UNIT V OPTICAL FIBER COMMUNICATION

SUDHEESH.S

7.2

Figure 7.1 Transmission medium and physical layer

7.3

Figure 7.2 Classes of transmission media

7.4

Figure 7.3 Twisted-pair cable

7.5

Figure 7.7 Coaxial cable

Fiber-optic CableMany extremely thin strands of glass or plastic bound

together in a sheathing which transmits signals with light

beams

Can be used for voice, data, and video

Introduction to Optical Fibers. Fibers of glass

Usually 120 micrometers in diameter

Used to carry signals in the form of light over distances up to

50 km.

No repeaters needed.

Fiber v. Copper

Optical fiber transmits light pulses

Can be used for analog or digital transmission

Voice, computer data, video, etc.

Copper wires (or other metals) can carry the same types of

signals with electrical pulses

Optical Fiber & Communications System

FREQUENCIES

Frequency refers to the modulating message signal

Frequency.

The rapid exchange of energy from the beam to the dot

excites the phosphor into the radiating photon of energy

which agitate at 4.2857×10^64 times/sec.

Fiber Optics are cables that are made of optical fibers that

can transmit large amounts of information at the speed of

light.

Glass Fibers

Characteristics

Glass Core

Glass Cladding

Ultra Pure Ultra Transparent Glass

Made Of Silicon Dioxide

Low Attenuation

Popular among industries

Plastic Fibers

Optical Fiber

Core

Glass or plastic with a higher index of refraction than the

cladding

Carries the signal

Cladding

Glass or plastic with a lower index of refraction than the core

Buffer

Protects the fiber from damage and moisture

Jacket

Holds one or more fibers in a cable

Total Internal Reflection

Optical fibers work on the principle of total internal

reflection

With light, the refractive index is listed

The angle of refraction at the interface between two

media is governed by Snell’s law:

n1 sin1 n2 sin2

Reflection

Refraction

When a ray of light crosses from

one material to another, the amount

it bends depends on the difference

in index of refraction between the

two materials

17.1 Index of refraction

The ability of a material to bend rays of light is described by the

index of refraction (n).

Refraction & Total Internal Reflection

Total Internal Reflection

Optical fibers work on the principle of total internal

reflection

The angle of refraction at the interface between two

media is governed by Snell’s law:

n1 sin1 n2 sin2

Numerical Aperture

The numerical aperture of the fiber

is closely related to the critical angle and

is often used in the specification for

optical fiber and the components that

work with it

The numerical aperture is given by the

formula:

The angle of acceptance is twice that

given by the numerical aperture

2

2

2

1.. nnAN

7.23

Figure 7.12 Propagation modes

Figure 7.13 Modes

1. Single-mode fiber

Carries light pulses

along single path.

2. Multimode fiber

Many pulses of light

travel at different

angles

Multi-Mode vs. Single-mode

Singlemode Fiber Singlemode fiber has a core diameter of 8 to 9 microns, which

only allows one light path or mode

Images from arcelect.com (Link Ch 2a)

Index of

refraction

Singlemode FIber

Best for high speeds and long distances

Used by telephone companies and CATV

Multimode Step-Index Fiber Multimode fiber has a core diameter of 50 or 62.5 microns

(sometimes even larger)

Allows several light paths or modes

This causes modal dispersion – some modes take longer to pass

through the fiber than others because they travel a longer distance

See animation at link Ch 2f

Index of

refraction

Step-index Multimode

Large core size, so source power can be efficiently coupled to

the fiber

High attenuation (4-6 dB / km)

Low bandwidth (50 MHz-km)

Used in short, low-speed datalinks

Also useful in high-radiation environments, because it can be

made with pure silica core

Multimode Graded-Index Fiber The index of refraction gradually changes across the core

Modes that travel further also move faster

This reduces modal dispersion so the bandwidth is greatly increased

Index of

refraction

Graded-index Multimode

Useful for “premises networks” like LANs, security systems,

etc.

62.5/125 micron has been most widely used

Works well with LEDs, but cannot be used for Gigabit Ethernet

50/125 micron fiber and VSELS are used for faster networks

7.31

Table 7.3 Fiber types

In multimode step-index fiber, the density of the core remains constant from the

center to the edges. A beam of light moves through this constant density in a straight

line until it reaches the interface of the core and the cladding. At the interface, there is

an abrupt change due to a lower density; this alters the angle of the beam's motion. The

term step index refers to the suddenness of this change, which contributes to the

distortion of the signal as it passes through the fiber.

