Mike Harrop
mike.harrop@exfo.com
Advanced Optical Measurements
in Next Generation Networks
October 2007
Agenda
Introduction Digital Transmission
Dispersion in optical Networks.
Dispersion challenges for 40G
OSA challenges for 40G/ROADM’s
What is the fundamental of digital
transmission…?
Tx
Rx
101010001001010101010101010000100101010011001010101001010
The Rx circuit is clocking at the system line rate and ‘simply’
needs to discern between a 1 and a 0 to recover the original
signal.
The need for speed…
SONET
OC-1
OC-3
OC-12
OC-24
OC-48
OC-192
OC-768
SDH
Transmission Rate
51.84 Mb
STM-1
155.52 Mb
STM-4
622.08 Mb
1244.16 Mb (1.2 Gb)
STM-16
2488.32 Mb (2.4 Gb)
STM-64
9953.28 Mb (10 Gb)
STM-256 39,813.12 Mb (40 Gb)
Bit Period
19.29 ns
6.43 ns
1.61 ns
803.76 ps
401.88 ps
100.47 ps
25.12 ps
Eye diagram at Rx demonstrates signal quality
Low BERT
Intermediate
BERT
Unacceptable
BERT
BERT causes a lot of pain to transmission
groups
Typical values for acceptable BERT levels:
 >> 1 x10
-12
 (or 1 bit error per 1,000,000,000,000 bits sent)
In terms of QoS measurements:
 single BIT error = 1 error second on the network
Conclusion of high BERT:
 Networks inability to operate at high speed
 Poor QoS figures
What’s important in Optical Networks
Source : British Telecom Laboratories Technical Journal 2003 (authors Sikora, Zhou and Lord),
Advanced network parameters which have to be properly evaluated
What is Dispersion?
In
Out
TX
RX
Dispersion is the time domain spreading or broadening
of the transmission signal light pulses - as they
travel through the fibre
Types of Dispersion
•Chromatic Dispersion:
•Different wavelengths travel at different velocities
Pulse
Pulse Spreading
•Polarization mode dispersion:
•Different polarization modes travel at different velocities
Pulse
Pulse Spreading
Types of Dispersion
•Chromatic Dispersion:
•Is deterministic
•Is linear
•Is not affected by environment
•Can be compensated
•Polarization mode dispersion:
•Is stochastic
•Is not linear
•Is affected by the environment
•Cannot be easily compensated
Mike Harrop
mike.harrop@exfo.com
Chromatic Dispersion
October 2007
Chromatic Dispersion Issue
Source wavelengths = do not propagate at the same
speed, thus arrive at different times
A pulse transmitted in such way suffers a spread,
dispersion, limiting the transmission bandwidth.
Pulse
Pulse Spreading
l1 l2 l3
l1
l1
l2
l3
l3
Visualizing CD
Let’s visualize a light pulse travelling into a
fiber and segment it into 9 quadrants
(easier to visualize, and to draw!!!)
Visualizing CD
Fiber length:
Light pulse:
Pulse width
Effects of Dispersion
Why is Measuring Dispersion so important?
As transmission speeds go up, the residual dispersion allowable
at the receiver to give a fixed system penalty goes down.
Receiver Tolerance for a 1dB power penalty
2.5 Gb/s
16,000ps/nm
10 Gb/s
1,000ps/nm
40 Gb/s
60ps/nm
e.g. An 80km link at 1550nm will build up 17ps/(nm.km) x 80km =
1360ps/nm. Therefore at data rates at 10Gb/s and higher it is
necessary to compensate for the chromatic dispersion.
To compensate effectively you need to measure the
dispersion of the link.
16 times less CD, cause 1
Time slot 125 us
Time slot 125 us
Faster means less time
