Transcript Multiplexing Techniques in Optical Networks: WDM

Multiplexing Techniques
in Optical Networks: WDM
Dr Manoj Kumar
Professor & Head(ECE)
DAVIET, Jalandhar
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)
Sub Carrier Multiplexing
Widely used in CATV distribution
A Closer Look….
Baseband
Data
Baseband-RF
Modulation
RF-Optical
Modulation
Two different Modulations
for each RF Carrier !
Baseband
Data
RF-Baseband
Demodulation
Gain
BPF
1.8 GHz
Transm
itting
End
Single
Mode
Fiber
Optical - RF
Demodulation
200 THz
Receiving
End
Sub Carrier
Multiplexing
Unmodulated
(main) carrier
f2
f2
f1
f1
f0
Frequency
Sub-carriers
• Each modulating RF carrier will look like a subcarrier
• Unmodulated optical signal is the main carrier
• Frequency division multiplexed (FDM) multi
channel systems also called as SCM
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
CATV Distribution
50-88 MHz and 120-550 MHz spectrum is
allocated for CATV
Either AM or FM technique for RF  Optical
conversion
AM: Simple implementation, but SNR > 40 dB
for each channel, high linearity required
FM: The information is frequency modulated
on RF before intensity modulating the laser,
better SNR and less linearity requirement
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.)
Wavelength Division multiplexing
Each wavelength is like a separate channel (fiber)
SONET
TDM Vs WDM
Wavelength Division Multiplexing
• Passive/active devices are needed to
combine, distribute, isolate and amplify
optical power at different wavelengths
Why WDM?
• Capacity upgrade of existing fiber
networks (without adding fibers)
• Transparency: Each optical channel can
carry any transmission format (different
asynchronous bit rates, analog or digital)
• Scalability– Buy and install equipment for
additional demand as needed
• Wavelength routing and switching:
Wavelength is used as another dimension
to time and space
Evolution of the Technology
Review of Modes
Multimode Fiber: There are several electromagnetic modes that are stable within the fiber,
Ex: TE01, TM01
The injected power from the source is distributed
across 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 – making
Wavelength Division Multiplexing possible
Multimode Laser Spectrum
Multimode Lasers
are not suitable
for DWDM systems
(two wide spectrum)
Photo detector Responsivity
Photo detectors are
sensitive over wide
spectrum (600 nm).
Hence, narrow optical
filters needed to
separate channels
before the detection
in DWDM systems
Optical
Amplifiers
are key in
DWDM
systems
WDM, CWDM and DWDM
• WDM technology uses multiple wavelengths
to transmit information over a single fiber
• Coarse WDM (CWDM) has wider channel
spacing (20 nm) – low cost
• Dense WDM (DWDM) has dense channel
spacing (0.8 nm) which allows simultaneous
transmission of 16+ wavelengths – high
capacity
WDM and DWDM
• First WDM networks used just two wavelengths,
1310 nm and 1550 nm
• Today's DWDM systems utilize 16, 32,64,128 or
more wavelengths in the 1550 nm window
• Each of these wavelength provide an
independent channel (Ex: each may transmit 10
Gb/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
ITU-T Standard Transmission DWDM
windows
 c 
   2  
 
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 and
active devices
 c 
   2  
 
Ex. 10.1
Nortel OPTERA 640 System
64 wavelengths each carrying 10 Gb/s
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
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 to
provide long transmission
distances without repeaters
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
Summary
• DWDM plays an important role in high
capacity optical networks
• Theoretically enormous capacity is possible
• Practically wavelength selective (optical
signal processing) components decide it
• Passive signal processing elements like FBG
are attractive
• Optical amplifications is imperative to realize
DWDM networks