Optical fiber Communication

Introduction
• Depending on medium process happens
– Reflection.
– Refraction: Travel through the medium.
• Law of Reflection (Snell’s Law).
• Optical Fiber Communication
– Optical - Light.
– Fiber - Non-conducting wave guide.
• Total Internal Reflection (TIR).
• K-FACTOR : An audio is coupled with laser by using the transistor in common collector
mode which acts as the impedance match hence laser light intensity follows the message
signal. 1Dept. of EC&E, JIT, Davangere.

Reflection
• Medium should be opaque.
• Types:
– Specular reflection.
– Diffuse Reflection.
2Dept. of EC&E, JIT, Davangere.

Refraction
• Medium should be transparent.
• Types: Positive and negative.

Law of Reflection
• Holds good for both rough and smooth
surface.
• Smooth Surface:
– Incident, reflected and normal to the surface all lie
in the same plane.
– Angle of reflection is equal to incidence angle.
• Rough Surface (diffuse reflection)
– If the incident wave falls on rough smooth the
reflected ray follows the different angle.
Dept. of EC&E, JIT, Davangere. 4

Principle of TIR.
• This is the phenomenon which occurs when a
propagated wave strikes a medium boundary
at a angle larger than critical angle.
• Reflected light bends towards normal when
n2>n1. Because light travels at slower rate.
• Ex: Vehicles move at slower rate in traffic.

Application:
Telephonic communication.
Automotive Rain sensors.
Shining of Diamond.
Optical fingerprint.
Camera

Sky color
• Is sky having color?
• If yes, which color?

Conti.
• Scattering- blue color has low wavelength
(frequency is more). Red (wavelength is more)
• If wavelength is more it won’t come in contact
with small particles but travels for larger
distance. ( For stop red color is used)
• Ex: radio waves: penetrates the tree and
moves forward.

Photo-Diode
• It is used to convert the light into current
based on equipments requirements.
• Principle: When photon of energy > 1.1ev hit
the diode a electron-hole pair are created.
• If absorption occurred in P-N junction, hole
pairs are swept towards respective junction.

Electromagnetic Spectrum

An optical fiber is a hair thin
cylindrical fiber of glass or any
transparent dielectric medium.
The fiber which are used for optical
communication are wave guides
made of transparent dielectrics.
Its function is to guide visible and
infrared light over long distances.
What is Fiber optic ?

Evolution of fiber OPTIC
• 1880 – Alexander Graham Bell.
• 1930 – Patents on tubing.
• 1950 – Patent for two-layer glass wave-guide
• 1960 – Laser first used as light source
• 1965 – High loss of light discovered
• 1970s – Refining of manufacturing process
• 1980s – Optical Fiber technology becomes
backbone of long distance telephone networks.

Historical Development of OFC.
• Fiber optics used for point to point
transmission using light waves.
• The conventional carrier frequencies;
frequency is proportional to creation of the
bandwidth.

First generation The first generation of light
wave systems uses GaAs semiconductor laser
and operating region is 0.8 μm.
i) Bit rate : 45 Mb/s
ii) Repeater spacing : 10 km
Second generation i) Bit rate: 100 Mb/s to 1.7
Gb/s ii) Repeater spacing: 50 km iii) Operation
wavelength: 1.3 μm iv) Semiconductor: In
GaAsP(Gallium Arsenide Phosphide)- emits
red, orange and yellow LED.

Third generation i) Bit rate : 10 Gb/s ii) Repeater spacing: 100 km iii)
Operating wavelength: 1.55 μm
Fourth generation uses WDM(Wavelength Division Multiplex)
technique. i) Bit rate: 10 Gb/s ii) Repeater spacing: > 10,000 km.
iii) Operating wavelength: 1.45 to 1.62 μm .
Fifth generation
i) Bit rate: 40 - 160 Gb/s ii) Repeater spacing: 24000 km - 35000 km iii)
Operating wavelength: 1.53 to 1.57 μm

General System

•Normally the Communication system is designed to transfer the
message signal from source to destination.
•Information source
•Modulator
•Medium
•De-modulator
•Medium is Optical Fiber Communication.
•Without the use of repeaters with less or no attenuation the signal the
transmit.
•Long Distance is achieved by using amplifiers while joining two fibers.
•Information source which gives an electrical signal.
Example: real time audio to voltages.
•Electrical Transmitter : A voltage or current is converted to accurate
format.

