MODULE-1

Laser and Optical Fibers

LASER
LASER: Light Amplification by Stimulated Emission of Radiation.

• It was invented by American Scientist Maiman in the year 1960.

• Today there are about hundred different kinds of lasers.

Characteristics of Laser beam

The following important properties of laser make it different from other ordinary
source of light.

1) Laser is highly monochromatic.

The laser beam is emitted in a very narrow frequency band.
2) Laser light is spatially coherent.
The laser is highly coherent due to stimulated emission of radiation.
3) Laser light extremely high directionality or unidirectionality.
The laser beam has very small divergence due to the resonant cavity. Hence
light intensity does not decrease as fast with distance as it does in ordinary
source of light.
4) The laser beam is extremely bright or intense.
Light from laser is much brighter than other ordinary sources of light.

Principle and Production of Laser:

Radiation interacts with matter under appropriate conditions. The interaction

leads to an abrupt transition of the Quantum system such as an atom or molecule
from one energy state to another. If the transition is from a higher state to a lower
one, the system gives out a part of its energy and if the transition is in reverse
direction, then it absorbs the incident energy.
In order to understand the manner in which radiation can interact with
matter, consider two energy states E1 and E2 of a system. If the energy difference
between the two energy levels is ΔE,
Then ΔE = E2 – E1
Max planck suggested that if an electromagnetic radiation of frequency ‘ν’ with
value
∆𝐸 𝐸2 − 𝐸1
ν= = -------(1)
ℎ ℎ
is incident on the system which is in the energy state E1 , then the system moves
from E1 to the energy state E2 (from lower to higher) by absorbing the energy. On
the other hand, if the system is in state E2, then it emits an electromagnetic
radiation of frequency ‘ν’ given by eq(1) and changes to E1.
There are three possible ways through which interaction of radiation and
matter can takes place:

• Induced Absorption
• Spontaneous Emission
• Stimulated Emission

Induced Absorption:
It is a process in which an atom in the ground state undergoes transition to
the higher energy state by absorbing an incident photon.
The process can be represented as
Atom + Photon ➔ Atom *
Where Atom* indicates an excited atom

Spontaneous Emission:
It is a process in which an atom in the excited state undergoes transition to
the ground state by emitting a photon without any aid of external agency.
As shown in the figure, consider an atom in the excited state E2. It makes a
transition to the ground state E1 by the emission of a photon of energy hν. It may
be represented as
Atom* ➔ Atom + Photon

Stimulated Emission:
It is a process in which an atom in the excited state undergoes transition to
the ground state by the influence of passing photon. During this process a
stimulated photon is emitted along with the incident photon and these photons are
found to be coherent.
Atom * + Photon → atom + 2 Photons (Photon + Photon)
This principle is used for laser action.
Einstein’s Coefficients:
(Expression for energy density of photons in terms of Einstein’s Coefficients
under thermal equilibrium condition)
* Consider two energy states E1 and E2.
* Let E1 be the lower energy state and E2 be the higher energy state.
* Let N1 be the number of atoms per unit volume in the energy state E1 and
N2 be the number of atoms per unit volume in the energy state E2.
* Let 𝑬𝝂 be the energy density of photons.
1. Induced Absorption:
In this case, an atom in the lower energy state E1 undergoes transition to the higher
energy state E2 by absorbing a photon.
The number of such absorptions per unit time per unit volume is called Rate of
induced absorption.
Rate of induced absorption ∝ N1 𝑬𝝂
= B12 N1 𝑬𝝂 …………. (1)
Where N1 is number of atoms in the state E1,
𝑬𝝂 is the energy density in frequency range 𝜈 and 𝜈 + d𝜈 and
B12 is called Einstein coefficient of induced absorption.

2. Spontaneous Emission:
In this case, an atom in the higher energy state E2 undergoes transition to the lower
energy state E1 by emitting a photon without any aid of external agency. The
number of such Spontaneous emissions per unit volume per unit time is called Rate
of spontaneous emission.
Rate of spontaneous emission ∝ N2
= A21 N2 -------- (2)

Where, A21 is called Einstein coefficient of spontaneous emission.

3. Stimulated Emission:
In this case, an atom in the higher energy state E2 undergoes transition to the lower
energy state E1 under the influence of passing photon.
During this process a stimulated photon is emitted along with the incident photon.

