Fiber Optics Guide

rement
Guide to Fiber Optic Measu
Reference: 901GFOM/00

Reprinted: September 2001

2001 Acterna

The information contained in this document is the property of Acterna. It is only provided for the operation and maintenance of the instrument. It must not be duplicated without the prior written permission of Acterna.

Acterna Saint-Etienne 34 rue Necker42000 Saint-EtienneTel. +33 (0) 4 77 47 89 00Fax +33 (0) 4 77 47 89 70Web www.acterna.com

http://www.acterna.com

Technical Specifications OFI 2000

11-8 USER MANUAL 720000992/03

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Acterna shall not be liable for errors contained herein.This document must not be photocopied, reproduced, or translated into another language without the written consent of Wavetek.

Printed in France

Authors J. LaferrièreR. TawsS. Wolszczak

ii Guide to Fiber Optic Measurements

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1Table of contents

Fiber Principles ................................................................................ 1-1Types of fibers....................................................................................... 1-1

Multimode fiber ........................................................................... 1-7Singlemode fiber .......................................................................... 1-8Fiber standards and recommendations ....................................1-10

Optical Testing ................................................................................ 2-1Families of optical fiber tests ............................................................... 2-1Transmission tests................................................................................. 2-2

Field tests ..................................................................................... 2-3Different families of optical testers ..................................................... 2-7

Sources, Power meters and Attenuators ....................................2-7Mini-OTDR ...............................................................................2-11Mainframe or full-featured OTDR ..........................................2-12Monitoring systems ...................................................................2-13Other general test equipment ..................................................2-16

Principles of an OTDR ..................................................................... 3-1Fiber Phenomena.................................................................................. 3-1

Rayleigh scattering ..................................................................... 3-2Fresnel reflection ......................................................................... 3-4

OTDR block diagram........................................................................... 3-5Laser diodes ................................................................................. 3-6Pulse generator with laser diode ................................................. 3-6Photodiode ................................................................................... 3-7Time base and control unit ......................................................... 3-7

OTDR specifications ............................................................................ 3-8Dynamic range ............................................................................. 3-8Dead Zone ..................................................................................3-11Resolution ..................................................................................3-14Accuracy .....................................................................................3-15Wavelength ................................................................................3-16

Using an OTDR ................................................................................. 4-1Acquisition ............................................................................................. 4-1

Guide to Fiber Optic Measurements iii

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Injection level .............................................................................. 4-2OTDR wavelength ..................................................................... 4-3Pulse width .................................................................................. 4-4Range ............................................................................................ 4-6Averaging ..................................................................................... 4-6Smoothing .................................................................................... 4-8Fiber parameters ......................................................................... 4-8

Measurement ..................................................................................... 4-10Slope or fiber section loss ......................................................... 4-14Event loss ................................................................................... 4-14Reflectance and Optical Return Loss ...................................... 4-17

Measurement artifacts and anomalies ............................................... 4-19Ghosts ......................................................................................... 4-19Splice "Gain" .............................................................................. 4-21

Getting the most out of your OTDR ................................................ 4-26Using launch cables ................................................................... 4-26Verifying continuity to the fiber end ....................................... 4-28Fault location ............................................................................. 4-29Effective refractive index ......................................................... 4-30

Glossary ........................................................................................... A-1

Notes ................................................................................................ N-1

Index ................................................................................................... I-1

iv Guide to Fiber Optic Measurements

Chapter

1

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1Fiber Principles

1.1 Types of fibersAn optical fiber is made of very thin glass rods composed of two parts:the inner portion of the rod or core and the surrounding layer or cladding.Light injected into the core of a glass fiber will follow the physical pathof that fiber due to the total internal reflection of the light between thecore and the cladding. A plastic sheathing around the fiber provides themechanical protection.

Fibers are classified into different categories based on the way in whichthe light travels in them, which is closely related to the diameter of thecore and cladding.

Principle of the transmission (simplified version):

• a ray of light enters into the fiber at a small angle α. • the capability (maximum acceptable value) of the fiber cable to

receive light on its core is determined by its numerical aperture NA:

where: α0: maximum angle of acceptance(i.e limit between reflection andrefraction)n1: core refractive indexn2: cladding refractive index

Note : 2 α0 is the full acceptance angle.

NA αsin 0 n12

n22

–= =

α0 arc n12

n22

–sin=

n2

n1

Cladding

Coreα0

Full acceptanceangle

Guide to Fiber Optic Measurements 1-1

Fiber Principles


Light propagation

• If α > α0: the ray is fully refracted and not captured by the core.

• If α < α0: the ray is reflected and remains in the core

Velocity

The velocity at which light travels through a medium is determined by therefractive index of the medium. The refractive index (n) is a unitless numberwhich represents the ratio of the velocity of light in a vacuum to the velocityof the light in the medium.

where:n: Refractive Indexc: Speed of light in a vacuum (approximately 3 x 108 m/s)V: Speed of light in the transmission medium

Typical values of n lie between 1.45 and 1.55.

Light entering with different angles does not follow the same path. Lightentering the center of the fiber core at a very low angle will take a relativelydirect path through the center of the fiber. Light injected at a high angle ofincidence or near the outer edge of the fiber core will take a less direct,longer path through the fiber and therefore travel more slowly down thelength of the fiber. Each path resulting from a given angle of incidence andentry point can give rise to a mode. As they travel along the fiber, all themodes are attenuated.

n1

n2

n2

αr

αi

Refraction : n1 sin αi = n2 sinαr

α0

αi αr n1

n2

n2

α0

Reflection : αi = αr

ncV----=

1-2 Guide to Fiber Optic Measurements

Types of fibers


Attenuation

The attenuation in a fiber is caused by different factors:

• light absorption. Absorption may be defined as the conversion of light energy to heat, and is related to the resonances in the fiber material. There are intrinsic absorptions (due to fiber material and molecular reso-nance) and extrinsic absorptions (due to impurities such as OH- ions at around 1240 nm and 1390 nm). In modern fibers, extrinsic factors are almost negligible.

• Rayleigh scattering. Scattering, primarily Rayleigh scattering, also contrib-utes to attenuation. Scattering causes the light energy to be dispersed in all directions, with some of the light escaping the fiber core. A small por-tion of this light energy is returned down the core and is termed «backs-cattering».

Note Forward light scattering (Raman Scattering) andbackward scattering (Brillouin scattering) are two additionalscattering phenomena that can be seen in optical materialsunder high-power conditions.

Backscattering effect

• bending losses which are caused by light escaping the core due to imper-fections at the core/clad boundary (microbending), or the angle of inci-dence of the light energy at the core/cladding boundary exceeding the Numerical Aperture (internal angle of acceptance) of the fiber due to bending of the fiber (macrobending).Singlemode fibers (for example) may be bent to a radius of 10 cm with no significant losses, however after the minimum bend radius is exceeded, losses increase exponentially with increasing radius. Mini-mum bend radius is dependent on fiber design and light wavelength.

Backscattered light

Scattered light

Incident light


Fiber Principles


For a fiber optic span, passive components and connection losses have to beadded to obtain the total signal attenuation.

Loss mechanisms

The attenuation, for a given wavelength, is defined as the ratio between theinput power and the output power of the fiber being measured. It is gener-ally expressed in decibels (dB).

This attenuation depends on the fiber and on the wavelength. For example,Rayleigh scattering is inversely proportional to the fourth power of thewavelength. If we look at the absorption spectrum of a fiber against thewavelength of the laser, we can notice some characteristics.

The following graph illustrates the relationship between the wavelength ofthe injected light and the total fiber attenuation resulting from the contribu-tion of all the loss mechanisms:

Input

Optical Fiber

Impurities

HeterogeneousStructures

InjectionLoss Absorption

Loss

DiffusionLoss

JunctionLoss

CouplingLoss

Output

BendingLoss

Macro or

micro bending


Types of fibers


Attenuation versus wavelength

The main telecommunication transmission wavelengths correspond to thepoints on the graph where the attenuation is a minimum. These wave-lengths are known as the “telecom” windows and are typically as follows:

• first window from 820 to 880 nm• second window from 1285 to 1330 nm• third window from 1525 to 1575 nm

Another factor affecting the signal during transmission is dispersion. Thisreduces the effective bandwidth available for transmission.

Two main types of dispersion are defined.

• Modal dispersion : when a very short pulse is injected into the fiber within the numerical aperture, all of the energy does not reach the end of the fiber at the same time. Different modes of oscillation carry energy down the fiber down different paths and thus travel further. As an exam-ple, a 50 µm core multimode fiber may have several hundred modes. This pulse spreading by virtue of different light path lengths is called modal dispersion or more simply modal dispersion.

• Chromatic dispersion : the pulse sent down the fiber is usually com-posed of a small spectrum of wavelengths. This means they go through the fiber at different speeds. Because propagation speed is dependent on the refractive index and therefore the wavelength, this effect is known as chromatic dispersion. It explains why it is important to use test equip-

850 1300 1550

Attenuation (dB)Scattering OH-absorption peak

Wavelength (nm)

Infrared absorption loss


Fiber Principles


ment which are at the same small spectrum of wavelengths as the wave-length of operation. Chromatic dispersion is expressed in picosecond per nanometer perkilometer: ps / (nm x km). This coefficient, at a given wavelength,represents the difference after one kilometer between the propagationtime of two wavelengths which differ by a given number of nanometers.Chromatic dispersion is the dominant dispersion mechanism insinglemode fibers. In singlemode fibers there is a minimum or zero(chromatic) dispersion wavelength determined by fiber design andmanufacture, and this wavelength is generally chosen to be near theoperating wavelength of the system. Historically (in standardsinglemode fiber), this was near 1310 nm, but for newer systems, so-called dispersion shifted fibers are used with the zero dispersionwavelength moved closer to 1550 nm to take advantage of the lowerfiber attenuation at that wavelength. In some systems, for example,Dense WDM (Wavelength Division Multiplexing) applications, a slightpositive chromatic dispersion is desirable and fiber designs are availableto accommodate this.This fiber is ideal for submarine cables because of the increased repeaterspacing and reduced cost. The maximum repeater spacing for high bitrate transmission is found by measuring the ratio between the maximumchromatic dispersion tolerated by the system (in ps/nm) and the fiber inps / (nm x km). The attenuation of the fiber must also be taken intoaccount.

Bandwidth limitation


Types of fibers


The two major classes of fibers are those that exhibit modal dispersion (mul-timode) and those that do not (singlemode) :

• Multimode fibers have much larger core (> 50 µm) than singlemode fibers permitting many modes of light to travel through the core.

• The core of a single mode fiber is generally 10 µm or less and will allow only one mode of light (at 1310 or 1550 nm) to propagate, greatly reduc-ing total dispersion.

1.1.1 Multimode fiber

Multimode fiber, due to its large core, enables different paths (multi-modes)to transmit the light along the link. This is the reason why this fiber is quitesensitive to the modal dispersion.

The primary advantages of multimode fiber are it’s ease of coupling to lightsources and to other fibers, reducing the cost of light sources (transmitters),connectorization and splicing. However, it’s relative higher attenuation and/or low bandwidth limit it to short distance and low speed applications.

Multimode fiber

A. Step index multimode fibers

Step-index fiber guides light rays through total reflection on the boundarybetween core and cladding. The refractive index is uniform in the core.Step-index fibers have minimum core diameter of 52.5 µm and 62.5 µm,

CoreDiameter: from 50 µm to 100 µm

CoatingDiameter: 250 µm

Cladding refractive index < core refractive index

CladdingDiameter: 125 µm and 140 µm


Fiber Principles


cladding diameter of 100/140 µm and numerical aperture between 0.2 and0.5.

