Transcript 40 Gig Challenges

May 19, 2011
Bill Reynolds
Technical Support Engineer
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2008 EXFO Electro-Optical Engineering Inc. All rights reserved.
Agenda
Fiber Optic Theory
Connector Cleaning
OTDR Testing
OLTS Testing
CD Testing
PMD Testing
CWDM/DWDM
Fiber Optic Theory
Spectrum
Optical fiber domain
 850 nm  353 000 GHz
 1650 nm  182 000 GHz
λ (nm)=c (m/s) / f (Hz)
Units
Micrometers (mm) - 10-6 m
Nanometers (nm) - 10-9 m
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Index of Refraction
 The speed of light = c = 299793 km/s under vacuum
 Any material that can transmit light has it’s own index of refraction represented
by n
Example
Index (n)
1.000
1.400
1.333
Material
Air
Fiber Optic
Water
n = c vac / c mat
 In a given index of refraction, the speed of light gets slower


Speed of light in water ≈ 225 000 km/s
Speed of light in fiber optic ≈ 215 300 km/s
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Fiber Optic
Coating
Acrylate, Teflon,
polyimide
Cladding
Glass index n2
Core
Glass index n1
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Fiber Optic
Coating diameter = 250 µm
Cladding diameter = 125 µm
Core diameter = 9, 50, 62.5 µm
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Reflection

Reflection
 When a light beam I hits a material with a different index of refraction, a portion of

the beam is reflected R
The angle of this reflected beam is the same as the incident beam
n1
Core
I
θi
θR
R
n2
Cladding
θi = θR
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Refraction

Refraction
 For the same light beam hitting a different material, another portion is refracted
 This occurs when a the light goes through a material with a different index of

refraction
The angle of this beam changes because the speed of propagation changes
n1
Core
n2
I
θi
θR
θr
R
T
Cladding
n1 sin(θi) = n2 sin(θr)
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Total Internal Reflection

Critical angle (Total internal reflection)
 There is a certain angle where 100% of the light is reflected and no light is
refracted, we call this angle, the critical angle
 Fiber optics use this concept to propagate light
Cladding
Core
I
θC
θR
R1
R2
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Fiber Types
 There are 2 fiber optics types in telecommunications
Singlemode
Multimode
Core
Core
Cladding
Core
Cladding
Cladding
62.5/125 (µm)
50/125 (µm)
9/125 (µm)
ITU-T G.652D
For telecommunications
applications
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Agenda
Fiber Optic Theory
Connector Cleaning
OTDR Testing
OLTS Testing
CD Testing
PMD Testing
CWDM/DWDM
Inspecting & Cleaning
Inspection
 Inspection techniques:
 A microscope or fiber probe can be used to inspect connectors
 A microscope acts as a magnifying glass. If you inspect a connector on a live fiber,

permanent damage can be done to your eyes!
Using a fiber probe is the safest was to inspect a connector:
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Cleaning
 Cleaning Techniques:
 The best way to clean connectors can be done by following these easy steps:
1.
2.
3.
4.
Clean the outside of the ferrule with a wet pad
Clean the ferrule using a dry pad
Inspect the connector using a fiber probe
If the connector is still dirty, repeat the 2 previous steps with a wet pad
Dirty ferrule
Clean ferrule
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Why Clean?
80% of network problems are related to dirty connectors!
Permanent damage can occur on dirty connectors
on high power systems
RF video may reach +23 dBm @ 1550nm
MDUs now includes MPO connectors
MPO are tricky to clean
Clean
Permanently burnt –
combined high power and dirt
View of an multi-fiber angle-polished connector
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Cleaning
 Bad cleaning results
Broken surface
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Damaged
Dirty
Clean
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Agenda
Fiber Optic Theory
Connector Cleaning
OTDR Testing
OLTS Testing
CD Testing
PMD Testing
CWDM/DWDM
OTDR Testing
Attenuation

