Applied Physics for EEE stream Module 3- study material

MODULE 3- LASERS AND OPTICAL FIBERS

LASERS
The word Laser stands for Light Amplification by Stimulated Emission of Radiation. It is a device
which amplifies light. It has properties like Coherence, Unidirectional, Monochromatic, Focus
ability, etc.

Interaction of an electromagnetic wave with matter leads to transition of an atom or a molecule

from one energy state to another. If the transition is from lower state to higher state it absorbs the
incident energy. If the transition is from higher state to lower state it emits a part of its energy.

Emission or Absorption takes through quantum of energy called photons. h is called quantum
energy or photon energy.

h = 6.626×10-34 Joules Second is Planck’s constant and ‘’ is the frequency.

If ∆E is the difference between the two energy levels,

Then ∆E = (E2 - E1) Joule

According to Max Planck, ∆E = h = (E2-E1)

 = (E2 - E1)/h Hz.

Three types of interactions, which are possible, are as follows:

1) Induced Absorption:

Induced absorption is the absorption of an incident photon by system as a result of which the
system is elevated from a lower energy state to a higher state, wherein the difference in energy of
the two states is the energy of the photon.

Consider the system having two energy states E1

and E2, E2 > E1. When a photon of energy h is
incident on an atom at level E1, the atom goes to a
higher energy level by absorbing the energy.

When an atom is at ground level (E1), if an electromagnetic wave of frequency  is applied to the
atom, there is possibility of getting excited to higher level (E 2). The incident photon is absorbed.
It is represented as

Atom + Photon → Atom*

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2) Spontaneous Emission:

The emission of a photon by the transition of a system from a higher

energy state to a lower energy state without the aid of an external
energy is called spontaneous emission.

Let ‘E1’ and ‘E2’ be two energy levels in a material, such that E2>E1.
E1 is ground level and E2 is the higher level. h=E2-E1 is the difference in the energy. The atom at
higher level (E2) is more unstable as compared to that at lower level (E1).

The life time of an atom is less in the excited state, In spontaneous emission atom emits the photon
without the aid of any external energy. It is called spontaneous emission. The process is
represented as

Atom* → Atom + Photon

The photons emitted in spontaneous emission may not have same direction and phase similarities.
It is incoherent.

Ex: Glowing electric bulbs, Candle flame etc.

3) Stimulated Emission:

Stimulated emission is the emission of a photon by a system under the influence of a passing
photon of just the right energy due to which the system transits from a higher energy state to a
lower energy state.

The photon thus emitted is called stimulated photon and will have the same phase, energy and
direction of movement as that of the passing photon called the stimulation photon.

Initially the atom is at higher level E2. The incident photon of energy h forces the atom to get de-
excited from higher level E2 to lower level E1.

i.e. h=E2–E1 is the change in energy.

The incident photon stimulates the excited atom to emit a photon of exactly the same energy as
that of the incident photons. The emitted two photons have same phase, frequency, direction and

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polarization with the incident photon and results in coherent beam of radiation. This kind of action
is responsible for lasing action.

Atom* + Photon → Atom + (Photon + Photon)

Expression for energy density in terms of Einstein’s Coefficients

Consider two energy levels E1 and E2 of a system of atoms with N1 and N2 are population of energy
levels respectively.

Let U be the energy density of incident beam of radiation of frequency γ. Let us consider the
absorption and two emission process

1) Induced absorption:

Induced absorption is the absorption of an incident photon by system as a result of which the
system is elevated from a lower energy state to a higher state.

The rate of absorption is proportional to N1U


Rate of absorption = B12N1U....................................... (1)

Where ‘B12’ is the proportionality constant called Einstein Coefficient of induced absorption.

2) Spontaneous emission:

The emission of a photon by the transition of a system from a higher energy state to a lower energy
state without the aid of an external energy is called spontaneous emission.

Spontaneous emission depends on N2 and independent of energy density.

