Design of the physical
layer in Metro DWDM
networks
Alessandro Barbieri
abarbier@cisco.com
bock-bock.cisco.com/~abarbier
© 1999, Cisco Systems, Inc.
Confidential
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Agenda
• WDM system overview
•Loss Management:
The problem
The solution
The limitation
•Dispersion Management:
The problem
The solution
The limitation
• The role of PMD and nonlinear effects in Metro Optical
Networks design:
The problem
The solutions
The limitations
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Basic Elements of a WDM system
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Wavelength Division Multiplexing
Systems
Client
Equipment
λ1
λ3
Pump
Pump
λ3
EDFA
Transponder-Based
WDM System
OEO
OEO
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Mux/DeMux
OEO
850nm
1310nm
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Managing Optical Power Loss
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Loss Management: Problem
Fiber Attenuation
S-Band:1460–1530nm
Loss (dB)/km vs. Wavelength
2.0 dB/Km
L-Band:1565–1625nm
OH- Absorption Peaks in
Actual Fiber Attenuation Curve
Rayleigh
Scattering
IR Absorption
0.5 dB/Km
UV Absorption
0.2 dB/Km
800
900
1000
1100
1200
1300
1400
Wavelength in Nanometers (nm)
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1500
1600
C-Band:1530–1565nm
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Loss Management: Solution
Erbium Doped Fiber Amplifier
EXCITED
STATE
Pump Photon
980 or 1480 nm
FUNDAMENTAL STATE
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METASTABLE STATE
SIGNAL PHOTON
1550 nm
Amplified
Signal
1550 nm
FUNDAMENTAL STATE
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Erbium Doped Fiber Amplifier
Isolator
Coupler
Coupler
Isolator
Erbium-Doped
Fiber (10–50m)
Pump
Laser
Pump
Laser
“Simple” device consisting of four parts:
• Erbium-doped fiber
• An optical pump (to invert the population).
• A coupler
• An isolator to cut off backpropagating noise
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Loss Management: Limitations
Erbium Doped Fiber Amplifier
Each EDFA at the Output Cuts at Least in a Half
(3dB) the OSNR Received at the Input
Noise Figure > 3 dB
Typically between 4 and 6
• Each amplifier adds noise, thus the
optical SNR decreases gradually
along the chain; we can have only
have a finite number of amplifiers and
spans and eventually electrical
regeneration will be necessary
• Gain flatness is another key
parameter mainly for long amplifier
chains
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Loss Management: Limitations
EDFA (Cont.)
Constraint:
Ge-αL < 1
San Francisco
= EDFA
= DWDM
Equipment
Phoenix
San Diego
In an ADD/DROP WDM Ring Topology Noise Can Build up (Positive Feedback) Until It Overcomes All
the Signals If the Overall Attenuation Is Not Bigger Than the Gain Provided by the Amplifier Chain
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Loss Management: Solution
Photodetectors
• A photodetector is a device that measure
optical power by converting the energy of
the absorbed photons into electrical
current
• Photodetectors for optical communication
are basically semiconductor diodes (pn
junctions)
• The link budget can be tuned by choosing
the appropriate type of photoreceiver:P-I-N
or Avalanche Photo Diode
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Photodiode Basic Principle
O-E Converter
The Electron-Hole Pair Give Rise
to an Electrical Current
Conduction Band
Electron
ΔE<hc/λ
Absorption
E=hc/λ
Photon
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Hole
Valence Band
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Photodetectors Types
There are two type of photodiodes:
• P-I-N photodiodes: This type employs an
intrinsic (not doped) layer of semiconductor
between the p-doped and n-doped side in order
to extend the usable area to receive photons
• Avalanche Photo Diode (APD): This is a strongly
biased (reverse biasing) pn diode that creates
many electron-hole pairs per each photon
received; an APD amplifies the signal, therefore
it has improved sensitivity (+8/10dBm over a
PIN), but even higher noise and saturates with
less input power than the PIN diode
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PIN Photodiode
Absorptive
p
InP
i
n
InGaAs
InP
Optical
Input
Transparent
VR
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Managing Chromatic Dispersion
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Dispersion Management: Problem
Chromatic Dispersion (CD)