Due to modal dispersion, the drawback to this design is its very low band-width, expressed as bandwidth-length product in MHz x km. This fiber’sbandwidth of approximately 20 MHz x km indicates that it is suitable forcarrying a 20 MHz signal only a distance of 1 km, or a 10 MHz signal a dis-tance of 2 km, or a 40 MHz signal a distance of 0.5 km, etc.

Step-index fibers have been implemented in plastic; their application fieldis mostly in short distance links which can accommodate high attenuations.

B. Graded-index multimode fibers

Graded-index (GI) fibers are obtained by giving to the core a non-uniformrefractive index, decreasing gradually from the central axis to the cladding.This index variation of the core forces the rays to progress in the fiber in asinusoidal manner.

The highest order modes will have a longer travel, but outside of the centralaxis, in areas of low index, their speeds will increase and the speed differ-ence between the highest and lower order modes will be smaller than forstep-index fibers.

Typical attenuations are : 3 dB/km at 850 nm 1 dB/km at 1300 nm.

The numerical aperture of graded-index fibers is typically about 0.2.

The bandwidth-length product for Graded index fibers is approximately:160 MHz x km at 850 nm500 MHz x km at 1300 nm.

Typical values of the group index :1.49 for 62.5 µm at 850 nm1.475 for 50 µm at 850 nm1.465 at 1300 nm.

1.1.2 Singlemode fiberThe advantage of singlemode fiber is its higher performance with respect tobandwidth and attenuation. The reduced core diameter limits the light topropagation of only one mode, eliminating modal dispersion completely.

With proper components, a singlemode fiber system can carry signals inexcess of 10 GHz for over 100 km. The system carrying capacity may be fur-


Types of fibers


ther increased by injecting multiple signals of slightly differing wavelengths(Wavelength Division Multiplexing) into one fiber.

The small core size generally requires more expensive light sources andalignment systems to achieve efficient coupling and splicing and connector-ization is also somewhat complicated. Nonetheless, for high performancesystem or systems over a few kilometers, singlemode fibers remain the bestsolution.

The typical dimensions of single mode fibers range from 5 to 12 µm for thecore and 125 µm for the cladding. A typical core-cladding angle is 8.5degrees.

The group index is typically 1.465 for the singlemode fiber.

Singlemode fiber

The small core diameter decreases the number of propagation modes. In asingle mode fiber, only one ray propagates down the core at a time.

Mode field diameter

The mode field diameter (MFD) of a single mode fiber can be expressed asthe section of the fiber where the majority of the light energy passes.

The MFD is larger than the physical core diameter i.e. an 8µm physical corecould yield a 9.5 µm MFD. This also shows that some of the light energyalso transits through the cladding.

CoreDiameter: 5 to 10 µm

CladdingDiameter: 125 µm

CoatingDiameter: 250 µm

Cladding refractive index < core refractive index


Fiber Principles


1.1.3 Fiber standards and recommendationsThere are many international and national standards governing optical cablecharacteristics of which only some are cited below.

International standards

For just the international standards, there are 2 main groups :

• The IEC has several standards of which we find:

• IEC 60793-1 and -2 Optical fibers (containing several sections)• IEC 60794-1, -2, and -3 Optical fiber cables

• The ITU-T (formerly the CCITT) has more standards such as:

• G650 Definition and test methods for the relevant parameters of sin-gle-mode fibers,

• G651 Characteristics of 50/125 µm multimode graded index optical fiber

• G652 Characteristics of singlemode optical fiber cable• G653 Characteristics of singlemode dispersion shifted optical fiber

cable• G654 Characteristics of 1550 nm loss minimized singlemode optical

fiber cable

National standards

• The CEN is preparing the following recommendations for Europe: EN 186000 (Optical fibre connectors), EN 187000 (Optical fibres), and the EN 188000 (Optical fibre cables);

• The ETSI provides additional recommendations for Europe;

• The EIA/TIA provides additional recommendations for the USA (FOTP).

Many other standards organizations exist in other countries.

Test equipment standards

• IEC 61350: Power meter calibration

• IEC 61746: OTDR calibration


Chapter

2

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2 Optical Testing

2.1 Families of optical fiber tests

When analyzing a fiber optic cable over its product life, a series of mea-surements have to be performed:

• mechanical tests, • geometrical tests, • optical tests • transmission tests.

The three first measurements are only performed once, as there isminor variation of these parameters during the fiber's life.

Several measurements are made on optical fibers or cables in order tocharacterize them before their use for transmission. Many of these mea-surements are described in the FOTP (Fiber Optic Test Procedure)propositions of the EIA (Electronic Industries Association) and aredefined by the ITU-T G650 recommendations or the EN 188 000 docu-ment.

Different kinds of test

Mechanical Geometrical Optical Transmission

Traction Concentricity Index Profile Bandwidth

Torsion Cylindricity Numerical aper-ture

Optical Power

Bending Core diameter Spot size Optical Loss

Temperature Cladding diameter

Reflectometry


Optical Testing


2.2 Transmission tests

The main measurements implemented on optical fibers and optical fibersystems in order to qualify their use for information transmission purposesare:

• End-to-End Optical Link Loss

• Rate of attenuation per unit length

• Attenuation contribution to splices, connectors, couplers (events)

• Length of fiber or distance to an event

• Linearity of fiber loss per unit length (Attenuation discontinuities)

• Reflectance or Optical Return Loss

Other measurements such as bandwidth or polarization mode dispersionmay also be done, but they are less important, except for some specificapplications.

Whereas some measurements may require access to both ends of the fiber,others require only one end. Measurement techniques which require accessto one end are particularly interesting for field applications since it willreduce the time spent travelling from one end of the fiber cable system tothe other.

If we focus on field testing on optical cables, we can see that there are threemain tasks - Installation, Maintenance and Restoration - where testing isrequired.


Transmission tests


2.2.1 Field tests

Below is a non-exhaustive list of the various tests that can be performed dur-ing each task (Installation, Maintenance, Restoration). The exact nature of atesting program will depend on the system design, system criticality andcontractual relationship between the cable and components suppliers, sys-tem owner, system installer and system user.

Installation testing is performed to ensure that fiber cables received fromthe manufacturer are conform to specifications (length, attenuation, etc.)and have not been damaged in transit, and that they are not damaged duringcable placement. Tests also determine the quality of cables splices and cableterminations (attenuation, location, reflectance) and that the completedcable subsystem is suitable for the intended transmission system (end-to-end loss, system optical return loss) and provide complete documentation ofthe cable link for maintenance purposes.

Maintenance testing involves periodic evaluation of the cable system toensure that no degradation of the cable, splices or connections has occurred(cable attenuation, attenuation and reflection of splices and terminations).In some systems, maintenance tests may be performed every few monthsand compared to historical test results to provide early warning of degrada-tion. In very high capacity or critical systems, automated testing devicesmay be employed to test the integrity of the system every few minutes togive immediate warning of degradation or an outage.

During cable restoration, testing is first performed to identify the cause ofthe outage (transmitter, receiver, cable, connector) and to locate the fault inthe cable if the outage is caused by the cable. Testing is then used to assessthe quality of the repaired system (permanent splices), similar to the testingperformed at the conclusion of cable installation.


Optical Testing


Pre-installation test on a drum

When installing a fiber network, network topology and equipment specifi-cations have to be taken into consideration. One of the major parameters tomeasure is optical loss budget or end-to-end optical link loss. When calculatingthe budget of a fiber link, the following must be considered: the source, thedetector and the optical transmission line. The transmission link includesthe source-to-fiber coupling loss, the fiber attenuation loss, and the loss ofall components along the line (connectors, splices, passive components,etc.).

Optical loss budget

An optical loss budget lies within maximum and minimum values:

• the maximum value is defined as the ratio of the minimum optical power launched by the transmitter to the minimum which may be received by the receiver whist still maintaining communication;

• the minimum value is defined as the ratio of the maximum optical power launched by the transmitter to the maximum which may be received by the receiver whist still maintaining communication.


Transmission tests


A typical example of a multimode system is described below.• Transmitter output power (typical) for multimode fiber

(GI) = -12 dBm ±2 dB• Optical Receiver sensitivity ≤ -27 dBm• Optical Receiver Dynamic Range ≥ 18 dB

The transmitter specification provides the maximum (-10 dBm) and mini-mum (-14 dBm) power levels that will occur.

The receiver sensitivity gives us the minimum power level that will bedetected.

The receiver dynamic range provides the maximum power level that can bedetected (-27 dBm + 18 dBm = -9 dBm).

In this example, the maximum optical loss budget is 13 dB :• Minimum optical power of the transmitter (-14 dBm) • Minimum receiver sensitivity (-27 dBm)

Example of a typical budget loss

Optical loss budget

Optical Budget

B max = L min - RminB min = L max - Rmax

Tx Rx

L max (dB)

R max (dB)

L min (dB)

R min (dB)

Launchedopticalpower (L)

Receivedopticalpower (R)

Minimum Optical loss budget (B min )

Maximum Optical loss budget (B max)

Opticalnetwork


Optical Testing


Optical loss budgets should take into account the cable and equipment mar-gins, which covers allowances for the effect of time and environmental fac-tors (launched power, receiver sensitivity, connector or splice degrada-tion...). In order to calculate this budget, typical values of attenuations of thedifferent fiber components are given, for example:

• 0.2 dB/km for singlemode fiber loss at 1550 nm;• 0.35 dB/km for singlemode fiber loss at 1310 nm;• 1 dB/km for multimode fiber loss at 1300 nm;• 3 dB/km for multimode fiber loss at 850 nm;• 0.05 dB for a fusion splice• 0.1 dB for a mechanical splice;• 0.2 - 0.5 dB for a connector pair;• 3.5 dB for a 1 to 2 splitter (3 dB splitting loss plus 0.5 dB excess loss).

Once this analysis is performed, the cable installation can be made.

Example of a typical budget loss

NETWORK SHORT HAUL MEDIUM HAUL LONG HAUL

Distance (km) 30 80 200

Fiber loss (dB/km) at 1550 nm 0.25 0.22 0.19

Total Fiber loss (dB/km) 7.5 17.6 38

N° of splices 15 40 25

Average splice loss 0.1 0.1 0.05

Total splice loss 1.5 4 1.25

N° of connectors 2 2 2

Average connector loss 0.5 0.5 0.5

Total connector loss 1 1 1

TOTAL LOSS 10 22.6 40.25


Different families of optical testers


2.3 Different families of optical testers

2.3.1 Sources, Power meters and Attenuators

The most accurate way to measure overall attenuation in a fiber is to inject aknown level of light in one end and measure the level when it comes out theother end. Light sources and power meters are the main instruments recom-mended by the ITU-T (G651) and the IEC 61350, to measure insertionloss.

This method required access to both ends of the fiber which is not alwayspossible.

Light source, power meter and talk set


Optical Testing


Light sources

A light source is a device used as a continuous and stable source (CW) forattenuation measurements.

It includes a source - either an LED or a laser - that is stabilized throughsome type of Automatic gain Control:

• LED’s are mainly used for multimode fibers. Lasers are used for single-mode applications.

• The light output of either an LED or laser source may also have the option to be modulated (or "chopped") at a given frequency. The power meter can be set up to detect this frequency. This improves ambient light rejection. A 2 kHz modulated light source can be used with certain types of detectors to "tone" the fiber for fiber identification or confirma-tion of continuity.

