FIBER OPTICS

Fiber optics is a branch of physics based on the transmission of light through transparent filters of glass or
plastic.

These optical fibers can carry light over distances ranging from few meters to hundreds of kilometers.

John Tyndall, a British Physicist demonstrated in 1870, that light can be guided along curved stream of water,
employing the phenomena of total internal reflection.

Optical frequencies are extremely large (~1015 Hz) compared to radio and micro waves. A light beam acting
as a carrier wave is capable of carrying more information than radio and microwaves.

In order to have efficient and dependable communication system, one would require a guiding medium in
which the information carrying light waves would be transmitted.

Te guiding medium is the optical fiber which is hair thin and guides the light beam from one place to another.

The fibers fabricated with recent developed technology are characterized by extremely low losses ~ 0.2
dB/Km and possible send 4.5 lakh voice channels.

Discovery of gas and solid state lasers gave momentum for the development of fiber optic communication
technology.

Commercial communication systems based on optical fiber cables made their appearance by 1977.

The advantages of fiber optics make them to be used in fiber optical communication.

Some of them are:

Low attenuation losses

High information carrying capacity.

Small diameter of the individual fiber channel.

Low weight of high capacity channel.

Material cost in bulk is low.

High security for tapping.

No dangers of short circuits.

Structure, Principle of Optical Fiber:-

 Optical fiber is a cylinder of transparent dielectric medium designed to guide visible and infrared light, over
long distances.
 Optical fibers work on the principle of total internal reflection.
 When a ray of light travels from a denser
medium into a rarer medium and if the angle of
incidence is greater than the critical angle then
the light gets totally reflected.
 Optical fiber is a very thin and flexible medium
having a cylindrical shape.
 The various parts are :-
i) Core: - A typical cylindrical glass material
of diameter 50 µm surrounded by a clad.
ii) Clad: - A glass material of slightly lower
refractive index than core.
iii) Silicon Coating: - It is provided between
clad and buffer jacket in order to improve the
quality of transmission of light.
iv) Buffer jacket: - It is covered over silicon
coating which is made up of plastic material
and protects from moisture and abrasion.
v) Strength members: - This layer is arranged over the buffer jacket to provide necessary toughness and
tensile strength to the fiber.

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vi) Outer jacket: - Finally a black polyurethane outer jacket to avoid damages during hard pulling, bending,
stretching or rolling of the fiber in the real field.
 When a light ray passes from an optically denser medium into an optically rarer medium, the refracted ray
bends away from normal.
 When the angle of incidence is increased,
angle of refraction also increases and a stage
is reached when the refracted ray just grazes
the surface of seperation of denser and rarer
i.e., core and clad respectively.
 At this position the angle of refraction is 900.
 This angle of incidence in the denser medium
is called the critical angle (θc) of the denser
medium with respect to rarer medium and it is
shown below.
 If the angle of incidence is further increased

then the ray is totally reflected.

 Here refractive index of core and clad
materials are n1 & n2.
 When a light ray, travelling from
optically denser medium to optically
rarer medium, if it is incident at an
angle greater than the critical angle for
the two media, the ray is totally
reflected back into the same medium by
obeying laws of reflection. This
phenomenon is called total internal
reflection.
 According to law of refraction
n1 sinθ1 = n2sinθ2
If θ1 = θC => θ2 = 900
So, n1sinθC = n2sin900
=> sinθC = n2/n1 => θC = sin-1(n2/n1)
 As there is no loss of light energy during reflection optical fibers are designed to guide light wave over long
distances.

Acceptance angle and acceptance cone: -

When light beam is launched into a fiber at its end by using a focusing lens, the entire light may not pass
through the fiber.

Only the rays which make angle of incidence greater than critical angle will undergo total internal reflection.

The other rays are refracted into the cladding material and are lost.

So, we have to know at what angle called acceptance angle we have to launch the beam. The maximum angle
of launch is called ‘acceptance angle’.

Suppose that the light is launched from the medium of refractive index n0 (n0= 1 for air) into the core of
refractive index n1.

αi is the angle of incidence and angle of refraction is αr .

After refraction the ray proceeds with angle of refraction and undergoes total internal reflection.

From triangle ABC αr + θ + 900 = 1800 so, αr + θ = 900 => αr = 900 – θ;

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From Snell’s law we have

We have n0 sin αi = n1sinαr
=> n0 sin αi = n1sin(90-θ) => n0 sin αi = n1cosθ

If θ is less than critical angle θC, the ray
will be lost by refraction.

