Fiber Optic Theory and Testing
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Objective
• 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
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Optical Fiber• 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
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Optical Fiber Types
• Single Mode
• 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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• LowerCost
• Very small core• Lower Attenuation• Higher Bandwidth
125 µ
Inexpensive Cable
Expensive Splicing
Longer Distance High
Capacity
8 - 9 µ 62.5 µ
Single Mode• Higher
Cost• Very Large Core• Higher Attenuation• Lower Bandwidth
Expensive Cable Inexpensive Splicing Shorter Distance Lower
Capacity
Multi-mode
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Transmission Sources • Fabry-Perot (FP) and Distributed Feedback (DFB) Lasers
– 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)
• Light Emitting Diodes (LED)– 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– 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
Wavelength
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Wavelength Windows Operation
800 900 1000 1100 1200 1300 1400 1500 1600
Att
enu
atio
n (
dB
/K
m)
L Ban
d
1st Window
2ndW
indo
w
5thW
indo
w
4thW
indo
w
3rdW
indo
w
C - Band 1530 - 1560L - Band 1565 - 1610
Wavelength (nm)
0.1
0.7
1.3
1.9
2.5
Reference Point: Visible Light is between 450 and 650 nm
Theoretical Minimum Attenuation of Single Mode
Fiber
C B
and
O B
and
E Band
S B
and
6thW
indo
w
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Factors Affecting Optical Fiber Performance• Factors Affecting Light Losses or Attenuation
– Intrinsic– Bending Losses– Splice Losses
• Factors Affecting Light Pulse Broadening (Bandwidth)– Chromatic Dispersion– Modal Dispersion– Polarization Mode Dispersion
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Attenuation
• Typical Attenuation for various types of optical fiber
Fiber Type 850 nm 1310 nm 1550 nm
Single Mode N/A 0.35 dB/km 0.25 dB/km
Multimode 3.5 dB/km 1.0 dB/km N/A
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Sources of Attenuation• Intrinsic
– Raleigh Scattering– Water Peak Absorption (except of zero water peak fiber)
• 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
• 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
• 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
• 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
• 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
• 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
• Over-Filled Launch (OFL)– Uses LED– Completely fills all modes of multimode fiber
• 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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Network might not work with “wrong” source
Over-filled launch = over estimates loss
Under-filled launch = under estimates loss
Pessimistic result
Optimistic result
Effects of Modal Dispersion
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1. Reduces link loss variation2. Was developed to keep up with components used in high
speed networks (850 nm VCSEL, OM3/4 fiber)3. Was intended for >1GbE4. Targets 850 nm and 50 um cabling5. Can be used for all sources and links6. Improves supplier to supplier consistency
EF tightly controls the number of mode groups
EF is a new multimode launch condition metric that:
The “Encircled Flux” standard
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How is EF measured?
Measured at output of test cord
Source
Reference grade test cord
mandrel
Near field measurement
EF output Test cord
output
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Encircled Flux solves the problem by:
1. Controlling the number of mode groups launched from the test cord
2. Requiring better test cords from suppliers3. Formulating a tight standards-based template4. Advising all test equipment suppliers to use the
same template.
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Launch Controller in use
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Differential Modal Dispersion
Standard 62.5 µ vs. Laser Optimized 50 µ Fiber:Received pulse at 10 Gb/s over 300 meters
62.5 µ fiber
10 Gb/sBit Period
Laser Optimized 50µ Fiber
10 Gb/sBit Period
FiberCore
Center
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10 GB Ethernet
– Approved by TIA in June 2002– A trend finds it’s continuation
10004 10 16 100 10 52 266 100
10000
02000400060008000
1000012000
Toke
nRi
ng 4
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DI/T
PPM
D10
BASE
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Fibr
eCh
anne
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0BAS
E-FX 10
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GBA
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1986 1987 1989 1992 1993 1993 1994 1995 1998 2002
Mbp
s Higher Speeds
0500
100015002000250030003500
Toke
nRi
ng 4
Mb
10BA
SE-
FOIL
Toke
nRi
ng 1
6
FDDI
/TP
PMD
10BA
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FL ATM
Fibr
eCh
anne
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100B
ASE-
FX 1000
BASE
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10G
BASE
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1986 1987 1989 1992 1993 1993 1994 1995 1998 2002
Met
ers
Shorter Distances
13 12.5 13
1112.5
10
6
11
3.562.60
2468
101214
Toke
nR
ing
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100B
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1986 1987 1989 1992 1993 1993 1994 1995 1998 2002
dB
Smaller loss budgets
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Smart Testing & Troubleshooting
• 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
• 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)
RemoteMain
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Tier 1 Fiber Certification• 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?
CladdingCore
Light
Under-filled launch provides under-estimated loss testing.
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Tier 2 Test with OTDR• Single-ended testing of
fibers• Increase the quality of
fiber link installation• Troubleshoot faulty fiber
links
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Tier 2 Fiber Certification• 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 7 m 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) certification100 m 7 m 110 m
Location(m)
850nm(dB)
Event Pass/Fail
0 .18 Reflect Pass100 .14 Reflect Pass107 1.4 Reflect Fail217 .19 Reflect Pass
1.91db loss at connections plus .76db loss for cable = 2.67db total link loss
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Questions?