Demystifying OTDR Event Analysis OTDR... · 2 Training Abstract An Optical Time Domain...

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 OpticsHistorical 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

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• 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

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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 SiO2crystalline 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 #$ – #&'(

#$ + #&'(

*



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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.


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).


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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)

Lin

ear

Pow

er

(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

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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 = 10��2$ -&13

Loss (

Arb

Units)


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.


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.


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


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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.


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‘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.

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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.


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.


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.


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.


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.


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 ORLCalculate 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


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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.


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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.


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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


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Demystifying OTDR Event Analysis OTDR... · 2 Training Abstract An Optical Time Domain...

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