Outline

1
Outline
1. Introduction to Reconfigurable
Networks 2. Degrading Effects in
Systems 3. Optical Amplifiers 4. Dispersion
Compensation 5. Polarization Mode
Dispersion 6. Modulation Formats 7. Performance
Monitoring 8. Optical Switching
2
Polarization Mode Dispersion Outline

• Characteristics of PMD
• PMD effects on different transmission formats
• PMD Emulation
• PMD Measurement
• PMD Compensation (Electrical and Optical )
• Combined effects of PMD and PDL
• PMD Induced RF Power Fading

3
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4
Physical Origin of PMD
5
Polarization Mode Dispersion (PMD)

• Caused by slight fiber asymmetry
• Different polarizations propagate at different
  speeds
• Critical limitation at high data rates (OC-192
  and above)

6
Characteristics of PMD in Real Fiber
Evolution of polarization through the fiber
Circular polarization
Linear polarization
Elliptical polarization
Input
Output
Short Fiber
Short Fiber
Short Fiber
SOP
SOP

• PMD is a random process
• The probability density function of DGD is
  Maxwellian.

7
Differential Group Delay (DGD)
Vary input SOP of a given
PSP
PSP
1
1
l
signal into two PSPs
t
t
Polarization
dependent
delay
element
t
t
Dt
PSP
PSP
2
2
Dt
Differential Group Delay
8
Higher Orders of PMD
Poincaré Sphere
DGD and PSPs are frequency dependent
Poincaré Sphere
Higher orders must be considered when data bits
become comparable to the DGD.
C.D. Poole et al, PTL, 1991
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10
Characteristics of PMD
BER fluctuations due to changes in temperature
C.D. Poole et al, PTL, 1991
3
, 68, 1991
11
Maxwellian Distribution
System degradation occurs due to the non-zero
tail of the distribution.
12
PSP changes with frequency
S1,S2,S3 are the Coordinates of PSP on Poincare
Sphere
13
Maxwellian distribution of DGD
Mechanical,
Birefringence
Statistical
Randomly
thermal
adds

treatment
varing
vibration or
stochastically
birefringence
of PMD
Wavelength drift
(i.e. randomly)
35 km long installed fiber
Corresponding distribution
PM delays
of the delays
8
0.4
Maxwellian fit
6
0.2
4
Frequency
PM delay ps
2
0
0
1280
1300
1320
0
2
4
6
8
Wavelength nm
PM delay ps
1
Dt

Dt

PMD lt
gt
in long fiber lt
gt
L
N. Gisin, PTL, 1993
14
Statistical Distribution of Q-factor
10 Gb/s NRZ 40 ps total PMD
Probability Density
Region of System Failure
Q-factor (dB)

• PMD statistics result in Q-factor distribution
• The tail of the distribution is the limiting
  factor

15
PMD Effect on NRZ Transmission
10 Gbit/s
Input
Normalized Eye Opening
1
00

200

Transmission Length (km)
1/2
PMD 10 ps/(km)
High PMD limits NRZ
transmission
16
PMD Emulation
An accurate PMD Emulator is needed for RD
because

• The High PMD fiber (1-100 ps/km1/2) was
  installed in 1980s but present fibers have less
  than 0.5 ps/km1/2.
• High PMD fiber is extremely difficult to acquire.

• PMD is a random process, and an emulator can be
  used to probe all system possibilities in a short
  time.

17
The Need for PMD Emulators (I)
Opinion PMD emulators are even more important
for systems that will not deploy PMD compensators.
18
The Need for PMD Emulators (II)
DGD of Old and New Fibers

• PMD state of fiber changes slowly and
  uncontrollably.
• System may perform well for 364 days of the year
  and fail on day 365.
• Tail of distribution causes system outages.
• The fiber installed in the 80s and early 90s has
  high PMD.
• Today, high-PMD fiber is extremely difficult to
  acquire for laboratory testing.

