝅𝒉𝝂𝟑 𝝁𝟑
= 5
𝑩𝟏𝟐 𝒄𝟑
Known as
𝑩𝟐𝟏 Einstein relations
= 𝟏 𝒐𝒓 𝑩𝟏𝟐 = 𝑩𝟐𝟏 6
𝑩𝟏𝟐 21
 𝐴 8𝜋ℎ𝜈 𝜇
= 5
𝐵 𝑐

𝐵
= 1 𝑜𝑟 𝐵 =𝐵 6
𝐵

Therefore coefficients are related through

𝒄𝟑
𝑩𝟏𝟐 = 𝑩𝟐𝟏 = 𝑨 7
𝟖𝝅𝒉𝝂𝟑 𝝁𝟑 𝟐𝟏

Eq (6) shows that the coefficients for both absorption and stimulated emission are
numerically equal.
i.e. when an atom with two energy levels placed in the radiation field, the probability of
upward transition is equal to the probability for a downward transition.

22
Light Amplification
• Absorption and spontaneous emission always occur together with stimulated emission
• Light amplification requires that stimulated emission occur almost exclusively
Condition for stimulated emission to dominate spontaneous emission:
𝑆𝑡𝑖𝑚𝑢𝑙𝑎𝑡𝑒𝑑 𝑡𝑟𝑎𝑛𝑠𝑖𝑡𝑖𝑜𝑛 B21 N2 ρ(𝜈) 𝐵
= = 𝜌(𝜈)
𝑆𝑝𝑜𝑛𝑡𝑎𝑛𝑒𝑜𝑢𝑠 𝑡𝑟𝑎𝑛𝑠𝑖𝑡𝑖𝑜𝑛 A21 N2 𝐴

• This indicate that stimulated transitions will dominate the spontaneous transitions if the
radiation density is very large
• Thus, the presence of a large number of photons in the active medium is required
• However, it will lead to more absorption transitions and therefore large photon density
alone is not sufficient and need the ratio B21/A21 to be a large
• So to increase the probability of stimulated emission, the lifetime of atoms at the excited
state should be larger
• It means that there is a requirement of states of larger lifetime 23
Condition for stimulated emission to dominate absorption transitions:
• Taking the ratio of these two transitions yields,

B21 N2 ρ
12N1 ρ

• It indicates that the stimulated emission will be larger than absorption process if
N2 > N1

• A medium amplifies light only when following three conditions are fulfilled:
i) High photon density in the active medium
ii) Larger lifetime excited states
iii) N2 > N1
24
Population Inversion
• If material is in thermal equilibrium, the population ratio is given by Boltzmann factor,
𝟐 𝑬𝟐 𝑬𝟏 /𝒌𝑻
𝟏
• It means that population N2 at the excited level E2 is far smaller than the population N1 at
the level E1 at normal or equilibrium condition (N1>>N2)
• Population inversion is a non-equilibrium state in which population of the upper energy
level N2 far exceeds the population of the lower level N1 (N1<<N2)
• Population inversion can be obtained by employing pumping techniques

25
Metastable States
• Metastable state is an excited state of atoms where atoms remains excited for an
appreciable time (10-6 to 10-3 sec) which allows accumulation of a large number of excited
atoms at this level
• Metastable state can be obtained by incorporating impurity atoms in the crystal system.
These levels lie in the forbidden band gap

“There could be no population

inversion and hence no laser
action, if metastable states do
not exist”

26
Components of Laser
The essential components of laser are
i) Active medium
ii) Pumping agent
iii) Optical resonator

27
Active medium
• Active medium is the material in which the laser action takes place
• Most important requirement of laser medium is that population inversion should be possible
• The atoms, which cause laser action, are called as active centres. The rest of the medium acts as
host and supports active centres
• This medium after excitation reaches to population inversion and promotes stimulated emissions
leading to the light amplification
• Depending upon the active medium the lasers are classified as solid state, liquid state, gaseous
state and semiconductor lasers