•Optical source electrical to optics.
•Optical Detector used to convert optical to electrical.
•Example Photo Diode, PIN, Avalanche.
•Optical Carrier can be obtained using ANALOG or DIGITAL
•Analog: AM modulation based on the intensity the
received signals are analyzed.
•Digital: Collection of 1’s and 0’s.
•Disadvantage of Analog Modulation.
•SNR value should be more to received error free signals.
•Linearity is unable to achieve.
•O/p should change based on the input and follow
ohm’s law.

Digital Optical Fiber Communication.
•Encoder: Digital to compactable format for Optical Transfer.
• Laser communication is used to transfer the information.
• Photo Diode is used to sense the amplitude of the input
signal.
•Amplifiers are used to boost the signal level that the
decoders are accepted.
•Decoders are used to obtain the information from received
signal.

Advantages of Optical Fiber
•Enormous potential bandwidth: The carrier frequency is about 10^13 to 10^16 range.
•Boosters are absent; it have achieved 100 MHz for 100Km, 100 GHz for 300 Km.
•Transmission loss is less: TIR has efficient.
•Cost for overall achievement of communication reduces as the number of repeaters are
less.
•The range is fixed for optical carrier frequency in order to increase the bandwidth i.e.
information carrying capacity instead of single beam need to multiplex the several optical
beam in parallel with the fiber center.
• Large Information Capacity: More number of bandwidth allocation for the users can be
performed.
•Small Size and less weight:
• Posses less weight (about 130 per Km) and dimension of core as low as 50 microns.
• By considering size and weight it is used in applications of military, ships etc.
• Metropolitan cities are connected as these are handy.
Material Coverage(without repeater) Speed Voice Channels
Copper 2.5 Km 1.5 Mb/s 24
Optical fiber 300 Km 2.5+ Gb/s 32000

•Immunity to electrical interface (Electrical Isolation)
•Light is used for communication.
•The materials used are glass or plastic which is electrical insulators
hence no earth or ground loop, short circuit, spark hazards.
•Immunity to interference and cross talk (ex: keeping watch near TV)
•Immune to interface loss, inductive pick up from near by
components or equipment.
• Enhanced Safety
•No risk of voltage or ground as in copper wire.
•Emission of laser beam should be carefully injected as of eye
damage.
• Signal Strength
•Light is secured inside the fiber and cladding is opaque in nature
which observes emission.

•Signal Security
• The light from the fiber doesn’t radiate significantly hence security is
more.
•Without drawing optical power the hacking is not possible in OFC.
•If the power is extracted it is detectable saying that message signal is
theft.
•Low transmission loss
•0.2 db per Km.
•Ruggedness and flexibility
•Manufactured with high tensile strength.
•As it is glass also it can be bend with small radii.
•System Reliability and Maintenance cost
•Low loss of signal without repeaters the communication is achieved to
larger distance. The system can be used reliable for 30 years.
• Is considerably less or no maintenance cost.

Disadvantage of Optical Fiber
• Cost: Initial cost
•Fragile :Care should be taken while installing
•Protection: extra protection is essential compared to the
copper wires.
•Applications of Optical Fiber
•Data transfer 10Gbps
•Cable TV
•Summary

Wave Guide
It is defined as the structure which is used to guide the EM waves and
confines the waves in parallel to the fiber axis.
Optical Wave Guide
oGuides the EM waves in Optical Spectrum.
oIn 1910 a Debye[Dih-bahy] proposed a theoretical arrangement
and after ten years it came into practical.
oThe arrangement was with core [1.5] and air. Interface loss is more.
oIn 1950 a new structure was proposed.
oRefractive index of core will be always higher than cladding.
12/23/2017 24Dept. of EC&E, JIT, Davangere

The cladding is thick hence it reduces the radiation loss into air,
provides the mechanical strength to fiber.
The interface loss is less because most of the time the light rays travels
in core area.
Researchers are concluded that sources with higher wavelength has
lesser loss.{Silica Wavelength is 1.1 to 1.6 um}

Ray Theory Transmission
The propagation of the light using Ray theory is based on the change on the refractive index
of the medium.
It is the ratio of velocity of light in vacuum to medium.
In denser medium the speed of light is less.
The refraction occurs when light travels from rarer to denser or vice versa.

Acceptance Angle
It is defined as the angle of incidence of ray should be within the
conical half angle rest of the rays radiates out.
All the rays entering the fiber won’t reach the end of fiber.
Acceptance angle is the geometrically provided shape(conical) within
which the specific waves undergoes TIR.

