optical fiber communication (by a k)

7/31/2019 Optical Fiber Communication (by a k)

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OPTICAL FIBER COMMUNICATION

&

Its Applications

By

AMOD KUMAR

Dy.DIRECTOR(Engg.)

STAFF TRAINING INSTITUTE(Technical)

DELHI


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Transmission ofinformation(audio,video,data,text

,graphics or all)

from one point to another point


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Optical Fiber Communication Systemconverts electrical signal into light signal

witch after passing through optical fibercable is reconverted into electrical signalby using optical Receiver.


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1 Two wire trans.line AF to UHFregion Current Carriersareelectrons

2 Co-axial cable AF to UHFregion

CurrentCarriers

areelectrons

3 Optical fiber Opticalfrequencyregion

NO CurrentCarriers butPhotons/light waves


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Wavelength Region


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It lies in the range of 1THz to 10PHz(infrared+visible light+ultraviolet)

It is very high as compared to the radio

frequecies(1MHz to 100MHz) or Micro Wave(1GHz to100GHz)

We know that a large no. of channels& higher BWtransmission is possible with high frequency carrier.

Hence , Optical communication is better it is calledBroad Band Optical Fiber Communication .


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InformationSignal

Point to Point Optical Communication Link

Light Source /Modulator

Receiver /Photodetector

User Display

Transmitter Receiver

OpticalFibre


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It consists of following devices :(1)Optical Transmitter : It consists of light source , modulator & multipexerIt changes electrical signal to optical signal

Light source : LED/Laser DiodeModulator : The information (like speech , music ,digital code etc.) can generally be made available inthe form of electrical signal.

We know that the light intensity in LED or Laser Diodevaries with the applied voltage or current throughthe device.

Hence the applied information signal voltage producesa modulated light signal.


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Light Sources

LED and LASER Diodes are two main sources of light in opticalcommunication system, both are made as p-n junction diodes.These diodes are working under forward biased condition.

LED and LASER Diodes offer many advantages like compactsize, high efficiency, good reliability, right wavelength range,small emissive area compatible with fiber-core dimensions, andpossibility of direct modulation at relatively higher frequencies.

LED is suitable for short-distance and low-bandwidth networks(LAN). The material dispersion is higher due to larger spectral

width and limits the length of fiber link. LEDs are mainly usedalong with multimode fibers

LED has certain disadvantages in comparison to LD like lowintensity, poor beam focus, low-modulation bandwidth, andincoherent radiation.


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LASER Diode (LD) is suitable for long-haul communication linksand used along with single mode fibers. LD radiation properties as

Brightness, Directivity,

Narrow spectral width,

Coherence High Speed (turned off/on)

make them the best light sources for long-haul fiber-optic links.


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(2)Optical Receiver It consists of photo-detector, demodulator &

demultiplexer.It changes light signal back into electrical signal.

Photo detector : PIN diode / Avalanche diode(photo diode)

Detector : The optical signal reaching the receiving endhas to be detected by a detector which converts light intoelectrical signal so that the transmitted information maybe detected.


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Semiconductor Photo DetectorsPhoto detectors are the devices which convert light energy intoelectrical energy

Semiconductors (Si, Ge, GaAs, AlGaAs etc.) are mainly used asphoto detectors in optical communication systems.

PN photodiode, PIN photodiode and Avalanche photodiode (APD)are commonly used photo detectors in optical receives

Photodetector Principle

Particle nature : light exhibit particle nature, i.e. light consists of photons having energy . There is quantum interaction between

photons and electrons.

h E

Absorption: When photons fall on semiconductor, they are absorbed.The absorption of photons excites electrons from valence band toconduction band, resulting in electron-hole pairs generation


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(3) Optical Fiber

A basic optical fiber consists of two concentriclayers.Thinner layer, called core, has arefrective index(n1) higher than the outer

layer,called cladding,has a refrective

index(n2) .

Light injected into the core & striking the core-to-cladding interface at an angle greater thanthe critical angle is reflected back into thecore.