Bit 1
Bit 2
Bit 1
Bit 2
Bit 1
Bit 2
Bit 1
Bit 2
Bit 1
Bit 2
• The optical pulse tend to spread as it propagates
down the fiber generating Inter-Symbol-Interference
(ISI) and therefore limiting either the bit rate or the
maximum achievable distance at a specific bit rate
• Physics behind the effect
The refractive index has a wavelength dependent factor, so
the different frequency-components of the optical pulses are
traveling at different speeds
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Dispersion Management: Problem
Fiber Dispersion Characteristic
Dispersion Coefficient ps/nm-km
Normal Single Mode Fiber
(SMF) >95% of Deployed Plant
17
l
0
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1310 nm
1550nm
Dispersion Shifted Fiber (DSF)
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Dispersion Management: Problem
Increasing the Bit Rate
• Higher Bit Rates experience higher signal
degradation due to Chromatic Dispersion:
1)
Time Slot
2.5Gb/s
Dispersion
Dispersion
16 Times Greater
10Gb/s
OA
OA
Dispersion Scales as (Bit Rate)2
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Dispersion Management: Solution
Direct vs. External Modulation
Direct Modulation
Iin
External Modulation
DC Iin
Electrical
Signal in
Electrical
Signal in
Optical
Signal out
• Laser diode’s bias current is
modulated with signal input to
produce modulated optical
output
• Approach is straightforward and
low cost, but is susceptible to
chirp (spectral broadening) thus
exposing the signal to higher
dispersion
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Mod.
Unmodulated
Optical
Optical Signal
External Signal
Modulator
• The laser diode’s bias current
is stable
• Approach yields low chirp and
better dispersion performance,
but it is a more expensive
approach
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Dispersion Management: Solution
Dispersion Compensation
In the Normal(1) Dispersion Regime
Shorter Wavelengths Travel Slower
(BLUE Is Slower Than RED)
(1)
In the Normal Dispersion Regime the
Dispersion Coefficient Is D > 0
While in the Anomalous Regime It Is D < 0
Note: f = c/l
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Dispersion Management: Solution
Dispersion Compensation (Cont.)
• Dispersion
Compensating Fiber:
By joining fibers with CD
of opposite signs and
suitable lengths an
average dispersion close
to zero can be obtained;
the compensating fiber
can be several kilometers
and the reel can be
inserted at any point in the
link, at the receiver or at
the transmitter
Note: Although the Total Dispersion Is Close to Zero, This Technique
Can Also Be Employed to Manage FWM and CPM Since at Every
Point We Have Dispersion Which Translates in Decoupling the
Different Channels Limiting the Mutual Interaction
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Dispersion Management: Limitation
Chromatic Dispersion
• CD places a limit on the maximum distance a
signal can be transmitted without electrical
regeneration:
For directly modulated (high chirp laser)
LD = 1/ B Dl (1)
D dispersion coefficient (ps/km-nm): 17ps/nm*km
@1.55μm
l source line width or optical bandwidth (nm): 0.5nm
B bit rate (1/T where T is the bit period): 2.5Gb/s
LD ~ 47 km (*)
For externally modulated (very low chirp laser f ~ 1.2B )
LD ~ 1000 km @ 2.5Gb/s (*)
@1.55μm and 17ps/nm*km
LD ~ 61 km @ 10Gb/s (*)
(*) Source: Optical Fiber Communication IIIA, Chap. 7
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The role of Polarization Mode Dispersin
and
Nonlinear effects in WDM systems
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Polarization Mode Dispersion
(PMD)
Ey
nx
Ex
ny
Pulse As It Enters the Fiber
Spreaded Pulse As It Leaves the Fiber
• The optical pulse tends to broaden as it travels down
the fiber; this is a much weaker phenomenon than
chromatic dispersion and it is of some relevance at
bit rates of 10Gb/s or more
• Physics behind the effect
If the core of the fiber lacks a perfect circular symmetry, the two
components (along the x and y axis) of the electric field of the light
pulse travel with different speeds
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Transmission Limitations Due to
Polarization Mode Dispersion
• PMD accumulates with a squared root
dependence on the fiber length:
 = DPMD L
 differential delay between the x and y component of the electric
field
DPMD PMD coefficient
• The distance versus bit rate limit can be
determined using:
B2L ~ 0.02/(DPMD)2 (*)
DPMD typical values between 0.5 and 2 ps/km (**)