between pulses
16 times less CD, cause 2
P
The chirp effect
P
modulation
l
@ 2.5 Gb/s
l
P
@ 10 Gb/s
Pulse before modulation
l
P
Faster means broader pulses
@ 40 Gb/s
l
Dispersion Compensation
Good News : CD is stable, predictable, and controllable.
Dispersion compensating fiber (“DC fiber”) has large
negative dispersion -85ps/(nm.km)
DC fiber modules correct for chromatic dispersion in the
link
delay [ps]
d
0
Tx
Rx
fiber span
DC modules
Dispersion Compensation for DWDM
SMF-28
-D
18.5
17.0
16.2
+D
SMF after 80km
1296ps/nm @ 1530nm
1360ps/nm @ 1550nm
1480ps/nm @ 1570nm
Using 16km of DCF @
85 ps/(nm.km).
0
1300
1530 1550 1570
Wavelength (nm)
-85
DCF
Slope = 0ps/nm^2/km
Consider 3 channel SMF system
Distance
Gives a residual dispersion of
-64ps/nm @ 1530nm
0ps/nm @ 1550nm
120ps/nm @ 1570nm
Dispersion Compensation for DWDM
Dispersion compensation modules can only compensate exactly for
one wavelength
DWDM system design requires knowledge of end-to-end CD as a
function of wavelength… especially for long-haul
-D
+D
-D
+D
+D
10 Gb/s
Tolerance
40 Gb/s
Tolerance
Transmission path
For 40Gb/s transmission slope compensators will be required.
CD: Bad compensation
Dispersion Compensation for DWDM
Note. In practise system vendors don’t compensate perfectly for CD at each stage.
Usually a system will be pre-compensated and then not brought back to zero
during transmission. This is to avoid additional non-linear penalties such as Four
Wave Mixing and Cross Phase Modulation.
-D
-D
+D
+D
+D
DRes
Transmission path
D Accumulated
Z
Types of Dispersion
•Chromatic Dispersion:
•Different wavelengths travel at different velocities
Pulse
Pulse Spreading
•Chromatic Dispersion:
•Is deterministic
•Is linear
•Is not affected by environment
•Can be compensated
Chromatic Dispersion - Conclusion
For 10Gbits/s and higher DWDM systems we
need to measure both the dispersion and the
slope accurately.
Many ways to measure CD in fibre but with the
tolerances required for accurate compensation
– the only accepted method for making this
measurement with this sort of accuracy is the
Phase shift method
Mike Harrop
mike.harrop@exfo.com
Measuring Chromatic Dispersion
October 2007
Chromatic dispersion
Measurement Method- Phase Shift FOTP-169
Patented FTB-5800 method:
Phasemeter
Source
Oscillator
Optical filtering
DUT or FUT
Chromatic dispersion
Measurement Method- Phase Shift FOTP-169
RGD 1
Ref l
Test l1
Few kms of fiber
Chromatic dispersion
Measurement Method- Phase Shift FOTP-169
RGD 2
Ref l
Test l2
Few kms of fiber
Chromatic dispersion
Measurement Method- Phase Shift FOTP-169
RGD 3
Ref l
Few kms of fiber
Test l3
ADVANTAGES: - More points: more resolution
- Ideal for compensation
- Ideal for complex networks
Reference and Measured Spectral Regions
The system compares spectral regions about 1 nm
width (A,B,…) with a reference to find the relative
group delay and compute CD
Measuring CD
Delay points are acquired
Delay
(ps)
Lamdba
Points are fitted
according to models
Delay
(ps)
60
Lamdba
50
40
30
20
CD (ps/nm)
10
0
Slope of Delay gives CD
Lamdba
RGD Fitting
The by-default or user selected mathematical model is
fitted to the RGD point using the generalized least
square method.


