Applied Physics Sem - II

Module-2
Lasers
2.1 Motivation
There are many applications of lasers viz. A laser light show, Barcode readers, Laser pointers,
Writing subtitles onto picture films, Metal Cutting , welding, marking, in laser printers lasers
play a key role in manufacturing high resolution printing and in image scanning equipment,
Rangefinder , surveying using LIDAR in military(light detection and ranging) and Optical
communications.

2.2 Syllabus
Sr. Contents Duration Self
Study
1. Quantum processes as absorption, spontaneous emission and 1 2 hours
stimulated emission; metastable states, population inversion,
pumping,
2. resonance cavity, Einstein’s equations; Helium Neon laser; 1 2 hours
3. Nd: YAG laser; Semiconductor laser, 1 2 hours
4. Applications of laser- Holography (construction and 1 2 hours
reconstruction of holograms) and other applications

2.3 Weight age: 12-15 marks

2.4 Learning Objective
• Absorption, spontaneous emission, stimulated emission, Population inversion, and
metastable states thoroughly to understand basic operation of a laser.
• Those ideas are applied to describe He-Ne laser, Nd: YAG laser, semiconductor
diode laser.
• Their applications in construction and reconstruction of holography, Memory
reading and writing, other industrial and medical applications.
2.5 Theoretical Background.
• Rutherford and Bohr atomic models, which explains the electron behavior in an atom
when it can absorbs energy and emits radiation, wave theory of light in particular the
properties of polarized and un polarized light.
• Semiconductor PN junction Diode fabrication, forward reverse biasing for I-V
characteristics. Based on these theories one can go ahead to learn more about lasers.

2.6 Key Notation

Nd-YAG laser (Neodymium-Yttrium Aluminum Garnet)
He-Ne laser,

2.7 Key Definitions

Active Medium: Collection of atoms or molecules which can be stimulated to a population
inversion, and emit electromagnetic radiation in a stimulated emission. Amplification The
process in which the electromagnetic radiation inside the active medium within the laser
optical cavity increase by the process of stimulated emission.
Brightness: The brightness of a light source is defined as the power emitted per unit surface
area per unit solid angle.
Hologram: An interference phenomena captured on a plate (or film). It can contain enormous
amount of information and a 3 dimensional image can be constructed from it.

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Module 2: Lasers

Population Inversion: Normally the number of atoms at high energy level ((E1) is less than
those in low energy level (E1), N2(E2) < N1(E1). If N2>N1, we say population inversion exists,
which is a necessary condition for lasing.
Pumping: The process to raise atoms from lower level to upper level is called pumping.
Solid State Laser: A laser in which the active medium is in solid state (usually not including
semiconductor lasers).
Semiconductor Lasers: Lasers which use semiconductor as active medium. The majority of
semiconductor materials are based on a combination of elements in the third group of the
Periodic Table (such as Al, Ga, In) and the fifth group (such as N, P, As, Sb) hence referred to
as the III-V compounds.
YAG: Yttrium-Aluminum Garnet arnet

2.8 Theory

Learning Objective – Learner will be able to understand the characteristics of laser beam,
difference between laser light and ordinary light, interaction of radiation with matter.

Lecture: 8
2.8.1 Important Characteristics of Laser Beam
1. Highly coherent
The light is coherent with waves all exactly in phase with another. It means an interference
pattern can be obtained by using two laser sources.
2. Highly monochromatic
A laser produces light in more or less single wavelength i.e tthe
he line width associated with llser
beams is exactly narrow.
3. Highly directional
A laser beam diverges hardly at all. Such a beam sent from the earth to a mirror left by
Apollo-III expedition , remained narrow enough to be detected on its return to the
earth.(Distance between moon and earth is around 3,84,000 km).
4. Brightness
The laser beam is highly intense as compared to ordinary source of light.
5. Highly energetic
The laser beam is highly intense. To understand it clearly here is an example
example. To achieve an
equal energy density to that in laser beam, a hot object would have to be at temperature of
1030 K.
This makes laser suitable for applications like cutting, drilling, and w
welding.

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Applied Physics Sem - II

2.8.2 Difference between LASER and ordinary light

(Based on1st edition, 14th chapter, 1st edition, page no.395.)

