By: Dr. N. Ioannides (Feb 2010)CT0004NI - L.06 – Fibre Optic Communications - pp 1/28 Fibre Optic...

By: Dr. N. Ioannides (Feb 2010)

CT0004NI - L.06 – Fibre Optic Communications - pp 1/28

Fibre Optic CommunicationsFibre Optic Communications

Saroj RegmiSaroj Regmi

Lecture 06Lecture 06

CT0004NICT0004NIPrinciples of Comms SystemsPrinciples of Comms Systems



Last Lecture: 05Last Lecture: 05Satellite CommunicationsSatellite Communications

• Introduction to Satellites,

• Components of a Human-Made Satellite,

• Launching a Satellite,

• Orbital Altitudes,

• Satellites in Orbit,

• Satellite Systems, GSO, MEO and LEO Satellites,

• Satellite Payload.



Today’s Lecture: 06Today’s Lecture: 06Fibre Optic CommunicationsFibre Optic Communications

• Introduction to Optical Communications,

• Technological Developments,

• System & Data Link Considerations,

• System Components,

• Optical Fibre Principle of Operation,

• Types of Optical Fibre,

• Optical Fibre Transmission Characteristics,

• Wavelength Division Multiplexing.



• Reflection from sunlight, smoke signals, etc.

• “Flashing light signalling” used by the military to transmit information (Morse Code).

Optical CommunicationsOptical Communications



Technological DevelopmentsTechnological Developments

• The invention of laser during the 1950s,

• The invention of glass optical fibres during the 1960s,

Initially α = 1000 dB/km

1970s α = 20 dB/km

1980s α < 1 dB/km (for single mode glass optical fibres),

1990s α 0.1 dB/km (for single mode glass optical fibres).

Where: α = attenuation



Optical Fibre Communication SystemsOptical Fibre Communication Systems

• Initial installations in the late 1970s to early 1980s,

• Transcontinental and Sub-sea communications,

• In lakes and around continents under the sea.



Fibre Optic Data Link Design ConsiderationsFibre Optic Data Link Design Considerations

• Bandwidth / Data Rate,

• Power Budget:

Source power and detector sensitivity minus losses.

• Component Compatibility based on the wavelength used.



Optical Fibre Communication System ComponentsOptical Fibre Communication System Components

TransmitterTransmitter ReceiverReceiver



Generic Optical Fibre Communication SystemGeneric Optical Fibre Communication System



Active Opto-Electronic ComponentsActive Opto-Electronic Components

• Light Sources used are:

Laser (Light Amplification by Stimulated Emission of Radiation),

LED (Light Emitting Diode).

• Light Detectors:

With internal amplification - Avalanche Photodiodes (APD),

Without internal amplification.



Comparison Between Light SourcesComparison Between Light Sources

ParameterParameter LEDsLEDs LasersLasersEfficiency Lower Higher

Response Time Slower Faster

Data Transmission Rate Lower Higher

Output Spectrum Broader Narrower

Light Beam Not coherent Coherent

Bit Rate Lower Higher

Launch Power Lower Higher

Distortion at Output Higher Lesser

Transmission Distances Shorter Longer

Dispersion Higher Lower

Heating Problems Lower Bigger

Construction Simpler Complicated

Life Time Longer Shorter



Principle of Operation of an Optical FibrePrinciple of Operation of an Optical Fibre

• Based on the laws of refraction and reflection:

n2

n1

NormalIncident light

n1 > n2

cr

cr

1

02 90

Reflectedlight

inc

refl

reflinc

crinc

Refractedlight

12

1

2

Where: n1 and n2 are the refractive indices of the two media,

= critical angle,

= angle of incidence.

cr

inc



Core

CladdingJacket

n1n2

• Core made of glass or plastic,

• Cladding made of glass or plastic,

• Jacket made of plastic elastic material.

• The refractive index of the core (n1) is higher than the refractive index of the cladding (n2) to ensure total internal reflection:

n1 > n2

The Structure of an Optical FibreThe Structure of an Optical Fibre



Core

CladdingJacket

Numerical Aperture

AcceptanceAngle

• Light is injected in the core of the fibre up to a maximum angle, known as the acceptance angle,

• The acceptance angle in within a cone shaped zone, known as Numerical Aperture, NA.

Acceptance Angle & Numerical ApertureAcceptance Angle & Numerical Aperture

Where: n0 = the refractive index of air (n0 ≈ 1),

n1 = the refractive index of the core,

n2 = the refractive index of the cladding.