In multimode graded-index fiber, decreases this distortion of the signal through the

cable. The word index here refers to the index of refraction. As we saw above, the index

of refraction is related to density. A graded-index fiber, therefore, is one with varying

densities. Density is highest at the center of the core and decreases gradually to its

lowest at the edge. Figure 7.13 shows the impact of this variable density on the

propagation of light beams.

Optical Fiber Cable Construction

How are Optical Fibre’s made??

Three Steps are Involved

-Making a Preform Glass Cylinder

-Drawing the Fibre’s from the preform

-Testing the Fibre

Modified Chemical Vapor

Deposition (MCVD)

Fiber and Acrylate Coating

Optical fiber is covered by an acrylate

coating during manufacture

Coating protects the fiber from moisture and

mechanical damage

Advantages of Optical Fibre

Thinner

Less Expensive

Higher Carrying Capacity

Less Signal Degradation& Digital Signals

Light Signals

Non-Flammable

Light Weight

Areas of Application

Telecommunications

Local Area Networks

Cable TV

CCTV

Optical Fiber Sensors

Type of Fibers

Optical fibers come in two types:

Single-mode fibers – used to transmit one signal per fiber

(used in telephone and cable TV). They have small cores(9

microns in diameter) and transmit infra-red light from laser.

Multi-mode fibers – used to transmit many signals per fiber

(used in computer networks). They have larger cores(62.5

microns in diameter) and transmit infra-red light from LED.

Splices and Connectors In fiber-optic systems, the losses from splices and connections can be

more than in the cable itself

Losses result from: Axial or angular misalignment

Air gaps between the fibers

Rough surfaces at the ends of the fibers

How are Optical Fibre’s made??

Three Steps are Involved

-Making a Preform Glass Cylinder

-Drawing the Fibre’s from the preform

-Testing the Fibre

Testing of Optical Fiber

Tensile Strength

Refractive Index Profile

Fiber Geometry

Information Carrying Capacity

Operating temperature/humidity range

Ability to conduct light under water

Attenuation

Optical Fiber Laying

Mechanical Linking Includes coupling of two connectors end to end

Optical distribution frames allow cross connect fibers from by means of connection leads and optical connectors

Soldering: This operation is done with automatic soldering machine that ensures:

Alignment of fiber’s core along the 3 axis

Visual display in real-time of the fibers soldering

Traction test after soldering (50 g to 500 g)

Optical Fiber Laying (Cont…)

Blowing

Used in laying optical cables in roadways.

Cables can be blown in a tube high density Poly Ethylene

Optical fiber is then blown in the tube using an air compressor

which can propel it up to 2 kilometers away.

Tools of Trade Cleaning fluid and rags

Buffer tube cutter

Reagent-grade isopropyl alcohol

Canned air

Tape (masking or scotch)

Coating strip

Microscope or cleaver checker

Splicer

Connector supplies

Fiber Optics Test Kit

Features

Includes Smart FO Power Meter and Mini LED or laser source

FO test lite software for data logging

Tests all networks and cable plants

New versions of Gigabit Ethernet

Low Cost

Applications

Measure optical power or loss

Trouble shooting networks

Protecting Fibers

Tougher than copper wires

Designed in three concentric layers

Core – Cladding – Buffer

Two basic buffer types

Tight buffer

Loose tubes

Implementation of Different LANs

IEEE 802.3

FOIRL

Fiber optic inter repeater link

Defines remote repeaters using fiber optics

Maximum length – 1000 meters between any two repeaters.

IEEE 802.3 (Cont…) 10BASEF Star topology with hub in the center

Passive hub:

Short cables

No cascading

Reliable Active hum:

Synchronous

May be cascaded

Do not count as one repeater

Any 10BASEF active hub must have at least two FOIRL ports

Token Ring

Advantages

Long range

Immunity to EMI/RFI

Reliability

Security

Suitability to outdoor applications

Small size

Compatible with future bandwidth requirements and future LAN standards

Token Ring (Cont…)

Disadvantages

Relatively expensive cable cost and installation cost

Requires specialist knowledge and test equipment

No IEEE 802.5 standard published yet

Relatively small installed base.

Fiber Distributed Data Interface

Stations are connected in a dual ring

Transmission rate is 100 mbps

Total ring length up to 100s of kms.

Conclusion

This concludes our study of Fiber Optics. We have

looked at how they work and how they are made. We have

examined the properties of fibers, and how fibers are

joined together. Although this presentation does not

cover all the aspects of optical fiber work it will have

equipped you knowledge and skills essential to the fiber

optic industry.