When θ = θC ; αI = αm = maximum α
value



Therefore, sin αm = cosθC


-1
And we have θC = sin (n2/n1)


So, cosθC = 1  
 = 1 



Therefore,


n0 sin αm = n1 1 



so, n0 sin αm =  


if n0 = 1 {
medium is air}

Sin αm =  


This maximum angle αm is called acceptance

angle (or) the acceptance cone half angle of the
fiber.

The acceptance angle may be defined as the
maximum angle that a light ray can have with
axis of the fiber and propagate through the
fiber.

Rotating acceptance angle about the fiber axis describes acceptance cone of the fiber.

Numerical Aperture: -

 The numerical aperture defines the acceptance angle more clearly.

 The sine of acceptance angle is called numerical aperture.
 The value of Numerical Aperture, NA is

 

NA = Sin αm =  


=   
  
 =   
  Δ Where Δ = is fractional


difference in Refractive indices, n1 and n2 are refractive indices of core and clad respectively.
As n1 ~ n2 we can take n1 + n2 = 2n1 so, NA = n12Δ

 Numerical aperture is a measure of the amount of light that can be occupied by a fiber.
 NA values ranges from 0.1 to 0.5.

Types of Optical Fibers: -

 Depending on the relation between refractive indices of core and clad they are divided into two types. Step
index fiber and graded index fiber.
 Additional to these there are also single mode fiber and multimode fiber.

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Step-index fiber: -
 In the step-index fiber, the
refractive index of core is
uniform and undergoes an
abrupt change (or step) at
the cladding boundary.
 In the step-index fiber,
there is a core of constant
refractive index n1 and a
cladding of slightly less
refractive index n2.
 The refractive index profile may be defined as
    

n(r)=   "

     !

$%
 The step-index
index fibers have a core radius ‘a’ and n2 = n1( 1-∆) (or) ∆ #


 A step-index
index fiber may be single or multimode fiber.
 For short distance communication single mode
m fiber is used.
Transmission of signal in step-index
index fiber: -
 Generally the signal sent through the fiber is in
digital form i.e., in the form of pulses
representing 0’s and 1’s.
 Let us consider the propagation of one such
pulse through the multimode step
tep index fiber.
 The same pulsed signal travels in different
paths.
 If we see the diagram it is clear that ray ‘1’
travels shorter distance and ray ‘3’ travels
longer distance.
 Hence the three rays reach the received end at different times. Therefore, the pulsed signal received at the
other end gets broadened. This is called intermodal dispersion.
 This difficulty is over come in grad fiber.
 The ray propagating along the axis i.e., ray 1 is called HE11 mode and is the fastest mode and takes minimum
time to reach the receiving end.

Graded-index fiber: -
 In graded-index
index fiber, the core refractive index varies as a function of radial distance from the centre of the
fiber.
 In the graded index fiber the core refractive index decreases

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continuously with increasing radial distance ‘r’ from the centre of the fiber, but is generally constant in the
cladding.
 The most commonly used construction for the RI variation in the core is given by
 '1  2∆⁄* +/
    
n(r)= & .
 '1  2∆+/
#  1  ∆ -
     !
 Where n1 = RI of the core at the centre
a = radius of core
α = Grading profile of RI in the core material

$%
∆= = fractional total change of the core RI


α=2
 ‘α’ defines the shape of the index profile and it is a
dimension less parameter.
 The input and output pulses for GI fiber are shown
below. The Graded-index fiber is multimode fiber.
 These fibers have several modes or paths of
transmission through the core, but they are much
more orderly and predictable.
 The typical paths of GI fiber are shown in figure.

Transmission of signal in GI fiber: -

 Let us now consider signal pulses travelling

through graded index fiber in two different
trajectories represented by 1 and 2 as shown
below.
 The pulse represented by 1, travelling along the
axis of the fiber, though travels along shorter
route it travels through a medium of higher
refractive index.