The number of such stimulated emissions per unit time per unit volume in called
the Rate of stimulated emission.
Rate of stimulated emission ∝ N2 𝑬𝝂
= B21 N2 𝑬𝝂 ------ (3)
Where, B21 is called the Einstein coefficient of stimulated emission
At thermal equilibrium, the number of upward transitions must be equal to the
number of downward transitions.
Rate of absorption = Rate of spontaneous emission + Rate of stimulated
emission
B12 N1 𝑬𝝂 = A21 N2 + B21 N2 𝑬𝝂

𝑬𝝂 (B12 N1 – B21 N2) = A21 N2

𝑨𝟐𝟏 𝑵𝟐
𝑬𝝂 =
𝑩𝟏𝟐 𝑵𝟏 − 𝑩𝟐𝟏 𝑵𝟐

𝑨𝟐𝟏 𝑵𝟐
𝑬𝝂 = 𝑩 𝑵
𝑩𝟐𝟏 𝑵𝟐 [ 𝟏𝟐 𝟏 −𝟏]
𝑩𝟐𝟏 𝑵𝟐

𝑨𝟐𝟏
𝑬𝝂 = 𝑩𝟏𝟐 𝑵𝟏
………… .(4)
𝑩𝟐𝟏 [ −𝟏]
𝑩𝟐𝟏 𝑵𝟐

By Boltzmann law, we have

𝑬𝟐−𝑬𝟏 𝒉𝝊
𝑵𝟏
= 𝒆( 𝑲𝑻
)
= 𝒆(𝑲𝑻)
𝑵𝟐

∴ (4) becomes,

According to Planck’s law, the equation for energy density of radiation at given
temperature, 𝑬𝝂 is

Comparing equation (5) and (6), we get

This means that the probability of induced absorption is equal to the probability of
stimulated emission. By neglecting the subscripts, A21 and B21 can be represented
as A and B respectively i.e., A21 = A and B21 = B.
Then at thermal equilibrium, the equation for energy density is

Energy states of atoms:

Ground state: It is the lowest possible energy state of an atom which is the most
stable state. Atoms can remain in this state for unlimited time.
Excited state: These are the possible energy states of an atom which are higher
than the ground state. Atoms remain in these energy states for a very short time
called the lifetime typically of the order of 10-8 s to 10-9 s.

Metastable State: These are excited states of an atom with relatively large lifetime
of the order of 10-3 s.
CONDITION FOR LASER ACTION:
(Population inversion and metastable state)

“Population inversion is the state of a system at which the population of a

particular higher energy state is more than that of a lower energy state”. To achieve
population inversion a special kind of excited state called metastable state are
used and it can be explained as follows.
Atoms in the ground state undergo transition to the higher energy state E3 by
absorbing incident photons. Since E3 state is ordinary excited state, atoms in the E3
state don’t stay over a long time, as a result the atoms immediately undergoes
spontaneous downward transitions to the E2 state. Since E2 is metastable state,
Atoms in the E2 state stay over a long duration of about 10-2 to 10-3 seconds.
Because of this, population of E2 state increases and at a particular stage
population inversion takes place. Once population inversion takes place the
stimulated photons are emitted which gives laser beam. Hence the condition for
laser action is achieved by means of population inversion.
REQUISITES OF A LASER SYSTEM:
There are three requisites of laser systems.
1. An Excitation source for pumping action
2. An Active medium to achieve population Inversion
3. An Optical resonant cavity or laser cavity

1. An Excitation source for pumping action: The process of supplying energy

to the medium to excite an atom from lower energy state to a higher energy
state is called pumping.
Energy can be supplied to atoms in different forms Optical pumping,
Electrical pumping and Chemical Pumping

2. An active medium to achieve population Inversion:

Active medium refers to the medium in which the laser action takes place. The
energy levels of the atoms or molecules which are involved in laser action are
identified. Accurate information about the energy levels and their lifetimes helps
in identifying the level between which the population inversion can be achieved.
3. An Optical resonant cavity or laser cavity:

Semi transparent
mirror

100%
Active LASER
Reflecting
Medium
Mirror

A laser device consists of an active medium bound between two mirrors. The
mirrors reflect the photons to and fro through the active medium. A photon moving
in a particular direction represents a light wave moving in the same direction.
Thus, the two mirrors along with the active medium form a laser cavity.
SEMICONDUCTOR LASER (GALLIUM ARSENIDE LASER):

Principle: A Semiconductor diode laser is a specially fabricated p-n junction diode

which emits light when it is forward biased. ‘n’ junction is the active medium.
Construction:

• GaAs diode is a single crystal of Ga and As.

