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Demystifying OTDR Event Analysis
TTI Technical Training Presentation
for CCTA July 15, 2014

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Training Abstract
An Optical Time Domain Reflectometer
(OTDR) is the ubiquitous tool for fiber optic
network health, reflectance, loss and distance
measurements. For a new user, and even
seasoned users in modern systems, OTDR
event analysis can be a complicated and
somewhat confusing endeavor. We will
explain the nuances of OTDR manual and
automated discontinuity detection, and
provide a clear path to understanding the
OTDR results.
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us

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7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us 3
Training Objectives
• Introduction Into Optical Time Domain Reflectometry
• Review of Fiber Optic Fundamentals
• Understanding OTDRs
• Understanding FO Event Characteristics
• Understanding OTDR Results
• Applying Best Practices for Event Analysis

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Training Sections
• Fiber Optic Introduction
• Fiber Optic Fundamentals
• OTDR Introduction
• OTDR Fundamentals
• Event Analysis Fundamentals
• Conclusions
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us

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Fiber Optics
Historical Perspective
• Colladon / Babinet 1840s - Principle of total internal reflection in water jets & bent glass rods.
• Hopkins / Kapany /Snitzer 1950s – Light propagation with cladded fibers for applications of
medicine, defense, even television.
• Charles Kao / Standard Telecommunications Lab Team 1960s – proposed a perfected SiO2 fiber
light pipe for low loss transmission capabilities.
• Keck / Maurer / Schultz at Corning 1970s – proved Kao’s vision through designing and drawing
preforms of chemical vapor deposition glass.
• Kao the 2009 co-winner of the Nobel prize in physics. [courtesy Royal Swedish Academy of
Sciences]

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Fiber Optic Introduction
• FO medium is made of hair thin SiO2 glass material
with an inner core section, surrounded by outer
cladding section having differing optical densities.
• As the laser light enters into the FO core, differing
densities of the core/cladding interface trap the light via
a mechanism of total Internal reflection.
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us

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• IOR n=cvac/cmat
• Snell’s Law n1Sinθi=n2Sinθt
• Critical Angle sinθ = nclad/ncore
– Multimode Step Index
– Multimode Graded Index
– SingleMode
• Trapping Light From The
Source – Numerical Aperture
NA = sinθ = (n2
core-n2
clad)½
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© 2014 TTI / Mike Mazzatti / teratec.us
Fiber Optic FUNdamentals

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Fiber Optic FUNdamentals
• Losses - Attenuation
– dB or –dB
– Loss vs Wavelength (Loss vs Freq)
See Table
– Rayleigh Scattering Loss ≈ 1.7(0.85/λ)4
– Fresnel Reflections ρ = [(n-1)/(n+1)]2
– ORL Optical Return Loss
– Insertion Losses
– Connectorization Alignment
– Micro/macrobends & Absorption
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© 2014 TTI / Mike Mazzatti / teratec.us

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• Radiation travelling back towards the source has two
predominate classifications, Rayleigh scattering and
Fresnel reflections.
• Rayleigh scattering is the main source of loss of signal in
the fiber. Intrinsic material property caused by the
lightwave of ~1um, traveling though the sub-micron SiO2
crystalline matrix causes an elastic scattering.
• The received backscatter power P as a function of t :
= 0.5 α exp −
, where: S ≅
.
• A Fresnel reflection is caused by light traveling through a
glass density change, where reflected power is:
% ! = 100
#$ – #'(
#$ + #'(
*
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us
Fiber Optic FUNdamentals

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• An OTDR is one-dimensional Fiber Optic (FO) radar.
• OTDR development started in the 1970s with a simple analog
laser pulser, detection circuit, oscilloscope.
• Of FO test and measurement tools available, the OTDR is only
tool that provides time (or distance) measurements.
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us
Introduction Into
OTDR Technology

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Introduction Into
OTDR Technology
• Fiber Optic Radar – Launch an
ultra-short laser pulse monitor
the reflected light in sequential
time samples.
• A small amount of light is
scattered back to the source by
fiber impurities and crystalline
structure.
• Splices or connections cause
large reflections back to the
source.
• OTDR Measures Scattering
– Rayleigh – loss mechanisms of
the exponentially decaying signal
– Fresnel Reflections – high level
signals at discontinuities

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• Examines Events
– Distances – IOR fiber calibration – use reflecting events
– Splices - Slope Loss Analysis - 2pnt / splice / least square appr.
– Connectors – insertion loss and ORL
– Ends Reflections - Breaks
• What Gets In The Way ?
– Pulse Widths – Event Attenuation Dead Zone
– Dynamic Range – Noise
– Types Of Losses – Dirty Front End / High Level Reflection
Introduction Into
OTDR Technology

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OTDR Fundamentals
• FO speed of light, and
distance traveled is based
on optical density of the
glass, or the index of
refraction ‘n’.
• The light velocity ‘v’, and
distance ‘D’ to a
discontinuity is:
+ = , =
*
The reflection intensity verses the travel time (distance).
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us

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• The detected Rayleigh
scattering takes the shape
of an exponential signal.
• After every couple of
kilometers, the received
power is reduced
approximately in half.
• To make use of this
decaying signal the OTDR
provides a log conversion,
which basically contracts
the strong top signals and
expands the weak bottom. Distance (1km/div)
Linear
Power
(arb
units)
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us
OTDR Fundamentals

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• Typical un-averaged FO
trace shows
backscattered radiation
dropping across sections
of fiber containing
discontinuities.
• It not only is difficult to
discern the drops due to
pressure points or fiber
mismatches, also
discontinuities or ‘events’
very close together will
not be seen.
Distance (250m/div)
Loss
(
Arb
Units
)
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us
OTDR Fundamentals