Power meter

The power meter is the standard tester in a typical fiber optic craftsman’stoolkit. It is an invaluable tool during installation and restoration.

The power meter’s main function is to display the incident power on thephotodiode. Features found on more sophisticated power meters mayinclude temperature stabilization, ability to calibrate to different wave-lengths, ability to display power relative to "reference" input, ability to intro-duce attenuation, or high power option.

The requirements for a power meter vary depending on the application.Power meters must have enough power to measure the output of the trans-mitter being used (to verify operation) but be sensitive enough to measurethe received power at the far (receive) end of the link. Long haul telephonysystems and cable TV systems use transmitters with outputs as high as+16 dBm and amplifiers with outputs as high as +24 dBm. Receive powerscan be as low as -36 dBm in systems that use an optical pre-amplifier. Inlocal area networks, transmit powers are much lower, as are received power.the difference between the maximum input and the minimum sensitivity ofthe power meter is termed the Dynamic Range.

While the dynamic range for a given meter has some limits, the usefulpower ranges can be extended beyond that by the of well characterizedattenuators in front of the power meter input; this does limit the low end



de (-3


sensitivity. this high power mode can be an internal or external attenuator :if internal, it may be fixed or switched.

Typical Dynamic Ranges requirements for power meters are:

• +13 dBm to -70 dB for telephony applications1,

• +24 dB to -50 dB for CATV applications1,

• -20 dB to -60 dB for LAN applications.

Insertion loss and cut back measurements

• The cut back technique is the most accurate measurement, but is also destructive, and cannot be applied in the field. This is the reason why it is not used during installation and maintenance. Testing with the cut-back method requires first measuring attenuation of the length of fiber under test, then cutting back a part of the length from the source end, and measuring attenuation of this part as a reference, and then substrac-ting the two values: the result gives the attenuation of the cut fiber.

• The insertion loss technique is a non destructive method to measure the attenuation across a fiber, a passive component or an optical link. With the substitution method, the output from a source and a reference fiber is measured directly, then a measurement is realized with the fiber to be measured added to the system. The difference between the two results gives the attenuation of the fiber.The purpose of the first or "reference" measurement is to cancel out asfar as possible the losses caused by the various patch cables.

1. Most power meters meet this requirements through two modes of operation, a standard moto -70 dBm) and a "high power" mode (+23 to - 50 dBm).


Optical Testing


Insertion loss method (2 steps) to measure the attenuation across a fiber

Significant variations may occur in attenuation measurements if precautionsare not taken with the injection conditions.

Transmitted and received optical power are only measured with an opticalpower meter. For transmitted power, the power meter is connected directlyto the optical transmitter’s output.

In the case of received power, the optical transmitter is connected to thefiber system and then the power level is read with the power meter from thefiber cable at the point where the optical receiver should be.

Power meter / light source combinations (also defined as loss test sets) mea-sure cable continuity and cable attenuation.

Link losses are sometimes measured in each direction and averaged toimprove confidence in the measurements.

Calibratedlight source Power meter

Referencefiber

Fiber under test

Reference pigtail

Power meterCalibratedlight source

Measurement P1

Measurement P2

Total attenuation of the fiber :AdB = P1dBm - P2dBm




2.3.2 Mini-OTDR

Using the same basic technology as the OTDR (see page 2-12), a new classof instruments became available in the beginning of the 90’s. Known as"mini-OTDRs", these fiber test instruments are typically battery-powered,lightweight, and small enough to be carried in one hand.

The simplest and earliest designs were capable of fault location as a mini-mum and some rudimentary analysis (attenuation, rate of attenuation, dis-tance and reflectance) of fiber systems. Modern designs mimic the capabili-ties of mainframe OTDRs including sophisticated analysis (automatic eventdetection, table of events, optical return loss, trace overlay) of fiber links,data storage capabilities, additional functionality (light source, power meter,talk set, visual fault locator) and even the modularity formerly found only inmainframe OTDRs.

A mini-OTDR has become the popular choice for pre-installation and resto-ration tests where ease-of-use and mobility are important.

Mini-OTDR


Optical Testing


2.3.3 Mainframe or full-featured OTDR

OTDRs are the main test equipment used to analyze fiber optics.

Most mainframe OTDRs are modular in design and contain a mainframeand different plug-in modules which can be implemented to suit the appli-cation.

The OTDR mainframe contains the controller, display, operator controls,and optional equipment (such as printer/plotter, external interfaces,modem, disk drive, etc.). The optical module consists of the laser sourceand optical detector and can be changed to allow testing at various wave-length and fiber type combinations.

Mainframe OTDRs are being rapidly replaced by mini-OTDRs but remainthe choice for laboratory and benchop applications where data acquisitionfunctions are desired.

Mainframe OTDR




2.3.4 Monitoring systems

Test equipment can be integrated into an automated system and connectedto a Network Manager.

Remote systems usually consist of an access point switch, several remotetest units that sit at various central offices, and a centrally located controller.

With traditional field test equipment, it can typically take about six hoursfrom the failure until the repair is made. The centralized control of a remotesystem allows carriers to manage their networks with fewer people. Theyalso can avoid sending crafts people into the field unless there is an actualneed for service.

In a case of a failure, the system can report the exact location of the prob-lem, so crafts people and technicians can quickly and easily find the troublespot in the field.

Remote fiber test system

CCCC

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: Cable Center

PSTN

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CC

CCCC

CCCC

CCCC

NetworkOperationCenter


Optical Testing


Most network operators initially will use remote systems to look for and sec-tionalize catastrophic failure of a link. In this case, the monitoring system isconnected to only one or two fibers in a multifiber link, assuming that in theevent of a catastrophic break all the strands will be cut.

Out-of-service

Remote monitoring can also be accomplished simultaneously with live traf-fic being transmitted through the use of Wavelength Division Multiplexing(WDM) and test equipment operating at wavelengths differing from thoseof the transmission system.

Dark fiber

NTE NTEcable under test

Cable under test

Fiber under test

OpticalSwitchOutput Fiber not in use

for transmission




In-service

WDM orBlocking Filter

WavelengthDivision

Multiplexer

WDM

NTE NTEcable under test

Cable under test

Fiber under test

OpticalSwitchOutput

WDM

lambda test

lambda test


Optical Testing


2.3.5 Other general test equipment

Talk sets

Talk sets transmit voice over installed fiber cable, allowing technicians splic-ing or testing the fiber to communicate, even when they are in the field.

Both singlemode and multimode talk sets exists.

They can be used to replace mobile or land-based telecommunicationsmethods which may not be cost-effective or which may not operate at thedistances common to fiber optic links.

OTS talk set

Visual Fault Locators

Visual Fault Locators are red light lasers which visually locate faults, up toaround 5 kilometers.

By sending visual light, the operator can easily see breaks and importantbends in the fiber, as the light escapes out. This function makes them use-ful for continuity testing of patch cords, jumpers, or short sections of fiber.




They can also be used in conjunction with:

• splicing machines to identify fibers to be jointed.

• OTDR to analyze failures which occur within the dead zone.

The most popular fault finders are made with a HeNe source.

Visual Fault Locators can use 635 nm, 650 nm or 670 nm lasers or LEDs,according to the application:

• 670 nm VFL provides long distance fault location and correct light inten-sity

• 635 nm VFL provides excellent visibility by shorter fault location.

Fiber Identifiers

Fiber Identifiers are test sets which can detect a modulated signal on a fiber(usually 2 kHz "tone").

Clip-on testers

These devices are used in conjunction with a suitable light source to enablepower measurements without disconnecting or damaging the fiber. Theclip-on tester is performing measurement by putting a controlled bend inthe fiber and measuring the level of light which escapes out of the fiber.The measurement can be performed non intrusively (low bend) or intru-sively (tight bend).


Optical Testing



Chapter

3

Otdr.bk : otdr_c3.doc Page 1 Mercredi, Mars 4, 1998 3:31 PM

3Principles of an OTDR

n OTDR (Optical Time Domain Reflectometer) is a fiber optictester characterizing fibers and optical networks. The aim of thisinstrument is to detect, locate and measure events at any loca-

tion in the fiber link.

One of the main benefits of the OTDR is that it can fully test a fiberfrom only one end, as it operates as a one dimensional radar system. TheOTDR is similar to an accurate radar as its resolution can be between6 cm and 40 meters.

The OTDR technique produces geographic information with regard tolocalized loss and reflective events thereby providing a pictorial andpermanent record which may be used as performance baseline.

3.1 Fiber Phenomena

The OTDR’s ability to characterize a fiber is based on detecting smallsignals returned back to the OTDR in response to injection of a largesignal, much like a "radar". In this regard, the OTDR depends on twotypes of optical phenomena: Rayleigh Backscattering and FresnelReflections.

The major difference between these two phenomena is as follows:

• Rayleigh scattering is intrinsic to the fiber material itself and is present along the entire length of the fiber. If Rayleigh scattering is uniform along the length of the fiber, then discontinuities in the Rayleigh backscatter can be used to identify anomalies in transmis-sion along the fiber length.

• On the other hand, Fresnel reflections are "point" events and occur

A


Principles of an OTDR


only where the fiber comes in contact with air or another media such as at a mechanical connection/splice or joint.

3.1.1 Rayleigh scatteringWhen a pulse of light is sent down a fiber, some of the photons of light arescattered in random directions from microscopic particles. This effect,referred to as Rayleigh scattering, provides amplitude and temporal informa-tion along the length of cable.

Some of the light is scattered back in the opposite direction of the pulse andis called the backscattered signal.

The scattering loss is the main mechanism for fibers operating in the threetelecom windows (850 / 1310 / 1550 nm). Typically, a singlemode fiber trans-mitting light at 1550 nm with a scattering coefficient (αs) of 0.20 dB/km, willlose 5 % of the transmitted power over a 1 km section of fiber.

The backscattering factor (S) describes the ratio between backscatteredpower and the scattered power. S is typically proportional to the square ofthe numerical aperture.

Depending on the fiber scattering coefficient (αs) and the fiber backscatte-ring factor (S), the backscatter coefficient (K) is the ratio of the backscatte-red power to the energy launched into the fiber.

The logarithmic value of the backscatter coefficient, normalized to a 1 nspulse duration, is given by:

Kns (dB) = 10 log K(s-1) - 90 dB

When Kns = - 80 dB, this means that for a 1 ns pulse duration, the backscat-ter power is - 80 dB below the incident pulse peak power.

Backscattered light1/1000 of scattered light

Scattered light5%/km at 1550 nm

Incident light


Fiber Phenomena


Note that -80 dB at 1 ns is equivalent to -50 dB at 1 µs, i.e. :Kµs (dB) = Kns(dB) + 30 dB

The Rayleigh scattering effect is like shining a flashlight in a fog at night:the light beam gets diffused -- or scattered -- by the particles of moisture. Athick fog will scatter more of the light because there are more particles toobstruct it.

The Backscattering depends on the launched power Po (Watt), the pul-sewidth used ∆t (seconds), the backscattering coefficient K(s-1), the distanced (meters) and the fiber attenuation (α) in dB/km:

A higher density of dopants in a fiber will also create more scattering andthus higher levels of attenuation per kilometer. An OTDR can measure thelevels of backscattering very accurately, and uses it to measure small varia-tions in the characteristics of fiber at any point along its length.

While Rayleigh scattering is quite uniform down the length of any givenfiber, the magnitude of Rayleigh scattering varies significantly at differentwavelength as shown in the following diagram and with different manufac-turer’s fiber.