Sr.no. LASER ORDINARY LIGHT

1 It is highly monochromatic. It is poly chromatic.
2 It is highly coherent. It is not coherent.
3 Stimulated emission is responsible for it. Spontaneous emission is responsible for it.
4 Highly direction. Not directional.
5 Highly energetic. Poor energy is associated.
6

2.8.3 Interaction of radiation with the matter

(Based on1st, edition, 14th chapter, page no.396
no.396-400)
1. Absorption Process
Absorption is the process by which a quantum system such as
an atom, molecule, nanocrystal or nucleus is excited to higher energy state

Figure 2.8.3(a): Absorption of a photon

A + hν = A*
Where A = Atoms in lower er energy state, A*= Atoms in higher energy state
The number of atoms Nab excited during the time ∆t is given by Nab= B12N1Qd dt
Where N1 = Number of atoms in state E1, Q = Energy density of the incident beam
B21= probability of an absorption transition
2. Spontaneous Emission
Excited state with higher energy is unstable because of a natural tendency of atoms to seek out
lowest energy configuration.
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Therefore excited atoms do not stay in the excited state for a relatively longer time but tend to
return to the lower state by giving up the excess energy (hv=E2 –E1) in the form
orm of
spontaneous emission

Figure 2.8.3(b): spontaneous emission

The excited atom in the state E2 may return to the lower state. During the transition the energy
is released as a photon of energy h
hν= E2 –E1.
This type of process in which photon emission occurs without any external agency is called
spontaneous emission or natural emission.
A* → A + hν
Where A*= *= Atoms in higher energy state,
state A = Atoms in lower energy state
The number of spontaneous transitions Nsp taking place in the medium during time dt
depends only on the number of atoms N2lying in the excited state E2.
It is given by,
Nsp= A21N2dt
Where, N2=number of atoms in the energy state E2 ,A21=probability of a spontaneous
emission from higher energy state to lower energy state
3. Stimulated Emission:

Figure 2.8.3(c): stimulated emission

A photon of energy=E2-E - 1 can induce the excited atom to make a downward transition
releasing the energy in the form of the photon.

Thus the interaction of a photon with an excited atom triggers the excited atom to drop
to the lower energy state giving up a photon.

This phenomenon is called forced emission or stimulated emission. The process may be
represented as
• A*+hν=A+2hν
Where A*= Atoms in higher energy state A = Atoms in lower energy state

The number of stimulated transition Nst occurring in the material during any time dt
may be written as
Nst = B21 N2 Q dt

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Applied Physics Sem - II

Where B21= Probability of a stimulated emission.

Einstein had predicted this probability and it is considered as one of the essential
requirements of laser.
The main features are:

The emitted photon is identical to the incident photon in all respects.

It has the same frequency (ν) as that of incident photon.

Both the photons travel in the same direction.

The process is controllable from outside.

Multiplication of photons takes place in the process.
Let’s check Take away from this Lecture
1. Laser is acronym for
a) Light Amplification by Spontaneous Emission
b) Light Amplification by Stimulated Emission
c) Light Amplification by Synchronous Emission
d) Light Amplification by Stopping Emission

2. Laser is a
a) Quantum device b) Mechanical device c) Electrical device d) Quantum Electronic Device.

Exercise
1. Explain characteristics of laser.
2. Differentiate between ordinary light and laser light
3. Explain absorption, spontaneous emission and stimulated emission in laser.
Questions/problems practice for a day
1. Explain interaction of radiation with matter as absorption process, spontaneous emission and
stimulated emission.

Learning from the topic - Learner can explain characteristics of laser, interaction of radiation with
matter and can differentiate between ordinary light and laser light.

Lecture: 9
Learning Objective – Learner will be able to understand the different terms involved in laser
system like active medium, resonant cavity, pumping, metastable state, population inversion,
types of laser and pumping schemes.
2.8.4 Different Terms
a. Active Medium
A medium in which light gets amplified is called an active medium. The medium may be a
solid, liquid or gas. Therefore the medium where we get population inversion and laser as
output is active medium.
b. Laser Resonator (Resonant Cavity)
• The most common way to achieve the necessary LASER ACTION is to use some sort of
mirrors in an open cavity configuration.
• one mirror with 100% reflection and the other mirror with less than 100% reflection.
• In the case of low gain medium (such as the helium-neon gas), these mirrors have to be
highly reactive to minimize losses.

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Module 2: Lasers

Figure 2.8.4(a): Basic laser schematic diagram

Learning outcome: Students will be able to explain Laser Resonator.

c. Types of Lasers
Solid state laser : Ruby laser, Nd-YAG laser
Gaseous state laser : He-Ne laser, CO2 laser
Liquid state laser : Dye laser
d. Population Inversion
(Based on1st, 1st edition, 14th chapter, page no.399-401)(May 97)
In order to understand population inversion, we must know the meaning of ,
i) Population of energy level ii) Boltzmann factor
i) Population of energy level
If we consider an assembly of identical atoms, then we can compare the energies of all those
atoms in a single energy level scheme. The number of atoms in a particular state is referred to
as its population.

