Multi Channel Systems
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Transcript of 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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Sub Carrier Multiplexing
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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Sub Carrier Multiplexing
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