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 The other pulse represented by 2, travelling away from axis and undergoes reflection.
 The path of this ray is sinusoidal for the
meridional rays which are nearly parallel to
the fiber axis in which paraxial
approximation is used.
 Though it travels longer distance, it travels
along a trajectory possessing relatively less
refractive index and hence both pulses reach
the other end simultaneously.
 So, the problem of intermodal dispersion is reduced to minimum.
 When a step-index fiber and parabolic index (α = 2) fiber having same core diameter and same total variation
of RI are illuminated by a source excites all modes equally, then the graded index fiber will propagate only
half the total power carried by step-index fiber.
/01
 Simply, if NSI & NGI be the number of modes in step index fiber and graded-index fiber then NGI =

Modes: -
 Based on the number of modes propagating through a fiber there are single and multi-modes.
 Mode is mathematical concept of describing, the nature of propagation of EM waves in a wave guide.
 In metallic modes there are only two modes TE (transverse electric) for which Ez = 0, and if Hz = 0 called as
TM (transverse magnetic mode).
 They are explained using quantum numbers l & m. so, TElm or TMml.
 Fibers have three modes, additional to above they have hybrid modes HE or EH modes.
Comparison between Step-index fiber & Grad-index fiber: -
S.N Step-index fiber S.N Grad-index fiber
O O

1 The RI of the core is uniform throughout and 1 The RI of the core is made to vary in parabolic
undergoes sudden change at cladding boundary. manner such that the maximum RI is present
at the centre of core.
2 The diameter of core is about 100 µm for 2 The diameter of the core is about 50µm in
multimode fiber and 10µm for single mode fiber. case of multimode fiber.

3 The light rays propagating through it are in the 3 The light rays propagating are mainly skew
form of meridional rays. rays, which follow a helical path.
4 Signal distortion is more in multimode step-index 4 Signal distortion is low because of self-
fiber since the rays reflected at high angles travel a focusing effect. Here light rays travel at
greater distance than the rays reflected at low different speeds in different paths of the fiber
angles. because the RI varies throughout the fiber.
5 Bandwidth of fiber is about 50MHz Km for 5 Bandwidth of fiber is from 200 MHz Km to 600
multimode step-index fibers, but for single mode, MHz Km even though theoretically it has
the bandwidth is more than 1000 MHz Km. infinite bandwidth.
6 Attenuation is more for multimode step index 6 Attenuation is less.
fiber but for single mode SI fiber it is very less.
7 Numerical aperture is more for multimode SI fiber 7 Numerical Aperture is less.
but for single mode it is very less.

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Differences between single and multimode fiber: -

Single mode fiber S.NO Multimode fiber

S.NO

1 In single mode fiber only one mode 1 Multimode fiber allows a large number of
(HE11) can propagate through the fiber. paths or modes for the light rays travelling
through it.
2 The condition for single mode operation 2 Here V > 2.405
is given by V-number, given as
V = 2π/λ n1 a√2∆ : V≤ 2.405
3 The single mode fiber has smaller core 3 In multimode fiber the core diameter and
diameter < 10 µm and the difference the relative refractive index difference are
between RI of core and cladding is very more than the single mode fiber.
small.
4 There is no dispersion. 4 There will be some dispersion.

5 The single mode fibers are more suitable 5 These fibers are used in local area
for long distance communication. networks.

6 Launching of light into single mode fiber 6 Launching of light into fiber and splicing of
and splicing of two fibers is very difficult. two fibers is easy.
7 Fabrication is very difficult and fiber is 7 Fabrication is easy and the fiber is
costly. cheaper.

Attenuation in Optical fibers: -

The main specification of a fiber optic cable is its attenuation.

Different mechanisms are responsible for the signal attenuation in the fiber.
The attenuation of the signal is measured in Decibel/KM.
Signal attenuation is defined as ratio of input optical power Pi into fiber to the output received optical power
PO from the fiber.
3 8
The attenuation coefficient of the signal per unit length is given as α =
4
log 8 9 dB/Km
:

Absorption losses: -

Absorption is basically a material property.

Absorption is a major cause of signal loss in an optical
fiber.
Absorption is defined as the portion of attenuation
resulting from the conversion of optical power into
another energy form, such as heat.
Absorption in optical fibers is explained by three factors.
o Absorption by atomic defects in the fiber
material
o Extrinsic absorption by impurity atoms in the
fiber material.

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o Intrinsic absorption by the basic constituent atoms of the fiber material.

(i) Absorption by atomic defects: -
Atomic defects such as vacancies, imperfections of the atomic structure of the fiber material and cluster of
atoms produce a small absorption loss. By careful fabrication these atomic defects can be reduced.
(ii) Extrinsic absorption by impurity atoms:
atoms -
Impurity absorption results from transition metal ions such as iron, chromium, copper, manganese and Nickel.
Impurity absorption is more for fibers drawn from direct melt technique.
nsition metal ions produce loss at λ = 0.8 µm.
The transition
Impurity absorption also results from OH- ions. The OH- ions which are present in the material due to trapping
of minute quantities of water molecules during manufacturing absorb energy.
(iii) Intrinsic absorption: -
Intrinsic absorption is associated with basic fiber material (SiO2). In the case of Silica fibers, tail of infrared
absorption by Si-OO coupling occurs at wavelengths higher than 1.5 µm.
Intrinsic absorption also results from the electronic absorption ban
bands
ds in the ultra violet region and from atomic
vibration bands in the near infrared region.
Ultraviolet absorption decays exponentially with increasing wavelength.