Figure 2.8.4(b): Population of energy level

ii) Boltzmann factor
The population of different energy states of any physical system is related to each other
provided the system is in thermal equilibrium. The relation is given by Boltzmann factor.
Among the various energy states, if we consider any two energy states E1 and E2 with
population N1 and N2 respectively, and if E2>E1 then Boltzmann factor is the ratio (N2/N1)
given by,
N2
= e− ( E2 − E1 ) / KT
N1
Where k is Boltzmann constant.
Since E2>E1, Therefore N2<N1.
Hence for a system in thermodynamic equilibrium, it is mandatory that the population of any
higher state is always lesser than that in any of the lower states.
Population inversion is the state of a system at which the population of a higher energy state
is more than that of a lower energy state
i.e.N2>N1

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Applied Physics Sem - II

Under normal condition population inversion condition does not exist. However it is possible
to achieve the population inversion condition in certain system, which has metastable states.
g) Metastable States
no.399-401)(May 97)
(Based on1st, edition, 14th chapter, page no.399
By providing energy, if an atom is made to go to one of its excited states, it stays there over a
brief interval of time not exceeding 10
10¯⁸ sec, and then returns to one of the lower energy states,
then the atom stays theree for unusually long time, which is of the order of 10-3 to 10-2seconds.
This property is essential for achieving population inversion.

Figure 2.8.4(c):
2 Metastable state

Metastable state is the state where an electron stays for maximum amount of time.

h) The Conditions For Laser Action

ction are:
are
Population inversion
• Rate of stimulated emission should be more than the rate of absorption.
• There should be a upper excited level having long life time and lower level that decays
fastly to the ground level.
• Active medium, pump and optical resonance are essential.
i) Pumping
The process of raising
ising large number of atoms from lower energy level to a higher energy level
is called pumping.

Types of Pumping
Optical pumping: Which uses strong light source for excitation?
Electrical pumping: Which uses electron impact for excitation?
Chemical pumping: Which uses chemical reactions for excitation?
Direct pumping: Which uses direct conversion of electric energy into
i light?

Figure 2.8.4 (d): pumping

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2.8.5 Pumping Schemes

Any atom has large number of energy levels but for pumping process only few are of some
use. Important pumping schemes which are ,
(a) Three level (b) Four level. (c) Three levels

We assume that all the atoms start out from the ground state.

Then most of atoms pumped up to a higher energy level E3.

They quickly fall from the short-lived
short level E3 to the metastable upper level E2, which
has a much longer lifetime (typically thousands of times longer).

The result is the population inversion between energy level E2 and depopulated level E1,
which are the laser transition levels.

Figure 2.8.5(a): Energy level diagram of a three-level laser.

(b) Four Level

As in the three-level
level laser, the excitation energy pumps electrons from the ground state to a
short-lived
lived highly excited level E4.

The atoms then trop quickly to a metastable upper laser level E3.

The laser transition then takes these atoms to a lower leve
level E2, rather than all the way down to
the ground state.

After they have dropped to level E2, the atoms loose se the rest of the excess energy by
spontaneous emission or other processes, and finally fall to the ground state.

Figure 2.8.5(b):: Energy level diagram of a four-level laser.

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Applied Physics Sem - II

Let’s check Take away from this Lecture

1. A medium in which light gets amplified is called an
(a)Inactive medium (b) Active medium (c) Intra active medium (d) Interactive medium
2. The type of solid state laser is
(a) He-Ne laser (b) CO2laser (c) Dye laser (d) Nd-YAG laser
3. Population Inversion is the state where
(a) N1> N2 (b) N2> N1 (c) N1 N2>1 (d) N1 N2< 1

Exercise
1. Explain Active medium, resonant cavity, population inversion.
2. Explain metastable state.
3. Describe pumping and pumping scheme.
Questions/problems practice for a day
1. Explain 3 level and 4 level pumping schemes.

Learning from the topic - Learner can explain active medium, population inversion, and
pumping, metastable state.

Lecture: 10
Learning Objective – Learner will be able to understand Einstein’s Co-efficient and Helium –
Neon laser.
2.8.5 Einstein’s Co-efficient
(Based on1ST, 1st edition, 4th chapter, page no.4.3-4.4.)
In section 2.8.1 we have discussed spontaneous and stimulated emission.
Einstein was the first to calculate the probability of such transition assuming the atomic
system to be in equilibrium with electromagnetic radiation.
Under thermal equilibrium
The number of downward transition = number of upward transition
Spontaneous emission + Stimulated emission = Absorption transition
∴   +   Q = Q ………………………(1)

∴   =   Q-  Q ………………………(2)

∴   =(   -   )Q ………………………(3)

From Equation (3) we get,



Q = …………………………..(4)

 
 

Divide equation (4) Nr and Dr by B21N2





Q= 
 …………………………….(5)
 



  
Now  = e x p ( 
) = e x p ( )



∴Q = 



 …………….…(6)


.  
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Module 2: Lasers

Equation (6) must agree with Planck’s energy distribution formula which is given by,
!" # 
Q= $#
.  ………………………..…(7)
 