3-term Sellmeier (Standard fiber)

5-term Sellmeier

Lambda Log Lambda

Cubic (Unknown fiber, flattened fiber and amplified links)

Quadratic (Compensating, DSF and NZDSF fibers)

Linear
Standard Fiber
Standard Fiber
Extrapolated l0 = 1320.14 nm
CD at 1550nm = 16.641 ps/nm.km
DSF Fiber
l0 = 1547.754 nm
NZDSF fiber (True Wave®)
Example of NZDSF
Analyzed with the help of the FTB-5800
Specifications
Good repeatability
Good accuracy
Measuring Chromatic Dispersion
EXFO FTB-5800







Industry leading accuracy on CD and Slope
Ideal for 10G-40G compensation
Source Shape insensitivity
EDFA testing
 time saving
Component characterisation
Fast measurement
Powerful but simple software
Mike Harrop
mike.harrop@exfo.com
Polarization Mode Dispersion
October 2007
Reminder
•Polarization mode dispersion:
•Different polarization modes travel at different velocities
Pulse
Pulse Spreading
•Polarization mode dispersion:
•Is stochastic
•Is not linear
•Is affected by the environment
•Cannot be easily compensated
Visualizing PMD
Let’s visualize a light pulse travelling into a
fiber and segment it into 9 quadrants
(easier to visualize, and to draw!!!)
Visualizing PMD
Fiber section:
Light pulse:
Pulse width
PMD Impact
If we transmit 1-0-1:
1
0
1
With PMD, this becomes:
1
0
1
The « 1 » is dimmer, the « 0 » can have light:
BER
What causes PMD
Asymmetries in fiber during fiber manufacturing and/or
stress distribution during cabling, installation and/or
servicing create fiber local birefringence.
A "real" long fiber is a randomly distributed addition of
these local birefringent portions.
What causes PMD?
Fiber defects
Geometric
Environmental
constraints
Internal Stress
Lateral Pressure
Wind
(aerial
fibers)
Heat
Bend
Small Birefringence
Small Birefringence
Small Birefringence
Small Birefringence
Small Birefringence
Small Birefringence
Small Birefringence
Small Birefringence
Fast
Fast
Slow
Slow
Large Birefringence
Large Birefringence
Large Birefringence
Large Birefringence
Large Birefringence
Large Birefringence
Fast
Fast
Slow
Slow
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Birefringence and mode coupling
Fast
Fast
Slow
Slow
Causes of PMD
Birefringence (Bad)
 Introduced during manufacture
 non uniform intrinsic fibre stresses ie core
concentricity
 non uniform extrinsic stresses ie pressure
Mode coupling (Good)
 fibre bend and twist
 in-built stress in “spun” fibre
 splices
PMD - Lower Bit Rate
T0
T
t
Dt
fast axis
z, t
slow axis
Dt
PMD - Higher Bit Rate
Dt
t
fast axis
z, t
slow axis
Dt
PMD vs Wavelength and Time
Pradeep Kumar Kondamuri and Christopher Allen
Information and Telecommunications Technology Center, The University of Kansas,
Lawrence, Kansas, 66045
Douglas L. Richards
Sprint Corporation, Overland Park, Kansas
1dB Penalty probability: Very low
Low PMD average
System
Tolerance
Average PMD
1dB Penalty probability: low
Limit PMD average
System
Tolerance
Average PMD
1dB Penalty probability: very high
Too high PMD average
System
Tolerance
Average PMD
PMD Power Penalty
A PMD outage is when the instantaneous DGD exceeds a
given threshold (Max DGD)
A factor 3 between Max DGD and Average PMD is taken
from a number of ITU-T Recommendations (including
G.959-1 OPTICAL TRANSPORT NETWORK PHYSICAL LAYER
INTERFACES) for 99.9954% of no PMD problems
Once you know the system tolerance (Max DGD), aim at
PMD < 1/3 of this value if you transmt Sonet/SDH
PMD Pass-Fail criteria
ITU-T G.959.1, version 7.6 defines Max DGD as 3*<DGD>
It also defines Max DGD as 30ps for OC-192
ITU-T G.650 places it at 25ps Max DGD, but this is based
of FIBER, with no allowance to components. Good for
Fiber Manufacturer, too tight for NSP
IEEE-802.3ae has Max DGD at 19ps (10 GigE), and with a
tolerance of 99.999987% (Corporation, Banks, etc need
higher security) Max DGD is divided by 3.73 for this
level
PMD vs Outage probability