)(sin 22

210 nnnNA



• Multimode Step Index Optical Fibre:

An optical fibre with refractive index for the core, n1, and the cladding, n2, where n1 > n2.

Uniform refractive index throughout.

It can support many hundreds of modes, i.e. rays of light.

Suffers from dispersion, i.e. broadening of the pulse.

Optical Fibre StructuresOptical Fibre Structures



Optical Fibre Structures (…2)Optical Fibre Structures (…2)

• Multimode Graded Index Optical Fibre:

An optical fibre whose core refractive index decreases as a function of the radial distance from its centre.

The refractive index of the core at its centre is the highest slowing the mode which travels parallel to the optical axis of the fibre.

This fibre causes the modes which have been injected closer to the critical angle of the fibre to travel faster thus resulting in all the modes reaching the end of the fibre at the same time.

Considerable decrease in dispersion effects.



• Singlemode Step Index Optical Fibre:

Step refractive index arrangement but the core is only 50μm.

Only a single ray can propagate thus no dispersion effects.

Optical Fibre Structures (…3)Optical Fibre Structures (…3)



Light Distribution Within the Fibre CoreLight Distribution Within the Fibre Core



Three Common Types of Optical FibreThree Common Types of Optical Fibre

Where:GOF = Glass Optical FibrePOF = Plastic Optical Fibre



Optical Fibre Transmission CharacteristicsOptical Fibre Transmission Characteristics

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

100.050.020.010.0

5.02.01.00.50.20.1

OH radicalabsorptionpeaks

Rayleighscattering

IR absorption tail

Wavelength, µm

Fibre attenuationdB/km

1st window(880 nm)

2nd window(1310 nm) 3rd window

(1550 nm)



Amplifier Amplifier AmplifierFibre Fibre

• Long distance communication is achieved using either electrical or optical amplifiers.

Electrical: Required the optical signal to be converted to electrical to be amplified and then back to optical again for transmission.

Optical: Can amplify the optical signal without needing to convert them to electrical.

AmplificationAmplification



Wavelength Division Multiplexing (WDM)Wavelength Division Multiplexing (WDM)

• A number of channels is used simultaneously to transmit a number of independent information channels over the same fibre.

23 4 5 6

• WDM is a viable system concept.

• Technology available.

• Very good design understanding.

• Systems are deployed continuously.

• Current configurations deliver 1.6 Tbps (160 x 10 Gbps).

• Worldwide research on 40 Gbps per wavelength.

• Future evolution towards 40 Gbps per wavelength and beyond.



• Information Capacity:

Optimum use of fibre,

Avoids “bottle neck” in electronics.

• Operational:

Dynamic routing,

Wavelength routing,

Management and control in the optical layer.

Advantages of WDMAdvantages of WDM

• Linear propagating conditions,

• Sufficient channel separation to prevent cross talk.

Requirements of WDMRequirements of WDM



WDM SystemsWDM Systems

Monitor Points

Dem

ux

2

n

1

n-1

Wavelength Converter

2

n

1

n-1

Mu

ltip

lexer

Wavelength Converter

NT

Netw

ork

Term

inals

NT

NT

NT

NT

NT

NT

NT

• Today, dense wavelength-division multiplexing (DWDM) systems combine 4 to 120 channels using a wavelength multiplexer (MUX).

• A demultiplexer (DEMUX) separates the received channels to individual receivers in network terminals (NTs).



WDM Systems (…2)WDM Systems (…2)

1 1

Multip

lexer

Dem

ul tip

lexer

n

n

Fibre Fibre

Optical Amplifier

Routing Node

• WDM is achieved using multiplexers and demultiplexers.



1565 nm1545 nm

Channels: 16

Spacing: 0.8 nm

DWDM SpectrumDWDM Spectrum



SummarySummary

• Introduction to Optical Communications,

• Technological Developments,

• System & Data Link Considerations,

• System Components,

• Optical Fibre Principle of Operation,

• Types of Optical Fibre,

• Optical Fibre Transmission Characteristics,

• Wavelength Division Multiplexing.

By: Dr. N. Ioannides (Feb 2010)CT0004NI - L.06 – Fibre Optic Communications - pp 1/28 Fibre Optic...

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Transcript of By: Dr. N. Ioannides (Feb 2010)CT0004NI - L.06 – Fibre Optic Communications - pp 1/28 Fibre Optic...