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• To Improve SNR without
reducing spatial resolution,
co-addition is used to maintain
event features and reduce the
uncorrelated noise.
• Co-addition develops a
significant SNR improvement
by reducing the noise as a
square root of the number of
averages.
• The SNR 2 way improvement
will take the mathematical
form (/2 for OTDR 1 way):
-'./(01 = 102$ -13
Loss
(
Arb
Units
)
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us
OTDR Fundamentals
Signal to Noise Ratio

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• The signal is typically buried deep in the noise, so increase the laser
power to increase the signal reduce the bandwidth to eliminate noise.
• Increasing pulse length increases optical power, but will reduce the
spatial resolution of the signal and capability to distinguish close events.
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us
OTDR Fundamentals
Pulse Width

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• Again to improve SNR, one should increase the number of
averages to eliminate noise.
• Increasing the average time by a factor of four improves the
visible noise, and the OTDR dynamic range increases by 1.5dB.
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us
OTDR Fundamentals
Co-addition / Averaging

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OTDR Apparatus
Pulsed Lased Driver
Analog Delay Line
1st Preamplifier
2nd Preamplifier
Synchronous ADC
Programmable APD
2 Port BFT Coupler
FUT
Acquisition Engine
Optics Control
Accumulator
TI DSP
Keyboard Flash Drives USB
Display Engine
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us

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Event Analysis
Fundamentals
• After co-addition of many pulses, and log conversion of exponential decaying
signal, further analysis is needed.
• We use a second derivative for slope change detection and monitor
reflective rises in the signal for saturation and multiple events.
• Further averaging for splice loss or LSA methods is used.
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us

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‘Tail’ from a dirty connection at
20.7 meters causes long ‘dead’
zone hiding the next reflection.
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us
Reflection Event Analysis
With a clean connection set the
cursor at the left hand side of the
reflection for accurate distance.

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• Set cursor at left of reflection for distance to event. Make certain
splice loss areas around cursors are on level backscatter.
• Below the green lines of the Least Square Approximation (LSA)
areas need to be adjusted properly for level and accurate results.
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us
Reflection Event Analysis

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• Set cursor before backscatter droop for distance to event. Make
certain splice loss areas around cursors are on level backscatter.
• Use SPL LSA method in lower noise areas for best accuracy.
Use SPL AVG for maximum noise reduction with good results.
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us
Splice Event Analysis

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• Need to adjust the SPL
areas further away before
and after closely spaced
multiple reflection areas.
• With reduced pulse width
and more averaging time,
sufficient resolution can be
accomplished to detect
distance and ORL at each
reflection. However, losses
could be combined.
• In multiple fusion splices,
use pulse knowledge to
understand where high
losses could be occurring.
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us
Multiple Event Analysis

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• Ghosts events are caused
by saturated (dirty)
connections and end
reflections.
• Gainers are caused by
splicing on higher
backscatter coefficient
fiber, examining the other
direction will show a high
loss.
• Use a bidirectional loss
method to average out the
backscatter coefficient
problems (gainers). Use
macro bend analysis to
detect macrobend issues.
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us
Event Analysis Oddities
Ghost Reflections at
Double Network Length
Macrobend Losses at
1550nm in Black

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• Link Loss can be calculated using
the 2 Point method. To analyze
end connectors a launch and
receive patchcord is required.
• System ORL can be calculated
using a CW Continuous Optical
Return Loss method.
• PONs – Passive Optical
Networks are tricky to evaluate.
Live networks require out-of-band
laser OTDR analysis.
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us
Other Event Analysis

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Automatic Event Analysis
• Set parameter variable for
loss and reflection thresholds
– Splice Loss
– Reflection Level
– Link Loss
– End Of Fiber
• Use proportional and
differential analysis to detect
and mark events.
• Analyze events against
threshold settings and smart
parameters (macrobends,
ghosts), generate schematic.
• Examine events for best end
detection algorithm:
– End of Fiber Threshold?
– Saturated Reflections?
– Last Event?
Calculate Past
Slope
Calculate
Forward Slope
Calculate ORL
Calculate
Standard
Deviation
Calculate Slope
Differential
Test For
Reflection
Test For
Saturation
Test For
Splice
Display
Events
Test For
Completio
n
Mark Event
Analyze
Parameters
YES
YES
YES
YES
NO
NO
NO
NO
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us

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Manual and Automatic
Advantages / Disadvantages
• Manual Advantages / Disadvantages:
– Evaluation is adjusted for each network condition.
Encountering new conditions such as ghosts,
multiple events, etc., improves user experience
and adaptability.
– Looking closely at event traces can uncover
problems such as dirty connectors or slight
micro/macrobend conditions, that would be
unrecognizable to the machine.
– Without proper training and experience results
could be confusing, unrepeatable and inaccurate.
• Automatic Advantages / Disadvantages:
– Accurate evaluation against preset parameters.
Can be very repeatable.
– Quickest response and documentation process.
– Unforgiving in complex situations, cannot learn
from new experiences.
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us

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Summary
• We introduced FO principles
and discussed measurement of
network losses and distance.
• We introduced OTDR
fundamentals and explored
measurement techniques.
• We analyzed FO network
discontinuities and
perturbations to distinguish and
localize events.
• We evaluated event detection
mechanisms and determined
methods to improve analysis
performance and accuracy.
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us

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Conclusion
Muchas Gracias! Merci! Dank U! Thank You!
Tenk Yuh!
To Learn More Visit:
Terahertz Technologies Inc.
teratec.us
1-888-US-OTDRS
315-736-3642
mmazzatti(at)teratec(dot)com
7/15/14 © 2014 TTI / Mike Mazzatti / teratec.us