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OPTICAL FIBER CABLE

CONTENTS Brief Introduction History Optical fiber construction Critical angle & Total Internal Reflection Types of fibers/transmission modes Acceptance angle & acceptance cone Numerical Aperture V-Number Dispersion & its types Attenuation and Losses in Fiber Fiber optic communication Fiber Vs Copper Cable Fiber VS Co-axial Cable Disadvantages Applications References

BRIEF INTRODUCTION

Optical fiber is flexible, transparent fiber made of silica or plastic slightly thicker than a human hair

It is a form of guided or wired non conducting medium

Its working is based on principle of Total Internal Reflection

It permits transmission over longer distances and at higher bandwidths than other forms of communication

HISTORY

In 1870, Tyndall introduced concept of Total Internal Reflection with a demonstration.

In the same year, Alexander Graham Bell, developed a optic voice transmission, which he named the photo phone.

HISTORY (contd.)

Later around 1954, Brien, Hopkins & Kapany achieved low loss transmission through 75 cm bundle of thousand fibers

Jun-ichi Nishizawa, was the first to propose the use of optical fibers for communications in 1963

Kao and Hockham were first to reduce attenuation in optical fibers below 20 (dB/km), making it a practical communication medium which earned Kao the Nobel Prize in Physics in 2009.

OPTICAL FIBER CONSTRUCTION

CoreGlass or plastic with a higher index

of refraction than the claddingCarries the signal

CladdingGlass or plastic with a lower index of

refraction than the core Buffer

Protects the fiber from damage and moisture

JacketHolds one or more fibers in a cable

CRITICAL ANGLE & TOTAL INTERNAL REFLECTION

Critical angle is angle of incidence in denser medium for which angle of refraction in rarer medium is 90 degrees.

Total internal reflection-If angle of incidence in denser medium is increased beyond critical angle , then ray of light is reflected back completely into denser medium.

OPTICAL FIBER TRANSMISSION MODES/ TYPES OF FIBERS

TYPES OF FIBERS (contd.) Multimode step-index fiber

the reflective walls of fiber move light pulses to receiver

Multimode graded-index fiber acts to refract light toward center of fiber

by variations in density

Single mode fiber the light is guided down center of an

extremely narrow core

TYPES OF FIBERS (contd.)

SINGLE MODE FIBER

Advantages: Minimum dispersion: all rays take same path,

same time to travel down the cable. A pulse can be reproduced at the receiver very accurately.

Less attenuation, can run over longer distance without repeaters.

Larger bandwidth and higher information rate

Disadvantages: Difficult to couple light in and out of the tiny core Highly directive light source (laser) is required Interfacing modules are more expensive

MULTIMODE FIBER

Multimode step-index Fibers: inexpensive easy to couple light into Fiber result in higher signal distortion lower TX rate

Multimode graded-index Fiber: intermediate between the other two types

of Fibers

ACCEPTANCE ANGLE

Acceptance angle is maximum angle at which a light ray enters into core and propagate through it in zigzag path

Acceptance angle

ACCEPTANCE CONE

If all possible direction of acceptance angle are considered at same time we get a cone corresponding to surface known as acceptance cone

NUMERICAL APERTURE

It defines gathering capability of fiber mathematically expressed as sine of acceptance angle

High Numerical Aperture increases dispersion hence low Numerical Aperture is desirable

V- NUMBER

No. of modes supported by optical fiber is obtained by cut-off condition known as normalized frequency or V-Number

Number of modes (N) = ½ V² V- number can be reduced either by

reducing numerical aperture or by reducing diameter of fiber

DISPERSION & ITS TYPES

Dispersion is the spreading out of a light pulse as it travels through the fiber

It is of two main types: Intermodal or Modal Dispersion Intra modal or Chromatic

Dispersion

INTERMODAL OR MODAL DISPERSION

Spreading of a pulse because different modes (paths) through the fiber take different times

Only happens in multimode fiber

Reduced, but not eliminated, with graded-index fiber

INTRA MODAL OR CHROMATIC DISPESRSION

Different wavelengths travel at different speeds through the fiber

This spreads a pulse in an effect named chromatic dispersion

Chromatic dispersion occurs in both single mode and multimode fiber

It is of two types1) Material Dispersion which is wavelength based effect caused by glass of which fiber is made2) Waveguide Dispersion occurs due to change in speed of wave propagating through waveguide

ATTENUATION

Modern fiber material is very pure, but there is still some attenuation The wavelengths used are chosen to avoid absorption bands

-850 nm, 1300 nm, and 1550 nm-Plastic fiber uses 660 nm LEDs

LOSSES IN FIBER

Absorption Losses- due to material, impurities & atomic defects in glass fiber

Geometric Effects- due to manufacturing defects like irregular diameter of core

Rayleigh Scattering- Change in local refractive index due to

local microscopic variation density It is a scattering loss

FIBER OPTIC COMMUNICATION

InputSignal

Coder orConverter

LightSource

Source-to-FiberInterface

Fiber-to-lightInterface

LightDetector

Amplifier/ShaperDecoder

Output

Fiber-optic Cable

Receiver

TX, RX, and Fiber Link

Transmitter

FIBER OPTIC COMMUNICATION (contd.)

Light source: Amount of light emitted is proportional to the drive

current Two common types:

LED (Light Emitting Diode) ILD (Injection Laser Diode)

Source–to-fiber-coupler (similar to a lens): A mechanical interface to couple the light emitted

by the source into the optical fiber Light detector: PIN (p-type-intrinsic-n-type) APD (avalanche photo diode) Both convert light energy into current.Note- For long links,repeaters are used to

compensate for signal loss

FIBER VS COPPER CABLE

Smaller size & weight Greater capacity Faster communication Transmit over Longer distances Can be used for both analog & digital

transmission Broader Bandwidth – more data per

second

FIBER VS COPPER CABLE (CONTD.)

Immunity to Electromagnetic Interference Low attenuation/transmission loss over

long distances Electrical Insulator Lack of costly metal conductor Dielectric waveguide Signal Security

FIBER VS CO-AXIAL CABLE

More information carrying capacity with higher data rates and fidelity

Greater transmission speed Smaller in size and light in weight Easier to handle and install Immune towards environmental hazards &

electromagnetic interference Higher Bandwidth Economical Low signal loss

DISADVANTAGES Cumulative losses due to large size of

fiber couplers Hazardous emissions like glass shards &

optical radiation Requires technicians with special expertise

for installation & maintenance

APPLICATIONS

Used in Cable T.V. , HDTV, LANs & CCTV systems

Used in Optic Fiber Communication for transmission of analog & digital data

Used in Imaging Optics & Spectroscopy Used in illumination applications Used in various military applications Fiber optic sensors & couplers

REFERENCES

Govind P. Agrawal, “Fiber Optic Communication Systems”, John Wiley, 3rd Edition,2004.

R J Hoss and EA Lacy, Fiber optics 2nd edition (Prentice Hall, New Jersey, 1993)

John M. Senior, “Optical Fiber Communications”, PEARSON, 3rd Edition, 2010.

Gerd Keiser, “Optical Fiber Communications”, TMH, 4th Edition, 2008.

Joseph C. Plais, “Fiber Optic Communication”, Pearson Education, 4th Ed, 2004.

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