If DPMD = 1.4 ps/km and B = 10Gb/s L is limited to 100km
If DPMD = 0.14 ps/km and B = 10Gb/s L is limited to 10000km
(*) Source: Optical Fiber Communication IIIA, Chap. 6
(**) Source: Optical Networks, Chap. 5
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Fiber Nonlinearities
• As long as optical power within an optical
fiber is small, the fiber can be treated as a
linear medium; that is the loss and
refractive index are independent of the
signal power
• When optical power level gets fairly high,
the fiber becomes a nonlinear medium;
that is the loss and refractive index
depend on the optical power
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Cross Phase Modulation
• In WDM systems intensity fluctuation of one channel
can affect the phase of other channels  CPM
induced chirp  dispersion induced distortion
• Chromatic dispersion limit the effect of CPM because
the interfering pulses of different channel tend to
“walk away” from each other limiting the reciprocal
interaction
• To limit CPM distortion, channel power should be
below 10mW for 5 channels and below 1mW for 50
channels(*)
• Decreasing the number of channels reduces CPM
effects
• Increasing the channel spacing reduces CPM effects
• Dispersion management can also be used by
dispersion management techniques
(*) Application
of Nonlinear Fiber Optics, Chap. 7
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Four Wave Mixing
• FWM is the dominant source of crosstalk and
loss in WDM systems; the beating among
many channels generates new tones as
sidebands; in the worst case of equally
spaced channels most new frequencies
coincide with existing channels and
generates interference; in the best case the
WDM channels experience just a power
depletion l l l
1
f113
f213
f123
f112
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f223
2
f312
f132
3
f221
f231
f321
f332
fijk - fi = fj - fk (i,j <> k)
f331
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FWM Performance Impact
• Like CPM in the presence of dispersion
FWM is less efficient because of the “walk
away” effect of different channels
• Using Dispersion Shifted Fiber greatly
enhances the FWM process
• Reducing the channel count and the
channel spacing also reduces FWM
penalties
• Adopting an unequal channel spacing
limits FWM cross-talk but the channel
power depletion is still present
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Managing CPM and FWM:
Non-Zero Dispersion Shifted Fiber
Dispersion ps/nm-km
Normal Single Mode Fiber
(SMF) >95% of Deployed Plant
~+3ps/nmkm
20
~-3ps/nmkm
Wavelength
l
0
1310 nm
1550nm
Dispersion Shifted Fiber (DSF)
Nonzero Dispersion Shifted Fibers (NZDSF)
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NZDSF Flavors:
Lucent TrueWave vs. Corning LEAF
• TrueWave fibers: Small amount of
chromatic dispersion throughout the
EDFA band (~1550nm); this dispersion
prevents phase matching among the
various signals reducing CPM and FWM
• Corning LEAF: Similar to TrueWave as far
as dispersion goes; however it has a large
effective area design that reduced the light
intensity and therefore all the nonlinear
effects
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Appendix
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Definitions:
refractive index & propagation
constant
• Relationship between frequency and wavelength in
electromagnetic radiations: c =x l
•The refractive index (n) of a material is the ratio of the
speed of light in the vacuum to the speed of light in that
material: n=c/v.
• The propagation speed of an electromagnetic radiation
depends on the refractive index and the wavelength and it
is determined by the propagation constant: β=2πn/λ
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The physics behind Chromatic
Dispersion
• An optical pulse S is composed of a series of
monochromatic waves:
S(l)=cos(2πc λ1t - β1z) + cos(2πc λ2t - β2z) + …
Where β1 != β2!= …  the different waves
composing the pulse propagates at different
speed
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Calculating Transponders CD
distance limitation
• Transponder Dispersion Tolerance (TDT) is
usually expressed in: [ps/nm]
• The dispersion coefficient D is expressed in
[ps/nm*km]
•To calculate the distance:
LMAX = TDT/D [ps/nm * nm*km/ps = km]
Eg. ONS 15540 has 1800ps/nm of TDT
on regular SMF D = 18 ps/nm*km
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LMAX = 1800/ 18 = 100km
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