Demystifying OTDR Event Analysis CCTA Presentation 070714.pdf

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

2
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

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

4
Training Sections
• Fiber Optic Introduction
• Fiber Optic Fundamentals
• OTDR Introduction
• OTDR Fundamentals
• Event Analysis Fundamentals
• Conclusions

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]

6
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
• 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
7/15/14 7
© 2014 TTI / Mike Mazzatti / teratec.us
Fiber Optic FUNdamentals

8
• 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
7/15/14 8
© 2014 TTI / Mike Mazzatti / teratec.us

9
• 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
#$ – #'(
#$ + #'(
*

10
• 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.
Introduction Into
OTDR Technology

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

• 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

13
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).

14
• 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)
OTDR Fundamentals

15
• 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
)
OTDR Fundamentals

16
• 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
)
OTDR Fundamentals
Signal to Noise Ratio

17
• 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.
OTDR Fundamentals
Pulse Width

18
• 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.
OTDR Fundamentals
Co-addition / Averaging

19
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

20
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.

21
‘Tail’ from a dirty connection at
20.7 meters causes long ‘dead’
zone hiding the next reflection.
Reflection Event Analysis
With a clean connection set the
cursor at the left hand side of the
reflection for accurate distance.

22
• 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.
Reflection Event Analysis

23
• 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.
Splice Event Analysis

24
• 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.
Multiple Event Analysis

25
• 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.
Event Analysis Oddities
Ghost Reflections at
Double Network Length
Macrobend Losses at
1550nm in Black

26
• 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.
Other Event Analysis

27
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

28
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.

29
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.

30
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

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