28
The Pump (Energy Source)
The pump is an external source that supplies energy needed to transfer the laser medium into the
state of population inversion
Pumping techniques:
1. Optical pumping
• A light source such as a flash discharge tube is used to illuminate the laser medium
• Optical pumping is used in solid laser. The solid materials have very broad absorption band, so
sufficient amount of energy is absorbed from the emission band of flash lamp to create
population inversion
• Xenon or Krypton flash tubes are used for optical pumping
• Examples: - Ruby laser, Nd:YAG Laser (Neodymium: Yttrium Aluminum Garnet), Nd: Glass Laser
2. Electrical Pumping
• This pumping is used only in laser materials that can conduct electricity. This method is limited to
gases medium
• A high voltage pulse ionizes the gas so that it conducts electricity. An electric current flowing
through the gas excites atoms to excited level
• Examples:- He-Ne laser, CO2 laser, Argon-ion laser, etc. 29
3. Injection current pumping
• This pumping mechanism is used in semiconductor lasers
• In semiconductor lasers, by passing high currents across the junction, the population
inversion will create
• In semiconductors lasers the population inversion always creates among majority and
minority charge carriers
• Examples:- InP and GaAs lasers

30
Pumping Schemes
1. Three-Level pumping scheme

• The pumping leads to a large number of atoms at E3 which further rapidly undergo
downward transition to metastable state E2 through non-radiative transitions
• The population inversion is achieved only when more than half of the ground state atoms
are pumped to upper state
• Thus, require very high pump power for population inversion
31
• Three level pumping scheme produces light only in pulses
2. Four-Level pumping scheme

• Population inversion takes place at energy level E3

• The level E2 is well above the ground state such that (E2-E1) > KT atoms can not
thermally excited to state E1 from E2 at normal temperature. Thus level E2 is virtually
empty. Therefore, population inversion is attained between the states E3 and E2
• Therefore, it requires less pump power for population inversion
• Four-level pumping scheme operates in continuous wave (CW) mode
32
Optical Resonator
• The active medium is enclosed between a fully reflective mirror and partially reflective
mirror. These mirrors constitute the optical cavity or resonator
• The reflectors enhance the stimulated emission process by reflecting the photons into the
active medium
• Laser is analogues to an electronic oscillator which is essentially an amplifier supplied
with positive feedback
• In laser, medium is an amplifier and optical resonator is a positive feedback
• Optical cavity selects and amplifies only certain frequencies causing the laser output to
be highly monochromatic

33
Threshold Condition for Lasing

• Let laser medium fills the space between the mirrors 1 and 2 which have reflectivity r1
and r2 respectively
• Let the intensity of light beam be 0 at mirror 1
• In traveling from mirror 1 to 2, the beam intensity increases from 0 to , given by

Where is the gain coefficient and the loss coefficient of the active medium
34
After reflection at M2, the intensity will be 𝑟2𝐼 𝑒
After complete round trip the final intensity will be
𝐼 2𝐿 = 𝑟1𝑟2𝐼 𝑒
The amplification obtained during the round trip is
𝐼(2𝐿)
𝐺= = 𝑟1𝑟2𝑒
𝐼
𝑟1𝑟2 represents the losses at the mirrors
𝛼 s represents losses due to scattering, diffraction and absorption occurring at the medium
The losses are balanced by gain, when G ≥ 1 ⟹ 𝑰(𝟐𝑳) = 𝑰𝟎
⟹ 𝑟1𝑟2𝑒 ≥1
𝑒 ≥
Taking log on both sides,
2𝐿 𝛾 − 𝛼 ≥ − ln 𝑟 𝑟
1
𝛾−𝛼 ≥− ln 𝑟 𝑟
2𝐿
1
𝛾≥𝛼 + ln 𝑟 𝑟
2𝐿
𝟏 𝟏
𝜸 ≥ 𝜶𝒔 + 𝒍𝒏 Condition for lasing 35
𝟐𝑳 𝒓𝟏 𝒓𝟐
• It shows that that the initial gain must exceed the sum of the losses in the cavity
• This condition is used to determine the threshold value of pumping energy for lasing
action

• depends on how hard the laser medium is pumped. At particular pump power, the
threshold value ( th) will be reached and the laser will starts oscillating

