Multi Channel Systems

7/29/2019 Multi Channel Systems

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Multiplexing Techniques

in Optical Networks: WDM

Dr Manoj KumarProfessor & Head(ECE)

DAVIET, Jalandhar


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Multiple Access Methods

TDMA Time Division Multiple Access

Done in the electrical domain

SCMA

Sub Carrier Multiple Access FDM done in the electrical domain

CDMA Code Division Multiple Access

Not very popular WDMA Wavelength Division Multiple

Access (The most promising)


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Sub Carrier Multiplexing

Widely used in CATV distribution


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Single

Mode

Fiber

Baseband

DataBaseband-RF

Modulation

RF-Optical

Modulation

Optical - RF

Demodulation

Gain

BPF

200 THz

1.8 GHz

RF-Baseband

DemodulationBaseband

Data

Receiving

End

Transm

itting

End

A Closer Look.

Two different Modulations

for each RF Carrier !


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Each modulating RF carrier will look like a sub-

carrier

Unmodulated optical signal is the main carrier

Frequency division multiplexed (FDM) multi

channel systems also called as SCM

Frequency

Unmodulated (main) carrier

Sub-carriers

f1

f2

f1

f2

f0


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Ability to both analog and digitally

modulated sub-carriers

Each RF carrier may carry voice, data,

HD video or digital audio

They may be modulated on RF carriers

using different techniques

Performance analysis is not

straightforward


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CATV Distribution

50-88 MHz and 120-550 MHz spectrum isallocated for CATV

Either AM or FM technique for RF Optical

conversionAM: Simple implementation, but SNR > 40 dB

for each channel, high linearity required

FM: The information is frequency modulatedon RF before intensity modulating the laser,

better SNR and less linearity requirement


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TDMA Signals are multiplexed in time

This could be done in electrical domain

(TDMA) or optical domain (OTDMA)

Highly time synchronized

transmitter/receiver

Stable and precise clocks Most widely used (SONET, GPON etc.)


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Wavelength Division multiplexing

Each wavelength is like a separate channel (fiber)


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TDM Vs WDM

SONET


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Wavelength Division Multiplexing

Passive/active devices are needed to

combine, distribute, isolate and amplify

optical power at different wavelengths


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Why WDM?

Capacity upgrade of existing fibernetworks (without adding fibers)

Transparency: Each optical channel can

carry any transmission format (differentasynchronous bit rates, analog or digital)

ScalabilityBuy and install equipment foradditional demand as needed

Wavelength routing and switching:Wavelength is used as another dimensionto time and space


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Evolution of the Technology


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Review of Modes

Multimode Fiber: There are several electro-magnetic modes that are stable within the fiber,Ex: TE01, TM01

The injected power from the source is distributedacross all these modes

WDM is not possible with multimode fibers

Single Mode Fiber: Only the fundamental mode will

exist.All the coupled energy will be in this mode. This

mode occupies a very narrow spectrum makingWavelength Division Multiplexing possible


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Multimode Laser Spectrum

Multimode Lasersare not suitable

for DWDM systems

(two wide spectrum)


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OpticalAmplifiers

are key in

DWDM

systems


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WDM, CWDM and DWDM

WDM technology uses multiple wavelengths

to transmit information over a single fiber

Coarse WDM (CWDM) has wider channelspacing (20 nm) low cost

Dense WDM (DWDM) has dense channel

spacing (0.8 nm) which allows simultaneous

transmission of 16+ wavelengths high

capacity


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WDM and DWDM

First WDM networks used just two wavelengths,1310 nm and 1550 nm

Today's DWDM systems utilize 16, 32,64,128 ormore wavelengths in the 1550 nm window

Each of these wavelength provide anindependent channel (Ex: each may transmit 10Gb/s digital or SCMA analog)

The range of standardized channel grids

includes 50, 100, 200 and 1000 GHz spacing Wavelength spacing practically depends on:

laser linewidth

optical filter bandwidth


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ITU-T Standard Transmission DWDM

windows

2

c


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Principles of DWDM

BW of a modulated laser: 10-50 MHz 0.001 nm Typical Guard band: 0.4 1.6 nm

80 nm or 14 THz @1300 nm band

120 nm or 15 THz @ 1550 nm Discrete wavelengths form individual channels that

can be modulated, routed and switched

individually

These operations require variety of passive andactive devices

2

c

Ex. 10.1


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Nortel OPTERA 640 System

64 wavelengths each carrying 10 Gb/s


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Key components for WDM

Passive Optical Components

Wavelength Selective Splitters

Wavelength Selective Couplers

Active Optical Components

Tunable Optical Filter

Tunable Source

Optical amplifier

Add-drop Multiplexer and De-multiplexer


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DWDM Limitations

Theoretically large number of channels

can be packed in a fiber

For physical realization of DWDM

networks we need precise

wavelength selective devices

Optical amplifiers are imperative toprovide long transmission

distances without repeaters


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Types of Fiber

Dispersion Optimized Fiber:

Non-zero dispersion shifted fiber (NZ-DSF) 4

ps/nm/km near 1530-1570nm band

Avoids four-way mixing

Dispersion Compensating Fiber:

Standard fiber has 17 ps/nm/km; DCF has -100

ps/nm/km 100 km of standard fiber followed by 17 km of

DCF zero dispersion


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Summary

DWDM plays an important role in highcapacity optical networks

Theoretically enormous capacity is possible

Practically wavelength selective (opticalsignal processing) components decide it

Passive signal processing elements like FBG

are attractive

Optical amplifications is imperative to realizeDWDM networks

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Multi Channel Systems