Now by comparing Equations (6) and (7), we get, B12=B21

Learning outcome: Students will be able to derive the derivation of Einstein’s Co-efficient

2.8.8 He-Ne Laser

(Based on1st, 1st edition, 14th chapter, 1st edition, page no. 409-410.)
Principle : Gas lasers are usually employ a mixture of two gases say A (He) and B (Ne) where
atoms of type A are initially excited by electron impact and they in turn transfer their energy
to atoms of type B which are actual active centers .
Here the energy transfer is done by atomic collisions between A and B where two of their
energy levels are equal. It can be expressed as
e1+A = e 2 + A*
A* + B = A + B*
A* represents the energy values of the atom of type A in metastable state and
A represents in ground state of type A
B represent the energy values of the atom of type B in ground state and
B* represent in excited state of type B
Construction
• The He – Ne laser consist of long and narrow discharge tube
• Filled with Helium and Neon gas in ratio of 10: 1 with pressure of 1 mm of mercury.
• Flat glass quartz plates which function as Brewster windows
• Two optical plane mirrors are fixed on either side of the tube
• One of the mirrors is fully silvered with 100% reflectivity, where as the silvering of the
other mirror is slightly less so that 1% of the incident laser beam could be trapped by
transmission.

Figure 2.8.8(a): Schematic representation of Helium-Neon Laser

Working
• When a voltage of about 1 kV is applied across the tube, a slow discharge of the gases is
initialized in the tube. During the discharge, many electrons are rendered free from the
gas atoms and are accelerated towards positive electrode and collide with helium atoms.
Since. He atoms are large in number, excitation of He takes place easily. This is
represented as
e1 + He = e2 + He*
• He atoms are excited to two energy levels F2 and F3 which are metastable states.
Therefore atoms remain there for a relatively long time, which leads to an increase of
population in each of them. For Ne energy states E6 and E4 are very close to metastable
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Applied Physics Sem - II

states of F3 and F2 of He* atom. Therefore when He atoms collides with a Neon atoms,
because of the matching of energy levels, resonant transfer of energy takes place from
He to Ne atoms. As a result, the Ne atoms get elevated to the E6 and E4 levels, whereas
the He-atoms return to ground state. This is represented as
He* + Ne = He + Ne*
• The population increases rapidly and population inversion takes place between E6 and
E4 with respect to E5 and E3.
Energy level diagram of a He-Ne laser
The laser process in a He-Ne laser starts with collision of electrons from the electrical
discharge with the helium atoms in the gas.

Figure 2.8.8(b): Energy Level Diagram of He-Ne

(a) E6 and E5 gives rise to a radiation of wavelength 33912A0, which is in infra-red region and
hence not visible.
(b) E6 and E3givesrise to a radiation of wavelength 6328A0, which is visible and of red colour.
(c) E4 and E3 gives rise to a radiation of wavelength 11523A0, which is also in infra-red region.
• From E5 and E3 levels, atoms undergo spontaneous transitions to E2 level at much faster
rate. But E2 level is metastable for Ne. The atoms will come down to ground state by
wall collision.
• Since the discharge in the tube is maintained continuously, the cycle of events also takes
place continuously and the emission of laser is also continuous, because of which He-Ne
laser is referred as a continuous wave laser.
Merits
1. Continuous output
2. Highly monochromatic
3. No separate cooling is required
Demerits
1. Very low output power
Applications

1. Holography
2. Research activities
3. Communication

4.

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5. Let’s check Take away from this Lecture

1. Metastable State where atom remain there for the time
a) 10-8 sec b) 10-13 sec c) 10-3 to 10-2 sec d) 10-10 sec.
2. The type of pumping in He-Ne laser is
a) Optical pumping b) electrical pumping c) chemical pumping d) direct pumping

Exercise
1. Explain principle, construction and working of He-Ne laser.
2. Explain merits .demerits and applications of He-Ne laser.
3. Derive Einstein’s Co-efficient.

Questions/problems practice for a day

1. Explain principle, construction and working of He-Ne laser..

Learning from the topic - Learner can derive Einstein’s Co-efficient and can explain He-Ne laser.

Lecture :11
Learning Objective – Learner will be able to understand the Semiconductor laser and Nd-
YAG laser.
2.8.9 Semiconductor laser
(Based on1st, 1st edition, 14th chapter, 1st edition, page no.413-414.)
• When a diode is forward biased, holes from the p-region are injected into the n-region,
and electrons from the n-region are injected into the p-region.
• If electrons and holes are present in the same region, they may radiatively recombine-
that is, the electron "falls into" the hole and emits a photon with the energy of the band
gap. This is called spontaneous emission, and is the main source of light in a light-
emitting diode.
• Photons emitted in precisely the right direction will be reflected several times from each
end face before they are emitted.
• Each time they pass through the cavity, the light is amplified by stimulated emission.
Hence, if there is more amplification than loss, the diode begins to "lase".
• The wavelength emitted is a function of the band-gap between p and n region energy
levels. No photons with higher energy than the band-gap will be emitted.

