Optical_fibers.ppt

7/27/2019 Optical_fibers.ppt

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FIBER OPTICS- LINES (CABLES)


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Introduction

An optical fibre is a glass or plastic fibredesigned to guide light along its length by

total internal reflection

Advantages Large Bandwidth BW f

BW at optical frequencies >

BW at Microwave freq

Low Loss

Good signal to noise ratio


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Comparison

Compared to other transmission media fiber

optics have infinite bandwidth ( more than

25 THz)

For radio transmission the useful band is

100 GHz

For coaxial cable the bandwidth is 800 to

1000MHz

For a pair of wires around 200-300 MHz.


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Comparison with other media


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Satellite Fibre Optics

Point to Multi-point Point to point

BW ~ GHz BW ~ THz

Maintenance free Needs Maintenance

Short life ~7-8 Yr Long life

No upgradeability Upgradeable

Mobile, air, sea On ground only


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Advantages of Fiber Optic Cable Over Copper

Speed: Operate at high speeds - up into the gigabits Bandwidth: Large carrying capacity

Distance: Signals can be transmitted further without

needing to be "refreshed" or strengthened.

Resistance: Greater resistance to electromagnetic

noise such as radios, motors or other nearby cables.

Attenuation: Low attenuation loss over long

distances. Better Signal security and no cross talk Maintenance: Fiber optic cables costs much less to

maintain

Light weight and small diameter cables


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Structure

Hair-thin fibers consist of two concentriclayers of high-purity silica glass the core and

the cladding, which are enclosed by a

protective sheath.

The light stays confined to the core because

the cladding has a lower refractive index

refractive indexa measure of its ability tobend light.


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Core: - It is made of highly purified glass. Most of

the light energy in confined to the core.

Cladding: - It is a concentric glass shell surrounding

the core. The cladding shields optical fields so as

not to get interfered by the outer layers of the

fibre. Buffer coating: - The cladding is surrounded by the

buffer layers. These layers have no role in

propagation of light. They are essentially there to

provide the mechanical support to the glass fibre

and to protect the fibre from external damage


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Refractive Index in Optical Fibers

The refractive index of a medium is a measure

for how much the speed of light (or otherwaves such as sound waves) is reduced insidethe medium.

Light rays change direction when they crossthe interface from air to the material .

The larger the angle to the normal, thesmaller is the fraction of light transmitted,until the angle when total internal reflectionoccurs.


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The refractive index of a medium is the ratio of

the phase velocity c of a wave phenomenon such

as light or sound in a reference medium to thephase velocity vp in the medium itself

=/

Total Internal Reflection


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If the angle of incidence, 1 is greater than

the critical angle, c given by

where n1 and n2 are the refractive indices ofthe two media, then the light is Total

Internally Reflected in medium 1.

There is no refracted ray in that case.


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Numerical Aperture (NA)of a Fiber Optic

NA, sin mmeasure of the power launched efficiently into

an optical fibre.

for good light launching efficiency, m shouldbe as large as possible and n12should be large

compared to n22

N.A = sin m=(

)

n0If the medium outside the fibre is air, n0 = 1


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Acceptance Angle (AA)

The cone of acceptance of light into the core Light rays approaching the fibre within this cone will

undergo total internal reflection at the core-cladding

interfaces, and will be trapped within the fibre.

Any rays which arrive at larger angles of incidence

will not be trapped properly, but will refract away

into the cladding and be lost.

AA= m = sin-1

N.A= sin -1 (

)

n0


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Function of the Fiber Cable System.

Fiber optic cable functions as a "light guide," guidingthe light introduced at one end of the cable through

to the other end.

The light source can either be a light-emitting diode

(LED) or a laser

Total internal reflection confines light within the

optical fiber, because the cladding has a lower

refractive index

light rays reflect back into the core if they encounter

the cladding at a shallow angle. A ray that exceeds a

certain "critical" angle escapes from the fiber


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The transmitter is the place of origin for

information coming on to fiber-optic lines. The transmitter accepts coded electronic pulse

information coming from copper wire.

It then processes and translates that information

into equivalently coded light pulses. A light-emitting diode (LED) or an injection-laser

diode (ILD) can be used for generating the lightpulses.

Using a lens, the light pulses are funneled intothe fiber-optic medium where they transmitthemselves down the line.


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Light pulses move easily down the fiber-optic

line because of a principle of total internalreflection.

The light source is pulsed on and off, and a

light-sensitive receiver on the other end of thecable converts the pulses back into the digitalones and zeros of the original signal.

Light strengtheners, called repeaters, may benecessary to refresh the signal in certainapplications.


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The light source is pulsed on and off, and a light-

sensitive receiver on the other end of the cable

converts the pulses back into the digital ones and

zeros of the original signal.

Light strengtheners, called repeaters, may benecessary to refresh the signal in certain

applications.


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Problems

1. A silica optical fibre has a core refractive

index of 1.5 and a cladding refractive index of

1.47. Calculate the Numerical Aperture of the

fibre and the critical angle at the core

cladding interface

2. An optical fibre has a numerical aperture of

0.15 and a cladding refractive index of 1.55.