The angle of incidence of the rays whose angle is
more than half conical half angle doesn’t undergo TIR.
The half conical angle is the maximum angle
through which all the ways entered undergoes TIR.
This geometrical structure is called as aperture angle
for the fiber.

Numerical Aperture
Numerical Aperture and refractive index of core, cladding and air are
related to each other by derivation.
These rays are called as meridional rays (i.e. rays passes through the
axis of fiber and ray always hit the axis of core.)

Skew Rays
Usually all rays propagate using axis of fiber in waveguide.
Skew rays is another type of communication in which light
rays follows the helical path to reach the destination.
The analysis in the two dimensions seems difficult hence in
fig (b) it is clear that rays follows the helical path.
The angle between projection of ray and radius of fiber core
during the time of reflection is given by Gamma.12/23/2017 30Dept. of EC&E, JIT, Davangere

The point of ray coming out from the fiber is depending on number
of reflection it undergoes.
The outputted ray is always uniform irrespective of the input (random)
The acceptance angle of the fiber is found by defining the ray in the
two perpendicular plane.

ATB one plane,
SBR another plane.

By observing the equations for the Numerical aperture for Meridional
rays and skew rays ; skew rays has the more accepted angles.
The input angle is minimum to the skew rays as observed in figure the
rays follows the helical path and doesn’t uses full portion of fiber.
The light gathering capacity is more compared to meridional rays.

12/23/2017 Dept. of EC&E, JIT, Davangere 34
Mode Field Diameter
Provides optical power per unit area is called as Mode field Diameter which is calculated at
the end of fiber.
Cylinder Fiber
The modes are used in the cylindrical fiber.
HE- the E field is not present in the core area.

Fiber Types
Fiber Mode and Configuration are discussed
Directly the cladding is not involved in the communication but it reduces the
scattering loss and provides the mechanical strength to fiber.
Based on the refractive index of the medium the types of the fiber are
Step Index fiber: The refractive index of the core remains constant.
Graded Index fiber: The refractive index of the core varies with distance from
center of the fiber increase.
Every type of fiber is further divided into single and multi-mode fiber.
The difference between the single and multi mode is number of propagation of the
waves into fiber.

Step Index Fiber
A fiber which contains the constant core refractive index with varying
cladding refractive index is called as step index fiber.
The step wise refractive index at core- cladding interface.
The radius of core is ‘a’ and refractive index is 1.48.
The refractive index of cladding is less than core given by
n2=n1(1-index difference)12/23/2017 36Dept. of EC&E, JIT, Davangere

The refractive index profile is given by
Advantage of single mode step index fiber
Has lesser broadening of transmitted light pulse( less intermod dispersion).
As dispersion of signal is less it supports more bandwidth allocation compared to multi-
mode step index.
Advantage of multi-mode step index fiber for lower bandwidth.
Posses larger core diameter, numerical aperture, facilitating easier coupling to optical
sources.
Fiber connectors posses low tolerance.
Disadvantage of multimode step index fiber
The number of light waves is more and hence the signal dispersion is more.
Normalized frequency is given by
Number of modes which need to be propagated inside the fiber (M) is given by
M=V2/2; V- frequency of the fiber
r- radial distance .
a-radius of core.

Need for the single mode fiber in communication
As discussed earlier these exhibit more transmission
bandwidth and less loss.
Superior transmission quality compared to others fiber
types as modal noise is absent.
Easily upgradable to future trends like making the
bandwidth wide by using the faster optical transmitter and
receivers making use of coherent technology (Receiver
sensitivity can be increased )

Graded Index Fiber
The refractive index of the fiber’s core decrease with increase in the
distance from radii of centre towards cladding.
Alpha =infinity- same as step index profile,
2– Parabolic , 1- Triangular.

Due to parabolic refractive index; the meridional rays
follows the curved path through the axis of the fiber.
The gradual decrease in the refractive index from core to
cladding many refraction of rays are possible.

The ray of light is gradually curved due to change in refractive index till the TIR
conditions are accepted the ray is refracted back to the same medium with different
angle of incidence.
Signal dispersion in the multimode graded index is less because of different
refractive index.
After emerging the ray travelling near to axis of fiber travels less distance as
compared to waves travelled at boundary of core.

The skew rays follows the helical path in graded index
fiber.
Modes of the graded index fiber is given by M= V2/4.

Mode Coupling
The improper propagation of light
Reasons: change in core diameter, bending of fiber, refractive
index in core and cladding.
Two cases of unevenness of fiber is shown: which changes the
direction of the refraction light rays.