System vendors give Max DGD. You choose Outage probabliity,
then calculate PMD to achieve
Digital Transmissions
PMD Specifications
Maximum PMD value to ensure 99.9954% probability that the
tolerable broadening will correspond to a mean power penalty of
1 dB.
SONET-SDH
Bit rate
(Gbit/s)
Average PMD*
(ps)
2.5
40
10
10
40
2.5
Digital Transmissions
PMD Specifications
Maximum PMD value to ensure 99.999987% probability that the
tolerable broadening will correspond to a mean power penalty of
1 dB
10 GigE
Bit rate
(Gbit/s)
10
Average PMD*
(ps)
5
Total PMD vs PMD Coefficient
Total link PMD (ps)
10ps over 400km
5ps over 50km
Which is better?
PMD Coefficient (ps/√km) used by fibre & cable
manufacturers, based on ITU recommendations that a
network will be 400km.
For 10G Total limit is 10ps, using our network length of
400km gives:
10ps
= 0.5ps/ √km
√400km
Typical values for new fibre.
G.652 Standard Single Mode
<0.1ps/km
G.655 NZDSF
<0.04ps/km
e.g. For a 80km SMF link you would expect to see 0.1 x sqrt(80km)
= 1ps Delay
For a 80km NZDSF link you would expect to see 0.04 x sqrt(80km)
= 0.36ps Delay
Installed base?
Installed Base
10G
40G
Source: John Peters, Ariel Dori, and Felix Kapron,
Bellcore
Reminder
•Polarization mode dispersion:
•Different polarization modes travel at different velocities
Pulse
Pulse Spreading
•Polarization mode dispersion:
•Is stochastic
•Is not linear
•Is affected by the environment
•Cannot be easily compensated
Pitfalls
•Chromatic Dispersion:
•Should be specified at the cable specs (install or rental of
dark fiber)
•Should be tested/compensated on installation or ahead of
system turn up
•Should be considered very deeply for DWDM systems
•Polarization mode dispersion:
•Should be specified at the cable spec level (install or rental
of dark fiber)
•Fibers should be tested and classified for suitability of
different lines speeds
•High levels could mean very costly re-engineering
Conclusions
Uncontrolled fiber dispersion leads to
increased BERT and lower QoS metrics
Dispersion should be considered mission
critical to any operator considering high speed
digital transmission
Accurate measurement and interpretation of
those data are critical…
Mike Harrop
mike.harrop@exfo.com
Measuring Polarization Mode
Dispersion
October 2007
TIA/EIA FOTP 124 : Polarisation Mode Dispersion for
Single-mode fibres by Interferometry.
Interferometer
Traditional Interferometric Method (TINTY)
Limitations
FUT
Gaussian Interferogram
Broadband
Polarizer
Smooth ripple free,
SourceGaussian like source
Ideal random coupling DUT
Analyzer
Mirror
Detector
Autocorrelation
Peak
Cross correlation
Gaussian fit
Half width
FOTP-124: Are these Gaussian???
Saudi Arabia:
South Africa:
FOTP-124: Are these Gaussian???
USA:
UK:
FOTP-124: Are these Gaussian???
UK:
UK:
Autocorrelation: source shape
Source Shape
Auto-correlation
Infinitely broad source
Infinitely thin line
Add Autocorrelation
to Crosscorrelation
Broad uniform
Very thin peak
?
Odd-looking spectrum
Broad peak, humps, ripple, etc…
TIA/EIA FOTP 124a : Polarisation Mode Dispersion for
Single-mode fibres by Interferometry.
Interferometer
Generalised Interferometic Method (GINTY)
No Limitations
Polarizer
No reliance onBroadband
Gaussian Interferogram
Source
Any fibre or component can be measured
Any source shape acceptable
FUT
Analyzer
Mirror
PBS
Detectors
FOTP-124
6.1.2 PMD Calculation for Fibers with Strong Mode
Coupling
The PMD delay, <Dt>, is determined from the half width
parameter, se, of the Gaussian curve fitting applied to the
interferogram according to:
Where se is the RMS width of the Gaussian calculated from
the interferogram…
6.2 Accuracy
Accuracy is related to the capability to precisly fit the
interferogram with the Gaussian function…
What do the standards say?
Ref. IEC 61282 Fibre Optic communication system design guides – Part 9: Guidance on PMD measurements and theory
Measuring PMD
FTB-5500B:

Highest accuracy and resolution


Source Shape insensitivity





Ideal for 10G-40G compliance & certification
Test the whole link
EDFA, OADM testing
Fast measurement time
Powerful but simple software
Same source as FTB-5800 CD Analyzer
Mike Harrop
mike.harrop@exfo.com
Polarization Optical Time Domain Reflectometer
October 2007
What to do with a link with high PMD?
Frequent PMD problems (not measured when built)
Need a way to find high PMD sections:
PMDTOT =N(PMDN)2
Example: 15ps,
2ps,
1ps,
6ps
225ps2 + 4ps2 + 1ps2 + 36ps2 = 266ps2
2661/2 = 16.31ps
Find the 15ps section, replace it, problem solved…
Birefringence & Mode Coupling
PMD 
L
L
Fibres with short (h) where Fast & Slow axis
change frequently, tend to have low PMD
h
Fibres with long (h) where Fast & Slow axis
Change infrequently, tend to have high PMD
fast
fast
slow
fast
slow
h
slow
slow
fast
DOP Polarization-OTDR
PMD 
Pulsed
DFB
Laser
SOP1/SOP2
fiber under test
L
L
l/4
4x2 OTDR acquisitions for
characterizing SOP(z)
Detector
Polarizer l/4
Polarimeter
DOP 
S12  S22  S32
Quantitative  = not measured  PMD value not measured
DOPSOP1, DOPSOP2, h and L = all measured  Tendency for High PMD
S0
h
Example of Measurement and Validation (1)
29 km
5 km 7 km
Link Length ~ 41 km
PMD = 9.8 ps
PMDcoefficient ~ 3 ps/km
Cable opened and
PMD measured with
EXFO FTB-5500B PMD
test set:
29 km, PMD = 4.3 ps
High Contrast
5 km, PMD = 17.4 ps
7 km, PMD = 6.9 ps
Example of Measurement and Validation (2)
35 km
6 km
Link Length ~ 41 km
PMD = 9.8 ps
PMDcoefficient ~ 1.53 ps/km
Cable opened and
PMD measured with
EXFO FTB-5500B PMD
test set:
6 km, PMD = 9.25 ps
High Contrast
Bi-directional Measurements
Quite similar results
Fiber Mapping in a Cable
km
0
3
6
9
12
15
18
21
24
27
30
33
36
39
42
45
48
51
54
57
60
63
Fiber PMD
# (ps)
1
1
7.6
2
2
19.4
3
3
12.4
4
4
3.7
5
5
8.4
6
6
8.8
7
7
8.2
8
8
15.7
9
9
2.5
10
10
28.1
11
11
9.5
Open and test
Source: Connibear, A.B. and Leitch,
A.W.R., Uni. Port Elizabeth, “Locating High
PMD Sections of an Overhead Cable
Unsing Polarization OTDR”
PMD
Fiber #
(ps)
Replace
PMD (ps)
and retest
40.6-49.6km
fiber#
1
7.6
1.7
2
19.4
18.5
3
12.4
7.2
4
3.7
2.7
2.9
Questions?