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WDM
Piotr Turowicz
Poznan Supercomputing and Networking Center
[email protected]
9-10 October 2006
1
Agenda
Dense Wavelength Division Multiplexing
–
–
–
–
The traditional and emerging challenges
How does DWDM work?
What are the enabling technologies?
The evolution of optical fibres
2
Optical Networking Challenges
Traditional Challenges
Faster
Further
More Wavelengths
3
Optical Networking Challenges
Traditional Challenges
Emerging Challenges
Faster
Access
(FTTN, FTTC, FTTH)
Further
Switching
More Wavelengths
Muxing
4
What is a Wavelength Mux?
Tributaries are sent in their own timeslots
Time
Division
Mux
5
What is a Wavelength Mux?
Tributaries are sent in their own timeslots
Time
Division
Mux
Tributaries are buffered and sent when
capacity is available
Statistic
al Mux
6
What is a Wavelength Mux?
Electrical inputs
Tributaries are sent in their own timeslots
Time
Division
Mux
Tributaries are buffered and sent when
capacity is available
Statistic
al Mux
Tributaries are sent over the same fibre,
but at different wavelengths
Wavelength
Division Mux
Tributaries may arrive on different
fibres, and at "grey" wavelengths
7
Early WDM Deployment
• Two transmission
wavelengths, most
common...
 1310nm
 1550nm
• Coupler used to
combine streams into
the fibre
• Splitter (another coupler)
and filters used to
separate and detect
specific streams
8
Dense WDM
How many
channels?
• Many more than 2 channels!
• Initial ITU Grid allows 32 channels with 100GHz Spacing
• Proprietary systems with up to 160 channels are
currently available as slideware
Be very, very careful regarding manufacturer claims!
(c.f. Never ask a barber if he thinks you need a haircut)
9
Question...
Why don't the streams
on different wavelengths
get "mixed up"?
10
Dense WDM:
ITU Channel Spacing
1565
1560
1555
1550
1545
1540
1535
1530
1525
0.6
Attenuation (dB/km)
0.5
0.4
ITU Channel
Spacing 100GHz
(Currently)
0.3
0.2
0.1
1200
1300
1400
1500
Wavelength (nm)
1600
1700
11
A Basic Answer
• Light is sent into the fibre on a very narrow range of
wavelengths…

A typical DFB laser peak width is ~10MHz (~1pm at 1500nm)
• Different channels are spaced so that they don't "overlap"



In this context, "overlap" implies a power coupling (ie. interference)
between one channel and its neighbours
Typical spacing "rule of thumb"…take the transmission rate in Gbps,
multiply by 2.5, and you have the minimum channel spacing in GHz
(eg. 100GHz at 40Gbps)
Another "rule of thumb": each time you double the transmission rate or
the number of channels, an additional 3dB of transmission budget is
needed
• Need to know the range of available wavelengths in the fibre
12
DWDM Channel Spacing
• Must have enough channel spacing to prevent
interaction at a given transmission rate…



40Gbps 100GHz
10Gbps 25GHz
2.5Gbps 6GHz
• Must test lasers from large batch, ensure temperature
stability, and include margins for component ageing
• Total range of wavelengths must be able to be
consistently and reliably amplified by EDFA

"Accepted" EDFA range is 1530 to 1565 nm (C-band)
• Must be aware of fibre limitations (see later)
13
Why (and Where) DWDM?
• DWDM increases capacity on a given point to point link
Bandwidth is multiplied by factor of 4, 8, 16 etc.
• Typical 1st generation DWDM is deployed in point to
point topologies, over long-haul distances
• In Metro installations, there is an active debate between
mesh and ring-based topologies
• Economics of Metro DWDM are not clear-cut
Often is cheaper to deploy more fibre
These markets are…

Changing rapidly
 Are sensitive to nature of installed fibre
 Are very sensitive to disruptive technologies
…more later!
14
DWDM Enabling Technologies
•
•
•
•
•
•
•
•
•
The notion of "Service Transparency"
Laser sources
Receivers
Tuneable filters
Fibre gratings
Modulation and Modulators
Wavelength couplers and demuxers
Optical amplifiers
Points of flexibility
Optical Cross-Connect (OXC)
Optical Add-Drop Mux (OADM)
15
Service Transparency
• Each Lambda can carry any serial digital service
for which it has an appropriate physical interface




SONET/SDH
Which can be carrying ATM, PoS and other services
ESCON
c.f. SCSI, which is a parallel communication channel
(parallel to serial converters are available for SCSI)
Fast/Gigabit Ethernet
• Each channel can be transmitting at different rates
16
Why Lasers?
• Lasers in general...