Figure 2.8.9 (a): Energy level diagram

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Applied Physics Sem - II

Figure 2.8.9(b):: Emission of laser beam from semiconductor diode laser

Merits
1. Simple and Compact
2. Requires very little power and more efficient
3. Output can be controlled by controlling the junction current
4. Metastable state is not required
Demerits
1. Highly Temperature sensitive
2. Less monochromatic

Applications
1. Laser printer and copier
2. CD Players
3. Optical communications (as light source)

2.8.10 Nd-YAG laser

(Based on1st ,1st edition, 14th chapter, 1st edition, page no. 407-408.)
• Nd:YAG (Neodymium
eodymium Y Yttrium AluminiumGarnet-Nd+3:Y3Al5O12) is a crystal that is
used as a lasing medium for solid-state lasers.
• The dopant,, triply ionized neodymium, typically replaces yttrium in the crystal
structure of the yttrium aluminum garnet,, since they are of similar size.
• Generally the crystalline host is doped with around 1% neodymium by wei weight.
Construction
• An elliptically
lliptically cylinder reflector
• Both
oth of its axis occupied by a flash lamp
• Nd: YAG rod
• The light leaving one focus of the ellipse will certainly pass through the other focus after
reflection from reflecting surface.
• Hence
ence entire light generated flash tube is focused on the Nd YAG rod.
Working
• Nd: YAG lasers are optically pumped using a flash lamp or laser diodes.
diodes
• Nd: YAG lasers operate in both pulsed and continuous mode.
• Pulsed Nd: YAG laserss are typically operated in the so called Q-switching
switching mode: An
optical switch is inserted in the laser cavity waiting for a maximum population
inversion in the neodymium ions before it opens.

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Module 2: Lasers

Eliptical Reflector

Figure 2.8.10(a): Nd-YAG laser

• Nd:YAG absorbs mostly in the bands between 730730–760 nm and 790–820820 nm.
• Nd:YAG lasers typically emit light with a wavelength of 1064 nm,, in the infrared.

Figure 2.8.10 (b):

(b) Energy Level Diagram of a Nd-YAG Laser
Nd-YAG Laser applications
1. Welding
2. Cutting
3. Drilling
4. Let’s check Take away from this Lecture
1. Laser beam is
a) Incoherent b) polychromatic c) less intense d) highly directional
2. The ratio of He-Ne gas in He-NeNe laser is
a) 50 :50 b) 60:50 c) 70:30 d) 10:1
3. Nd : YAG laser is a
a) Solid state laser b) liquid state laser c) gaseous state laser
d) Liquid crystal laser
4. Nd represents
a) Niobium b) nimulidinc) neodymium d) Neobutane
5. for semiconductor laser, the materials used
a) Silicon b) Germanium c) Gas d) Aluminum
Exercise
1. Explain Semiconductor laser.
2. Explain Nd-YAG laser
3. Write applications of semiconductor laser and Nd-YAG
Nd laser .
Questions/problems practice for a day
1. Explain Nd-YAG laser.

Learning from the topic - Learner can explain Semiconductor laser and Nd-YAG
YAG laser.
laser

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Applied Physics Sem - II

Lecture: 12
Learning Objective – Learner will be able to understand the applications of lasers like
holography, medical field, industry, scientific research.
2.8.11 Application of lasers
1. Holography
(Based on1st, 1st edition, 14th chapter, 1st edition, page no.415-416.) (Dec 2008)
(May 96) (May 2002) (Dec 2002) (June 2003)
• Both the phase and intensity wave are recorded and when viewed the photograph
shows a three dimensional image of the object. This technique is named as holography.
• Holography is referred to as 3D photography.
• In holography, some of the light scattered from an object or a set of objects falls on the
recording medium.
• A second light beam, known as the reference beam, also illuminates the recording
medium, so that interference occurs between the two beams.
• The resulting light field generates a seemingly random pattern of varying intensity
which is recorded in the hologram. Interference and diffraction
• The two beams interfere with one another to form an interference pattern.

Figure 2.8.11(a) : Holographic recording process

Reconstruction of hologram
• If the hologram is illuminated by the original reference beam, the reference beam is diffracted
by the hologram to produce a diffracted light field which is identical to the light field which
was scattered by the object or objects.

Figure 2.8.11(b): Holographic reconstruction process

Learning outcome: Students will be able to describe the Holography construction and
reconstruction
2. CD-ROM
(Based on1st, 1st edition, 14th chapter, 1st edition, page no.415.)