Determine the critical Angle and acceptanceangle of the fibre in water whose refractive

index is 1.33


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Types of Fibers

Single mode Step Index Fibre

Multimode Step Index Fibre

Multimode Graded Index Fibre


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Single-mode optical fiber

the lowest order bound mode only can

propagate at the wavelength of interest

typically 1300 to 1320nm.

It is a single strand (most applications use 2

fibers) of glass fiber with a diameter of 8.3 to10 microns that has one mode of

transmission.

It carries higher bandwidth than multimodefiber.


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The amount of the electromagnetic spectrum

that a laser beam covers is called as spectral

width .

Single mode fiber requires a light source with

a narrow spectral width.

gives a higher transmission rate and more

distance than multimode

more costs.


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Single-mode fiber.

Single Mode Fiber has a relatively narrow

diameter and much smaller core thanmultimode.

eliminate any distortion that could result from

overlapping light pulses provide the least signal attenuation and the

highest transmission speeds of any fiber cable

type. Used in broadband ISDN communication


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Single-mode fiber has a narrow core (eight

microns or less)

the index of refraction between the core and

the cladding changes less than it does for

multimode fibers.

Light thus travels parallel to the axis, creating

little pulse dispersion.


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Multi-mode cable

A little bit bigger diameter, with a common diameters in

the 50-to-100 micron range for the light carry component Multimode fiber gives high bandwidth at high speeds (10

to 100 MBS - Gigabit to 275 m to 2 km) over medium

distances.

Light waves are dispersed into numerous paths, or

modes, as they travel through the cable's core typically

850 or 1300 m

In long cable runs (greater than 3000 feet [914.4

meters), multiple paths of light can cause signal

distortion at the receiving end, resulting in an unclear

and incomplete data


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Step-Index Multimode Fiber

This has a large core, up to 100 microns in diameter.

some of the light rays that make up the digital pulse

may travel a direct route, whereas others zigzag as

they bounce off the cladding. These alternative pathways cause the different

groupings of light rays, referred to as modes, to

arrive separately at a receiving point.


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The pulse, an aggregate of different modes,

begins to spread out, losing its well-definedshape.

The need to leave more spacing between

pulses to prevent overlapping This limits bandwidth that is, the amount of

information that can be sent.

best suited for transmission over shortdistances.


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Graded-Index Multimode Fiber

This contains a core in which the refractive indexdiminishes gradually from the center axis out toward

the cladding.

The higher refractive index at the center makes the

light rays moving down the axis advance more slowly

than those near the cladding.

Also, rather than zigzagging off the cladding, light in

the periphery curves helically because of the gradedindex, reducing its travel distance.


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The shortened path and the higher speed

allow light at the periphery to arrive at a

receiver at about the same time as the slow

but straight rays in the core axis.

The result: a digital pulse suffers less

dispersion.


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Wavelength Division Multiplexing

Multiplexes multiple optical carrier signals on a single

optical fiber by using different wavelengths (colours)

of laser light to carry different signals.

This allows for a multiplication in capacity, in

addition to enabling bidirectional communications

over one strand of fiber


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WDM-applies to an optical carrier (which istypically described by its wavelength),

Frequency-Division Multiplexing (FDM)-

applies to a radio carrier (which is more oftendescribed by frequency).

since wavelength and frequency are inversely

proportional, and since radio and light areboth forms of electromagnetic radiation, the

two terms are equivalent.


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WDM (Wavelength Division Multiplexing): several

baseband-modulated channels are transmitted alonga single fiber but with each channel located at a

different wavelength

The WDM channels are separated in wavelength toavoid cross-talk when they are (de)multiplexed by a

non-ideal optical fiber.

The wavelengths can be individually routed through

a network or individually recovered by wavelength-selective components.


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Wavelength Division Multiplexing


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Advantages

Transmission speed and bandwidth capacityincreases with the number of wavelengths.

Can be used for longer distances

Different wavelengths can carry data at different bit

rates. Signals arrive at the destination at the same time

and not in time slots as in TDM.


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Disadvantages

Designing optical amplifiers for WDM systemsis much difficult.

Separate terminating equipment for each

wavelength. Cannot monitor the bit error rates or frame

errors in the data.


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Time-Division Multiplexing (TDM)

Each lower-speed channel transmitting a bit (or

allocation of bits known as a packet) in a given a time

slot and waiting its turn to transmit another bit (or

packet) after all the other channels have had their

opportunity to transmit

It is limited by the speed of the time-multiplexing

andde multiplexing components.


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Comparison between WDM and TDM


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Additional method is the Code-Division Multiplexing

(CDM) Instead of each channel occupying a given

wavelength, frequency or time slot, each channel

transmits its bits as a coded channel-specific

sequence of pulses.

This coded transmission typically is accomplished by

transmitting a unique time-dependent series of short

pulses. These short pulses are placed within chip(fragment) times within the larger bit time.


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Solitons

The term "soliton" suggests, these solitary waves

behave like "particles".

any optical field that does not change during

propagation because of a delicate balance between

nonlinear and linear effects in the medium. A soliton is non dispersive pulse that makes use of

nonlinear dispersion properties in a fiber to cancel

out chromatic dispersion effects.

When they are located mutually far apart, each is

approximately a travelling wave with constant shape

and velocity.

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Optical_fibers.ppt