High power output (compared to beam diameter)
Narrow transmission spectrum
High spatial quality beam (diffraction limited)
Well-defined polarisation state
• Semiconductor lasers


Small Size
To improve efficiency with fibre coupling
To allow high density port counts
Industrial scale production
Needs lots of them!
17
A Basic Semiconductor Laser
Reflective coating
P
N
Partially
reflective
coating
18
How Do Lasers Work?
"High"
energy
level
Energy
absorbed
(pump)
Electron
Electrons exist in a stable "low" energy
state until we pump in energy to promote
them to a higher state
"Low"
energy
level
"High"
energy
level
Energy
emitted
High energy state is unstable and electron
will soon decay back to the low energy
state, giving out a characteristic level of
energy in the process
"Low"
energy
level
Electron
Characteristic
energy
19
A Laser Cavity
Reflective
Surface
Containment
Layer
Electrodes
Atom in "high" energy state
Photon of characteristic energy
Atom in "low" energy state
Gain Medium
Reflective
Surface
Atom will emit photon and
return to "low" energy state.
The emitted photon has
exactly the right energy to
stimulate emission in the
other high energy atoms
Photons that travel parallel to
sides of resonant cavity are
returned to stimulate further
emissions
20
Tuneable Lasers
What and Why?
• The ability to select the output wavelength of the laser…

The primary sources are fixed wavelength
• What happens if one of these lasers fails?


How many backup lasers would we need?
What is the range of wavelengths over which we need to
operate?
• We could use one tuneable laser to back up all of the
primary sources
21
Tuneable Lasers
What and Why?
• There are three parameters that we
trade-off in a tuneable laser…



Tuning range (goal 35nm)
Power output (goal 10mW)
Settling latency (app. specific)
• Tunable lasers with a "slow" settling
speed can be used in service
restoration applications
-5
-10
-15
-20
Output (dB rel.)
• Tunable laser with a "fast" settling
speed can also be used in next
generation optical switching designs
Module 9831L Tuning Comb; Superimposed Spectra
0
-25
-30
-35
-40
-45
-50
-55
-60
1525
1530
1535
1540
1545
1550
1555
1560
1565
1570
Wavelength (nm)
22
1575
Signal Modulation
• Notion of imposing a digital signal on a carrier wave



Amplitude Modulation
Frequency Modulation
Phase Modulation
• In Optical Communications, typically Amplitude
Modulation

NRZ and RZ encoding
• Directly modulated lasers
• Externally modulated lasers
23
Modulation Schemes
• NRZ: non-return to zero

Most common modulation
scheme for short-mediumlong haul
• RZ: return to zero

Signal
1
1
1
0
Signal
1
1
0
0
0
0
1
Ultra-long haul
0
24
A Traditional Optical Repeater
•High speed electrical components
High
cost, lower reliability
•Single wavelength operation
•Regenerator will make amplifier rate-specific
This system is not Service-Transparent!
25
OEO Amps in a DWDM System
~40km
TX
Amp
RX
TX
Amp
RX
TX
Amp
RX
RX
Amp
TX
RX
Amp
TX
RX
Amp
TX
26
Solution:
Broadband, All-Optical Amplifier
•Single amplifier for multiple wavelengths
•No electrical components
Cheaper, more reliable, not rate-dependent
Gain element
27
The EDFA
What is "Erbium Doped"?
• Fibre is "doped" with the element Erbium

Controlled level of Erbium introduced into silica core
and cladding
Cladding
Core
28
The EDFA
How Does It Work?
• Energy is "pumped" into the fibre
using a pump laser operating at
980nm
• Erbium acts as lasing medium,
energy transferred to signal
• Not specific to wavelength
(operates in the EDFA Window)
• Not specific to transmission rate
29
The EDFA
How Does It Work?
30
The EDFA Window
Region of "flat gain"
5
EDFA Window: 1530-1565nm
Attenuation (dB/km)
4
3
-
-
OH
2
OH
-
OH
1
0
700
800
900
1000
1100
1200
1300
Wavelength (nm)
1400
1500
1600
First window
Second window
Third window
Fourth window
Fifth window
1700
31
CWDM
32
CWDM
Coarse wavelength division multiplexing (CWDM)
is a method of combining multiple signals on laser beams at
various wavelenghts for transmission along fiber optic cables,
such that the number of chanels is fewer than in dense
wavelength division multiplexing (DWDM) but more than in
standard wavelength division multiplexing (WDM).
33
CWDM
CWDM systems have channels at wavelengths spaced 20 nanometers
apart, compared with 0.4 nm spacing for DWDM. This allows the use of
low-cost, uncooled lasers for CWDM.
In a typical CWDM system, laser emissions occur on eight channels at
eight defined wavelengths:
1610 nm, 1590 nm, 1570 nm, 1550 nm, 1530 nm, 1510 nm, 1490 nm, 1470 nm.
But up to 18 different channels are allowed, with wavelengths ranging down
to 1270 nm
34
CWDM
35
CWDM
36
CWDM
System CWDM
Coarse Wavelength Division Multiplexing
37
CWDM
System CWDM
Coarse Wavelength Division Multiplexing
38
The Evolution of Fibre
• Fibre properties