Fiber Material
Long, thin and flexible.
Transparency.
Refractive index between core and cladding should be
different.
Glass or plastic is the best option.
Glass is preferred material and it has ranging between
low to high loss.
Plastics produces more attenuation, short distance and
used in extensive network.

Glass Fiber
A composite of metal oxide, silfides or selenides.
Random structure of the molecules are generated because of
composite structure but not definite structure.
High melting point greater than 1200 C due to composite of glass.
Optically transparent glass because of oxides.
Silica is the main molecule with refractive index 1.48 and 850 nm.

CORE CLADDING
GeO2-SiO2 SiO2
P2O5-SiO2 SiO2
SiO2 P2O3-SiO2
GeO2-B2O3-SiO2 B2O3-SiO3

Properties offered
High raise of temperature resistant for 1000 C
Low thermal expansion
Durability in chemical composition, high temperature even
in visible and infrared rays.

Active Glass Fibers
Doping rare earth materials into normal glass gives new optical and
magnetic properties.
These new material are capable to perform amplification, attenuation
and phase retardation on the light passing through it.
Commonly used materials for fiber lasers is erbium.
The ionic concentration of the rare elements are less(0.005 to 0.05)
which causes non-linear efforts (Clustering efforts).
By observing the spectra of material the optical source is chosen in
such a way that to emit the light in wavelength which excites the
electrons of material to higher state later a external photon used to
make fall it to lower state.

Spectra of the materials

Plastic Optical Fiber
The diameter of the core is larger[10-20 times] as compared to glass
fiber.
The connecting losses are less as core has larger diameter.
Growing demand for high speed service to workstations has led to
develop high bandwidth graded index optical fiber using polymer.
Core is polymethylmethalate polymer.
These polymers are referred as PMMA POF and PF POF respectively
They are tough and durable compared to glass fiber.
POF-Polymer Optical Fiber, PF-Plastic fiber.
PMMA- Graded Index Polymer Optical Fiber.

Photonic Crystal Fiber(PCF)
 Earlier it was called as Holy fiber and later became Photonic Crystal
Fibers
 The effective refractive index depends on pitch size and wavelength
of light.
 Cladding contains air holes and in some PCF core contains air holes
which run along the entire length of the fiber.
 Size and spacing (known as pitch ) of the hole in the structure and
the refractive index determine the light guiding characteristics of the
photonic crystal fibers
 Light transmission is happening because the change in refractive
index.
 Two types of PCF are 1. Index guided PCF 2. Photonic band gap

Index Guiding Photo Crystal Fiber
Consists of a solid core and surrounded by air holes running along the
length .
Holes have diameter -d and pitch A
Core and cladding are made of same material and the air holes have
effective index of refraction lower than the core and cladding i.e. n = 1
for air and n = 1.45 for core cladding.

Index Guiding Photo Crystal Fiber

Photonic Band gap Fibers
In contrast to index guided fiber these have hallow core and
surrounded by cladding with air holes running along the
length.
Again the diameter in the air hole is d and the pitch is A.
Principle of Photonic Band gap Fibers fiber is similar to the
role of periodic crystalline lattice in a semiconductor, which
blocks the electrons from occupying the band gap region.
In Photonic Band gap Fibers fiber the hallow core acts as a
region in which the light can propagate.

Photonic Band gap Fibers

Fiber Optic Cable
 Based on the application and place of installation the
structure of the fiber varies.
While manufacturing of these type of fiber a special care
should be taken because glass is used as material.
In practical applications the optical fiber to be incorporated
in some type of cable structures.
Structure will vary based on whether the optical fiber has to
be pulled underground, or interbuilding ducts, buried directly,
installed outdoor poles or submerged in water.

Cable Structure
One important property is that maximum allowable axial
load which determines the length of the fiber to be installed.
(Lorry axial load road are constructed)
In copper cables they themselves are the load-bearing
structures and they can be elongated to about 20% without
fracture.
Steel wire has been used for reinforcing the electric cables
and also used as strength member for optical fiber cables.
For some applications nonmetallic construction is desired to
avoid electromagnetic induction.