DGD (ps/km1/2)
DGD at 1000 km
DGD at 2500 km
Years
1980s
gt1
32 ps
50 ps
1990s
0.1 - 1
16 ps
25 ps
Today
lt0.1
3.2 ps
5 ps
1 dB penalty at 10 ps for 10 Gb/s 2.5 ps for
40 Gb/s
19
Requirements for PMD Emulators

• Performance characteristics of a good emulator
• Maxwellian distribution of DGD (??).
• Accurate higher-order PMD statistics.
• Quadratic falloff of the frequency
  autocorrelation function outside a limited
  frequency range

• Design goals for a practical emulator
• Stable over the measurement period
• Repeatable and predictable
• Simple and efficient
• Low loss and low PDL

20
PMD Emulator Concept
PMD can be modeled as randomly coupled sections
of birefringent fiber.
Two main ways to construct an adjustable PMD
emulator
Fixed birefringence, Variable polarization
coupling
??3
??4
??5
??6
??
??
??
Variable birefringence, Fixed orientations
??3
??4
??5
45?
45?
45?
?? polarization coupling between sections ??
DGD of each section ?? birefringence of each
section
21
Overview of Emulator Types
22
1st-Order PMD Emulators (DGD only)
Basic variable DGD element

• Easy to construct
• Unstable polarization
• output if fiber involved
• Slow speed

Fast programmable DGD module using polarization
switches
Digital tune binary Fast lt ms
L.-S. Yan et al, OFC, 2002
23
3-section PMD Emulator with Tunable Statistics

• Applied Maxwellian DGD distribution to each
  section
• Uniform polarization coupling between each
  section
• The output average PMD can be tuned by changing
• the average of each section

J.H. Lee et al, OFC, 2002 L.-S. Yan et al., OFC,
2003
24
Experimental Results of Tunable PMD Statistics
Generation
Second-order PMD 30 lower than real fiber
more sections required
First-order PMD match well with real fiber
25
Three-Section Emulator ExampleAutocorrelation
Function
Simulation
Simulation
10 for 15 sections
3 sections
15 sections

• In WDM systems over real fiber, the PMD of
  different channels is statistically independent.
• The correlation between PMD vectors for channel
  spacing gt0.15 nm
• 49 for 3 sections (too high not enough
  sections)
• 10 for 15 sections (good)

J. H. Lee et al, OFC, 2002
26
Emulators using Polarization Controllers or
Rotators - 3 sections
Three section emulators
with polarization scramblers
with polarization rotators
Three sections is not adequate
R. Khosravani et al, PTL, 2001
27
Emulators using Polarization Controllers or
Rotators - 15 sections
Fifteen section emulators
with polarization scramblers
with polarization rotators
Fifteen sections is adequate to obtain a good DGD
pdf
R. Khosravani et al, PTL, 2001
28
Emulator using Rotators Autocorrelation Function
Autocorrelation Functionusing polarization
rotators
Autocorrelation lt 10 for wavelength spacing gt
0.2 nm for the 15-section emulator.
R. Khosravani et al, PTL, 2001
29
Emulator using Rotators 2nd-order PMD
2nd-order PMDusing polarization rotators
Normalized magnitude of the 2nd-order PMD
pdf
15-sections is adequate to provide accurate
2nd-order PMD.
R. Khosravani et al, PTL, 2001
30
Recirulation Fiber Loop Transmission
Real Optical Link
2000 km Transmission
Polarization evolves randomly through entire
link.
Loop Transmission
200 km Recirculating Fiber Loop
10 Loops 2000 km
Odd Periodic Polarization Effects Not Really
Random
How do we solve this?
31
PMD Loop Experimental Setup
PMD Loop with Inter-Loop Polarization Decorrelator
PMD Loop without Inter-Loop Polarization
Decorrelator
Att.
Att.
AO Mod.
AO Mod.
Loop Control
Loop Control
OF
OF
AO Mod.
AO Mod.
Att.
Att.
EDFA
EDFA
EDFA
Computer
PM Fiber
5 section PMD emulator
EDFA
40 km SMF
84 km SMF
PC
DCF
LiNbO3 Pol. Scrambler
PC
DCF
PC
20 km SMF
21 km SMF
EDFA
EDFA
PMF Polarization Maintaining Fiber
S. Lee et al, OFC, 2001
32
DGD Distributions of PMD Loop
8 Loops with Inter-Loop Pol. Decorrelation and
1 section PMF inside
w/o Inter-Loop Pol. Decorrelation
6 Loops with 1 section PMF inside
8 Loops with 15-section PMD Emulator
Incorrect
Better
Solved
Maxwellian
Maxwellian
Maxwellian
0
20
40
60
80
120
0
40
80
0
20
40
60
80
DGD (ps)
DGD (ps)
DGD (ps)
S. Lee et al, OFC, 2001
33
Average DGD vs. of Loops
S. Lee et al, OFC, 2001
34
IMPORTANT SAMPLING