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CladdingCore

Buffer coating (Jacket)

Buffer coating (Jacket)

Refractive index n2

Refractive index n1 [ n1> n2]

A Schematic diagram of single optical fiber str


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Core : material are plastic ,glass, silica ,quartz etc.dia 10 micro metre to 100 micro metrePlastic core has high loss & hence glass cores arepreferred.

Cladding : The core is surrounded by a material likeglass, plastic ,silica etc is called cladding.

Buffer Coating : For providing safety and strength abuffer plastic coating or housing encapsules the core-cladding of the fiberes.


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Fiber Optic Structure


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OPTICAL FIBER PRINCIPLES

Light Principle is used in Optical fiber

i.e. Total Internal Reflection


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Refraction Cont..

Densermedium, n1

Raremedium, n2 2211

sinsin nn

Snells Law of Refraction

1

2

21nn


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Refraction Cont..

Densermedium, n1

Raremedium, n2 2211

sinsin nn

Snells Law of Refraction

1

2

21nn


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Critical Angle

Denser

medium, n1

Raremedium, n2

Critical Angle

1

21

021

sin

90sinsin

n

n

nn

c

c

c

o90

21nn

o

c 901

Condition of TotalInternal Reflection

Critical Angle

o

c

n

n

57.8048.1

46.1sin

46.1

48.1

1

2

1

For silica fiber


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Densermedium, n1

Raremedium, n2

1

21 nn

o

c 901

Condition for TotalInternal Reflection

Total Internal ReflectionAll the light-waves above critical angle will be reflected back in

the same medium. This is called Total Internal Reflection ,


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Cladding

Core

Lightwave is guided through opticalfiber by Total Internal Reflection

Optical Waveguide

Core with refractive index

Cladding with refractive index

Core diameter 2 a

1n

2n

2a


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Types of Optical Fiber

1. Step-Index Fiber

2. Graded-Index Fiber 3. Single-Mode Fiber

4. Multi-Mode Fiber

l f l b


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BUFFER

CLADDINGCORE

8-9 m 125 m 250 m

50 / 125 m, 62.5 / 125 m, 100 / 140 m

50 m,62.5 m,100 m

125 m

140 m250 m

Typical Dimensions of Optical Fiber

Multi-mode Fiber

Single-mode Fiber

Si l M d Fib


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Single Mode Fiber

Non-dispersion-shifted fiber (NDSF), G.652 used near 1310nm, orDWDM use in 1550nm (with dispersion compensators)

Dispersion-shifted fiber (DSF), G.653, used near 1550nm, notsuitable for DWDM due to non-linearity, but for TDM, support10Gbps Ethernet Non-zero dispersion-shifted fiber (NZ-DSF), G.655, at 1550nm,

used in DWDM and TDM


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CladdingCore

Propagation of light through multimode fiber

1 st Mode

2nd Mode

3 rd Mode

Optical fiber can support hundreds of modes dependingon its core diameter, refractive indices of core andcladding and the wavelength of operation


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A ray that is below a certain critical angle escapes from the fiber(Yellow ray)

Propagation of lightwave in graded index fiber

A A l /N i l A


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Acceptance Angle/Numerical Aperture

1

2

n

n

Sin c

c

o

c 90

)90sin(sin co

c

cc cossin

2

1

21cos

n

nc

2

1

21 1sin

n

nc

Only those rays falling within an angle will propagatethrough the fiber.This angle is call acceptance angle

a 2

ca n sinsin22 11

caa

nn sinsin1

1anIf

a NA sin22

21 nn

a 2

c 2

1n

2n

E l f t A l


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Example of acceptance Angle

oa

o

c

n

n

07.28)43.9sin(48.1sin22

43.948.1

46.11sin

46.1

48.1

1

21

2

1

For Silica fiber

For plastic fiber

2425.0sin a NA

5193.0sin a NA

54.622

402.1

495.1

2

1

a

n

n


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OPTICAL CABLES

Loose Tube Type

Center Tube Type


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Optical Cables: Loose Tube type metallic sheath cable