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Module 2: Lasers

• CD-ROM discs are identical in appearance to audio CDs, and data are stored and
retrieved in a very similar manner (only differing from audio CDs in the standards used
to store the data).
• Data is stored on the disc as a series of microscopic indentations. A laser is shone onto
the reflective surface of the disc to read the pattern of pits and lands (“pits”, with the
gaps between them referred to as “lands”).
• Because the depth of the pits is approximately one-quarter to one-sixth of the
wavelength of the laser light used to read the disc, the reflected beam’s phase is shifted
in relation to the incoming beam, causing destructive interference and reducing the
reflected beam’s intensity. This pattern of changing intensity of the reflected beam is
converted into binary data.
3. Scientific
In science, lasers are used in many ways, including:
• A wide variety of interferometric techniques
• Atmospheric remote sensing
• Laser based Light Detection and Ranging (LIDAR) technology has application in
geology, seismology, remote sensing and atmospheric physics.
4. Material processing
Laser cutting, laser welding, laser brazing, laser bending, laser engraving or marking, laser
cleaning, weapons etc.
When the material is exposed to laser it produce intense heat thus the material is heated and
melted.
5. Military
Military uses of lasers include applications such as target designation and ranging, defensive
countermeasures, communications and directed energy weapons. Directed energy weapons
are also in use, such as Boeing’s Airborne Laser which was constructed inside a Boeing 747.
It disrupts the trajectory of shoulder-fired missiles.
6. Medical applications
• Cosmetic surgery (removing tattoos, scars, stretch marks, sunspots, wrinkles,
birthmarks, and hairs): see laser hair removal. Laser types used in dermatology include
ruby (694 nm), alexandrite (755 nm), pulsed diode array (810 nm), Nd:YAG (1064 nm),
Eye surgery and refractive surgery
• Soft tissue surgery: CO2, Er:YAG laser
• Laser scalpel (General surgery, gynecological, urology, laparoscopic)
• Photo bio modulation (i.e. laser therapy)
• “No-Touch” removal of tumors, especially of the brain and spinal cord.
• In dentistry for caries removal, endodontic/periodontal procedures, tooth whitening,
and oral surgery
•
• Let’s check Take away from this Lecture
1. The invention of holography is by
a) Newton b) Einstein c) Thomson d) Gabor
2. Holography means
a) Complete recording or writing b) incomplete recording
c) Incomplete writing d) complete diffraction
3. In holography the type of light used is
a) Ordinary light b) laser light c) led light d) Unpolarised light

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Applied Physics Sem - II

Exercise
1. Explain Holography.
2. Explain Medical applications of laser.
3. What are industrial applications of laser.
Questions/problems practice for a day
1. Explain holography in detail.

Learning from the topic - Learner can explain applications of laser like holography, medical
field and scientific research.

Add To Knowledge (Content beyond syllabus)

Laser in defence
• The Boeing YAL-1 weapons system is a megawatt-class chemical oxygen iodine laser
(COIL) mounted inside a modified Boeing 747- 400
• Primarily designed as a missile defense system to destroy tactical ballistic missiles.
• Besides the COIL, the system also includes two kilowatt-class Target Illuminator Lasers for
target tracking.
Know:
1. Learner should be able to define the different terminology used in laser such as
active medium, pumping, population inversion, detestable state etc.
2. Able to explain types of lasers .
3. Applications of lasers like cutting, drilling, welding , scientific research, medical
fields.
Comprehend:
4. Learner should be able to describe the principle of He-Ne laser ,Nd-YAG and
semiconductor laser .
5. Explain the concept holography.
6. Compare the laser light with ordinary light.
7. Derive formula for Einstein’s Coefficients.
Apply, Analyze and synthesize:
8. Learner should be able to calculate the Einstein’s Coefficients.
9. Analyze the characteristics of laser.

Self –Assessment
Q 1. What is laser? [Level 1]
Q 2. Differentiate between Spontaneous emission and Stimulated emission process related to laser
operation [Level 2]
Q 3. Explain the concept of holography [Level 3]
Q.4 Explain He-Ne laser [Level 4]
Q 5. Classify Helium Neon laser; Nd:YAG laser; Semiconductor laser. [Level 5]

2.10 Short Answer Questions

1. What does LASER stands?
Ans. LASER is acronym for Light Amplification by Stimulated Emission of Radiation.
2. Which laser is the first successful laser?
Ans. The first successful laser He-Ne gas laser .
3. What are the types of laser?
Ans. There are four types of lasers namely