Attenuation versus wavelength

Backscattering = Po . ∆t . K . 10 -α.d/5

OTDR parameters

850 1300 1550

Attenuation (dB)Scattering OH-absorption peak

Wavelength (nm)

Infrared absorption loss




3.1.2 Fresnel reflectionFresnel reflection is due to the light reflecting off a boundary of two opticaltransmissive materials, each having different index of refraction. Thisboundary can occur either at a joint (connector or mechanical splice), at annon-terminated fiber end, or at a break.

The magnitude of the Fresnel reflection is dependent upon the incidentpower and the relative difference between the two indices of refraction.The amount of light reflected depends upon the boundary surface smooth-ness and the index difference.

Reflected light from a boundary between a fiber and air has a theoreticalvalue of -14 dB. This value can be over 4000 times more powerful than thelevel of the backscatter. This means that the OTDR detector must be ableto process signals which can vary in power enormously. Connectors using gelcan reduce the Fresnel reflection. The gel acts as an index matching mate-rial minimizing the glass/air index difference.

n1 n2

Fiber Pi

Pr

Reflection is:

Pr : reflected powerPi : injected powern1, n2 : index of refraction

R = Pi

Pr =

(n1 + n2)2

(n1 - n2)2

From fiber to air R= 4% (-14 dB)


OTDR block diagram


3.2 OTDR block diagram

OTDR block diagram

The OTDR injects light energy into the fiber through a laser diode andpulse generator. The returning light energy is separated from the injectedsignal using a coupler and fed to the photodiode. The optical signal is con-verted to an electrical value, amplified, sampled and then displayed on ascreen.

Time Base Control

Unit

AveragingProcessing

Display unit

Amplifier

Sampling & ADC

Photodiode

Laser diode

PulseGenerator

CouplerFiber




3.2.1 Laser diodesLaser diodes are selected according to the wavelength of the test.

The current wavelengths for OTDR are 850 nm, 1300 nm for multimode,and 1310 nm, 1550 nm for singlemode.

1625 nm laser diodes are sometimes also used, particularly in remote moni-toring systems which are carrying live traffic. The purpose of using 1625 nmis to avoid interference with traffic at 1310 and 1550 nm.

3.2.2 Pulse generator with laser diodeA pulse generator controls a laser diode which sends powerful light pulses(from 10 mW to 1 Watt) into the fiber. These pulses can have a width in theorder of 2 ns up to 20 µs and a recurrence of some kHz.

The duration of the pulse (pulse width) can be selected by the operator fordifferent measuring conditions. The repetition rate of the pulses is limitedto the rate at which the pulse return is completed, before another pulse islaunched. The light goes through the coupler/splitter and into the fiberunder test.

The OTDR measures the time difference between the outgoing pulse andthe incoming backscattered pulses hence the word "time domain". Thepower level of the backscattered signal and the reflected signal is sampledover time. Each measured sample is called an "acquisition point" and thesepoints can be plotted on an amplitude scale with respect to time relative totiming of the launch pulse. It then converts this time domain informationinto distance based on the user entered index of refraction of the fiber. Theindex of refraction entered by the user is inversely proportional to the veloc-ity of propagation of light in the fiber. The OTDR uses this data to converttime to distance on the OTDR display and divide this value by two to takethe round trip (or two way) into account. If the user entered refractive indexis incorrect or inaccurate, the resulting distances displayed by the OTDRcan be in error.


OTDR block diagram


Propagation or group delay in fiber :

V (Gp delay) = c/n ~ 3.108 / 1.5 = 2.108 m/s

c = speed of light in vacuum (the real value of c is 2.99792458 m/s)n = refractive index.

OTDR time to distance conversion (round trip):

L (distance) = V(Gp delay). t/2 = c.t. / 2.n ~ 108 x time (seconds)

E.g. for a 10 ns pulsewidth: L = 108 x 10 ns = 1 m

3.2.3 PhotodiodeOTDR photodiodes are especially designed to measure the extremely lowlevels of backscattered light, at 0.0001% of what is sent by the laser diode.

As previously stated, the diodes must also be able to detect the relativelyhigh power of reflected pulses of light. This causes some problems whenanalyzing the results of an OTDR (see "Dead Zone" on page 3-11).

The bandwidth, sensitivity, linearity and dynamic range of the photodiodeand its amplification circuitry are carefully selected and designed to be com-patible with the pulsewidths used and the levels backscattered from thefiber.

3.2.4 Time base and control unitThe control unit is the brain of the OTDR. It takes all the acquisitionpoints, performs the averaging, plots them as a log. function of time andthen displays the resulting trace on the OTDR screen.

The time base controls the pulsewidth, the spacing between subsequentpulses and the signal sampling. Multiple passes are used to improve the sig-nal to noise ratio of the resulting trace. Since noise is random, by acquiringmany data points at a given distance and averaging them, the noise will tend

P (Injection )

P (Reflection)P (Reflection)




to average out toward zero and the remaining data will more accurately rep-resent the backscatter or reflection level at that point. An OTDR mayacquire up to 32,000 data points and fire thousands of pulses, so the OTDRprocessor must be very powerful to deliver fast performance to the user.

The display shows a vertical scale in dB and an horizontal scale in km (orfeet), and plots numerous acquisition points which represent the backscat-ter "signature" of the fibers under test.

Typical OTDR trace

3.3 OTDR specifications

3.3.1 Dynamic rangeThe dynamic range is one of the most important characteristics of anOTDR, since it determines the maximum observable length of a fiber andtherefore the OTDR suitability for analyzing any particular network. Thehigher the dynamic range, the higher the signal to noise ratio and the betterthe trace will be, with a better event detection. This dynamic range is rela-tively difficult to determine since there is no standard computation methodused by all the manufacturers.

Connector pair

Distance (km)

Fusion Splice

Connector pair

Fiber bend

Mechanical splice

OTDR

Attenuation (dB)

Fiberend


OTDR specifications


Definitions of the dynamic range

One method of determining dynamic range (approved and endorsed by theIEC 61746) is to take the difference between the extrapolated point of thebackscatter trace at the near end of the fiber (taken at the interceptionbetween the extrapolated trace and the power axis) and the upper level ofthe noise floor at or after the fiber end.

• The upper level of the noise is defined as the upper limit of a range which contains at least 98% of all noise data points.

• The level is expressed in decibels (dB).

• This measurement is performed with a 3 minute period for the averag-ing.

• This value of the dynamic range was also recommended by Bellcore.

Other definitions of the dynamic ranges are given by different manufactur-ers, which makes the values comparison very difficult:

Dynamic range

• RMS. The RMS (Root Mean Square) also termed SNR=1 dynamic range is the difference between the extrapolated point of the backscatter trace at the near end of the fiber (taken at the intersection between the extrapolated trace and the power axis) and the RMS noise level. You can compare this value to the IEC 61746 definition by substracting 1.56 dB from the RMS dynamic range if the noise is gaussian.

• N=0.1 dB . This dynamic range definition gives an idea of the limit to

Dyn

amic

IEC

(98

%)

Dyn

amic

ran

ge

(RM

S)

~6.6 dB

1.56 dBSNR=1

Peak noise level

N = 0.1 dB

dB

km




which the OTDR can measure when the noise level is 0.1 dB on the trace. The difference between N=0.1 and SNR=1 RMS definition is approximately 6.6 dB. This means that an OTDR which has a dynamic range of 28 dB (SNR=1) can measure a fiber event of 0.1 dB up to 21.5 dB.

• End detection : The dynamic range end detection is the one way differ-ence between the top of a 4% Fresnel reflection at the start of the fiber and the RMS noise level. This value is approximately 12 dB higher than the IEC value.

• Bellcore measurement range : The Bellcore measurement range is defined as the maximum attenuation that can be placed between the OTDR and an event for which the instrument will still be able to mea-sure the event within acceptable accuracy limits. The event can be reflective or non-reflective, or a fiber break. For example, an event can be a 0.5 dB reflective splice (> 40 dB).

• 4% Fresnel : This is more an echometric parameter than a reflectome-tric parameter. It represents the ability of the instrument to perceive the peak of a Fresnel reflection for which the base cannot be perceived. It is defined as the maximum guaranteed range over which the far end of the fiber is detected, sometimes with a minimum of 0.3 dB higher than the highest peak in the noise level;

• Peak level plus 0.3 dB : the dynamic range is the difference between the front-end backscattered trace and 0.3 dB more than the peak noise level.

The value of the dynamic range , for each definition can also be givenaccording to different conditions:

• typical value : this represents the average or mean value of the dynamic range of the OTDRs which come out of production. An increase of around 2 dB is usually shown in comparison with the specified value.

• specified value : this is the minimum dynamic range specified by the manufacturer for its OTDR.

• over a temperature range or at room temperature . At low and high temperature, the dynamic range decreases usually by 1 dB.


OTDR specifications


3.3.2 Dead Zone

OTDR dead zone example

Why do we have dead zone ?

The OTDR is designed to detect the backscattering level all along the fiberlink. It measures backscattered signals which are much smaller than the sig-nal sent to the fiber. The component which receive those values is the pho-todiode. It is designed to receive a given level range. When there is a strongreflection, then the power received by the photodiode can be more than4000 times higher than the backscattered power and can saturate the photo-diode. The photodiode requires time to recover from the saturated condi-tion; during this time, it will not detect the backscatter signal accurately.The length of fiber which is not fully characterized during the recoveryperiod is termed the dead zone.

This effect is similar to the one when you are driving a car at night, and thatanother car’s headlights dazzle your vision momentarily.

Attenuation dead zone

The attenuation dead zone (defined in IEC 61746) for a reflective or atten-uating event is the region after the event where the displayed trace deviatesfrom the undisturbed backscatter trace by more than a given vertical value∆F (usually 0.5 dB or 0.1 dB). Bellcore specifies a reflectance of - 30 dB, a

Dead zone




loss of 0.1 dB and gives different locations. In general, the higher thereflected power sent back to the OTDR, the longer the dead zone.

The attenuation dead zone depends on the pulsewidth, the reflectance, theloss, the displayed power level and the location.

The attenuation dead zone usually indicates the minimum distance after anevent where the backscatter trace can be measured.

Attenuation Dead Zone measurement

At short pulse widths, the recovery time of the photodiode is the primarydeterminant of the attenuation dead zone and can be 5 to 6 times larger thanthe pulse width itself. At long pulsewidths, the pulsewidth itself is the dom-inant factor, and the attenuation deadzone is, in effect, equal to the pul-sewidth itself. The dead zone specified in the literature is generallymeasured at the shortest pulsewidth.

Bellcore specifies objectives for two attenuation dead zone, the "front end"dead zone and the "network" dead zone. Historically, the connectionbetween the OTDR was highly reflective; this an other factors often causedthe dead zone seen at the front end of the OTDR, to be much longer thanthe dead zone resulting from a reflection in the network. Currently, theOTDR connection has been engineered to have very low reflectance andthere is little difference between the front end dead zone and network deadzone.

ADZAttenuation dead zone

∆F = 0.5 dB or 0.1 dB


OTDR specifications


If the front end attenuation dead zone of the OTDR in use is large, theeffect can be minimized using a launch cable (see "Using launch cables" onpage 4-26).

Event dead zone

Event dead zone is the minimum distance on the trace, where two separateevents can still be distinguished. The distance to each event can be mea-sured, but the separate loss of each events cannot be measured.

This parameter usually gives an indication of the minimum distance inorder to distinguish between reflective events which occur in close proxim-ity.

• For a reflective event, the event dead zone definition is the distance between the two opposite points which are 1.5 dB (or FWHM) down from the unsaturated peak.