Scattering losses: -

(i) Linear Scattering: -

Linear scattering transfers linearly the
optical power
ower in one propagating mode
to a different mode.
This linear scattering process may
cause the attenuation of operating
mode power by means by transferring
power to leaky mode or radiation
mode which will not continue to
propagate within the core of fiber but
is radiated from the fiber.
Rayleigh scattering:: Rayleigh
scattering loss is the dominant loss
mechanism in the UV region. Its tail
extends upto infrared region.
Rayleigh scattering loss is inversely
proportional to the fourth power of wavelength. It arises
a from the microscopic in-homogeneities
homogeneities present in the
material of the fiber.
The in-homogeneities
homogeneities may arise from the density fluctuations, RI fluctuations and compositional variations.
The transmission loss due to Rayleigh scattering is α = exp (-αscat L).
This occurs for low wavelengths.
Mie Scattering: - Mie scattering is a linear scattering which arises from the in-homogeneities
in homogeneities which are
comparable in size to the guided wavelength.
Further, it is also due to the imperfect cylindrical structure of the
the waveguide; irregularities in the core
core-
cladding interface, core-cladding
cladding RI difference along the fiber and density fluctuations.

(ii) Non-linear scattering losses: -

When we use high optical power levels the non-linear
non linear scattering losses occur. This scatterin
scattering causes the
optical power in one mode to the other mode at different frequency in either forward or backward direction.
These are observed in single mode fiber at high optical power densities.

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Stimulated Brillouin scattering:: - stimulated Brillouin scattering

ring is defined as the modulation of light
through molecular vibrations within the fiber.
The scattered light contains a upper and lower side bands along with the incident light frequency. The
frequency shift varies with the scattering angle.
Stimulated Raman scattering: - Here the scattering light consists of a scattered photon and a high frequency
optical phonon. Further, stimulated Raman scattering occurs in both forward and backward directions in the
optical fiber.
Generally the scattering losses are maximum
maximum in multimode fibers than in the single mode fibers due to their
large diameter and compositional vibrations.

Bending losses: -

Whenever the optical fiber contains bends, then the bends produce radiative losses. There are two types of
bending losses.
Macroscopic bending losses: - these occur when the radius of curvature of bend is greater than fiber diameter.
This situation arises when a cable turns round a corner.
Microscopic bending losses: - these occur due to
bends in the fiber axis. This situation
situat arises when
the fibers are incorporated into cables.
Macrobending losses are minimized by (i) fibers
with small relative refractive index and (ii)
operating at the highest wavelength possible.
Microbends are due to small scale fluctuations in
the radiuss of curvature of the fiber axis.
Fluctuations in the radius of curvature either by non-uniformities
non ities in the manufacturing of fiber or by non
non-
uniform lateral pressures
sures created during the cabling of the fiber.
These are minimized by careful cabling.

Advantages
es of Fiber Optic communication: -

Enormous Bandwidth: -
In the coaxial cable transmission the bandwidth is upto around 500 MHz only whereas in fiber optical
communication it is as large as 105 GHz. Thus, the information carrying capacity of optical fiber system is
far superior to the best copper cable system.

Electrical isolation: -
Since fiber optic materials are insulators, unlike their metallic counter parts, they do not exhibit earth loop
and interface problems. Hence communication through fiber even in electrically hazardous environment
do not cause any fear of spark hazards.

Immunity to interference and cross talk: -
Since optical fibers are dielectric waveguides, they are free from any electromagnetic interference (EMI)
and radiofrequency interference ce (RFI). Hence fiber cables do not require special shielding from EMI.
Cross talk is very negligible.

Signal security: -
Unlike the situation with copper cables a transmitted optical signal cannot be drawn from a fiber without
tampering it such an attempt will affect the original signal and hence can be easily detected.

Small size and weight: -
Since fibers are very small in diameter the space occupied by the fiber cable is negligibly small compared
to metallic cables. Optical cables are light in weight; these
t merits make them more useful in aircrafts and
satellites.