• The threshold value th is given as

𝒔
𝟏 𝟐
This is a threshold condition for lasing

36
Modes of the Laser Beam
• The light waves oscillating at some modes that match the oscillation modes of the cavity
are sustained
• There are two types of resonant modes
1. Longitudinal modes: governed by the axial dimensions of the resonant cavity
2. Transverse modes: determined by the cross-sectional dimensions of the laser cavity

37
Longitudinal Modes

• The two waves of the same frequency and amplitude moving in opposite directions creates a
standing wave in the cavity
• To create standing wave, a phase must be same at the mirror
• Therefore, the optical path from one mirror to other and back must be an integer multiplication of
wavelength
Thus
Or
Light waves are amplified strongly if, and only if, they satisfy the above condition in the cavity38
• The resonator may support several standing waves called as longitudinal modes

• These are the allowed frequencies inside the laser cavity of length L
• The frequency difference between two adjacent modes ( is

• Only those frequencies (modes) that have the amplification above the lasing threshold will be
emitted out of the laser

Gain curve of a laser and

allowed frequencies
39
Transverse Modes
• The transverse modes illustrate the intensity
distribution across the cross-section of the laser
beam
• The shape of the optical cavity determines the
transverse modes
• The allowed modes in an optical cavity are
designated as TEMmn, where T,E and M stands for
transverse, electric and magnetic modes
respectively
• Integers m and n stand for minima between the
edges of the beam in two perpendicular directions
• The simplest transverse mode is TEM00 and most of
the cavities are designed to produce only this mode

40
Types of Laser
Based on the material used, some of the important lasers are

i) Solid-state lasers (Ruby laser, Nd:YAG laser)

ii) Gas lasers (He-Ne laser, CO2 laser)

iii) Semiconductor diode laser (GaAs)

41
Nd:YAG laser
• Nd:YAG laser stands for Neodymium: Yttrium
Aluminium Garnet (Y3Al5O12)
• One of the most popular solid state laser with
four-level laser system
• The system consist of an elliptically cylindrical
reflector having laser rod along one focus line and
a flash lamp along the other focus line
• Doping concentration of Nd is typically of the
order of 0.725% by weight, where Y3+ ions are
replaced by Nd3+ ions
• The crystal atoms do not participate in the lasing
action serve as a host lattice in which the active
centres Nd3+ ions resides
• The YAG crystal rods are typically of 10 cm in
length and 12 mm in diameter
42
 Working:

• The Krypton flash lamp pump the Nd3+ ions to excited state E4. These ions make non-radiative transition to
metastable state E3 leading to accumulation of Nd3+ ions at this energy level
• Therefore population inversion is readily achieved between the E3 level and E2 level and hence stimulated
emission initiated triggering many excited atoms to emit photons
• The laser emission occurs in infra red (IR) region at a wavelength of about 1.06 µm 43
Silent features of Nd:YAG laser
• Uses four-level pumping scheme
• The active centres are Nd3+ ions
• Krypton or Xenon flash lamp is used for pumping
• Low efficiency (1%) and moderate power output
• Operate in CW/pulsed mode

Applications of Nd:YAG laser:

• widely used for cutting, drilling, welding in the industrial products
• long haul communication systems
• Surgery and endoscopic application in medical field

44
Helium-Neon Laser
• He-Ne laser is the first gas laser invented in 1961 by Ali Javan and co-workers
• It is made of a long discharge tube filled with mixture of helium and neon gases in the ratio 10:1
• Neon atoms are the active centres and have energy levels suitable for laser transitions while
helium atoms help in exciting neon atoms

45
Role of Helium in He-Ne laser
• The Helium atoms are lighter. So more
Energy Transfer readily excitable than neon atoms
E3 E6 (20.61 eV) • Helium atoms in excited energy levels E2
E5 and E3 collide with Neon atoms in the
E2 ground level. Neon atoms are excited to
E4 (19.81 eV)
energy levels E4 and E6 and Helium atoms
come back to the ground state (Resonant
E3 energy transfer)
E2 • The Neon atoms are much heavier and
could not be pumped efficiently without
E1 E1 Helium atoms
• The role of Helium atoms is to excite Neon
He Ne atoms and cause population inversion
Levels Levels
46
 Why the population of He : Ne is 10:1 ?
• Many He atoms excited to
Energy Transfer higher energy levels E2 andE3
E3 E6 will fall down due to
E5 spontaneous emission and
won’t be able to transfer their
E2 E4 energy to Ne atoms