The rate of spontaneous emission = A21N2 .................................. (2)

Where ‘A21’ is called proportionality constant called Einstein coefficient of spontaneous

emission.

3) Stimulated emission:

Stimulated emission is the emission of a photon by a system under the influence of a passing
photon of just the right energy due to which the system transits from a higher energy state to a
lower energy state

The rate of stimulated emission is directly proportional to N 2Uγ.

The rate of stimulated emission = B21N2U .....................................(3)

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Where ‘B21’ is the proportionality constant called Einstein’s Coefficient of stimulated emission.

At thermal equilibrium,

Rate of absorption = (Rate of spontaneous emission + Rate of stimulated emission)

B12N1U = A21N2 + B21N2U

U (B12N1 - B21N2) = A21N2

A21N2
U =
(B12N1−B21N2)
A21 N2
i.e. U= [ ]
B
B21 ( 12N1−N2)
B21

A21 1
U = [ ] → (4)
B12N1
B21 ( ) −1
B21N2

E −E
−( 2KT 1)
By Boltzmann’s law, N2= N1e = N1 e-h/KT

i.e., N1/N2 = eh/KT

 
 
Eqn. (4) becomes A21  1  → (5)
U    
 
  
B21  B


e h 
 kT
1 
 12
 
 B 
  21  

   
   
8h 3  1 
By Planck’s law, U    → (6)
c
3   kT 
 h 

 
 e

1 

  

Comparing equation (5) & (6)


A21 B12
= 8πh3/c3 & =1 i.e. B12 = B21
B21 B21

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The probability of induced adsorption is equal to the stimulated emission.

Therefore A12is written as A and B12, B21 written as B.

Equation (5) becomes

𝐴 1
𝑈𝖯 = [ ℎ𝖯 ]
𝐵
𝑒𝑘𝑇 − 1
Above equation is the expression for energy density

Condition for laser action:

1) Meta Stable State:
It is the special type of excited state where in the life time of atom is more than the normal
excited state.
This state plays an important role in lasing action. In metastable state, atoms stay of the order of
10-3 to 10-2 second. In normal excited state other than metastable atom stay of order of 10 -8 to10-9
seconds. It is possible to achieve population inversion condition in certain system which possesses
a metastable state.

2) Population Inversion: It is the state of the system at which the population of a higher
energy level is greater than that of the lower energy level.

Let E1, E2, E3 be the energy levels of the system E 3>E2>E1. E2 is

the metastable state of the system. Atoms get excited from the
state E1 to E3 by means of external source and stay there for short
time. These atoms undergo spontaneous transitions to E2 and E1.
The atoms at the state E2 stay for longer time. A stage is reached
in which the number of atoms at state E2 is more than the number
of atoms at E1 which is known as population inversion.

Requisites of a Laser System:

1) The pumping process:
It is the process of supplying energy to the medium in order to transfer it to the state of population
inversion is known as pumping process
Optical Pumping: It is the process of exciting atoms from lower energy level to higher energy
level by using high intensity light or by operating flash tube as an external source called optical
pumping.
Electrical pumping: It is the process of exciting atoms from lower energy level to higher energy
level by using dc power supply as an external source called electrical pumping.

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2) Active medium:
It is a medium which supports population inversion and promotes stimulated emission leading to
light amplification
Active centers: In a medium consisting of different species of atoms only small fraction of the
atoms of a particular type are responsible for stimulated emission and consequent light
amplification they are known as Active centers

3) Laser cavity.
An active medium bounded between two mirrors is called as a laser cavity.

Vibrational modes of CO2 molecule:

A carbn dioxide molecule has two oxygen atoms between which there is a carbon atom. It
has 3 different modes of vibration.
1. Symmetric stretching mode : In this mode, carbon atom is stationary and the oxygen
atoms oscillate to and fro along the molecular axis. This state is referred as (100) state.