Attenuation in fiber is wavelength-dependent
C: 1310nm = 0.34 dB/km
Optical fiber is normally tested at
the same wavelength that the fiber
system will be operated.
Water peak
D: 1383nm = 0.50 dB/km
E: 1550nm
= 0.19 dB/km
Water peak
Corning SMF-28 SM Fiber
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OTDR Principle
 The OTDR can be compared to a submarine radar.
 Instead of sending RF or audio (submarine) signal to detect distant
objects, it sends short pulses of light to detect events in fiber.
 The OTDR locates and identifies events along the fiber.
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Setting Up the OTDR
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Rayleigh Backscattering
 Rayleigh Backscattering
 Comes from the “natural” reflection of the fiber
 The OTDR will use the Rayleigh back reflections to measure the fiber’s attenuation
(dB/Km).
 Back reflection level is around -75 dB (depends on pulse length)
 Higher wavelength will be less attenuated by the Rayleigh Backscatter
Dopant particules
Source
Ray of light
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Fresnel Reflections
 Fresnel back reflections
 Will come from abrupt changes in the IOR, ex: (glass/air)
Fiber break, mechanical splice, bulkheads, connectors
 Will show as a “spike” on the OTDR trace
 Reflection are typically UPC –45 dB and APC –65 dB (Typical OTDR results)
 Fresnel reflections will be approximately 20,000 times higher than the fiber’s

backscattering level
Will create a « Dead Zone » after the reflection
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Pulse versus resolution and dynamic range
Short Pulse : more resolution but less energy
Long Pulse width: more energy but less resolution
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Pulse Width
 Short pulses will give a better resolution but less dynamic range:
Two connectors 3 meters apart
Connectors
are
measured
for distance
and marked
as separate
events
End of link (patch panel)
5ns pulse
End of fiber is not reached due
to low power of short pulses
Long pulses will give a better dynamic range but less resolution
Connectors
are
« merged »
and
identified as
one event
30ns pulse
End of fiber is reached and located
when using a larger pulse
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10 us pulse
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20 us pulse
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OTDR Trace

Single ended measurement
Fusion splice
OTDR Connector
Connector (P.P.)
Connector (P.P.)
End of link
UPC
Reflection
Power (dB)
Loss
APC
Slope shows
attenuation
fiber
Distance (km)
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Macrobend

Loss in fiber is wavelength-dependent
Acquisition at 1310 nm
Shorter wavelengths
are more attenuated
by fiber’s scattering
Acquisition at 1550 nm
Longer wavelengths
tend to leak out of the
fiber more easily due
to bending
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Agenda
Fiber Optic Theory
Connector Cleaning
OTDR Testing
OLTS Testing
CD Testing
PMD Testing
CWDM/DWDM
OLTS Testing
Insertion Loss
Insertion testing is done in pairs
Dual ended measurement
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Measurement Units

dBm
 The dBm is use to measure the output power of a light source
Instrument
reading
Fiber optic
- 3.50 dBm
Detector
Laser source
 The laser output is -3.50 dBm
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Measurement Unit
Optical Power mW vs dBm
mW
How to convert the dBm in mW

dBm = 10*log(mW)
The laser seen on previous page was
emitting -3.50dB

-3.50 dBm so 0.45 mW
10000 mW
+40.00 dBm
1000 mW
+30.00 dBm
10 mW
+10.00 dBm
1 mW
0.00 dBm
500 µW
-3.00 dBm
100 µW
-10 dBm
10 µW
-20 dBm
1 µW
-30 dBm
100 nW
-40 dBm
10 nW
-50 dBm
1 nW
-60 dBm
100 pW
-70 dBm
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Measurement Units
dB (relative power)