• PMD effects are stochastic in nature
• Penalties depend in a complicated way upon the
  random fluctuations
• PMD induced power penalties are rare (one minute
  per year)

Importance sampling biases the events towards the
rare events
Important sampling enables reaching very low
probability events with less number of simulation
or experimental samples
W. Kath et al, OFC, 2001
35
Experimental Importance Sampling Using 3-section
Emulator
PMD Emulator
Output Distribution
Maxwellian Distribution
PC1
PC2
?1
?2
?3
pdf
pdf
DGD
DGD
Apply weights to measured samples
Apply Biased DGD Distributions
Biased DGD Distribution generate more output
samples in the region of interest
L.-S. Yan et al., OFC, 2003
36
Experimental Results with Uniform Biased
Distribution
Output pdf
Renormalized pdf
The tail of distribution extends to 10-24 with
only 1000 samples
L.-S. Yan et al., OFC, 2003
37
PMD Compensation Requirements
DGD and PSP determination for compensation (
PMD Monitoring)
Tunable compensation
Environment changes for point-to-point link
Reconfigurable and dynamic network
Speed of compensation
Environment change
Slow temperature change min
Abrupt stress in fiber 10s ms
Reconfigurable network ms
38

• Compensation schemes
• Electrical compensation
• Optical compensation
• First order compensation
• Higher order compensation
• Using variable DGD
• Using Forward Error Correcting Codes
  
• Simultaneous PMD Compensation in
  WDM Systems

39
Electrical PMD Equalizer
Electronic equalization (transversal filter
architecture)
Tc
Tc
PD
C1
CN
C0
?
CDR

• Electrical compensation may prove too cheap to
  beat.
• When you find yourself fighting Si, dont. A.
  Penzias

PMD monitoring and emulation still are key issues
even for E-Solution!
40
Feed Forward Decision Feedback Equalizer (FFE
DFE)
FFE (Feed Forward Equalizer)
with equalization (FFE only)
Optical Noise
Power penalty (dB)
Thermal Noise
0
40
60
20
80
Decision Feedback Equalizer
DGD (ps)
H. Bülow, invited, OFC, 2002
41
Electrical Compensation Experimental Results
w/o equalization
4 dB
DFE
4.3 dB
FFE
6.1 dB
FFEDFE
Feed Forward equalization (FFE) 4 dB improvement
with 8 taps Decision Feedback Equalizer (DFE)
4.3 dB improvement FFEDFE 6.1 dB improvement
H. Bülow, Electron. Lett., 2000
42
Performance of Electrical PMD Mitigation

• Cheaper than optical solutions because it can
  overcome all pulse distortions together without
  additional cost.
• Can not achieve zero penalty and exhibits a
  residual penalty worst case penalty of about 4dB.
  
• In long haul transmission where a PMD margin of
  1 or 2 dB can be allocated, it can only increase
  the maximum tolerable PMD.
• Electronic signal processing is possible at up
  to 10Gbits/s right now and not at 40Gbits/s.