2. Asymmetric stretching mode: In this mode, both the oxygen atoms moves in one
direction while the carbon atom moves in opposite direction along the molecular axis.
This state is referred as (001) state.

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3. Bending mode: In this mode, the carbon atom and oxygen atoms moves perpendicular
to molecular axis in the opposite direction. This state is referred to as (010)state.

CO2 LASER:

It was devised by C.K.N Patel in 1924. CO2 laser is molecular gas laser which operates in the IR
region involving a set of vibrational – rotational transitions. It is a four level laser producing both
continuous and pulsed laser.

Construction:

 It consists of discharge tube (quartz) of diameter 2.5cm and length of 5m.

 The tube is filled with a mixture of CO2, N2 and He gas in the ratio 1:2:3.

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 Sometimes water vapour is added because during discharge CO2 molecule breaks up into
CO and O. The water vapour additives help in deoxidizing CO to CO2.
 Brewster window made up of flat quartz are sealed to the tube at both of its ends to give
polarized light.
 The tube has got two parallel mirrors. One is partially silvered and the other is fully silvered
to function as laser cavity

Working:

CO2 laser Energy level diagram

 When an electric field is applied electrons rendered free from atoms, collide with N2 &
CO2 molecules in their path towards positive electrodes due to which N2 atoms are excited
to the higher energy level ν1.
 Likewise it happens to the CO2 molecule. This is collision of first kind
e1 + N2  e2 + N2*
e1 + CO2  e2 + Co2*
Where, e1 and e2 are the energies of electrons before and after collisions.
 Let the ground state, (010) state, (020) state,(100) state and (001) are represented as
E1,E2,E3,E4 and E5 levels respectively
 Because of matching energy levels, v = 1 state of N2 is equal to (001) state of CO2 , N2
molecule in the metastable state collide with the CO2 in the ground state and transfer of
energy takes place from N2 to CO2 . As a result of which CO2 molecule moved to (001) state
where as the N2 molecule moved to ground state. This is the collision of second kind.

N2* + CO2  N2 + CO2*

Where, CO2 and CO2* are the energies of CO2 in ground state and excited states.
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 Because of the resonant transfer of energy, the population inversion is achieved in (001)
state with respect to (100) and (020)
 The transition from E5 to E4 levels gives wavelength of 10.6μm (in far IR region)
 The transitions from E5 to E3 level gives wavelength of 9.6 μm (in far IR region)
 Following these transitions the CO2 molecules in E4 and E3 collide with the ground state
CO2 molecules (because of the matching energy levels) and arrive at E2 state.
 The molecules in the E2 state collide with He and water vapour molecules, so that come
down to the ground state.
 The cycle of operation gives both continuous and pulsed laser.

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Properties of laser:

1. Coherence: The emitted radiation after getting triggered is in phase with the incident
radiation.
2. Monochromaticity: The laser beam is highly monochromatic than any other radiations.
3. Unidirectionality: Laser beam travels in only one direction. It can travel long distance
without spreading.
4. Focusability: A laser beam can be focused to an extremely fine spot.

Applications of laser:

1) Lasers in Defense - Laser range finder in defense

A laser range finder works on the principle of time of flight, i.e., the time taken by the signal to
reflect off an object is often measured to determine the object's location. It is used in military and
some sports like archery and golf, where the distance measurement is done precisely. A high power
pulsed laser (Nd-YAG) beam is directed towards the enemy target from the transmitter. The beam
bounces back from the surface of the target as a reflection. A part of the reflected beam is received as
a signal by the receiver. The unwanted noise signal will be filtered by the optical filter and pure
signal is amplified by the photomultiplier in the receiver. The range finders high speed clock
measures the exact time of incident and reflection of the pulse and then convert it in to distance.

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 2. Laser Printer

Laser beam is used in laser printers to get printouts with better quality. Laser printers read the electronic
data from computer and beam this information onto a drum inside the printer, which builds up a pattern
of static electricity.This attracts a dry powder called toner onto the paper. A fuser roller generates heat
and pressure to permanently fix the image onto the paper.