E3 • The probability of energy

transfer from Helium atoms to
E2 Neon atoms is more as there
are 10 Helium atoms per 1
E1 E1 Neon atom in a gas mixture
He Ne
Levels Levels

47
Working of He-Ne Laser

48
Silent features of He-Ne Laser
• The laser is simple, practical and less expensive
• Uses four-level pumping scheme and hence emits continuous laser beam
• The laser beam is highly collimated, coherent and monochromatic compared to solid
state laser
• Widely used in interferometer, laser printing, bar code reader, etc.
• He-Ne laser is highly stable and require no external cooling

• The overall gain of He-Ne laser is very low and is typically about 0.010 % to 0.1 %
• The output power of He-Ne laser is very low ranging from 0.5 to 50 mW in the red
portion of the visible spectrum

49
Carbon Dioxide Laser: Definition, Principle

Definition
In carbon dioxide laser, CO2 gas molecules are used as the active medium
and population inversion is achieved by the electrical pumping method.

Principle
The active medium is a gas mixture of CO2, N2 and He. The laser transition
takes place between the vibrational states of CO2 molecules.
Energy states of CO2 molecules Carbon dioxide (Co2) is a symmetric
molecule (O=C=O) and it has three modes of vibration:

•Symmetric stretching.
•Bending.
•Antisymmetric stretching is shown in the figure

0.163 eV

0.078 eV

0.276 eV
Discharge tube : 2.5 cm dia; 5 m in
Length

Discharge tube filled with mixture

of CO2: N2:He
1 : 4:5
0.33: 1.2: 7 torr

Energy source : electric discharge

E5 to E4 : 10.6um (IR)
E5 to E3 : 9.6 um (IR)
Components of Co2 Laser
Active medium ( or Carbon dioxide laser gain medium )

The main component of a Carbon dioxide laser is a medium in the form of CO2 gas molecules called
an active medium. The main characteristics of the active medium are as follows:

•It must have a pair of energy levels separated by a certain amount of energy. The energy level
having energy is known as an upper energy level or higher excited energy level and the energy level
having low energy is known as low energy or ground state.

•It must allow a population inversion between two energy levels.

Working of Carbon Dioxide Laser
To have a CO2 laser, a mixture of CO2 and N2 in the ratio of about 0.8:1 is filled in a gas discharge tube.
Also, helium is part of the mixture. Helium is more than N2 in the mixture. CO2 molecule act as an active or
laser medium and N2 molecule help in achieving the population inversion in the same way as helium is used
in He-Ne laser.

When an electric discharge is passed through the tube, the number of electrons is emitted, which pumps
nitrogen molecules to V = 1 state.

In CO2 laser, the energy difference between the vibrational energy levels of Nitrogen and carbon dioxide is
very small ( i.e. about 0.3 eV ) and hence there are a large number of electrons in the gas discharge tube
having the energy of more than 0.3 eV.
Application of CO2 laser

•Due to the high power of CO2 laser, it has frequently used in industrial areas such as for cutting and
welding.

•it is used for soft tissue surgery.

•it is used in fabricating.

•used in skincare problems to treat different non-cancerous (benign) and cancerous (malignant).

•It is used to perform microsurgery and bloodless operations.

 APPLICATIONS
Holography

The method of producing 3D images of an object due to interference

phenomena of coherent light waves on a photographic plate is known as
Holography.
Holo (greek) : complete
Gramma (greek) : writing

Holography is a lens less photography

In conventional photography, only amplitude is recorded in 2-D projection of an

object onto the plane of the photograph. Complete information of the object is
not obtained because the phase distribution of the object is lost during
recording.