Attenuation
Dispersion
Non-linearlity
• Fibre Evolution




Dispersion-Unshifted Fibre (USF)
Dispersion-Shifted Fibre (DSF)
Non-Zero Dispersion-Shifted Fibre (NZDF)
Emerging fibre types
• Soliton Dispersion Management
39
Optical Fibre Properties
Traditional Challenges
Faster
Fibre Properties



Further


More Wavelengths
Attenuation
Modal Dispersion
Chromatic Dispersion
Polarisation Mode
Dispersion
Non-linearity
» Self-phase modulation
» Cross-phase
modulation
» 4-wave mixing
40
Fibre Optic Properties
Signal Attenuation
5
~190THz
Attenuation (dB/km)
4
1
~50THz
3
-
-
OH
2
2
OH
5
3
4
-
OH
1
0
700
800
900
1000
1100
1200
1300
Wavelength (nm)
1400
1500
1600
First window
Second window
Third window
Fourth window
Fifth window
1700
41
Fibre Optic Properties
Modal Dispersion
• In multimode cable, different
modes travel at different
speeds down the fibre
Result: signal is "smeared"
 Solution: single mode fibre

Signal in
Signal out
42
Fibre Optic Properties
Chromatic Dispersion
Different wavelengths travel at
different speeds down the cable
Result: signal is "smeared"
 Solution: narrow spectrum lasers
 Solution: avoid modulation chirp
 Solution: dispersion management

Signal in
Signal out

43
Fibre Optic Properties
Polarisation Mode Dispersion
Different polarisation components travel at different speeds
down the cable
Result: signal is "smeared"
 Solution: design and installation experience, good test equipment

Pulses start journey in phase
Fast
Slow
PMD delay time
After travelling down
fibre, pulses are now
out of phase
44
Fibre Optic Properties
Non-Linear Effects
• Self Phase Modulation
• Cross Phase Modulation
• 4-Wave Mixing
Effects are "triggered" when
power level of signal exceeds a
certain threshold
45
Self Phase Modulation (SPM)
• Non-linear effect
• Occurs in single and multi
wavelength systems

Spectral
broadening
In DWDM system, SPM will
occur within a single
wavelength



Spectral broadening
Pulse compression
• Solution is positive
dispersion in signal path
Intensity
• Two main effects…
Time
46
Cross-Phase Modulation (XPM)
• Pulses in adjacent WDM
channels exchange power

ie. only happens in multichannel systems
• Primary effect is spectral
broadening
• Combined with high
dispersion, will produce
temporal broadening
• Low levels of positive
dispersion will help prevent
inter-channel coupling

47
Four Wave Mixing
Case 1: Intensity modulation between two primary
channels at beat frequency
Result is two "phantom" wavelengths
fp
2f1-f2 f1
f2 2f2-f1
fq
fr
fF
Case 2: Interaction of three primary frequencies
Result is a "phantom" fourth wavelength
fF = fp + fq - fr
48
Fibre Evolution
1st Generation: USF
20
1310nm
1550nm
10
0.4
0
0.3
-10
Dispersion (ps/nm-km)
Attenuation (dB/km)
0.5
Dispersion
0.2
-20
1300
1400
1500
Wavelength (nm)
Attenuation
1600
USF
49
Fibre Evolution
2nd Generation: DSF
20
1310nm
1550nm
10
0.4
0
0.3
-10
Dispersion (ps/nm-km)
Attenuation (dB/km)
0.5
Dispersion
0.2
-20
1300
1400
1500
Wavelength (nm)
Attenuation
1600
USF
DSF
50
Fibre Evolution
3nd Generation: NZDSF
20
1310nm
1550nm
10
0.4
0
0.3
-10
Dispersion (ps/nm-km)
Attenuation (dB/km)
0.5
Dispersion
0.2
-20
1300
1400
1500
Wavelength (nm)
1600
USF
DSF
NZDF
Attenuation
51
Next Generation Fibres...
• Remove OH- interaction to open 5th window

Example: Lucent "All Wave" Fibre
• Minimise intrinsic PMD during manufacture
PMD is the "2.5Gbps speed bump"
 Example: Corning LEAF
 PMD is very dependent on installation stresses

• Reduce loss at higher wavelengths (>1600nm)
Selctive doping using chalcogenides
(Group VI elements)
 Fibre bend radius becomes significant

52
References
Reichle & De-Massari
http://www.porta-optica.org
53