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OPTICAL FIBERS
An optical fiber is a cylindrical wave guide made of transparent dielectric material (glass or plastic)
which guides light waves along its length by total internal reflection.

Principle

The propagation of light in an optical fiber from one end to the other end is based on the principle
of Total internal reflection (TIR). They are used in optical communication.

When a light enters one end of the fiber, it undergoes successive total internal reflections from side
walls and travels down the length of the fiber along zigzag path.

Construction
 A practical optical fiber is cylindrical in shape and has three regions.
 The innermost cylindrical region is the light guiding region called as core which is usually made
up of glass or plastic.
 The outer part which is a concentric cylinder surrounding the core is called as cladding and is
also made up of similar material but of lesser refractive index.
 The outermost region is called a Sheath or Protective buffer coating, nothing but the plastic
coating providing a physical and environmental protection for the fiber. Number of such fibers
is grouped to form a cable.

Total Internal Reflection

 When a ray of light travels from denser to rarer medium it bends away from the normal.
 As the angle of incidence increases in the denser medium, the angle of refraction also increases.
For a particular angle of incidence called the “critical angle” (θc), the refracted ray grazes the
surface separating the media or the angle of refraction is equal to 90°.
 If the angle of incidence is further increased beyond the critical angle, the light ray is reflected
back to the same medium. This is called “Total Internal Reflection”.
 In total internal reflection, there is no loss of energy. The entire incident ray is reflected back.

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CBCS-2021 Scheme
Let XXl is the surface separating medium of refractive index n1 and medium of refractive index
n , n > n . AO and OAl are incident and refracted rays. θ and θ are angle of incidence and angle
2 1 2 1 2
l
of refraction, θ2> θ 1. For the ray BO, θ cis the critical angle. OB is the refracted ray which grazes
l
the interface. The ray CO incident with an angle greater than θc is totally reflected back along OC .

From Snell's law,

n1sinθ1 = n2sinθ2

For total internal reflection, θ1=θc and θ2=90°

n1sinθc = n2 (because sin90°=1)

In total internal reflection there is no loss or absorption of light energy. The entire energy is
returned along the reflected light. Thus is called Total internal reflection.
Propagation mechanism

 The cladding in an optical fiber always has a lower refractive index than that of the core.
 The light signal which enters into the core and strikes the interface of the core and cladding with
an angle greater than the critical angle will undergo total internal reflection.
 Thus the light signal undergoes multiple reflections within the core and propagates through the
fiber.

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CBCS-2021 Scheme
 Since each reflection is a total internal reflection, there is no absorption of light energy at the
reflecting surface.
 Therefore the signal sustains its strength and also confines itself completely within the core
during the propagation.

 After series of such total internal reflection, it emerges out of the core. Thus the optical fiber
works as a waveguide. Care must be taken to avoid very sharp bends in the fiber because at
sharp bends, the light ray fails to undergo total internal reflection.

Expression for Numerical aperture and condition for propagation

Consider a light ray AO incident at an angle ‘θ0’ enters into the fiber. Let ‘θ1’ be the angle of
refraction for the ray OB. The refracted ray OB incident at a critical angle (90˚- θ1) at B grazes the
interface between core and cladding along BC. If the angle of incidence is greater than critical
angle, it undergoes total internal reflection. Thus θ0 is called the waveguide acceptance angle and
sinθ0 is called the numerical aperture.

Let n0, n1 and n2 be the refractive indices of the medium, core and cladding respectively.