dB is the difference between 2 power measurements
Take the -3.50 dBm laser shown previously
Laser output = -3.50 dBm
-3.50 dBm
There is an event on the fiber and the
detector reads -4.25 dBm
To calculate this difference:
(-3.50 dBm) – (-4.25 dBm) = 0.75 dB
We have lost 0.75 dB
-3.50 dBm
-4.25 dBm
So the insertion loss is -0.75dB
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Reflectance
 Will come from abrupt changes in the IOR:
Fiber break, mechanical splice, bulkheads, connectors, etc.
 We use the term « reflectance » when speaking of the amount of energy returned by
specific points within the network
 Expressed as a negative value
Connector 1 = -40dB
Connector 2 = -50dB
Reflectance [dB] = Preflected [dBm] - Pincident [dBm]
Q: Which one has the best
reflection value?
Connector
Fiber section
Patch Panel
Mec. Splice
Fiber section
Connector
reflectance: -55dB
Fiber section
Patch Panel
Connector
reflectance: -45dB
Mechanical splice
reflectance: -45dB
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Optical Return Loss (ORL)
 Comes from the amount of energy lost within components and fiber due to back
reflections
 We use the term « ORL » when speaking of the amount of energy returned by a
section or an entire link
 Expressed as a positive value
Link ORL = 35dB
Section 2 & 3 ORL = 45dB
Connector
Fiber section
Mec. Splice
Fiber section
Patch Panel
Fiber section
Patch Panel
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 Rayleigh Backscatter + Reflectance = ORL
Optical Return Loss (ORL) represents the sum of
all of the light returned to the source from the fiber
link under test.
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Consequences of ORL
• Less light is transmitted
• Causes interference with light source signals
• Creates higher bit error rate (BER) in digital systems
• Reduces signal-to-noise ratio (SNR) in analog systems
• Causes fluctuations in the light source’s central
wavelength
• Causes fluctuations in its output power
• Damages the light source permanently
UPC Connectors
SC/UPC
FC/UPC
Ultra Physical Contact
LC/UPC
UPC Connector
UPC Connector
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APC Connector
SC/APC
FC/APC
Angle Physical Contact
LC/APC
APC Connector
APC Connector
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• Mitigating ORL
– Backscatter
• Is an inherent property of fiber and therefore there is
nothing you can do about it.
• However backscatter will have minimal affect on ORL
– Reflectance
• Dirty connectors!!!
• Damaged endfaces
• Improper mating
– APC-UPC etc.
– Clean connectors, clean connectors, clean
connectors!!
Agenda
Fiber Optic Theory
Connector Cleaning
OTDR Testing
OLTS Testing
CD Testing
PMD Testing
CWDM/DWDM
Dispersion
Dispersion is the Fundamental limiting factor
in transmission links and determines the:
Data rate on long fibers!
Limits length on high data rate fibers!
Dispersions
Multimode Fiber
Optical
Paths
Optical
1
Frequencies 2
Difference in Arrival
Times
Chromatic Dispersion
Polarization Modes
Polarization Mode Dispersion
Input Pulse
Output Pulse
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• A light pulse will spread and lose peak power
as it travels down the Fiber.
The Pulse Broadening Causes intersymbol interference
and bit errors
Each light pulse representing a data bit is smeared out and runs into the
time slot allotted to the next bit, giving bit errors
Dispersion was never an issue because:
Low Transmission Rates (DS-0 to OC-48)
Switching equipment (OEO)
Regens were used, No EDFAs
Single transmission wavelength was near zero
dispersion point.
(SMF-28 1300nm = Lambda 0)
PMD Testing
PMD is Stochastic (random)
System
Tolerance
Average PMD
PMD for 99.9954% probability that the tolerable broadening will
correspond to a mean power penalty of 1 dB
SONET-SDH
Bit rate
(Gbit/s)
Average DGD*
(ps)
2.5 (OC-48)
40
10 (OC-192)
10
40 (OC-768)
2.5
Birefringence
 Speed varies with Index of Refraction (density)
 Fiber cross-section can have variable Index, which is called
Birefringence
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Birefringence

Fiber manufacturing
Fiber Geometry

Internal Stress
Core Concentricity
Lateral Pressure
Environmental constraint
Heat
Bend
Wind (aerial fibers)
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Birefringence
The causes
Asymmetries in fiber core geometry and/or stress distribution create fiber local birefringence.
A "real" fiber is a randomly distributed addition of these local birefringent portions.
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Visualizing PMD
 Let’s visualize a light pulse traveling into a fiber
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Polarization Mode Dispersion PMD
h
Coupling length
β
Bi-refringence
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T0
T
t
Dt
fast axis
z, t
slow axis
Dt
Dt
t
fast axis
z, t
slow axis
Dt
PMD Impact

When the detector receives this:
0

1
0
1
0
It sees: Clear 1 and clear 0
 When the detector receives this:
?