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(i) solid state laser (ii) gas lasers (iii) liquid state lasers (iv) solid state diode lasers
4. What is absorption?
Ans. Absorption is a process when an atom in lower energy state E1 absorb the incident photon and
excited to high energy level E2 .
5. What is spontaneous emission?
Ans. In this type of process in which photon emission occurs without any external agency is called
spontaneous emission.
6. What is stimulated emission?
Ans. A photon of energy hv = E2-E1 can induce the excited atom to make a downward transition
releasing the energy in the form of a photon. Thus the interaction of a photon with an excited
atom triggers the excited atom to drop to the lower energy state giving up two photons in
coherence. This phenomenon is called forced emission or stimulated emission.
7. What is the meaning of coherent?
Ans. The waves traveling in the same direction with a common frequency no phase difference, or
in constant phase difference are called coherence.
8. What is active medium?
Ans. A medium in which light gets amplified is called an active medium.
9. What is metastable state?
Ans. State in which atom can remains there for longer time of the order of 10-6 to 10-3 sec. is said to
be metastable state.
10. What is population inversion?
Ans. If there are more atoms in the upper level than in the lower level. That is a situation in which
the population distribution between the levels E1 and E2 is said to be inverted, and the
medium is said to have gone the state of population inversion.
11. What is pumping?
Ans. The process of supplying energy to the medium with a view to transfer it into the state of
population inversion is known as pumping.
12. Define lasing transition.
Ans. The transition between the two levels that generate stimulate demission is called a lasing
transition.
13. What are the types of pumping scheme?
Ans. There are two types of pumping scheme Three-level pumping scheme and four-level pumping
scheme
14. What are the types of solid state laser?
Ans. The types of solid state lasers are Ruby laser, Nd-YAG laser.
15. Nd-YAG stands for what?
Ans. Nd-YAG stands for Neodymium-Yttrium aluminum garnet.
16. Who built Helium –neon Laser?
Ans. Helium –neon Laser built Ali Javan, W. Bennett and D. Herriot in 1961
17. What is the length and diameter of discharge tube in He-neon laser?
Ans. The length and diameter of discharge tube in He-neon laser about 50 cm and 1 cm
respectively.
18. What is the ratio of gas in He-Ne laser?
Ans. The ratio of He-Ne gas is 10:1
19. Which semiconductor laser emits light of wavelength in visible red region?
Ans. GaAsP semiconductor laser emits light of wavelength in Visiblered region.
20. What is the meaning of holography?
Ans. Both the phase and intensity attributes of the wave are recorded and when viewed the
photograph shows a three dimensional image of the object. This technique is named
holography.
21. Who invented holography?
Ans. holography invented by Scientist Dennis Gabor
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Applied Physics Sem - II

22. Define hologram.

Ans. The photographic plate carrying the interference pattern is
called a hologram.

2.11 Long Answer Questions

1. Explain the terms: Spontaneous emission, stimulated emission and population
inversion.(Based on1st ,1st edition, 14th chapter, page no.396-400Module-2, 2.8.1)
2. What is laser action? Explain the following terms with their specific importance in laser action.
(i) Optical pumping
(ii) Population inversion
(iii) Metastable state
(Based on1st ,1st edition,14th chapter, page no.399-401Module-2, 2.8.5,2.8.6)
3. Explain population inversion and stimulated emission in producing laser. Describe the
construction and working of He-Ne gas laser.
(Based on1st ,1st edition, 14th chapter, page no.397,398,400,409-410.Module-2, 2.8.13)
4. Write short Note: Holography.
(Based on1st ,1st edition, 14th chapter, 1st edition, page no.415,416.Module-2, 2.8.18(a))
5. Write short note: Semiconductor laser.
(Based on1st, 1st edition, 14th chapter, 1st edition, page no.413-414 Module-2, 2.8.14)
6. Describe the process of obtaining a three dimensional image by the holography method.
(Based on1st ,1st edition, 14th chapter, 1st edition, page no.413-414. Module-2, 2.8.18(a))
7. What is holography? How it differ from ordinary photographic technique? How interference
and diffraction phenomena is related with construction and reconstruction of the hologram?
Explain in detail with suitable diagram.
(Based on1st ,1st edition, 14th chapter, 1st edition, page no.413-414.Module-2, 2.8.18)
(b) Describe briefly some industrial applications of lasers.
(Based on1st ,1st edition, 14th chapter, 1st edition, page no.415. Module-2, 2.8.18)
8. Write short note: Principle of Light amplification.
(Based on1st ,1st edition, 14th chapter, 1st edition, page no.403-404. Module-2, 2.8.7)
9. Differentiate between spontaneous and stimulated emission. Give essential features of a
LASER.
(Based on1st ,1st edition, 14th chapter, 1st edition, page no.396-398. Module-2, 2.8.1)
10. Describe the construction and working of a He-Ne gas laser. What are its merits and demerits?
(Based on1st ,1st edition, 14th chapter, 1st edition, page no. 409-410. Module-2, 2.8.13)
11. Describe the construction and working of Nd:YAG laser.
(Based on1st ,1st edition, 14th chapter, 1st edition, page no. 407-408. Module-2, 2.8.15)
12. Explain applications of lasers to memory reading and writing & other applications.
(Based on1st ,1st edition, 14th chapter, 1st edition, page no.415. Module-2, 2.8.18)
13. Derive the expression for Einstein’s coefficients.(Based on2nd , 1st edition, 4th chapter, page
no.4.3-4.4.Module-2, 2.8.10)
14. What does LASER stand for? In what respects it differ from an ordinary source of light ?(May
2008)(Based on1st edition, 14th chapter, 1st edition, page no.395.Module-2, 2.8.12)
15. With neat energy level diagram describe the construction and working of He-Ne laser. What
are its merits and demerits? (May 2008)
(Basedon1st, 1st edition, 14th chapter, 1st edition, page no. 409-410. Module-2, 2.8.13)
16. What is holography? Give its construction and advantages over photographic technique?
( Dec 2008)
(Based on1st, 1st edition, 14th chapter, 1st edition, page no.415-416. Module-2, 2.8.18)