Applying Snell’s law at O, no sinθ0 = n1 sinθ1

𝑛1
𝑠𝑖𝑛 𝜃0 = 𝑠𝑖𝑛 𝜃1 . . . . . . . . . . . . . . . . . . (1)
𝑛0

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CBCS-2021 Scheme
Applying Snell’s law at B,

n1 sin (90-θ1) = n2 sin90

n1 cos θ1 = n2
𝑛2
⇒ 𝑐𝑜𝑠 𝜃1 = . . . . . . . . . . . . . . . . . . . . . . . . . . (2)
𝑛1
𝑛1
From expression (1) 𝑠𝑖𝑛 𝜃 = √1 − 𝑐𝑜𝑠2 𝜃
0 𝑛0 1

Substituting for cos θ1 from (2)

𝑛1 2
𝑠𝑖𝑛 𝜃0 = √1 − 𝑛2
𝑛0 𝑛21
2
𝑛1 √ 𝑛1 − 𝑛2
2

𝑠𝑖𝑛 𝜃0 = 𝑛12
𝑛0
√ 𝑛12 − 𝑛22
𝑠𝑖𝑛 𝜃0 =
𝑛0

If 𝑛0=1 i.e., surrounding medium if it is air

𝑠𝑖𝑛 𝜃0 = √𝑛2 − 𝑛2
1 2

Where, 𝑠𝑖𝑛 𝜃0 is called numerical aperture

𝑁. 𝐴. = √𝑛2 − 𝑛2
1 2

Condition for propagation:

If θi is the angle of incidence of the incident ray, then the ray will be able to propagate,

if 𝜃𝑖 < 𝜃0

⇒ 𝑖𝑓 𝑠𝑖𝑛 𝜃𝑖 < 𝑠𝑖𝑛 𝜃0

𝑜𝑟 𝑠𝑖𝑛 𝜃𝑖 < √𝑛2 − 𝑛2
1 2

𝑖. 𝑒. , 𝑠𝑖𝑛 𝜃𝑖 < 𝑁. 𝐴.

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CBCS-2021 Scheme
Acceptance angle is defined as the maximum angle that a light ray can have relative to the axis
of the fiber and propagate through the fiber.

Numerical aperture indicates the ability of the optical fiber to accept light i.e the light gathering
capability of the optical fiber. The sign of the acceptance angle also called numerical aperture.

Fractional index change (Δ)

The ratio of the difference in refractive index of core and cladding to the refractive index of core
of an optical fiber.
𝑛1−𝑛2
𝑖. 𝑒. , 𝛥 =
𝑛1

Relation between N.A. and Δ

𝑛1−𝑛2
Consider Δ =
𝑛1

n1  n2 n1
We have, N.A = √𝑛2 − 𝑛2
1 2

= √(𝑛1 + 𝑛2)(𝑛1 − 𝑛2)

Considering n1≈n2

= √(𝑛1 + 𝑛2)𝛥𝑛1

𝑁. 𝐴 = √2𝑛12𝛥

𝑁. 𝐴. = 𝑛1√2𝛥

Increase in the value of Δ enhances the light gathering capacity of the fiber. Δ value cannot be
increased very much because it leads to intermodal dispersion intern signal distortion.

V- number

The number of modes supported for propagation in the fiber is determined by a parameter called
V-number.

If the surrounding medium is air, then V-number is given by,

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CBCS-2021 Scheme
𝜋𝑑
𝑉= √𝑛2 − 𝑛2
1 2
𝜆

Where, d is the core diameter,

n1 and n2 are refractive indices of core and cladding respectively,

λ is the wavelength of light propagating in the fiber.

If the fiber is surrounded by a medium of refractive index n0, then,

𝑉 = 𝜋𝑑 √ 𝑛1 − 𝑛2
2 2

𝜆 𝑛0

𝑉2
For V ≫1, the number of modes supported by the fiber is given by, number of modes ≅
2

Types of optical fibers

Based on the refractive index profile and mode of propagation, There are three types of optical
fibers,

1. Single mode fiber

2. Step index multimode fiber
3. Graded index multimode fiber
(i) Single mode fiber

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CBCS-2021 Scheme
 Single mode fibers have a core material of uniform refractive index value.
 Cladding material also has a uniform refractive index but of lesser value than that of core.
 Thus its refractive index profile takes a shape of a step. The diameter of the core is about
8-10 µm and the diameter of the cladding is about 60-70 µm.
 Because of its narrow core, it can guide just a single mode as shown in above figure.
 Single mode fibers are the extensively used ones ant they are less expensive. They need
LASERs as the source of light.