Event Dead Zone measurement

• For an non-reflective event, the event dead zone can be described as the distance between the points where the beginning and ending levels at a splice or a given value (≤ 1 dB) are within ±0.1 dB of their initial and final values (this is not the definition).

Event dead zones can also be reduced using smaller pulsewidths.

1.5 dB

EDZEvent

dead zone

Reflective eventEDZ Definition

Non-reflective eventEDZ example

≤1 dB

Event dead zone

±0.1 dB

± 0.1 dB




Front end event dead zone effects can also be minimized from a fiber undertest using a launch cable (see "Using launch cables" on page 4-26).

3.3.3 ResolutionThere are four main resolution parameters: display (cursor), loss (level),sampling (distance) and distance.

Display resolution

The display resolutions are defined as follows:

• The readout resolution is the minimum resolution of the displayed value (e.g. an attenuation of 0.031 dB will have a resolution of 0.001 dB).

• The cursor resolution is the minimum distance or attenuation between two displayed points, where a line has been drawn. A typical value can be 6 cm or 0.01 dB

Loss resolution

The loss resolution is governed by the resolution of the acquisition circuit.For two near power levels, it specifies the minimum loss difference that canbe measured. This value is generally around 0.01 dB.

Sampling resolution

The sampling (or data point) resolution is the minimum distance betweentwo acquisition points.

This data point resolution can go down to centimeters depending on pul-sewidth and range.

In general, the more datapoints that an OTDR can acquire and process, thebetter the sampling resolution. The number of datapoints an OTDR canacquire is therefore an important performance parameter.

a typical value for a high resolution OTDR would be 1 cm sampling resolu-tion.

Distance resolution

Distance resolution is very similar to sampling resolution.


OTDR specifications


The ability of the OTDR to locate an event is affected by the sampling res-olution. If it only samples acquisition points every 1 meter, then it can onlylocate a fiber end within ± 1 meter. The distance resolution is then like thesampling resolution, a function of the pulse width and the range. This spec-ification must not be confused with distance accuracy which is discussedlater.

3.3.4 AccuracyThe accuracy of a measurement is the capacity of the measurement to becompared with a reference value.

Linearity (Attenuation accuracy)

The linearity of the acquisition circuit determines how close an optical levelcorresponds to an electrical level, across the whole range.

Most OTDRs have an attenuation accuracy of 0.05 dB/dB. Some OTDRscan go down to 0.02 dB/dB.

If an OTDR is non linear then with long fibers, the section loss values willchange significantly.

Distance accuracy

The distance measurement accuracy depends on the following parameters:

• Group index : Whereas index of refraction refers to a single ray in a fiber, group index refers to the propagation velocity of all the light pulses in the fiber. The accuracy of the OTDR distance measurements depends on the accuracy of the group index.

• Time base error. This is due to the inaccuracy of the quartz, which can vary from 10-4 to 10-5. In order to have an idea of the distance error, one has to multiply this uncertainty by the measured distance.

• Distance error at the origin.

A typical value for the MTS 5100 mini-OTDR is :± 5 x 10-5 x distance ± 1m ± sampling resolution ± group index uncertainties




3.3.5 WavelengthOTDRs measure according to a wavelength. The major wavelengths are850 nm, 1300 nm for multimode, and 1310 nm and 1550 nm for singlemode.A fourth wavelength is now appearing for monitoring live systems: 1625 nm.This occurs if the two singlemode wavelengths are used for transmission.

The wavelength is usually specified with a central wavelength and a givenspectral width. The standard spectral width is ±30 nm, but that can be ±10nm. Some OTDRs display the laser wavelengths used for the measurement.

The attenuation of optical fiber varies with the wavelength, and any mea-surement should be corrected to the transmission wavelength or to the cen-tral wavelength (850, 1310 or 1550 nm). Correction is most relevant in thefirst window at 850 nm.


Chapter

4


4Using an OTDR

he OTDR is very versatile and has many applications. Firstly, it’simportant to select an OTDR that has the proper specifications

(see chapter 3) for the task at hand. With recent breakthroughs in mod-ularity, some OTDRs, like the MTS 5100, can be configuredflexibly to perform testing on almost any kind of fiber optic network,singlemode or multimode, short or long haul.

We can broadly define the use of the OTDR as a two step process :

❏ Acquisition step where the unit acquires data and displays the results either numerically or graphically;

❏ Measurement step where the operator analyzes the data and makes a decision based on the results to either store, print, or go the next fiber acquisition.

4.1 Acquisition

Most modern OTDRs now automatically select the optimal acquisitionparameters for a particular fiber by sending out test pulses in a processknown as auto-conf iguration. Using the Auto-configuration feature, theuser would select the wavelength (or wavelengths) to test, the acquisi-tion (or averaging) time, and the fiber parameters (e.g. refractive indexif not already entered).

There are about three major approaches to configuration of the OTDR:

• A user might simply let the OTDR autoconfigure and accept the

T


Using an OTDR


acquisition parameters selected by the OTDR.

• A more experienced user might allow the unit to autoconfigure, analyze the results briefly and change one or more acquisition parameters to optimize the configuration for the purposes of his test.

• The experienced user may choose not to use the autoconfiguration fea-ture altogether and enter acquisition parameters based on his experience and knowledge of the link under test.

Typically, when testing multifiber cables, once appropriate acquisitionparameters are selected, they are "locked in" and the same parameters areused for every fiber in the cable (this speeds the acquisition process and pro-vides for consistency in the data which is helpful when analyzing or compar-ing fibers).

Below, various acquisition parameters and their effect on the resulting traceare discussed.

4.1.1 Injection levelDegrading the quality of the OTDR front panel connector through non-cleanliness will result in poor measurements.

The injection level is defined as the power level which OTDR injects intothe fiber under test. The higher this level, the higher the dynamic range. Ifthe injection level is low, traces will be noisy and measurement accuracy willbe degraded. Poor launch conditions resulting in low injection levels are theprimary reason for reductions in precision.

The presence of dirt on connector faces and damaged or low quality pigtailsor patchcords are the primary cause of low injection levels. It is importantthat all physical connection points are free of dust and dirt in an optical sys-tem. With core diameters of less than 10 µm in singlemode systems, thepresence of even a 4 µm speck of dirt or dust (approximately the size of theparticulate matter in cigarette smoke) can severely degrade injections lev-els.

Cleaning kits are available for optical systems from basic tools including iso-propynol cleaning solution, joseph paper, compressed-air spays, and ready-to-use impregnated wipes, to more advanced methods with cassette clean-ers.

Mating of dirty connectors to the OTDR connector, may scratch the OTDRconnector, permanently degrading launch conditions.


Acquisition


Some OTDRs, like the MTS 5100, will display the measured injection levelduring real time acquisition or just prior to averaging. The result is dis-played on a relative scale on a bar graph rating the injection level from"good" to "bad".

To determine the relative quality of the injection level, the OTDR "looks"out a short distance and observes the backscatter returned from the launchpulse and compares this to an expected value. It is sometimes possible forthe injection level to show "bad" when it is in fact acceptable. This will hap-pen if there is an attenuator in the system, near the OTDR or if there is asplitter near the OTDR; in this case, the backscatter level will be lower than"expected" by the injection level meter. Even though the injection levelincreases as pulsewidth increases, the scale displayed is calibrated sepa-rately for each pulsewidth so the scale is meaningful at any pulsewidth andincreasing pulsewidth will not change a bad injection level to a good one.

4.1.2 OTDR wavelengthThe behavior of an optical system is directly related to the wavelength oftransmission. Not only optical fiber will exhibit different loss characteristicsat different wavelengths, but splice loss values will also differ at differentwavelengths.

In general, the fiber should be tested with the same wavelength as that usedfor transmission. This means 850 nm and/or 1300 nm for multimode sys-tems, and 1310 nm and/or 1550 nm for singlemode systems.

If testing is only to be performed at one wavelength, the followingparameters need to be considered:

1. For a given Dynamic range, 1550 nm will see longer distances down the same fiber than 1310 nm due to the lower attenuation in the fiber:

• 0.35 dB/km at 1310 nm means that approximately 1 dB of signal is lost every 3 km.

• 0.2 dB/km at 1550 nm means that approximately 1 dB of signal is lost every 5 km.

2. Single mode fiber has larger mode field diameter (see MFD page 1-9) at 1550 nm than 1310 nm. Larger mode fields are less sensitive to lateral offset during splicing, but more sensitive to losses incurred by bending during either installation or in the cabling process.


Using an OTDR


• 1550 nm is more sensitive to bends in the fiber than 1310 nm. This is shown diagrammatically below. This can also be termed as macroben-ding.

• 1310 nm will generally measure splice and connector losses higher than 1510 nm. These results come from a Corning study of over 250 splices where the 1310 nm values were shown to be typically higher by 0.02dB over the 1550nm values for dispersion-shifted fiber.

Sensitivity to bending radius = 37,5 mm

Sensitivity to bending radius = 30 mm

4.1.3 Pulse widthThe OTDR pulsewidth duration controls the amount of light that will beinjected into the fiber. The longer the pulsewidth means the more the lightenergy injected. The more light injected means the more light backscatte-red or reflected back from the fiber to the OTDR.

Long pulsewidths are used to see long distances down a cable. Long pul-sewidths will also produce longer zones in the OTDR trace waveform where

Loss (dB)

1300 1400 1600 17001500 λ (nm)

0.5

0

13100.013

15500.042

15800.094

16200.048

Loss (dB)

13100.0051

15500.123

15800.489

16202.253

1300 1400 1600 17001500 λ (nm)


Acquisition


measurements are impossible. We call this the dead zone of an OTDR (seepage 3-11).

Short pulsewidths inject lower “levels” of light but reduce this dead zone.

Different pulsewidths

The pulse width duration is usually given in ns but can also be estimated in

meters according to the following formula: .

where c represents the speed of light in vacuum (3 x 108 m/s), T the pulseduration in ns, and n the refractive index.

As an example, a 100 ns pulse could be interpreted as a "10 m" pulse.

Time or Pulse width 5 ns 10 ns 100 ns 1µs 10 µs 20 µs

Distance or fiber length 0.5 m 1 m 10 m 100 m 1 km 2 km

10ns

30ns

100ns

1µs

3µs

10µs

Dc T×2n

------------=


Using an OTDR


4.1.4 RangeThe range on an OTDR is the maximum distance that the OTDR willacquire data samples. The longer this parameter, the more distance theOTDR will shoot pulses down the fiber.

This parameter is generally set at twice the distance of the end of the fiber.

If this parameter is incorrectly set, the trace waveform could contain somemeasurement artifacts (see "Ghosts" on page 4-19).

4.1.5 AveragingThe OTDR detector works with extremely low optical power levels (as lowas 100 photons per meter of fiber). Averaging is the process by which eachacquisition point is sampled repeatedly and the results averaged to improvethe signal-to-noise ratio.

By selecting the time of acquisition or the number of averages, the user con-trols this process within an OTDR.

The longer the time or the higher the number of average, the more signalthe trace waveform will display, in random noise conditions.

The relationship between the acquisition time (number of averages) andthe amount of improvement of the signal-to-noise ratio is expressed by theequation below:

N being the ratio of the two averages.

Note that the noise distribution is considered random for this formula.

As an example, an acquisition with 3 minutes averages will improve by 1.2dB the dynamic range compared to an acquisition with 1 minute. Averagingwill improve the signal to noise ratio by increasing the number of acquisi-tions, but the time taken to average the trace is increased. However, accord-ing to the equation, beyond a certain time, there is no advantage to begained as only the signal remains.