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Module 2: Lasers

2.13 University question June 2012

1. Explain the terms: stimulated emission and population inversion
2. Explain the construction and working of He-Ne laser with proper sketches energy level
diagram
4 What is holography? Explain the process of recording and reconstruction of hologram
5. Explain the construction and working of He-Ne laser with proper sketches energy level
diagram.

Dec2012
1. Draw the energy level diagram of He-Ne laser .What is its wavelength in visible range.
2. Write full form of LASER. Explain main three processes involved in the production of LASER
with appropriate figures.
3. Explain the construction and working of semiconductor diode laser with proper sketches.
What are merits and demerits of this LASER?
4. How the phenomenon of holography be explained with interference and diffraction of light.

June 2013
1. What is population inversion state? Explain its significance in the operation of laser?
2. Explain the construction and working of he-ne laser with proper sketches energy level
diagram

November 2013
1. Define Spontaneous and Stimulated emission. [3]
2. What is the difference of holographs and ordinary photograph? Discuss construction and
reconstruction of image in Holography with neat diagram. [8]

MAY 2014
1. What is the acronym of laser? How are they different than x-rays?
2. What is holography? Explain its construction and reconstruction with neat diagram.
December 2014
1. Define the terms
i) Total internal reflection
ii) Numerical aperture
iii) Acceptance angle
2. Differentiate Spontaneous emission and Stimulated emission
3. With neat energy level diagram describe the construction and working of a He-Ne gas laser.

MAY 2015
1. Explain construction and working of He-Ne laser. What are its merits?

DEC 2015
1. differentiate between Spontaneous emission and Stimulated emission process related to laser
operation
2. What is holography? Explain its construction and reconstruction with neat diagrams.
May 2016
1. Explain the term stimulated emission and population inversion.
2. Explain construction & working of Nd: Yag laser.

DEC 2016
1. What is pumping in Laser? Give the types of puming.
2. What is the fundamental principle of a Hologram? How is it produced and how is the image
constructed by it?
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Applied Physics Sem - II

MAY 2017
1. What is holography? Differentiate between holography & photography.
2. Explain construction & working of He-Ne laser with neat label diagram.
3. Explain the construction & reconstruction of hologram with neat diagram
DEC 2017
1. What is pumping in LASER? Give the types of pumping.
2. Explain construction & working of Nd: Yag laser.
3. What is holography? Differentiate between holography & photography.

MAY 2018

DEC 2018

Practice for Chapter No.2 LASERS(Based on University Patterns)

1. Attempt any five (15)
a) Spontaneous emission
b) Stimulated emission
c) Metastable state
d) Population Inversion
e) What is holography?
f) Any three applications of lasers
2. (a) Write a note on:
(1) What is pumping in LASER? Give types of pumping.(7)
(b) Derive the expression for Einstein’s coefficients.(8)
3. (a) Describe the construction and working of the solid state ruby laser.(8)
(b) Why x-ray and laser are so powerful than ordinary visible light.(7)
4. (a) Describe the construction and working of semiconductor laser.(7)
(b) Describe the construction and working of a He-Ne gas laser. What are its merits and
demerits?(8)
5. (a) Write short note : Principle of Light amplification.(5)
(b) With neat sketch explain principle, construction, energy diagram and specialtyof ND :
YAGlaser(10)

2.16 References
1) A textbook of engineering physics by M.N. Avadhanulu and P.G. Kshirsagar 1st edition
2) Engineering physics by Hitendra K Malik and A. K. Singh

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Module 2: Lasers

Self Evaluation:

1. Can you define total internal reflection, Numerical Aperture and maximum angle of
acceptance, absorption, metastable states, population inversion

a) Yes b) No

2. Are you able to distinguish between spontaneous emission and stimulated emission?

a) Yes b) No

3. Are you able to draw lasers energy level diagrams?

a)Yes b) No

4. Do you understand this module ?

a)Yes b) No

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