• Consists of heavily doped n and p sections.
• N-section is formed by doping with Tellurium and p-section with Zinc.
• Doping concentration is 1017 to 1019 dopant atoms/cm3
• Size of the diode is very small. Sides are 1mm and junction width is 1μm to
100μm.
• A pair of parallel planes is polished and these play the role of reflecting
mirrors. They provide sufficient reflection to sustain the lasing action.
• Other two sides are roughed surface to suppress the reflections of the
photons.
• End surfaces of p-n sections parallel to the plane of junction are provided
with the electrodes in order to facilitate application of a forward bias voltage
with the help of voltage source.

Working;
➢ Suitable forward bias voltage is applied to the diode to overcome the
potential barrier. Due to forward biasing, more and more electrons are
injected into the n-region. This leads to the increase in population of
electrons in n-region and population of holes in the p-region. When the
current crosses certain value called threshold current, electrons from n-type
come to higher energy level of the depletion region and population inversion
is attained.
➢ Once the populations of charge carriers in the depletion region increases,
the electrons are made to recombine with the holes in the lower energy level
of depletion region.
➢ At this stage, a photon released by spontaneous emission may trigger
stimulated emissions over a large no of recombination’s leading to the
buildup of laser radiation of high power.
Thus, the current flow provides pumping in semiconductor laser.
➢ The wavelength of emitted light is
 ℎ𝑐 6.626𝑋10−34 𝑋 3𝑋108
𝛌= = = 8874 Ao
𝐸𝑔 1.4𝑋1.6𝑋10−19

The energy gap of Ga As is 1.4 eV.

Applications of semiconductor laser:

1) Used in optical communication

2) Used as reading devices for compact disc players.
3) Semiconductor lasers are used in laser printers.
4) Semiconductor lasers are used in medicine, interferometry and barcode
scanners.

Applications of LASER

LASER has wide range of applications pertaining all disciplines of engineering

.Here in the syllabus only three applications are discussed relevant to computing

LASER bar code scanner

A barcode is a printed series of parallel bars or lines of varying width that is used
for entering data into a computer system.
A barcode scanner/reader is a device with lights, lenss, and a sensor that decodes
and captures the information contained in barcodes. Laser scanners use a laser
beam as a light source and typically employ oscillating mirrors or rotating prisms
to scan the laser beam back and forth across the barcode. A photodiode then
measure the reflected light from the barcode. An analog signal is created from the
photodiode. And is then converted into a digital signal.

Laser Printer

Laser printers were invented at XEROX in 1969 by researcher Gray Starkweather.

Laser printers are digital printing devices that are used to create high quality text
and graphics on plain printer. A Diode Laser is used in the process of printing in
LASER printer

Working Principle
1. A laser beam projects an image of the page to be printed onto an electrically
charged rotating photo sensitive drum coated with selenium.
2. Photo conductivity allows charge to leak away from the areas which are
exposed to light and the area gets positively charged.
3. Toner particles are then electrostatically picked up by the drum’s charged
areas. Which have been exposed to light.
4. The drum then prints the image onto paper by direct contact and heat. Which
fuses the ink to the paper.
Advantages
1. Laser printers are generally quiet and fast.
2. Laser printers can produce high quality output on ordinary papers.
The cost per page of toner cartridges is lower than other printers.
Disadvantages
1. The initial cost of laser printers can be high.
2. Laser printers are more expensive than dot-matrix printers and ink-jet
printers.
Laser Cooling
Principle of LASER cooling: Laser cooling is the use of dissipative light forces for
reducing the random motion and thus the temperature of small particles, typically
atoms or ions. Depending on the mechanism used, the temperature achieved can
be in the milli kelvin, micro kelvin, or even nano kelvin regime.

If an atom is travelling toward a laser beam and absorbs a photon from the laser, it
𝐸 ℎ
will be slowed by the fact that the photon has momentum p= = .
𝑐 𝜆
It would take a large number of such absorptions to cool the sodium atoms to near
0K. The following are the types of laser cooling
*Doppler cooling
*Sisyphus cooling
 OPTICAL FIBERS
➢ Fiber optics is an overlap of science and technology which deals with
transmission of light waves into optical fibers, their emission and detection.
➢ It is a waveguide through which light can be transmitted with very little
leakage through the sidewalls.
➢ These are essentially light guides used in optical communications as
waveguides.
➢ The principle behind the transmission of light waves an optical fiber is
TIR( Total internal Reflection)
➢ They are transparent dielectrics and able to guide visible and infrared light
over long distances.
CONSTRUCTION OF OPTICAL FIBER:

• An optical fiber is cylindrical in shape

• It has two parts a) inner part and b) outer part.
• The inner part is made of glass or plastic and its cylindrical in shape, it is
called core. Core is having high refractive index.
• Outer part is a concentric cylinder surrounding the core, and is called
cladding. Cladding is also made of same material with little lesser refractive
index.
• The polyurethane jacket is used to enclose cladding which safeguards the
fiber against chemical reaction with surroundings and also crushing.
• Many fibers which are protected by individual jackets are grouped to form a
cable. A cable may consist of one to several hundred such fibers.