In theory, four times more averaging equals + 1.5 dB gain in dynamic range.

5 N10log


Acquisition


Dynamic range versus averaging

10

10,5

11

11,5

12

12,5

13

13,5

20 40 60 80 100

120

140

160

180

Averaging Time (s)

Dyn

. IE

C Helios @ 5ns PW

Theoretical


Using an OTDR


4.1.6 SmoothingSmoothing is a technique whereby the signal-to-noise ratio is improved bydigitally filtering the acquisition points.

To improve accuracy at lower light levels an OTDR can use filters and aver-aging techniques to combine the measurements from many pulses.

Two identical fibers - top trace with a smoothing filter

A smoothing function can be performed on the acquisition points. This isperformed by using specific coefficients. A given true point value is modi-fied to another value which combines previous and subsequent acquisitionswith relevant coefficients.

4.1.7 Fiber parametersOther parameters related to the fiber can affect the OTDR results as fol-lows:

• Refractive Index n : this index is directly related to distance measure-


Acquisition


ments. When comparing distance results from two acquisitions, always be sure that the appropriate index is being used. It should be noted that using the refractive index reported by the fiber manufacturer will cause the OTDR to report fiber length accurately. However, often, particularly during fault location, the user wishes to determine the cable length. Fiber length and cable length are not identical and differ due to the overlength of the fiber in the buffer tube and the geometry (helixing) of the buffer tubes in the cable. The ratio between fiber length and cable length varies depending on cable fiber count and cable design, and even cable manufacturer. While it is possible to have this value (typically termed the "helix factor") reported by the manufacturer, the precision of the value still allows for large uncertainty in fault location. It is often recommended to measure a known length of similarly con-structed cable and determine an "effective refractive index" that willcause the OTDR to report cable length instead of fiber length. See "Get-ting the most out of your OTDR" on page 4-26 for more information onthis.

• Backscatter coefficient K : the backscatter coefficient K tells the OTDR the relative backscatter level of a given fiber. This coefficient is entered at the factory and generally the user will not change this param-eter. Changing it will affect the reported value of reflectance and optical return loss. While the assumption is made that the backscattered coeffi-cient for the entire span is consistent, it is possible that there will be very slight variations from one fiber span to the other. This variation can cause measurement anomalies such as splices with negative loss values (or gainers). See section Measurement artifacts and anomalies on page 4-19 for measurement techniques that minimize the impact of these.Typical backscatter coefficients at 1 ns are:- for standard single mode fiber: - 79 dB at1310 nm

- 81 dB at 1550 nm- for standard multimode fiber: - 70 dB at 850 nm

- 75 dB at 1300 nm


Using an OTDR


4.2 Measurement

Most modern OTDRs will perform fully automatic measurements with verylittle user intervention.

In general, there are two types of events: reflective and non reflective.

• Reflective events where a discontinuity in the fiber causes an abrupt change in the refractive index are either caused by breaks, connectors junctions, mechanical splices or the undeterminated end of fiber. Con-nector loss can be around 0.5dB and mechanical splices can range from 0.1dB up to 0.2dB

• Non reflective events occur where there are no discontinuities in the fiber and generally are produced by fusion splices or bending losses. Typical values would be from 0.02dB up to 0.1dB depending on the splicing equipment and operator.

The following measurements can be performed by an OTDR.

For each event: distance locationlossreflectance

For each section of fiber: section lengthsection loss in dB section loss rate in dB/kmORL (Optical Return Loss) of the section

For the complete terminated system:link lengthlink loss in dBORL of the link

The OTDR allows the user, at his discretion to perform measurements onthe fiber span in at least three different ways. The user can also use a combi-nation of these methods:

1. full automatic function : in this case, the OTDR will detect and measure automatically all the events, sections and fiber end, using an internal detection algorithm.


Measurement


Fully automatic trace and table of events (Table mode)

2. semi automatic function : when this is selected, the OTDR will measure and report an event at each location (distance) where a marker has been placed. These markers can be placed automatically or manually. This function is of high interest during span acceptance (after splicing), where the user desires to completely characterize all events along the span to establish baseline data. Automatic detection will not detect and report a non-reflective event with a zero loss, and therefore, a marker is placed at that location so that the semi-automatic analysis will report the zero loss. Further analysis of the trace using a PC software package such as WinTrace ® to perform bi-directional analysis of the span, then using semi-automatic measurement at fixed marker locations, will ensure consistency in the number of events from fiber to fiber and from measurements in the opposing direction.


Using an OTDR


Measurement with markers

3. manual measurement function : For even more detailed analysis or special conditions, the operator can completely control the measurement function manually. This means that the operator will place 2 or more cursors to control the way the OTDR measures the event or value. Depending on the parameter being measured, the operator may need to position up to 5 cursors to perform a manual measurement. While this is the slowest and most cumbersome method of measurement, it is important to have this capability available for those fiber spans whose design or construction are very unusual and difficult for automated algorithms to analyze accurately.


Measurement


Manual ORL measurement


Using an OTDR


4.2.1 Slope or fiber section lossThe slope of section of fiber, given in dB/km, can be measured either usinga 2-point method (described on page 4-14) or by using a least-squaresapproximation (LSA).

The least-squares approximation method tries to determine the measure-ment line that has the closest fit to the set of acquisition points. It is themost precise means to measure fiber loss but requires a continuous sectionof fiber, a minimum number of OTDR acquisition points, and a relativelyclean backscatter signal free of noise.

Least square approximation : fitting a straight line

The section loss can be reported either in dB or in dB/km. Typical sectionlosses range between 0.15 to 0.25dB/km for 1550nm systems, 0.25 to0.35 dB/km for 1310 nm singlemode, 0.5 to 1.5 dB/km for 1300 nm multi-mode, and 2 to 3.5 dB/km for 850 nm systems.

4.2.2 Event lossUsing manual measurements, there are two ways to measure an event loss:

2-point method

The operator must position a first cursor on the linear level before theevent, and a second cursor on the linear backscatter level after the event.The event loss is then the difference between these 2 cursor measurements.This method can be used for a reflective or a non-reflective event. How-ever, the precision of this method depends on the user’s ability to place thecursors at the correct positions and can be compromised if the trace has alarge amount of residual noise.

If the trace is very noisy or ‘spiky’, then the user should try to place the cur-sor on a data point on the trace that is not the top of a spike or bottom of a


Measurement


trough: this is a sort of visual “averaging” of the trace. If the user is using thetwo point method to measure a ‘point’ event (like a splice as opposed to alength of fiber), then the user should be aware that the result will alsoinclude the effects of any fiber losses between the cursors, because the dis-tance between the cursors is non-zero.

2-point measurement

5-point method

The purpose of the 5 point measurement method of ‘point’ events is toreduce the effects of noise on the fiber spans before and after the event byperforming a least squares analysis on the fiber spans, and to minimize theadditional fiber loss that is reported as event loss due to the non-zero dis-tance between the cursors. In order to do this, the software uses the positionof the 5 cursors to extrapolate the fiber data before and after the event andtake a zero distance measurement of the loss at the event location.

This method is used to measure the loss of both a non-reflective and reflec-tive events.

To accomplish this, first the operator must make a slope measurementbefore and after the event on the linear backscattered level of the trace. The


Using an OTDR


5th point of measurement is placed just before the event where the backs-catter trace suddenly deviates and the loss measurement is then made atthis event location. This method is more precise than the 2-point as theOTDR is comparing the difference between 2 linear backscatter levels.

5-point method

5-point measurement


Measurement


4.2.3 Reflectance and Optical Return LossThe reflectance of an event represents the ratio of the reflected power tothe incident power at that discrete location in a fiber span. It is expressed indecibels (-dB). The small negative value indicates a larger reflection than alarge negative value. That is, a reflectance of -33 dB is larger than a reflec-tance of -60 dB. The larger reflectance will show up as a higher peak on thetrace waveform.

Reflectance measurement

The amount of reflection at a connector, break or mechanical splicedepends on the difference in the refractive index in the fiber and the mate-rial at the fiber interface (another fiber, air, index matching gel), and geome-try of the break or connector (flat, angled, crushed, each of which will per-mit a different amount of reflection to be captured by in the fiber core).

Most mechanical splices use an index-matching gel or fluid to reduce theamount of change. Smaller changes in the refractive index produce smallerreflections. Some OTDRs can measure the amount of reflecting light auto-matically by placing one cursor just in front of the reflection, an another cur-


Using an OTDR


sor at the top of the reflection and by pressing the appropriate button on thecontrol panel.

The Optical Return Loss (ORL) represents the total optical power return-ing to the source from the complete fiber span. This includes the backscat-tered light from the fiber itself, as well as the reflected light from all thejoints and terminations.

ORL = -10 log (Pr/Pi) in dB

with: Pr = reflected powerPi = incident power

A high level of ORL will degrade the performance of some transmissionlinks. Analog transmission systems and very high speed digital transmissionsystems can be sensitive to ORL. If a system is sensitive to ORL, this isusually listed in the specifications for the link provided by the manufac-turer. The MTS 5100 can report a value for total link ORL, by selecting“ORL = Yes” in the setup menu. The manual ORL measurement is pro-vided to isolate the portion of the link contributing the majority of the ORL.

ORL of a link


Measurement artifacts and anomalies


4.3 Measurement artifacts and anomalies

From time to time, unexpected results and events can be seen on the backs-cattered trace.

4.3.1 GhostsFalse Fresnel reflections on the trace waveform can be observed from timeto time. They can be a result of either:

• strong reflective event on the fiber, causing a large amount of reflected light to be sent back to the OTDR

• or incorrect range setting during acquisition

Ghosts principle

In both cases, the ghost can be identified as no loss is incurred at the signalpasses through this event. In the first case, the distance that the ghost occursalong the trace is a multiple of the distance of that strong reflective eventfrom the OTDR.

Ghost

OTDR

OTDR


Using an OTDR


Example of ghost in the noise

In order to reduce the reflection, you can use index matching gel at thereflection, or reduce the injected power by selecting a shorter pulse width,or reducing the power (some OTDRs provide this option) or adding attenu-ation in the fiber before the reflection.

If the event causing the ghost is situated at the end of the fiber, a few shortturns around a suitable tool (pen, pencil, mandrel etc.) will sufficientlyattenuate the amount of light being reflected back to the source and elimi-nate the ghost. This is known as a mandrel wrap.

Caution: be sure to select a mandrel of the appropriate diameter for the type of cable, jacketed fiber, or coated fiber used, so as not cause permanent damage to the span! It is never recommended to bend a fiber or cable to introduce attenuation without the use of a suitable mandrel to prevent excess bending.

Ghosts can also be introduced on the OTDR trace waveform if we incor-rectly set the distance range.




The first pulse data overlaps with second and consequent pulses and intro-duces a ghost at 2 km. This distance corresponds to the fiber length minusthe OTDR laser distance range.

4.3.2 Splice "Gain" It must be remembered that an OTDR measures splice loss indirectlydepending on information obtained from backscattering to calculate spliceloss. It is assumed that the backscatter capture coefficient of the fibers inthe span are identical. If this is not the case, then measurements can beinaccurate. One common example of this is apparent splice ‘gains’ or ‘gain-ers’. The inaccuracy is quite small, but with today’s fusion splicing equip-ment and experienced operators making very low loss splices, it is possiblefor the effect to make the splice appear to be a gain.

OTDR laser distance range

Fiber length

OTDR laser pulses

The OTDR’s first pulse iscompleted at 20 km and thesecond pulse is launchedinto the fiber.

