advanced optical measurements in next generation networks october 2007 mike harrop...

Advanced Optical Measurements in Next Generation Networks

October 2007

Mike [email protected]

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…?

101010001001010101010101010000100101010011001010101001010

Tx Rx

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

SDH

Transmission Rate

Bit Period

OC-1 51.84 Mb 19.29 ns OC-3 STM-1 155.52 Mb 6.43 ns OC-12 STM-4 622.08 Mb 1.61 ns OC-24 1244.16 Mb (1.2 Gb) 803.76 ps OC-48 STM-16 2488.32 Mb (2.4 Gb) 401.88 ps OC-192 STM-64 9953.28 Mb (10 Gb) 100.47 ps OC-768 STM-256 39,813.12 Mb (40 Gb) 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?

Dispersion is the time domain spreading or broadening of the transmission signal light pulses - as they travel through the fibre

OutOut

RXRX

In

TX

Types of Dispersion

•Chromatic Dispersion:•Different wavelengths travel at different velocities

•Polarization mode dispersion:•Different polarization modes travel at different velocities

Pulse Pulse Spreading

PulsePulse 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

Chromatic Dispersion

October 2007


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.

Chromatic Dispersion Issue

1 2 3 1 23

13


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

The chirp effect

Pulse before modulation

@ 2.5 Gb/s

@ 10 Gb/s

@ 40 Gb/s

P

P

P

P

modulation

16 times less CD, cause 2

Faster means broader pulses

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]

0d

Tx Rx

DC modulesfiber span

Dispersion Compensation for DWDM

Consider 3 channel SMF system

Dis

per

sion

(ps

/(nm

.km

))

Wavelength (nm)1300 1550 15701530

18.5

17.0

16.2

0

-85

SMF-28

DCFSlope = 0ps/nm^2/km

Dis

per

sion

-D

+D

Distance

SMF after 80km1296ps/nm @ 1530nm1360ps/nm @ 1550nm1480ps/nm @ 1570nm

Using 16km of DCF @ 85 ps/(nm.km).

Gives a residual dispersion of -64ps/nm @ 1530nm 0ps/nm @ 1550nm120ps/nm @ 1570nm


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

Dis

pers

ion

Transmission path

10 Gb/sTolerance

-D+D

-D

+D +D

40 Gb/sTolerance

For 40Gb/s transmission slope compensators will be required.

CD: Bad compensation


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.

Dis

pers

ion

Z

D Accumulated

-D

+D

DRes

+D +D

-D

Transmission path

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

Measuring Chromatic Dispersion

October 2007


Patented FTB-5800 method:

Source

Oscillator DUT or FUT

Optical filteringPhasemeter

Chromatic dispersionMeasurement Method- Phase Shift FOTP-169


Ref

Test 1

Few kms of fiber

RGD 1


Test 2Few kms of fiber

Ref

RGD 2


Test 3

RGD 3

Few kms of fiber

ADVANTAGES: - More points: more resolution- Ideal for compensation- Ideal for complex networks

Ref

C Band Source Spectral Distribution

-50

-45

-40

-35

-30

-25

-20

1530 1535 1540 1545 1550 1555 1560 1565

nm

dB

m

Reference Filter 1562.25 nm

Grating Monochromator = 1 nm Pass Band Variable Filter

Scans All C and L Bands

A B

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

Lamdba

Delay

(ps)

Lamdba

Points are fitted according to modelsDelay

(ps)

Slope of Delay gives CD

Lamdba

0

10

20

30

40

50

60

CD (ps/nm)

The by-default or user selected mathematical model is The by-default or user selected mathematical model is fitted to the RGD point using the generalized least fitted to the RGD point using the generalized least square method.square method.

3-term Sellmeier (Standard fiber)3-term Sellmeier (Standard fiber)

5-term Sellmeier5-term Sellmeier

Lambda Log LambdaLambda Log Lambda

Cubic (Unknown fiber, flattened fiber and amplified links)Cubic (Unknown fiber, flattened fiber and amplified links)

Quadratic (Compensating, DSF and NZDSF fibers)Quadratic (Compensating, DSF and NZDSF fibers)

LinearLinear

RGD Fitting

Wavelength (nm)1530 1535 1540 1545 1550 1555 1560

14.5

15

15.5

16

16.5

17

17.5

18

18.5

0

5000

10000

15000

20000

25000

Standard Fiber

Extrapolated 0 = 1320.14 nm

CD at 1550nm = 16.641 ps/nm.km

Wavelength (nm)1250 1300 1350 1400 1450 1500 1550

-5

0

5

10

15

-80000

-70000

-60000

-50000

-40000

-30000

-20000

-10000

0

10000

20000

Standard Fiber

0 = 1547.754 nm

DSF Fiber

Example of NZDSF Analyzed with the help of the FTB-5800

NZDSF fiber (True Wave®)

Specifications

Good repeatability Good accuracy

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

Measuring Chromatic Dispersion

Polarization Mode Dispersion

October 2007


Reminder



•Polarization mode dispersion:•Is stochastic

•Is not linear



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

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

What causes PMD?