1. Central Strength member2. Water blocking yarn

4. Loose Buffer Tube

5. Plastic film tape (for binding loose tube)6. Water blocking tape

8. Rip cord7. Tensile filament yarn

3. Optical Fiber 9. Jacket (PE=polyethylene)10. Corrugated steel foil laminated

tape


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Optical Cables: Loose Tube type dielectric cable

1. Central Strength member2. Water blocking yarn3. Optical Fiber4. Loose Buffer Tube

5. Plastic film tape (for binding loose tube

6. Water blocking tape7. Rip cord8. Tensile filament yarn

Jacket (PE=polyethylene)

O i l C bl


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Optical Cables: Loose Tube type metallic sheath cable

1. Central Strength member2. Water blocking yarn

4. Loose Buffer Tube

5. Plastic film tape (for binding loose tube)6. Water blocking tape

8. Rip cord7. Tensile filament yarn

3. Optical Fiber 9. Jacket (PE=polyethylene)10. Corrugated steel foil laminated

tape

O i l C bl


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Optical Cables: center tube type metallic sheath cable

1. Optical Fiber2. Center core tube3. Water blocking yarn

4. Rip cord

5. Corrugated metallic armor

6. wire Strength member

7. Jacket (PE=polyethylene)


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O i l C bl 1000 fib l d d bl


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Optical Cables: 1000 fibers slotted rod type cable

1. Center strength member2. Slotted rod

3. Optical Fiber ribbon

4. Plastic film tape

6. Tensile filament yarn

7. Rip cord

8. Jacket (PE=polyethylene)

5. Rip cord


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Cross Section of Optical Cable


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Optical ModulationTechniques


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Why Modulation

A communication link is established by transmissionof information reliably

Optical modulation is embedding the information on

the optical carrier for this purpose The information can be digital (1,0) or analog (a

continuous waveform) The bit error rate (BER) is the performance measure

in digital systems The signal to noise ratio (SNR) is the performance

measure in analog systems


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Types of Optical Modulation

Direct modulation is done by superimposingthe modulating (message) signal on the drivingcurrent

External modulation , is done after the light isgenerated; the laser is driven by a dc currentand the modulation is done after that

separately Both these schemes can be done with either

digital or analog modulating signals


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Direct Modulation

The message signal (ac) is superimposed on thebias current (dc) which modulates the laser

Robust and simple, hence widely used Issues: laser resonance frequency, chirp, turn on

delay, clipping and laser nonlinearity


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Limitations of Direct Modulation

Turn on delay and resonance frequency are the twomajor factors that limit the speed of digital lasermodulation

Saturation and clipping introduces nonlineardistortion with analog modulation (especially in multicarrier systems)

Nonlinear distortions introduce second and thirdorder intermodulation products

Chirp: Laser output wavelength drift with modulatingcurrent is also another issue


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External Optical Modulation

Modulation and light generation are separated Offers much wider bandwidth up to 60 GHz More expensive and complex Used in high end systems


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WHY OPTICAL FIBER COMMUNICATION


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1. Need for ultra-high speed communications

Rapid access to very large databases

High-definition image transmission ( such as X-ray, MRIsand cat scans, for intercity medical tele-conferencing)

3D images for robotics and next generation surveillance andtracking systems.

Computer-computer communications Information superhighway communications

TV cables with a massive number of channels



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Cont..

12105

Low Loss coefficient : An improvement of at least threeorders of magnitude in the dB/km loss coefficient over

coaxial cables. Extremely low loss cable can be used in long-haulcommunications (~0.2 dB/Km at a wavelength of 1.55micrometer)

3. Extremely large bandwidth : ~ Hz at wavelength1.3 micrometer and Hz at 1.55 wavelengthmicrometer

4. Energy confinement : the energy is trapped so effectivelywithin the optical fiber that the fiber channel is virtuallyimmune to the effects of external fields and noise

5. Extremely light weight

121012


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optical fiber communication (by a k)

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