Fiber Optic Theory and Testing
Objective
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Understand Fiber Optic Basics
Understand Issues that Impact Fiber Optic
Link and Channel Performance
Understand How to Determine Installed Link
and Channel Performance.
Demonstrate Fiber Testing and
Troubleshooting
2
Optical Fiber
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Uses light pulses instead of electrical signals
Core & Cladding are composed of glass
n1 of the core > n2 of the cladding
Core diameter defines fiber type
Cladding diameter = 125 µm
Coating is UV curable urethane acrylate (2-Layers)
Coating diameter = 250 µm
3
Optical Fiber Types
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Single Mode
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Multimode
The radius, r, and index of refraction, n1, of the core determines the number of modes
allowed to propagate:
Number of Modes ≈ ∆(2πncorercore/λ)
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STEP-INDEX MULTIMODE FIBER
GRADED-INDEX MULTIMODE FIBER
SINGLE-MODE FIBER
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8-9µ
125 µ
62.5 µ
Single Mode
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Lower
Cost
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Very small core
Lower Attenuation
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Higher Bandwidth
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Inexpensive
Cable
Expensive
Splicing
Longer Distance
High
Capacity
Multi-mode
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Higher

Cost
Very Large Core 
Higher Attenuation
Lower Bandwidth
Expensive Cable
Inexpensive Splicing
Shorter Distance
Lower
Capacity
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Transmission Sources
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Fabry-Perot (FP) and Distributed Feedback (DFB) Lasers
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Light Emitting Diodes (LED)
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Used for singlemode: 1310 nm or 1550 nm
Narrow spectrum (can be less than 1 nm)
Narrow beam width (does not fill multimode fibers)
Highest power and fastest switching
Most expensive (especially DFB)
Wavelength
Used for multimode: 850 nm or 1300 nm
Wide beam width fills multimode fibers
Wider spectrum (typically 50 nm)
Inexpensive
Cannot modulate as fast as lasers
VCSEL’s
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Vertical Cavity Surface Emitting Laser
Used for multimode at 850 and 1300 nm
Quite narrow spectrum
Narrow beam width (does not fill multimode fibers)
Much less expensive than FP or DFB lasers
Wavelength
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Wavelength Windows Operation
Reference Point: Visible Light is between 450 and 650 nm
2.5
1st
Window
C - Band 1530 - 1560
L - Band 1565 - 1610
Window
th
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C Band
4
Window
rd
Window
th
6
S Band
Window
th
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Window
2
1.3
nd
Theoretical Minimum
Attenuation of Single Mode
Fiber
0.7
L Band
E Band
O Band
Attenuation (dB/Km)
1.9
0.1
800
900
1000
1100
1200
1300
1400
1500
1600
Wavelength (nm)
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Factors Affecting Optical Fiber
Performance
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Factors Affecting Light Losses or Attenuation
– Intrinsic
– Bending Losses
– Splice Losses
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Factors Affecting Light Pulse Broadening (Bandwidth)
– Chromatic Dispersion
– Modal Dispersion
– Polarization Mode Dispersion
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Attenuation
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Typical Attenuation for various types of optical fiber
Fiber Type
Single Mode
Multimode
850 nm
1310 nm
1550 nm
N/A
0.35 dB/km
0.25 dB/km
3.5 dB/km
1.0 dB/km
N/A
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Sources of Attenuation
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Intrinsic
– Raleigh Scattering
– Water Peak Absorption (except of zero water peak fiber)
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Splice Loss
– Fusion: core alignment
– Mechanical: core alignment, dirt on end face, reflection
– Mode Field Diameter in Single Mode Fibers
– Numerical Aperture Mismatch in Multimode Fibers
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Sources of Attenuation
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Macrobending (Single Mode Fiber)
– Bending radius ~ 2 – 15 mm
– Affects long wavelengths first
– Affected mostly by fiber design
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Sources of Attenuation
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Microbending (All Fiber)
– Bending radius ~ radius of core
– Can occur during optical fiber manufacturing process
– Can be induced during installation due to point
pressures
– Affects all wavelengths, but increases slightly with
wavelength
– Order of Sensitivity (least to highest): SM, 62.5 µ, 50 µ
– Affected by Coating and Cable Design
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Dispersion or Pulse Broadening
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Chromatic Dispersion (Single Mode Fibers)
– Laser output is distribution of wavelengths
– Different wavelengths travel different speeds
– Dispersion compensating fiber
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Dispersion or Pulse Broadening
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Polarization Mode Dispersion (Single Mode
Fibers)
– Radially imperfect core
– Causes delay in 1 of 2 Orthogonal Modes
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Dispersion or Pulse Broadening
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Modal Dispersion (Multi-mode Fibers)
– Mode is quantum level in light pulse
– Each mode occupies different area of core
– Imperfect core structure causes modes to have
different speeds
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Measuring Modal Dispersion
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Over-Filled Launch (OFL)
– Uses LED
– Completely fills all modes of multimode fiber
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Differential Modal Dispersion
– Uses Laser
– Injects pulses of light from one side of the core to the other at
micron intervals
– Measures Pulse Intensity and Time of Arrival
– Effective Modal Bandwidth (EFL) is determined from this test
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Pessimistic result
Effects of Modal Dispersion
Over-filled launch = over estimates loss
Optimistic result
Under-filled launch = under estimates loss
Network might not work with “wrong” source
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The “Encircled Flux” standard
EF is a new multimode launch condition metric that:
1.