1
?
1
0
It cannot differentiate 1 and 0, this is when we get BER!!!
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PMD in a cable
Cable 1 (18.6km):
Fiber 1:
Fiber 2:
Fiber 3:
Fiber 4:
Fiber 5:
Fiber 6:
Cable 2 (17.7km):
Fiber 1:
Fiber 2:
Fiber 3:
Fiber 4:
Fiber 5:
Fiber 6:
Fiber 7:
Cable 3 (18.4km):
7.82ps
23.15ps
7.84ps
9.72ps
9.74ps
10.40ps
3.16ps
3.67ps
3.03ps
8.82ps
6.80ps
13.07ps
3.90ps
Fiber 1:
Fiber 2:
Fiber 3:
Fiber 4:
Fiber 5:
Fiber 6:
Fiber 7:
Fiber 8:
Fiber 9:
Fiber 10:
0.17ps
6.49ps
1.29ps
0.11ps
0.29ps
8.93ps
0.40ps
0.22ps
0.10ps
0.88ps
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Dual ended or Single Ended measurement
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Cumulative PMD
PMD adds quadratically:
PMDTOT =N(PMDN)2
Example: 15ps + 2ps + 1ps + 6ps = 16.31ps
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Mitigation of PMD
CD Testing
Dispersions
Multimode Fiber
Optical
Paths
Optical
1
Frequencies 2
Difference in Arrival
Times
Chromatic Dispersion
Polarization Modes
Polarization Mode Dispersion
Input Pulse
Output Pulse
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Material Dispersion
 Each wavelength as it’s own
speed in the glass material
therefore it refracts at a
different angle, separating the
colors
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Material Dispersion
dBm
DFB Laser source
Central Wavelength
Zoom
Wavelength
Spectral width
The pulse is made out of all laser frequencies
Amplitude
Amplitude
Time
Time
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Waveguide Dispersion
With increasing wavelength, more power is
traveling in the cladding section of the fiber
Mode Field Diameter
N1
Wavelength 1
N2
Wavelength 2
Wavelength 3
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Waveguide Dispersion
Index
Index profile of step index
singlemode fiber G.652
Diameter
Index
Index profile of dispersion
shifted fiber G.653
Diameter
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1. Light will propagate both in core and cladding
2. Core and Cladding have different refractive indexes
3. Spectral width of source consists of several
wavelengths
4. Different wavelengths will propagate at different
velocities this is because the refractive index is
wavelength dependant.
Pulse Spreading occurs because different wavelengths of
light travel at different velocities
Sources are not truly mono-chromatic (one λ) so each
wavelength component will travel at a different speed
Single Mode Fiber
1
2
• The Laser has finite line width
• Spectral width is measured at FWHM (full width
at half maximum) really consists of several
wavelengths.
nm
Wavelength (nm)
• Therefore:
• Within the spectral width of the laser the
different wavelengths will propagate at
different velocities within the core and at
different velocities between the light in the
core and the cladding.
• This is the effect of both material dispersion
and waveguide dispersion, which combines
together into chromatic dispersion
• Chromatic Dispersion (CD) specified in ps/nm*km
• Total Chromatic dispersion of a link is specified in
ps/nm
• Lambda Zero or zero dispersion is the wavelength in
which there is no Chromatic Dispersion
• SMF-28 has about 17ps/nm*Km at 1550nm
CD Limits:
Chromatic Dispersion
 CD is an addition of 2 chromatic dispersion types
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Chromatic Dispersion
(ps/nm-km)
Fiber Types
+
+7
dispersion unshifted G.652
+

dispersion shifted G.653
non-zero
dispersion






non-zero dispersion shifted G.655


(nm)
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Chromatic Dispersion (ps/nm -km)
Corning LEAF
+4
Reduced Slope
Lucent
TrueWave
Balanced +
Lucent
TrueWave
G.653
+2
Corning LS
1530
1540
1550
-2
-4
S-band
EDFA C-band
1560
Lucent
1570TrueWave
Balanced Corning
MetroCor
EDFA L-band
Threshold
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Threshold
Cause #1
2.5 Gb/s
Time slot 125 us
Increasing the speed of
transmission by 4 will
decrease the threshold by
16. This is due to 2 reasons
10 Gb/s
Time slot 125 us
40 Gb/s
Faster bit rate means less
space between pulses
Time slot 125 us
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Threshold
Cause #2
The chirp effect
P
P
modulation