Six basic fiber-building blocks
Outer sheath protects the fiber cable from atmospheric and other
external effects.
Yarn strength member is a soft yellow synthetic nylon material. it is
used to avoid the effects of EMI or to reduce cable weight.
PVC jacket is tough polymer that provided crush resistance and
handles any tensile stresses applied to the cable so that the fibers
Inside are not damaged. it also prevents the fibers inside against
abrasion, oil, moisture and other contaminants.
The paper binding tape {Ex: Kevlar) encapsulates and binds the fiber
Groupings together.
 The Insulated copper conductor and the fiber building block are
wound loosely around the central buffered strength member

Six basic fiber-building blocks

Optical cable structure
We know that one important property is that maximum allowable
axial load which determines the length of the fiber to be installed.
Types are Tight buffered fiber cable and loose tube cable.
Tight- Indoor Applications.
Loose- Outdoor Applications.

Tight Buffered Fiber Cable
In Tight-buffered fiber cable, each fiber is
individually encapsulated within its own 900µm
diameter plastic fiber structure.
The 900µm protective coating provides excellent
moisture and temperature performance.
Permits wired termination with connectors.
These cables are used to indoor applications.

Tight buffered fiber cable

Ribbon Cable
It is an extension of tight-buffed Fiber cable and facilitates splicing.
The number of fibers In a ribbon, range from 4 to 12. These ribbons
can be stacked on top of each other to form a densely packed
arrangement.

Loose Tube Fiber Cable
In the Loose-tube fiber cable, one or more standard coated fibers are
enclosed in a thermoplastic tube that has an inner diameter which is
larger than the fiber diameter.
The fibers in the tube are slightly longer than the cable itself. The
purpose of this is to isolate the fiber from any stretching of the
surrounding cable caused by temperature changes, wind forces etc.
 They are used for outdoor applications.
Gels are filled inside the tube, which allows the tube for easy
movement and water resistant.

Loose Tube Fiber Cable

Indoor Cable(Tight Buffered Fiber)
Usage of Indoor Cable
Connections to printers or server
Distributing signals when computers are connected in LAN.
Short patch chords in telecommunication.
Three Types of indoor cable
Interconnect cable
Breakout or fan-out
Distribution Cable

Interconnect cable
It serves light-duty low-fiber count indoor. Applications,
such as fiber to the desk links. Patch chords etc.
The cable is flexible, compact and light weight with a tight-
buffered construction.
A popular indoor type cable is the duplex cable which
consists of two fiber that are encapsulated in a outer PVC
jacket.
Fiber Optics Patch cards. Used to
connect equipments.

Breakout or Fan-out Cable
This cable consists of up to twelve tight-buffered cables arranged
around a central strength member.
They allow easy installation of connectors on Individual fibers and
hence routing the individually terminated fibers to separate pieces of
equipment can be achieved easily.

Distribution cable
It consists of Individual groupings of tight-buffered fibers
arranged around a central strength member.
The main feature of distribution cable is that they enable
groupings within the cable to be branched to various
locations.
Distribution cables are designed for wide range of network
applications such as sending data, voice and video signals.

Outdoor Cables(Loose Tube Cable)
Used in duct, direct-burial and underwater applications.
It is the example loose tube structure.
Depending on the applications the sizes and designs of the fiber
changes.
Types of outdoor Cable
Aerial Cable.
Armored Cable.
Underwater Cable.

Aerial Cable
It is intended for mounting outside between
buildings or on poles.
Two popular designs of Aerial cables are
Self supporting cables- Contains an internal strength member
that permits the cable to be strong between poles without
any additional support mechanism.
Facility supporting cable- A separate wire or strength
member is strung between the pole and the cable is lashed or
clipped to this member

Armored cable
It consists of one or more layers of steel-wire
or steel-Sheath protective armoring below a
layer of polyethylene jacket.
Used for direct-burial or underground-duct
applications.

Armored cable

Underwater cable or submarine cable
It consists of various water-blocking layers,
protective inner polyethylene sheets and a heavy
outer armor jacket.
Used in rivers, lakes and ocean environments
 Cables that run under the ocean must have
additional layers of armoring and contain copper
wires to provide electrical power for optical
regenerators (Amplifiers).

Underwater cable or submarine cable

Summary
General System of Optical Fiber .
Advantage Slide 20.
Raytheory
 Comparison between Meridian and Skew Rays.
Fiber types Slide 34
Fiber MaterialSlide 43
Fiber structure.
Indoor Fiber (Tight) and outdoor fiber
Based on applications of cable the layers of fiber depends.
Problems.

Comparison between Meridian and
Skew Rays
Meridian Rays Skew Rays
Numerical Aperture=
Or acceptance Angle Numerical Aperture=
Or acceptance Angle
Step Index and Graded Index
Vc =2.405 for step index
Vc=2.405*

Optical fiber Communication

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