In holography, in addition to amplitude, phase is also recorded by interference

pattern.
late1940’s, Dennis Gabor an English physicist outlined
a new technique of photographing objects.
He called this technique wavefront construction.
A weak but broad beam of laser light is split into two
beams namely a reference beam and object beam.
The reference beam is allowed to reach the
photographic plate directly while the object beam
illuminates the object.
Part of the light scattered by the object travels towards
the photographic plate and interferes with the reference
beam and produces an interference pattern on the
photographic plate.
The photographic plate carrying the interference pattern is called a hologram.
Thus a hologram means complete recording.
Like ordinary photographic plate, a hologram is developed, fixed and stored.
A hologram does not contain a distinct image of the object. Its only a record
of the interference pattern formed by the superposition of two coherent light beams.
The interference pattern on a hologram consists of a complete pattern of alternate
regions of dark and bright fringes.
In reconstruction, the object is recreated by
directing the beam of light at the film as shown in
figure.
During reconstruction the hologram acts as a
diffraction grating and the secondary waves from
the hologram interfere constructively in certain
directions and interfere destructively in other
directions.

They form a real image in front of the hologram

and a virtual image behind the hologram at the
original site of the object
The reconstruction beam need not be laser light.
Ordinary sodium light can also be used.

By moving the hologram through the light until

the beam strikes the hologram at approximately
the same angles as the reference beam we can
.see the three dimensional virtual image if the
object just viewing through a window.
COMPACT DISC
Digital data are carved into the disc as pits (low spots) and lands (high spots).
As the laser shines into the moving pits and lands, a sensor detects a change
in reflection when it encounters a transition from pit to land or land to pit.
Each transition is a 1. The lack of transitions are 0s.
There is only one laser in a drive. Two are used here to illustrate the
difference in reflection.
 During reading when the radiation is reflected back
to a detector from both the upper metallic surface
and the bottom of pit, there is only one reflected
wave to the detector.
When the laser beam is reflected from a transition
between the upper and lower metallic surfaces, two
waves (with phase difference of π) are reflected to
the detector.
When the detector see the transition between the pit
and the upper surface, the detector reads one.
Otherwise the reflection is from metallic surface and
there is no transition (no destructive interference),
the detector reads zero.

Writing – high intense laser beam

Reading – Low intense laser beam (low
power 4 mW)
Blu-ray (not Blue-ray) also known as Blu-ray Disc (BD), is the name of a new
optical disc format jointly developed by the Blu-ray Disc Association (BDA), a
group of the world's leading consumer electronics, personal computer and media
manufacturers (including Apple, Dell, Hitachi, HP, JVC, LG, Mitsubishi,
Panasonic, Pioneer, Philips, Samsung, Sharp, Sony, TDK and Thomson).
The format was developed to enable recording, rewriting and playback of high-
definition video (HD), as well as storing large amounts of data.
The format offers more than five times the storage capacity of traditional DVDs
and can hold up to 25GB on a single-layer disc and 50GB on a dual-layer disc.

While current optical disc technologies such as DVD, DVD±R, DVD±RW, and
DVD-RAM rely on a red laser to read and write data, the new format uses a blue-
violet laser instead, hence the name Blu-ray.
The benefit of using a blue-violet laser (405nm) is that it has a shorter wavelength
than a red laser (650nm), which makes it possible to focus the laser spot with
even greater precision. This allows data to be packed more tightly and stored in
less space, so it's possible to fit more data on the disc even though it's the same
size as a CD/DVD. This together with the change of numerical aperture to 0.85 is
what enables Blu-ray Discs to hold 25GB/50GB. Storage capacity to 500GB (20
layers) is also achieved.
HOW CAN 50GB BE STORED ON ONE DISC?
The structure of a Blu-ray (BD) disc fundamentally differs from the structure
of a DVD/CD. Because the data layer on a Blu-ray disc is placed much
“closer” to the laser lens than on a DVD/CD, the laser beam provides more
precision. (fig. A)
The Blu-ray Drive uses a blue-violet laser and improved lens specifications
(wavelength, NA-numerical aperture) allowing for a laser beam focus that’s
approximately one-fifth smaller than the red laser used to burn DVDs.

This combination enables recording much smaller and higher density pits onto
BD discs.