(ii) Step index multimode fiber

 A step index multimode fiber is very much similar to the single mode fiber except that its
core is of large diameter. A typical fiber has a core diameter 50 to 200 µm and a cladding
about 100 to 250µm outer diameter.
 Its refractive index profile is also similar to that of a single mode fiber but with a larger
plane region for the core.
 Due to the large core diameter it can transmit a number of modes of wave propagation.
 The step index multimode fiber can accept either a LASER or an LED as source of light.
 It is the least expensive of all and its typical application is in data links which has lower
bandwidth requirements.

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CBCS-2021 Scheme

(iii) Graded index multimode fiber

 It is also called GRIN..

 The refractive index of core decreases in the radially outward direction from the axis of the
fiber and becomes equal to that of cladding at the interface but the refractive index of the
cladding remains uniform.
 Laser or LED is used as a source of light.
 It is the expensive of all. It is used in telephone trunk between central offices.

Signal attenuation in optical fibers

 Attenuation is the loss of optical power suffered by the optical signal as it propagates through a
fiber also called as the fiber loss.
 There are three mechanisms through which attenuation takes place.

Attenuation can be caused by three mechanisms.

(i) Absorption losses

 Absorption of photons by impurities like metal ions such as iron, chromium, cobalt and copper
in the silica glass of which the fiber is made of.
 During signal propagation photons interact with electrons of impurity atoms and the electrons
are excited to higher energy levels.
 Then the electrons give up their absorbed energy either in the form of heat or light energy.

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CBCS-2021 Scheme
 The re-emission of light energy will usually be in a different wavelength; hence it is referred as
loss of energy.
 The other impurity such as hydroxyl (OH) ions which enters into the fiber at the time of
fabrication causes significant absorption loss.
 The absorption of photons by fiber itself assuming that there are no impurities and in-
homogeneities in it is called as intrinsic absorption.
(ii) Scattering losses
 Scattering of light waves occurs whenever a light wave travels through a medium having
scattering objects whose dimensions are smaller than the wavelength of light.
 Similarly when a light signal travels in the fiber, the photons may be scattered due to the sharp
changes in refractive index values inside the core over distances and also due to the structural
impurities present in the fiber material.
 This type of scattering is called as Rayleigh scattering. Scattering of photons also takes place
due to trapped gas bubbles which are not dissolved at the time of manufacturing.
 A scattered photon moves in random direction and leaves the fiber.
(iii) Radiation losses
Radiation losses occur due to macroscopic bends and microscopic bends.
 Macroscopic bending: All optical fibers are having critical radius of curvature provided by the
manufacturer. If the fiber is bent below that specification of radius of curvature, the light ray
incident on the core cladding interface will not satisfy the condition of total internal reflection.
This causes loss of optical power.

 Microscopic bending: Microscopic bends are repetitive small scale fluctuations in the linearity
of the fiber axis. They occur due to non-uniformities in the manufacturing and also lateral
pressure built up on the fiber. They cause irregular reflections and some of them leak through
the fiber. The defect due to non-uniformity (micro-bending) can be overcome by introducing
optical fiber inside a good strengthen polyurethane jacket.

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CBCS-2021 Scheme

Attenuation co-efficient
 The attenuation of a fiber optic cable is expressed in decibels.
10 𝑃𝑜𝑢𝑡 𝑑𝐵
i.e., 𝛼=− 𝑙𝑜𝑔 [ ]
𝐿 𝑃𝑖𝑛 𝑘𝑚

 The main reasons for the loss in light intensity over the length of the cable is due to light
absorption , scattering and due to bending losses.