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Low transmissions loss: -

Since the loss in fibers is as low as 0.2 dB/Km, transmission loss is very less compared to best copper
conductors. Hence for long distance communication fibers are preferred. Number of repeaters required is
reduced.

Ruggedness and flexibility: -
Fiber cable structures are flexible, compact and extremely rugged.

Low cost: -
Since fibers are made of silica which is available in abundance, optical fibers are less expensive.

Applications of optical fibers: -

Fiber optic sensors: -

 Sensors are devices used to measure or monitor quantities such as displacement, pressure, temperature,
flow rate, liquid level, chemical composition etc.,
 A smoke detector and pollution
detector can be made from fibers.
 There are two types of fiber optic
sensors
 Intrinsic (or) active sensors: Here
guided light in the fiber gets
modulated by the variable to be measured and then demodulated.
 They are used as intensity modulated sensors, phase modulated sensors, Polarization modulated sensors,
and wavelength modulated sensors.
 Extrinsic (or) Passive sensors: - here modulation takes place outside the optical fiber which acts merely as
a convenient transmission channel for the light radiation.

Optical fibers in medicine: -

 Fiber scopes are employed widely in endoscopic applications. Bundles of optical fibers from part of gastro
scopes and other medical instruments.
 Fiber optic endoscopes are classified according to the application. Gastroscopic to examine stomach,
bronchoscope to view the upper passages lungs and arthoscope to study small spaces within joints.

Endoscopy: -

 An Endoscopy is an instrument designed to provide a direct view of internal parts of the body.
 It is also designed to perform tasks such as removal of samples, injection of fluids and diatherms.
 The long flexible shaft of the instrument is usually constructed of steel mesh.
 It is sheathed with a protective, low-friction covering of PVC.
 The shaft is about 10mm in diameter. It is about 0.6 to 1.8 long.
 The long flexible shaft
has
a) At least one non-
coherent fiber
optic bundle to
transmit light
from the distant
light source to the
distal tip.

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b) A coherent fiber optic bundle to transmit the image of inner part of the body from the obje objective
lens at the distal tip.
c) A flexible tube through which water can be pumped to wash the objective lens.
d) An operations channel for the performance of tasks.
e) Control cables.
 Applications: -endoscopic
endoscopic examination of the gastrointestinal tract for diagnosiss and treatment of ulcers,
cancers, constrictions, bleeding sites and so on.
 The heart respiratory system and pancreas can be investigated.
 By measuring absorption of light by the blood (by passing the light through one fiber and collecting the
light through
gh another fiber). We can estimate the proportion of hemoglobin in the blood.

Optical fiber communication system:

system -

 The above figure shows the important components of an optical fiber communication system.
 Transmitter consists of information encoder which converts the information signal into coded pulses.
Then there is modulaor which controls the operation of optical source.
 The emitted light from the source is coupled to the fiber optic cable which constitutes the transmission
medium.
 The light emerging
ing from the other end of the optical fiber cable is given to the optical detector which
converts light into electrical signal.
 The optical detector, amplifier and decoder or demodulator constitutes the receiver.
 Amplifier amplifies the detected electrical signal and then the amplified signals are decoded to
original information.
 The optical sources are semiconductor laser diodes and LED.
 The optical detectors are PIN photodiodes and avalanche Photodiodes.

Dispersion: -

There are two different types of dispersions,

disp intramodal & intermodal dispersions.
Ecah type of dispersion mechanism leads to pulse spreading. As a pulse spreads energy is overlapped.
The spreading of the optical pulse as it travels along the fiber limits the capacity of the fiber.
Intramodal dispersion: - Intramodal, chromatic, dispersion mainly depends primarily on fiber materials. The
intramodal dispersion may be a material dispersion or waveguide dispersion.
Intramodal dispersion occurs because different colours of light travel through different
different materials and different
waveguide structures at different speeds.
Material dispersion occurs because the spreading of light pulse is dependent on the wavelengths interaction
with RI of the fiber core.
Material dispersion is a function of the source spectral width.
Material dispersion is less at longer wavelengths.
Waveguide dispersion occurs because the mode propagtion constant is a function of the size of the fiber’s core
relative to wavelength of operation.

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In multimode fibers, waveguide dispersion and material dispersion are basically separate properties, where as
in single mode fiber they are inter-related.
Intermodal dispersion: -This occurs because each mode travels a different distance over the same time span.
The modes of a light pulse that enter the fiber at one time exit the fiber at different times.
This condition causes the light to spread.

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