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Reduces link loss variation
Was developed to keep up with components used in high
speed networks (850 nm VCSEL, OM3/4 fiber)
Was intended for >1GbE
Targets 850 nm and 50 um cabling
Can be used for all sources and links
Improves supplier to supplier consistency
EF tightly controls the number of mode groups
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How is EF measured?
EF
output
Test cord
output
Near field
measurement
Reference grade
test cord
Source
mandrel
Measured at output of test cord
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Encircled Flux solves the problem by:
Controlling the number of mode groups launched
from the test cord
2. Requiring better test cords from suppliers
3. Formulating a tight standards-based template
4. Advising all test equipment suppliers to use the
same template.
1.
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Launch Controller in use
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Differential Modal Dispersion
62.5 µ fiber
Laser Optimized 50µ Fiber
10 Gb/s
Bit Period
10 Gb/s
Bit Period
Fiber
Core
Center
Standard 62.5 µ vs. Laser Optimized 50 µ Fiber:
Received pulse at 10 Gb/s over 300 meters
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10 GB Ethernet
– Approved by TIA in June 2002
– A trend finds it’s continuation
12000
3500
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12.5
Higher Speeds
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10
4000
6
1500
loss
budgets
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2000 Smaller
Shorter
Distances
1000
4
10
16
10
100
2 0
500
0 0
6
52
266 1003.56
1000
2.6
10GBASE10GBASE10GBASESS S
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100BASE100BASE100BASEFX FX
FX
1000
1000
1000
BASE-SX
BASE-SX
BASE-SX
12.5
Fibre
Fibre
Fibre
Channel
Channel
Channel
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6000
2000
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ATM
ATM
ATM
8000
10
2500
10000
Token
Token
Ring
4
Mb
Token
Ring 4 Mb
Ring 4 Mb
10BASE10BASEFOIL
10BASEFOIL
Token
FOIL
Ring
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Token
Token
Mb16
Ring
Ring 16
FDDI/TP
FDDI/TP
FDDI/TP
PMD
PMDPMD
10BASE10BASE10BASEFL
FL FL
Meters
dB
Mbps
10000
3000
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1998 2002
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Smart Testing & Troubleshooting
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Eliminate common problems with good practices
during installation and maintenance
– Verify continuity, polarity, adequate end-face condition with
basic tools to ensure best termination and installation
practices
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Perform complete cable certification per TIA TSB140
– Basic certification (Tier 1):
– Extended certification (Tier 2):
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Tier 1 Test with LSPM (Light Source/Power Meter)
Main
Remote
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Tier 1 Fiber Certification
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Basic (Tier 1) certification of
fiber links
– Required for standards
compliance
– Uses absolute power/loss
measurement
– Best for measuring TOTAL
(end-to-end) loss of a fiber
channel
– Test against loss limits
based on industry
standards for current
application
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Tier 1 FAQ- What is the correct way to set a
reference?
1. Set the reference using the test reference cord (sets P0 to 0dB).
2. Attach tail cord to cable under test and measure P1 Loss = - (P1 - P0)
3. Measures loss of two connectors and cable (fiber).
Most accurate and repeatable reference method:
1Jumper Reference Method (also called “method B”)
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Tier 1 FAQ- Why am I required to use a
mandrel?
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What does the Mandrel do?
Mandrel wrap with LED allows testing 50um and 62.5um
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Tier 1 FAQ- When to use LED (MFM) and when to
use VCSEL (GFM) source?
Core
Cladding
Light
Under-filled launch provides under-estimated loss testing.
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Tier 2 Test with OTDR
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Single-ended testing of
fibers
Increase the quality of
fiber link installation
Troubleshoot faulty fiber
links
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Tier 2 Fiber Certification
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Extended (Tier 2)
certification of a fiber link
– Complements Tier 1
fiber certification
– Ensure that the fiber
link meets expectations
for current and future
applications
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Test Example: -Extended (Tier 2) certification
100 m
7m
110 m
Pass/Fail loss budget is 3.2 dB noted for Gigabit in 568-B.1, Annex E
Result of Tier 1 Certification is 2.67 dB
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Test Example: -Extended (Tier 2) certification
100 m
1.91db loss at connections plus .76db
loss for cable = 2.67db total link loss
7m
110 m
Location
(m)
850nm
(dB)
Event
Pass/Fail
0
.18
Reflect
Pass
100
.14
Reflect
Pass
107
1.4
Reflect
Fail
217
.19
Reflect
Pass
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Questions?
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