Fiber defects

Environmental constraints

Geometric Internal Stress

Lateral Pressure

Bend

Heat

Wind (aerial fibers)

Small Birefringence

Small Birefringence

Fast Fast

Slow Slow

Large Birefringence

Large Birefringence

Fast Fast

Slow Slow

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

fast axis

z, t

slow axis

t

T0 T

PMD - Higher Bit Rate

fast axis

z, t

slow axis

t

PMD vs Wavelength and Time

Pradeep Kumar Kondamuri and Christopher AllenInformation and Telecommunications Technology Center, The University of Kansas, Lawrence, Kansas, 66045 Douglas L. RichardsSprint Corporation, Overland Park, Kansas

1dB Penalty probability: Very low

Average PMD

System Tolerance

Low PMD average

1dB Penalty probability: low

Average PMD

System Tolerance

Limit PMD average

1dB Penalty probability: very high

Average PMD

System Tolerance

Too high PMD average

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 Power Penalty

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

Maximum PMD value to ensure 99.9954% probability that the tolerable broadening will correspond to a mean power penalty of

1 dB.

SONET-SDHBit rate

(Gbit/s)

2.5

10

40

Average PMD*(ps)

40

10

2.5

Digital TransmissionsPMD Specifications

Maximum PMD value to ensure 99.999987% probability that the tolerable broadening will correspond to a mean power penalty of

1 dB

10 GigEBit rate

(Gbit/s)

10

Average PMD*(ps)

5

Digital TransmissionsPMD Specifications

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

√400km= 0.5ps/ √km

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

Source: John Peters, Ariel Dori, and Felix Kapron, Bellcore

10G

40G

Reminder



•Polarization mode dispersion:•Is stochastic

•Is not linear



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…

Measuring Polarization Mode Dispersion

October 2007


TIA/EIA FOTP 124 : Polarisation Mode Dispersion for Single-mode fibres by Interferometry.

Traditional Interferometric Method (TINTY)

LimitationsGaussian Interferogram Smooth ripple free, Gaussian like sourceIdeal random coupling DUT

AutocorrelationPeak

Cross correlation

Half width

Gaussian fit

BroadbandSource

Polarizer

FUT

Interferometer

Mirror

Analyzer

Detector

FOTP-124: Are these Gaussian???

Saudi Arabia:

South Africa:Delay (ps)

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

10

20

30

40

Delay (ps)-20 -10 0 10 20

0

5

10


USA:

UK:Delay (ps)

-3 -2 -1 0 1 2 3

0

5

10

15

Delay (ps)-1.5 -1 -0.5 0 0.5 1 1.5

0

5

10

15


Delay (ps)-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

0

5

10

15

Delay (ps)-1.5 -1 -0.5 0 0.5 1 1.5

0

5

10

15

20

UK:

UK:

?

Source Shape Auto-correlation

Infinitely broad source Infinitely thin line

Broad uniform Very thin peak

Odd-looking spectrum Broad peak, humps, ripple, etc…

Add Autocorrelation to Crosscorrelation

Autocorrelation: source shape

TIA/EIA FOTP 124a : Polarisation Mode Dispersion for Single-mode fibres by Interferometry.

Generalised Interferometic Method (GINTY)

No LimitationsNo reliance on Gaussian InterferogramAny fibre or component can be measuredAny source shape acceptable

BroadbandSource

Polarizer

Interferometer

Mirror

Analyzer

PBS

Detectors

FUT

FOTP-124

6.1.2 PMD Calculation for Fibers with Strong Mode Coupling

The PMD delay, <>, is determined from the half width parameter, , of the Gaussian curve fitting applied to the interferogram according to:

Where is the RMS width of the Gaussian calculated from the interferogram…

6.2 AccuracyAccuracy 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 Ideal for 10G-40G compliance & certification

Source Shape insensitivity Test the whole link EDFA, OADM testing

Fast measurement time Powerful but simple software Same source as FTB-5800 CD Analyzer

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…

hL

LPMD

Birefringence & Mode Coupling

fast

slow

h

fastslow

fast

slow

fast

slow

Fibres with short (h) where Fast & Slow axis change frequently, tend to have low PMD

Fibres with long (h) where Fast & Slow axisChange infrequently, tend to have high PMD

DOP Polarization-OTDR

Quantitative =not measured PMD value not measured

DOPSOP1, DOPSOP2, h and L = all measured Tendency for High PMD

fiber under testPulsed DFB

Laser

Detector

Polarimeter

hL

LPMD

/4

Polarizer /4

SOP1/SOP2

4x2 OTDR acquisitions for characterizing SOP(z)

0

23

22

21

S

SSSDOP

Example of Measurement and Validation (1)

Link Length ~ 41 kmPMD = 9.8 psPMDcoefficient ~ 3 ps/km

Cable opened and PMD measured with EXFO FTB-5500B PMD test set:

29 km, PMD = 4.3 ps

5 km, PMD = 17.4 ps

7 km, PMD = 6.9 ps

29 km 5 km 7 km

High Contrast

Example of Measurement and Validation (2)

Link Length ~ 41 kmPMD = 9.8 psPMDcoefficient ~ 1.53 ps/km

Cable opened and PMD measured with EXFO FTB-5500B PMD test set:

6 km, PMD = 9.25 ps

35 km 6 km

High Contrast

Bi-directional Measurements

Quite similar results

Fiber Mapping in a Cablekm 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”

Fiber # PMD (ps)

PMD (ps) 40.6-49.6km

Replace and retest

fiber#

1 7.6 1.7

2 19.4 18.5 2.9

3 12.4 7.2

4 3.7 2.7

Questions?

advanced optical measurements in next generation networks october 2007 mike harrop...

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