Point to point optical fiber communication System

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 Engineering Physics Study Material Module 3- study material

CBCS-2021 Scheme
• In point to point communication analog information such as voice of a telephone coming out of
the transmitter section of the telephone are fed to the coder.
• The coder converts analog information into binary data which comes out as electrical pulses.
• The electrical pulses from the coder are fed to optical transmitter which converts signals into
pulses of optical power.
• These optical pulses are fed into the fiber. The incident light which is funneled into the core
within the acceptance angle propagate within the fiber by means of total internal reflection.
• The photo detector converts optical signals into electrical pulses in binary form and the decoder
converts the binary data into analogue signal which will be the same information such as voice.

Optical Fiber Sensors

Optical Fiber sensors are meant for measuring and sensing the rate of data transmission, change in
phase, intensity and wavelength and in the case of incentive conditions as noise, unstable
environment conditions, high vibration and extreme heat etc.

On the basis of operating principle, Optical Fiber sensors are classified into
1. Intensity based Displacement Sensor.
2. Temperature Sensor based on Phase Modulation.

1. Intensity based Displacement Sensor

Principle:
Light is sent through a transmitting fiber and is made to fall on a moving target. The reflected
light from the target is sensed by a detector. With respect to intensity of the reflected light from
the target, displacement of the target is measured.

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 Applied Physics for EEE stream Module 3- study material

CBCS-2021 Scheme

Description:
It consists of a bundle of transmitting fibers coupled to the laser source and a bundle of receiving
fibers coupled to the detector as shown in the figure.
The axis of the transmitting fiber and the receiving fiber with respect to the moving target can be
adjusted to increase the sensitivity of the sensor.

Working:
Light from the source is transmitted through the transmitting fiber and is made to fall on the
moving target. The light reflected from the target is made to pass through the receiving fiber and
the same is detected by the detector.

Based on the intensity of the light received, the displacement of the target can be measured, (i.e.)
if the received intensity is more than we can say that the target is moving towards the sensor and
if the intensity is less, we can say that the target is moving away from the sensor.

Merits of optical communication system:

• It carries very large amount of information in either digital or analog form due to its large
bandwidth.
• The materials used for making optical fiber are dielectric nature. So, it doesn’t produces or
receives any electromagnetic and R-F interferences.
• Fibers are much easier to transport because of their compactness and lightweight.
• It is easily compatible with electronic system.
• It can be operated in high temperature range.
• It does not pick up any conducted noise.
• Not affected by corrosion and moisture.
• It does not get affected by nuclear radiations.
• No sparks are generated because the signal is optical signal.

Demerits of optical communication system:

 Low power — Light emitting sources are limited to low power. Although high power emitters
are available to improve power supply, it would add extra cost.
 Fragility — Fiber optic cable is made of glass, which is more fragile than electrical wires such
as copper cabling. Not only that, but glass can be damaged by chemicals such as hydrogen gas
that can affect transmission. Particular care has to be taken with laying undersea fiber cabling
because of its fragility.

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Attenuation spectrum of an optical fiber with optical windows

Optical Windows are flat, optically transparent plates that are typically designed to maximize
transmission in a specified wavelength range, while minimizing reflection and absorption.

 The first optical window is defined from 800-900nm, where the minimum signal loss is
4dB/km. In early 1970’s this window was used for operation of optical sources and detectors.

 By reducing the concentration of hydroxyl ions and metallic impurities in the fiber material,
in 1980’s manufacturers were able to fabricate optical fibers with very low loss in the 1100-
1600nm region. This spectral band is called long wavelength region.

 The second optical window is centered at 1310nm also called O-band, which offers
0.5dB/km.

 The third optical window is centered at 1550nm also called C-band, which gives the loss of
0.2dB/km.

 Hence while designing optical systems for long distance applications the 1550nm wavelength
is preferred because loss offered at this wavelength is minimum than any other wavelength.

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