As the fiber is longer thanthe distance range, theOTDR’s first pulse is stillpresent on the fiber whilethe second pulse data isbeing acquired. The firstcontinues 2 km furtherdown the fiber until it hitsand reflects off the end.

20 km

22 kmFirst pulse Second pulse etc.

OTDR second pulse waveform

OTDR first pulse waveform

2 km


Using an OTDR


Gain theory

If fibers of different mode-field diameters (core size etc.) are joined, theresulting OTDR trace waveform can show a higher backscattering level.This is due to the increased level of backscattered signal reflected back tothe OTDR in the downstream fiber.

Normal splice

This phenomenon can occur when jointing different types of fiber in multi-mode or 2 fibers with different backscattering coefficients.

Positive splice from A to B

Negative splice from B to A

The sum gives the bidirectional or average splice loss value :

Ka = Kb = Backscatter coefficient S: S plice attenuation

OTDRA , Ka B, Kb

S

OTDRA , Ka B, Kb

Ka < Kb Kb-Ba= ∆k S1= S+∆K

S1

OTDRA , KaB, Kb

Ka < Kb Kb-Ba= ∆k S2 = S-∆K

S2

SS1 S2+

2-------------------=




Bidirectional Analysis

We all know that there is no such thing as a passive amplifier, and that wecan’t get a “gain” in optical power from a fusion splice, but the OTDR willsometimes report a gain caused by differences in backscatter coefficient.Note that while these backscatter differences will not always cause a gain tobe reported, they can cause erroneous splice loss readings even if the read-ing is still a loss.

Bidirectional analysis is a technique used to minimize the effect of backscat-ter coefficient differences along a span causing these erroneous splice read-ings. It is used where very accurate baseline data on a span is desired or dur-ing acceptance testing, where accurate measurement of splicing, oftenperformed by subcontractors, is desired.

The concept of bidirectional analysis is as follows: If there is a backscattercoefficient mismatch between two spliced fibers, the sense (algebraically) ofthat difference will change depending on the direction of measurement.That is, if measured in one direction, the difference will appear as a gain, ifmeasured in the opposing direction, it will appear as a loss. This differencewill combine with the actual splice loss during measurement. However, ifthe splice loss reading taken in the two directions is averaged, then thebackscatter effect will subtract out, yielding the actual splice loss.

While the concept is presented here in detail and the manual calculationspresented, in actuality, this analysis is usually performed using programssuch as WinTrace ® which will automatically perform this analysis on muchmore complex spans than that shown here.

Example of Bidirectional analysis on a hypothetical span

Span Architecture

The hypothetical span comprises three fiber sections, fusion splicedbetween Connector West and Connector East.

ConnectorWest

ConnectorEast

Fusion splice

A

Fusion splice

B

Fiber 2Fiber 1 Fiber 3


Using an OTDR


The relative backscatter profile of the fibers is shown. In this model, we aretemporarily ignoring the loss in the fiber to show, that if the backscattercoefficient was sampled at many points along the span, the coefficientwould be higher in the second or middle section.

Backscatter Profile of span

In this case, let’s say that the effect of the backscatter mismatch appears tothe OTDR to be about 0.05 dB. Remember, and this is very important, thatthe effect will appear as a gain if going into fiber 2, but as a loss if exitingfiber 2.

Apparent loss/gain at junction due to backscatter coefficient difference

This span has been fusion spliced and the actual fusion splice loss happensto be -0.03 dB at SPLICE A between fiber 1 and fiber 2, and -0.07 dB atSPLICE B between fibers 2 and 3. For this example, we will consistentlyuse the minus sign to represent a loss and no sign to represent a gain.

Actual Fusion Splice Loss

Fiber 2Fiber 1 Fiber 3

0.05 dB 0.05 dB

ConnectorWest

ConnectorEast-0.03 dB -0.07 dB




What the OTDR sees……

Measurement one (W -> E)

When measuring from West to East, and we are showing the fiber loss now,SPLICE A appears to be a “gain” of 0.02 dB (the actual -0.03 dB plus theapparent 0.05 dB gain due to backscatter). SPLICE B appears to be a -0.12loss (the actual -0.07 loss plus the apparent -0.05 loss due to backscatter).

Measurement two (E -> W)

ConnectorWest

ConnectorEastOTDR

A

B

-0.12 dB

+0.02 dB (West)

(East)

ConnectorWest

ConnectorEast

A

-0.08 dB

-0.02 dB

(East) (West)

OTDR

B


Using an OTDR


When measuring from East to West, remember that SPLICE B is now onthe left of the OTDR screen and SPLICE A is on the right, then:

• SPLICE A appears to be a loss of 0.08 dB (the actual -0.03 dB plus the apparent -0.05 dB loss due to backscatter).

• SPLICE B appears to be a -0.02 loss (the actual -0.07 loss plus the appar-ent 0.05 “gain” due to backscatter).

After taking the two measurements, we can now make a simple chart show-ing the loss/"gain" of Splices A and B taken in each direction. We can sumthe two readings and then divide by two to take the average. Note that theresult now accurately represents the actual splice losses of the two events.

Bidirectional analysis

4.4 Getting the most out of your OTDR

4.4.1 Using launch cablesThe use of a launch cables in an OTDR measurement enable a number ofeffective tasks:• correct measurement of the insertion loss of the system end connectors • moves the dead zone caused by the OTDR front panel connector out-

side of the system under test trace waveform• improves modal equilibrium characteristics in multimode systems so

that measurement are more precise• allows the user to control the OTDR injection level into the system

under test.

OTDR

W → E E → W Sum Average Actual loss

Splice A +0.02 -0.08 -0.06 -0.03 -0.03

Splice B -0.12 -0.02 -0.14 -0.07 -0.07


Getting the most out of your OTDR


The typical length of a launch cable will depend on the system being testedbut generally is between 500 and 1000 m for a multimode test, and 1000 mfor a singlemode test.The fiber used in the launch cable should match thefiber being tested (core size etc.) and the cable connectors should be of highquality.

Trace without launch cable

Note If a helper is available at the far end of the span under test, orif both ends of the span are accessible, some operators use a“receive cable” (a sufficiently long span of fiber mated to thefar end of the span) to measure the loss of the far endconnector as well


Using an OTDR


Trace with launch cable and receive cable

4.4.2 Verifying continuity to the fiber endSometimes a multifiber cable is installed and you wish to verify that thecable is continuous between the two exposed ends. You can make anOTDR measurement on the cable in each direction and that will confirmthat it is continuous. You can also make an OTDR measurement in onedirection and observe the length of the cable as represented on the trace,however, the length of each fiber in the cable will often vary by a few metersdue to slightly different buffer tube overlength or helix geometry within thecable. It is difficult, if not impossible, to distinguish a fiber with a muchlower overlength, from a fiber that is broken inside the cable, 1 meter fromthe far end.

A simpler way to verify continuity, without having to do a complete OTDRtest from both ends can be accomplished as follows. In this case, you needaccess to both ends of the cable, or a helper and communication to thehelper.

Simply hook the OTDR to one of the fibers in the cable, say Fiber 1. Turnthe OTDR on Real Time mode and observe the end of the trace. If the

System endconnectors




length looks approximately right then do the following (if the length isgrossly short, you know it’s broken).

If you can’t see an end spike (reflective event at the unterminated glass/airinterface at the end of the cable), then have the helper cleave the fiber endsquarely with a hand cleaver. The end spike or end reflection shouldbecome apparent, if it does not, then the helper is not holding the end ofFiber 1, Fiber 1 is broken somewhere inside the cable near the end. If atfirst you do see a large end spike, have the helper dip the end of the fiber inindex matching gel, or alcohol, or wrap the fiber around a small mandrelnear the end. Doing any of these will attenuate the end spike. If it does not,then the fiber is broken somewhere else near the end of the cable.

4.4.3 Fault locationThe OTDR can be an invaluable tool for fault location. Accurate fault loca-tion depends on careful measurement technique with the OTDR and oncomplete and accurate system (cable) documentation. While entire coursesare often taught on the subject of fault location, following the few recom-mendations below may make the process more accurate and efficient shouldthe need ever arise.

Cable breaks can be partial or complete (catastrophic). The most commoncause of cable breaks is «dig-ups» (over 40 % of all breaks are dig ups). Inthe case of a dig up, fault location does not need to be extremely precise asthe damage can usually be easily located once one is in the vicinity. Othertypes of breaks including ballistic (from hunting weapons) or rodent damageare difficult to find and accurate location with an OTDR can save a greatdeal of time and money.

When a cable is damaged the resulting break may be highly reflective ornon reflective. It is generally much easier to determine an accurate distanceto a reflective event. Therefore, it is sometimes helpful to measure severalbroken fibers until a reflective break is found. If the break is non-reflective,it is usually best to let the OTDR software determine the distance to eventsusing automated analysis. Placing a marker or cursor visually can be inaccu-rate.

The operator may wish to calibrate the OTDR to display distance in Cableor Sheath distance by using an «effective refractive index».This is impor-tant, while the OTDR can accurately determine distances to 5 meters in10,000, the helix factor of the cable will contribute up to 600 meters of inac-curacy over a 10,000 meter span. An alternate method of determining actual


Using an OTDR


distance from optical distance is to measure the break from both endpointsand determining the position of the break relative to the total span length.This ratio of the optical distance to the break to the total optical length ofthe span will be the same as the ratio of the sheath distance to the break tothe total sheath length.

It is important to remember any locations that cable slack is stored. If theOTDR reads 1800 meters to the break but there are 200 meters of slackstored at an intermediate handhole, manhole or pole, then the distance tothe break will be similarly shorter.

It is important to remember sag in aerial plant Sheath distance will differsomewhat from pole distance. After the location of the break is determined,it should be correlated to a cable sequential marking. Then, when excavat-ing the cable or examining the aerial plant with binoculars, the correct sec-tion of cable can be quickly confirmed.

It is always best to measure the distance to the break from the last eventwhose physical location is known on the OTDR signature using the cursors.In this manner, the shortest possible measurement is made on the OTDRreducing the OTDR contribution to measurement inaccuracy.

During initial cable documentation, take advantage of some of the OTDRfeatures that permit the addition of notes to events or files. Geographic orGPS data can be entered here that will be very useful during fault location.

Again, there is absolutely no substitute for complete, detailed, accuratecable documentation records during fault location.

4.4.4 Effective refractive indexThe user is reminded that the OTDR determines the distance to the eventbased on time. The refractive index serves as a correlation factor betweentime and distance allowing the OTDR to display distance.

If the user knows the refractive index provided by the fiber manufacturer,he can enter this value on the OTDR thus improving the accuracy of theoptical distance displayed.

In most cable designs, the length of the fiber is greater than the length ofthe cable. This can be caused by fiber overlength in the buffer tubes (in“ loose ” buffer designs) and/or “ helixing ” of the buffer tubes or ribbonsinside the cable. The cable length or physical distance can therefore varysignificantly from the fiber length or optical distance.




In some cases, notably fault location, the users wishes the OTDR to displaycable or physical distance instead of optical distance. This can be accom-plished by entering a different value of refractive index, sometimes termedthe “effective refractive index” that is adjusted for fiber overlength.

There are two ways to determine effective refractive index :

1. Using cable records or knowing the cable or physical distance (Leff) between two known events on the OTDR trace, the user must obtain from the OTDR the following data :

Optical distance between 2 known events (Lopt) Refractive index used by the instrument (RIopt)

The effective refractive index (RIeff) can then be calculated using theformula : RIeff = (Lopt* RIopt) / Leff

2. On some OTDR’s like the MTS 5100, the RIeff can be calculated automatically by delimiting the two known events with two cursors and changing the refractive index until the OTDR reports cable or physical distance instead of optical distance.