@ 2.5 Gb/s

P
@ 10 Gb/s
Pulse before modulation
Faster bit rate means broader pulses

P
@ 40 Gb/s

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Threshold
Mask Threshold
Increases BER
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FLS-5800 & FTB-5800
Dual ended or single ended measurement
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Results Table
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Mitigation of CD:
Compensation could be preformed:
• Per span: Negative dispersion fiber
• Per channel: DCM
• Per path: Tunable Dispersion Compensator
Summary: CD & PMD
 PMD and CD can increase BER and distortion.
 PMD and CD are expressed in ps
 PMD depends on the cable type and age and may change with environmental conditions,

so it is important to test PMD on a regular basis.
To correct PMD, faulty fiber sections must be identified and changed
 CD is a stable phenomenon as it is relative to refractive index of the fiber used
 To correct CD, compensators may be used
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Overview
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Agenda
Fiber Optic Theory
Connector Cleaning
OTDR Testing
OLTS Testing
CD Testing
PMD Testing
CWDM/DWDM
CWDM/DWDM
Difference
testing
WDM Wavelength Division Multiplexing
WDM technology supports simultaneous
carriers/wavelengths/Protocols traveling
along one fiber
WDM: Wavelength Division multiplexing
In order to increase the number of bits transmitted per fiber,
multiple different wavelengths could be transmitted in the same
fiber or system.
Mux/ demux and filters will combine and select the wavelength
WDM: Wavelengh Division Multiplexing
• Class of WDM :
– Wide WDM Devices: Access
• channel spacing > 50nm
• 1310 , 1490 , 1550, 1625...
– Coarse WDM : Metro
• channel spacing > 200 GHz (1.5nm @1550nm)
• but < 50 nm (typical less than 18 lambdas)
• Common spacing 20nm
– Dense WDM : Long haul
• channel spacing  200 GHz
WDM bands
CWDM Application
• CWDM is a WDM technology using 20nm between
channels for a total count of 18 Channel in the O,
E, S, C,L bands.
• This technology is deployed in Access and metro
networks where the cost per bit is critical.
CWDM Specifications
– Chromatic dispersion transmission rates are a LOT lower than
for DWDM
– CWDM is for now limited to 2.5Gbps
– Reason: Direct modulation causes CHIRP
– CD is important to test, even for 2.5Gbps
– CHIRP is induced by the modulation, the higher the bit rate
the wider the signal
mw
Ghz
DWDM technology
O-Band
1270
1290
1310
E-Band
1330
1350
1370
1390
1410
S-Band
1430
1450
1470
1490
1510
C-Band
1530
1550
L-Band
1570
1590
1610
1549.72 1550.12 1550.52 1550.92 1551.32 1551.72 1552.12 1552.52 1552.93 1553.33 1553.73 1554.13 1554.54 1554.94 1555.34 1555.75 1556.15 1556.55 1556.96 1557.36 1557.77 1558.17
These are only a small number of the ITU-T DWDM
channel plan. There are hundreds more!
DWDM technology
• DWDM is usually deployed where high bit rate
and long reach are required
Testing
CWDM OTDR
Channel Verifier
Optical Spectrum Analyzer
CWDM Channel Verifier
OSA Optical Spectrum Analyzer
System Critical Parameters
Channel
Power Level Spacing
OSNR
Center
Wavelength
Sensitivity
Sensitivity
Power meter versus OSA
Wavelength
Wavelength
A power meter will mesure the An OSA will mesure the
TOTAL POWER
POWER versus wavelenght
+5dBm
CWDM Solution for Multiple Users
Head-end




Each customer has a dedicated wavelength
Each customer has different distance from head-end
Conventional OTDR cannot test through CWDM system
Need CWDM OTDR for system installation, maintenance and trouble
shooting
Troubleshooting CWDM
Testing CWDM
Specialized
test
equipment
with
wavelengths
that match the
CWDM filters
are capable of
testing end-toend loss and
optical return
loss thus
assuring the
Thank You