43
Generic optical PMD Compensation
Polarization Controller
Tap
DGD Element
Feedback
Det.
PMD Compensator
? One section (Variable or Fixed) 1st order
(DGD) ? Multi-section 1st and higher order
44
Optical PMD compensation

First Order Compensation
Higher Order
Compensation

Monitoring
Control
Monitoring
Control
PC
PM Fiber
PD
PC
PC
PC
PD
PM Fiber
PM Polarization maintaining
PMD equalizing optical circuit
Takeshi Ozeki, TuN4, OFC 94
T. Takahashi, Elec. Lett.,
30
, 348, 1994
Advantage Simple

Advantage Higher Order Compensation
Disadvantage

First Order Compensation
Disadvantage Very Complex
45
Significance of Higher-order PMD (I)
It is sometimes stated that once the signal
bandwidth is large enough for 2nd-order PMD to be
important, then all higher-order terms become
important too
-H. Kogelnik, et al., JLT 2003
Autocorrelation Function of PMD Vector
PSP Bandwidth
DwPSP
Higher-orders important if signal BW gt DwPSP
ACF (ps2)
Theory
Measurement
M. Karlsson, et al., Optics Letters, 1999
BUT
46
Significance of Higher-order PMD (II)
If this were strictly true, then higher order
PMD compensation would be a hopeless taskthere
is a need for closer examination of these
bandwidth limitations.
-H. Kogelnik, et al., JLT 2003
Relative rms Magnitude of Terms in Taylor
Expansion of PMD Vector
2nd order
3rd order
4th order
5th order
Order
DGD
BW
Dw (0.5) DwPSP
1
.25
.06
.014
.003
Dw DwPSP
1
.045
.11
.24
.49
Dw (2) DwPSP
1
.98
.72
.87
.97
(H. Kogelnik, et al., JLT 2003)
47
Effect of Frequency Dependence of PMD

• Second order approximation vastly overestimates
  the actual penalties
• Considering frequency dependence of PMD in terms
  of discrete orders is not entirely valid

M. Shtaif and M. Boroditsky, PTL, Oct 2003
48
Variable vs. Fixed DGD for PMD Compensation

• Typical Optical PMD compensator

PC
D
Fixed
t
Feedback Control

• PMD Compensator with variable ?t

PC
D
Variable
t
Feedback Control
Whats the advantage of using a variable DGD(?t)?

49
Upper-Bound Compensator Performance
-
2
1
0
Fixed Dt (50 ps)
E
x
a
c
t

1
s
t
-
o
r
d
e
r
-
3
1
0
c
o
m
p
e
n
s
a
t
i
o
n

x
7
Outage Probability

Variable Dt

-
4
1
0
-
5
1
0
3
0
3
5
4
0
4
5
A
v
e
r
a
g
e

P
M
D

(
p
s
)
Q. Yu et al, OFC, 2001
50
System Penalty
6
5
4

3
Q-factor Penalty (dB)

w
/
o

c
o
m
p
.

f
i
x
e
d

4
0

p
s
2

f
i
x
e
d

5
0

p
s

f
i
x
e
d

6
0

p
s
1

v
a
r
.