TIR (TOTAL INTERNAL REFLECTION):

It is the principle behind the transmission of light waves in an optical fiber which
is a well-known optical phenomenon in physics.

A ray AO, travelling in a medium of refractive index n1 is separated by the boundary

XX1 , from another medium of lower refractive index n2. So n1 > n2
The incident ray AO makes an angle θ1 with the normal in the medium of refractive
index n1. The same AO undergoes refraction into the medium of refractive index
n2 and it bends away from the normal, since n1 > n2 . θ2 is the angle made by the
refracted ray with the normal.

If we increase θ1 for certain value of θ1= θc called critical angle, θ2 = 90, for such a
case, the refracted ray grazes along the boundary of separation along OB1 while
incident ray is along BO.

If θ1> θC , incident ray CO always gets reflected back into the same medium in
which it is incident on the boundary. These takes place as per the law of reflection.

For refraction, we have the Snell’s law

n1sin θ1 = n2 sin θ2
For θ1= θC and θ2 = 900
n1sin θC = n2 sin 900 = n2 (sin 900 = 1)

𝒏
θC = 𝒔𝒊𝒏−𝟏 [ 𝟐]
𝒏𝟏

PROPAGATION MECHANISM:
Explain mechanism of light propagation in optical fibers.
(Or)
Explain how optical fibers work as waveguides.
“Optical fibers are the devices used to transmit light effectively along any desired
path.”

• Optical fibers work on the principle of total internal reflection (TIR)

• For total internal reflection there are two essential conditions, they are
1) The light ray must pass from denser to rarer medium.
2) The angle of incidence must be greater than the critical angle i > c
• A waveguide is a tubular structure through which energy of some sort could
be guided in the form of waves.
• The waveguide as a light guide. also called fiber waveguide or fiber light
guide
• An optical fiber consists of a core and cladding.
 • In any optical fiber the refractive index of cladding is always lesser than that
of its core to achieve TIR, i.e. R.I cladding < R.I of core
• When a light is incident at one end of the fiber, it undergoes total internal
reflection and finally emerges at the other end of the fiber. It is found that
intensity of emergent light is almost same as that of incident light. In this way
optical fibers transmit light effectively along any desired path.

RAY PROPAGATION IN THE FIBER. ANGLE OF ACCEPTANCE AND NUMERICAL

APERTURE.

Q: Explain with a neat diagram Acceptance angle and numerical Aperture of

an optical fiber. Hence derives an expression for numerical aperture.

• Consider a ray AO entering into the core at an angle θ0 to the fiber axis. Then
it is refracted along OB at an angle θ1 in the core and further falls at critical
angle of incidence (equal to 90 - θ1) at B on the interface between core and
cladding. Since the incidence is critical angle of incidence, the ray is refracted
at 900 to the normal drawn to the interface i.e. grazes along BC.
• Any ray that enters into the core at an angle of incidence less than θ0 will
have refractive angle less than θ1 because of which its angle of incidence 900-
θ1 at the interface will become greater than the critical angle of incidence
and hence undergoes total internal reflection.
• On the other hand any ray that enters at an angle of incidence greater than
θ0 , will have to be incident at the interface at an angle less than the critical
angle, it get refracted into the cladding region. Then it travels across the
cladding and emerges into the surroundings and will be lost.
• If now OA is rotated around the fiber axis keeping θ0 same, it describes a
conical surface.
• Therefore if a beam converges at a wide angle into the core, then those rays
which are funneled into the fiber with in this cone will only be totally
internally reflected, and thus confined within for propagation..
θ0 is called waveguide acceptance angle or the acceptance cone half angle.
Sinθ0 is called Numerical aperture (N.A) of the fiber.
“The light gathering capacity of an optical fiber is known as Numerical
aperture.’
Condition for propagation;
Let n0 , n1 , n2 be the refractive indices of surrounding medium, core and cladding
respectively.
For refraction at the point of entry of the ray “AO” into the core, we can apply
Snell’s law, i.e., at point A
nosin θ0 = n1 sin θ1 -------- (1)
At the point B
The angle of incidence =900-θ1
Apply Snell’s Law
n1 sin (900-θ1) = n2 sin 900
n1cos θ1 = n2 (sin 900= 1)
𝑛2
Cos θ1= ------- (2)
𝑛1
Equation (1) can be written as
𝑛1
Sin θ0 = sin θ1
𝑛0
𝑛1
= √(1 − 𝑐𝑜𝑠 2 θ1)
𝑛0
𝑛1 𝑛
= √1 − ( 𝑛 )2
2
𝑛0 1

𝑛1 𝑛1 2 −𝑛2 2
= √
𝑛0 𝑛1 2

√𝑛12 −𝑛22
𝑛1
=
𝑛0 𝑛1

𝑛1 2 −𝑛2 2
Sin θ0 =√
𝑛0

If the medium surrounding the fiber is air then no=1

Therefore, Sin θ0 =√𝑛12 − 𝑛22
i.e.
N.A = √𝒏𝟐𝟏 − 𝒏𝟐𝟐

If θi is the angle of incidence, then the ray will propagate if θi < θ0

(Or) sin θi < sin θ0
(Or) sin θi < √𝑛12 − 𝑛22
sin θi < N.A
→Condition for propagation
Note: for light propagation, angle of incidence is less than θ0
FRACTIONAL INDEX CHANGE (∆):
The fractional index change ∆ is the ratio of the refractive index difference between
the core and cladding to the refractive index of core of an optical fiber.
𝒏𝟏 − 𝒏𝟐
∆= ---(3)
𝒏𝟏
MODES OF PROPAGATION:

MODE is,
• The pattern of motion in a vibrating body.
• The light ray paths along which the waves are in phase inside the fiber.
• In simple terms these modes can be visualized as the possible number of
allowed paths of light in an optical fiber.
Through it is expected that all the rays which enter into the core at an angle less
than the acceptance should travel in the core, it is not even theoretical. By the
application of Maxwell’s equation, we can get to know that , out of the light that
enters into the core within the waveguide acceptance angle, only the light waves
in terms of certain number of modes will be sustained for propagation in the fiber.

V-NUMBER:
“The number of modes supported for the light propagation in the optical fiber is
known as V- number.”
V – Number is given by
𝝅𝒅
V= √𝒏𝟏 𝟐 − 𝒏𝟐 𝟐
𝝀
Where, d is the diameter of the core, 𝛌 is the wavelength of light
n1 is the R.I of the core n2 is the R.I of the cladding
𝝅𝒅
(Or) V= N.A
𝝀
If the fiber is surrounded by a medium of R.I n0, then the expression is
𝝅𝒅 𝒏𝟏 𝟐 −𝒏𝟐 𝟐
V= √
𝝀 𝒏𝟎
𝑽𝟐
Number of modes =
𝟐

TYPES OF OPTICAL FIBERS:

• Optical fibers are classified into 3 major categories based on the materials
used for making optical fibers, number of modes transmitted and the R.I
profile of the fibers.
• In any optical fiber, the whole material of the cladding has a uniform
refractive index value but the refractive index of core material may either
remain constant or subjected to variation in a particular way. (R.I of the
core changes in graded index multimode fiber)
• The curve which represents the variation of refractive index with respect to
the radial distance from the axis of the fiber is called the Refractive Index
Profile.
Optical fibers are classified into 3 categories namely:
a) Single mode fiber
b) Step index multimode fiber
c) Graded index multimode fiber
Single mode fiber:

• Here core material has uniform refractive index value.

• Cladding also has uniform refractive index but of little lesser value than
that of core. This results in a sudden increase in the value of R.I from
cladding to core.
• R.I profile takes the shape of a step.
• Diameter of the core is 8 to 10 𝜇m.
Diameter of the cladding is 60- 70 𝜇m
Since the core is very narrow, it can guide just a single mode. Hence it is
called single mode fiber.
• These are the most extensively used ones and constituent 80% of all the
fibers that are manufactured.
• They need lasers as the source of light
• It is less expensive, but very difficult to splice.
• Used in submarine cable system.
Step index multimode fiber:

• Here, the core material has uniform refractive index value.