D
G
D
0
1
0
2
0
3
0
4
0
5
0
A
v
e
r
a
g
e

P
M
D

(
p
s
)
Variable compensator is always the optimal
solution.
Q. Yu et al, OFC, 2001
51
Local Minimum Problem
Variable compensator reduces the risk of the
feedback loop trapping a local minimum, not a
true global minimum.
52
Forward-Error-Correction (FEC) Coding
Errors
Data
Encoder
Transmission
Decoder
FEC Overhead
Recovered Data
FEC has been shown to be effective in correcting
errors due to optical noise, fiber chromatic
dispersion and nonlinearity
53
Using FEC Coding for High PMD Systems
10.7Gbit/s transmission, RS(255,239) code
FEC method can correct a long term BER of 10-6
to near error free for PMD degradation
ltDtgt43ps
Without FEC
ltDtgt38ps
ltDtgt33ps
Log(out put average BER)
But FEC method cant reach the code capability
for Poisson errors because due to DGD
fluctuations, error distribution is not Poisson.
Static DGD
Fluctuating DGD
Poisson error correcting
capability of RS(255,239)
Log(Input average BER)
M. Tomizawa et al, ECOC, 2000
54
Combining FEC with PMD Compensation
Transmission distance limited by PMD can be
doubled.
System margin can be improved by 5 dB at 43 ps
of average PMD
Y. Xie et al, OFC, 2001
55
Simulation Results PMD Induced Distortion as a
Function of Average PMD
12
Comp.
FEC
None
Tolerable Distortion
8
Performance Distortion (dB)
4
FEC Comp.
0
10
20
30
40
50
Average PMD (ps)
50 improvement on PMD tolerance. Transmission
distance can be doubled.
Y. Xie et al, OFC, 2001
56
Polarization Dependent Loss (PDL)
(PMD0)
Slowly varying changes in the input SOP
Optical components containing
PDL
in a long amplified sytem
Fluctuation in ASE noise due to local signal
power fluctuation
Slow varying changes in SNR at RCVR
8910 km with 270 EDFAs spaced 33 km apart
3
2
Simulation
Mean penalty (dB)
1
Theory
0
0
0.25
0.5
0.75
PDL (dB)
E. Lichtman et al, JLT, 1995
57
Combined Effects of PMD and PDL
Pulse Broadening
?
??
Polarization Mode Dispersion
OSNR Fluctuation
Optical Components (PDL? dB)
Polarization Dependent Loss
58
Standard Deviation of Combined Effect of PMD and
PDL
L.-S. Yan et al, OFC, 2001
59
PDL in High PMD Link
Distributed PDL in a high PMD link changes the
characteristics of the PMD

• Principal States of Polarization (PSP) wont be
  orthogonal anymore
• Pulse spreading can be much larger
• DGD distribution will not be Maxwellian anymore

Interaction between PMD and PDL must be
considered and new measurement methods should be
developed
60
What is polarization scrambling?
Scrambled SOPs
Arbitrary Input SOP
Polarization Scrambler

• Polarization scrambler randomizes the input SOP
• on the Poincarè sphere (DOPlt1)

61
Applications of Polarization Scrambling

• Optical Communications
• PDG reduction in long EDFA chains
• SNR and Q improvement
• PMD/PDL monitoring compensation
• Measurement
• Reduction of measurement uncertainty due to PDL
• Potential problem
• Eye closure due to intensity modulation in the
  presence of PDL

62
Polarization Scrambling in Optically-Amplified
Long-Haul Systems
Q Improvement
Scrambling
Polarization scrambling (DOP lt 1) reduces the
effects of polarization-hole-burning and PDG in
optically-amplified long-haul systems
F. Bruyere, et al., PTL, 1994
63
PMD-Induced RF Power Fading
SOP
SOP?
? Problem in wireless photonic networks
PMD-induced RF power fading
? Example In 60-GHz optical wireless systems,
the RF power is completely faded with 8-ps
instantaneous DGD (PMD tolerance lt
0.2ps/km1/2 for a fiber link of 100 km)
64
Power Fading in SSB Subcarrier Systems
??Carrier ? ??SSB cause Carrier and SSB in
different polarization direction.
PMD induce different phase delay (??Carrier,
??SSB) between two SOPs.
SOPCarrier SOPSSB
Carrier
(SOP)
SSB
?
??SSB
??Carrier
SOPCarrier ? SOPSSB
(SOP?)
Carrier and SSB have same state-of-polarization
(SOP).
Carrier and SSB have different polarization
state. Fading is produced.
65
Mitigating RF Power Fading using a PMD
Compensator (Simulation Results)
Results
SSB modulation cannot reduce PMD-induced RF power
fading
66
PMD Limits in 40-Gbit/s Systems
Fiber PMD (ps/km1/2)
Maximum Transmission Distance (km)