• Cladding also has uniform refractive index but of little lesser value than
that of the core. This results in a sudden increase in the value of R.I from
cladding to core.
• R.I profile takes the shape of a step.
• Diameter of the core is 50 to 200 𝜇m.
Diameter of cladding is 100- 250 𝜇m
• Here the core material has a much larger diameter, which supports
propagation of large number of modes.
• R.I profile is also similar to single mode optical fiber.
• Uses LED or laser as source of light.
• It is least expensive all and is used in data links which has lower band
width requirements
Graded index multimode optical fiber:

• It is also denoted as GRIN.

• The geometry of GRIN is same as that of step index multimode fiber.
• The special feature of the core is that its R.I value decreases in the radially
outward direction from the axis, and becomes equal to that of the cladding
at the interface. But the R. I of the cladding remains uniform.
• Diameter of the core 50 to 200 𝜇m.
Diameter of cladding 100- 250 𝜇m
• Uses LED or laser as source of light
• Application is in the telephone trunk between central offices.

ATTENUATION (POWER LOSS OR FIBER LOSS):

The power loss suffered by the signal when it propagates through the fiber is
called Attenuation. It is also known as fiber loss.
Types of losses in fiber are:
i) Absorption
ii) Scattering
iii) Radiation loss
i) Absorption loss:
a. Absorption by impurities: Iron, Chromium, Cobalt and Copper are
some of the impurities generally present in the glass fiber. When signal
propagates through the fiber, a few photons associated with the signal are
absorbed by the impurities present in the fiber. This results in power loss.
ii) Scattering loss:
a. Rayleigh scattering:
When a signal propagates through the fiber, a few photons associated with
the signal are scattered by the scattering objects such as impurities present
in the fiber. The dimensions of the scattering objects are very small
compared to the wavelength of light. This type of scattering is similar to
Rayleigh scattering. It is found that the co-efficient of scattering is inversely
proportional to the wavelength of the object.

iii) Radiation loss:

It is due to the bending of fibers and it can be explained as follows:
a) Macroscopic bending: They are the bends with radii much larger
compared to fiber diameter. It occurs while wrapping the fiber on a spool
or turning it around a corner. If the bending is too sharp then the power
loss becomes very high.

b) Microscopic bending: It occurs due to the non-uniformity in the fibers

while manufacturing. Because of this a few modes undergo leakage which
results in power loss.

EXPRESSION FOR ATTENUATION CO-EFFICIENT (𝜶):

𝟏𝟎 𝑷
α=- log10 [ 𝑷𝒐𝒖𝒕 ] dB/Km
𝑳 𝒊𝒏
Applications
FIBER OPTIC COMMUNICATION
Optical fiber communication is the transmission of information by propagation of
optical signal through optical fibers over the required distance which involves
driving optical signal from electrical signal at the transmitting end and
conversion of optical signal back to electrical signal at the receiving end.

➢ Firstly, we have analog information such as voice of a telephone user. The

voice gives rise to electrical signals in analog form coming out of the
transmitter section of the telephone.
➢ The analog signal is converted to binary data (digital) with the help of an
electronic system called Coder.
➢ These electrical pulses are converted into optical pulses by modulating the
light emitted by an optical source, in the binary form. This unit is called
optical transmitter (converts electrical signals into light signals)
➢ This optical Signal is fed into the fiber.
➢ Out of the incident light which is funneled into the core within the half
angle acceptance cone, only certain modes will be sustained for
propagation within the fiber by means of total internal reflection. While
propagating signal undergoes attenuation and delay distortion.
Delay distortion is the reduction in the quality of signal with time. These
effects cause degradation of the signal as the light propagates and may
reach a limiting stage beyond which it may not be possible to retrieve the
information from the light signal.
➢ The receiver section uses Photo detector which converts the optical signal
into corresponding electrical signal then electrical signal is amplified and
recast in the original form by means of an electrical regenerator, which is
part of receivers’ section.
➢ Lastly using the Decoder, the binary electrical signal is converted back to
analog electrical signal, which will be same information such as voice,
which was there at the transmitting end.
FIBER OPTIC NETWORKING

Local Area Network

A Local Area Network (LAN) is a type of computer net- work that interconnects
multiple computers and computer- driven devices in a particular physical location.
Tradition ally copper coaxial cables are used for LAN.

Passive Optical LAN

Passive here refers to the unpowered condition of the fiber and splitting/combining
components. Passive optical LANs are built entirely using Optical fiber cables. The
passive optical LAN is complicated as it works on the concept of optical network
terminals (ONT) and passive optical splitters. Network switches act as passive splitters
and the commercial media converters act as optical net- work terminals in a real-time
application of passive optical LAN.

Advantages
1. High speeds and bandwidth

2. Longer distances are possible

3. Less chance of errors