Using an OTDR



Chapter

5

Otdr.bk : otdr_a1.doc Page 1 Mercredi, Mars 4, 1998 3:31 PM

5Glossary

ased on IEC 50 chapter 731, EIA-440-B and other documents.

Absorption : in an optical fiber, loss of optical power resulting fromconversion of power into heat.

Adaptor : female part of a connector in which one or two connectorplugs are inserted and aligned.

APD (Avalanche Photodiode): photodiode which operates in theavalanche mode, providing internal gain that is advantageous inreception.

Architecture : The protocol that defines computer communicationnetworks. With respect to optical fiber cabling, this term refers to thelayout of the cabling in star or ring configuration, for example.

Armored cable : A fiber cable that includes a layer of corrugated steelto prevent rodent ingress. Primarily for direct buried applications,occasionally used in aerial applications where squirrels are a severeproblem

Attenuation dead zone : for a reflective or attenuating event it is theregion after the event where the displayed trace deviates from theundisturbed backscatter trace by more than a given vertical value ∆F(usually 0.5 dB or 0.1 dB). Bellcore specifies a reflectance of - 30 dB, aloss of 0.1 dB and gives different locations. In general, the higher thereflected power sent back to the OTDR, the longer the dead zone.

The attenuation dead zone depends on the pulsewidth, the reflectance,the loss, the displayed power level and the location. It usually indicates

B

Guide to Fiber Optic Measurements A-1

Glossary


the minimum distance after an event where the backscatter trace can bemeasured.

Attenuation Dead Zone measurement

ATM (Asynchronous Transfer Mode): A network standard that specifiesfixed length cells to transmit data, voice and video information. ATM isscalable in that it can operate at different transmission speeds such as 51,100, 155, 622 Mb/s and beyond.

Attenuation : in optical fibers, loss of average optical power due toabsorption, scattering and other radiation losses. It is generally expressed indB without a negative sign.

ADZAttenuation dead zone

∆F = 0.5 dB or 0.1 dB

A-2 Guide to Fiber Optic Measurements


Attenuation coefficient : The rate of optical power loss with respect todistance along the fiber, usually measured in decibels per kilometer (dB/km) at a specific wavelength. The lower the number, the better the fiber’sattenuation. Attenuation is specified at 850 and 1300 nm for multimodefiber. and 1310 and 1550 nm for singlemode fiber, over a temperature rangeof -60°C to +85°C.

Backscattering : portion of scattered light which returns in a directiongenerally reverse to the direction of propagation.

Bandwidth : difference, expressed in Hertz (Hz), between the highest andthe lowest frequencies passing through the fiber. Note : This term is often used to specify the bandwidth (MHz x km) of amultimode fiber.

(µm)


Glossary


Attenuation versus frequency

Bend Radius (minimum) : The radius a fiber can bend before increasedloss or mechanical damage occurs.

Broadband : A signal technique that involves modulating the signal on acarrier before transmission. This allows multiple information signals to betransmitted simultaneously on different carrier frequencies.

Buffer tube : A thermoplastic tube which is a component of fiber opticcables serving to segregate the fibers into “groups” and to mechanicallydecouple mechanical forces on the cable from the fibers by permitting thefibers to “float” in the tube. The tubes can be filled (in outdoor cables) orunfilled (in indoor cables).

Building backbone cable : A cable that connects the building distributorto a floor distributor. Building backbone cables may also connect floordistributors in the same building.

Building Distributor : A distributor in which the building backbonecable(s) terminate(s) and at which connections to the campus backbonecable(s) may be made.

Attenuation

Frequency0

5

10

15

20

Monomode

λ = 1.3 µm

Twistedscreened pair

TG22U CoaxialRG217U

CoaxialRG220U

Graded Index

λ = 0.85 µm

Graded Index

λ = 1.3 µm

0.1 1 10 100MHz 1 10 100 GHz



Cable : A structure carrying multiple fibers, usually more than 4 (less than 4fibers in a structure is usually referred to as “CORD”) and providingmechanical and environmental protection, tensile strength and fireresistance.

Two different design concepts exist :

Loose tube cable: This design allows primary coated optical fiber or bundles ofprimary coated optical fibers to lie loosely inside a polymer tube or "former"thus taking advantage of the minimum strain configuration within the tubeor former whist protecting them from abrasion and other external forces.

The tube or former may be filled with compounds to prevent ingress andpropagation of moisture which may affect the optical fibers.

These cables are designed to withstand the mechanical stresses involvedwhen cables are pulled through extensive duct systems and are particularlysuitable for external use.

Loose tube cable

Tight buf fered tube cable: This design features secondary coated (buffered)optical fibers within a flexible and durable construction. The cables are ofgenerally low fiber count with aramid strength element protection layersand a polymer outer sheath. This design is particularly suited to internalapplications.

Outer jacketpolyethylene

Central FRPstrength member

Flooded core

Aramid strengthelement

Bloistureblocking gel

Multiple 250micron fibers

Thermoplastictube


Glossary


Tight buffered tube cable

Cabling system :

Campus backbone cable : A cable that connects the campus distributor tothe building distributor(s). campus backbone may also connect buildingdistributors directly.

Campus Distributor: The distributor from which the campus backbonecabling emanates.

Outer jacketpolyethylene

Centralmember

900 micronTight Bufferedfibers

Aramidstrengthelement

Thermoplastic jacket

Overallpolyester tapebarrier

Generic cabling system

Campus backbone cabling

subsystem

Building backbone cabling

subsystem

Horizontal cabling

subsystem

Work area cabling

Campus distributor

Building backbone

Floor distributor

TelecommunicationOutlet

TerminalEquipment



Chromatic dispersion : A type of dispersion that causes broadening ofinput pulses along the length of the fiber. Chromatic dispersion is due to thedifferent wavelengths of light traveling at different speeds through the fiber.It is at a minimum value at the fiber zero dispersion wavelength.

Cladding : The glass layer surrounding the core of an optical fiber. Thelower index of refraction of the cladding as compared to the core causes thelight within the core to be totally internally reflected and remain in the core.

Coating : An acrylate polymer material put on a fiber during the drawprocess to protect it from the environment and rough handling.

Connector : A junction which allows an optical fiber or cable to berepeatedly connected or disconnected to a device such as a source or adetector.

Coupling ratio/loss (Cr, Cl) : ratio /loss of optical power from one outputport to the total output power, expressed as a percent.

Core : The central region of an optical fiber through which light istransmitted.

CPE: Customer Premises Equipment.

Cutoff wavelength : In singlemode fiber, the shortest wavelength at whicha single mode can be transmitted. Beyond this wavelength, several modestransmit simultaneously, and the fiber becomes multimode.

CW: Abbreviation for continuous wave.

Dense WDM (Wavelength division multiplexing) : Technique used tomultiplex several signals on the same fiber within a narrow wavelengthband.

Dead Zone : Distance sections in trace, which are associated with everyreflective event and represent the distance between the beginning of theevent and the point where a consecutive event can be detected.


Glossary


DFB: Abbreviation for distributed feedback laser. This laser has a Braggreflection grating in the active region in order to suppress multiplelongitudinal modes and enhance a single-longitudinal mode.

Dispersion : The cause of bandwidth limitation in a fiber. The spreading(or broadening) of a light pulse as it spreads along a fiber. Major types are :• modal dispersion cause by differential optical path lengths in a

multimode fiber, • chromatic dispersion caused by a differential delay of various wavelengths

of light passing through a fiber.

Distributor : The term used for the functions of a collection of components(e.g. : patch panels, patch cords) used to connect cables.

DTE: Data Terminal Equipment, generally.

Dynamic range :

IEC Dynamic Range (introduced by Bellcore)

The difference between the extrapolated point of the backscatter trace atthe near end of the fiber (taken at the interception between the extrapo-lated trace and the power axis) and the upper level of the noise floor at orafter the fiber end. The upper level of the noise is defined as the upperlimit of a range which contains at least 98% of all noise data points. Thedynamic range is expressed in decibels (dB). This measurement is per-formed for 180 seconds usually with largest pulsewidth of the OTDR.

RMS Dynamic Range

The difference between the extrapolated point of the backscatter trace atthe near end of the fiber (taken at the intersection between the extrapolatedtrace and the power axis) and the RMS noise level.



RMS dynamic range

EDFA : Abbreviation for Erbium Doped Fiber Amplifier. Device whichamplifies an optical signal without employing O/E and E/O conversions.

Electromagnetic Spectrum : It is a term used to describe the entire rangeof light radiation, from gamma rays to radio.

Electromagnetic spectrum

Ethernet : A network protocol specified for operation to 10 Mbit/s.Standards are being developed for 100 Mbit/s and beyond.

Event dead zone : minimum distance on the trace, where two separateevents can still be distinguished. The distance to each event can be mea-sured, but the separate loss of each events cannot be measured. This param-

Type of Radiation Frequency Range Wavelength Range

Gamma-rays 1020 - 1024 <10-12 m

X-rays 1017 - 1020 1 nm - 1 pm

Ultraviolet 1015 - 1017 400 nm - 1 nm

Visible 4.1014 - 7.5x1014 750 nm - 400 nm

Near infrared 1012 - 4.1014 300 µm - 750 nm

Infrared 1011 - 1012 300 µm

Microwaves 108 - 1012 3m - 300 µm

Radio waves 100 - 108 >1 mm

Dyn

amic

IEC

(98

%)

Dyn

amic

ran

ge

(RM

S)

~6.6 dB

1.56 dBSNR=1

Peak noise level

N = 0.1 dB

dB

km


Glossary


eter usually gives an indication of the minimum distance in order todistinguish between reflective events which occur in close proximity.

• For a reflective event, the event dead zone definition is the distance between the two opposite points which are 1.5 dB (or FWHM) down from the peak. The reflectance of the event shall be specified: as an example Bellcore gives a reflectance of -30 dB.

• For an non-reflective event, the event dead zone definition is the dis-tance between the points where the beginning and ending levels at a splice or a given value (≤ 1 dB) are within ±0.1 dB of their initial and final values. Usually this dead zone is a fixed value and depends only on the pulsewidth and the fiber. This definition is not often used.

Event Dead Zone measurement

Ferrule : A mechanical fixture, generally a rigid tube, used to confine andalign the polished or cleaved end of the fiber in a connector. Generallyassociated with fiber-optic connectors.

Fiber Distributed Data Interface (FDDI) : A standard for 100 Mbit/s fiber-optic local area network.

Fiber optic span : A series of one or more terminated optical fiber elementswhich may contain complex passive components.

1.5 dB

EDZEvent

dead zone

Reflective event Non reflective event

≤1 dB

Event dead zone



Floor distributor : The distributor used to make connections between thehorizontal cabling, other cabling subsystems and active equipment.

FTTB: abbreviation for Fiber-To-The-Building

FTTC / FTTK: abbreviation for Fiber-To-The-Curb / Kerb

FTTH: abbreviation for Fiber-To-The-Home

FTTO: abbreviation for Fiber-To-The-Office

Fusion Splice : A permanent joint accomplished by the application oflocalized heat sufficient to fuse or melt the ends of the optical fibertogether, forming a continuous single fiber.

Fusion splice by electrical arcing

Graded-index fiber : Fiber design in which the refractive index of the coreis lower toward the outside of the core and increases toward the center withthe peak at the centerline. This multimode fiber design reduces the timedifference between the arrival of different modes, minimizing modaldispersion and maximizing bandwidth.

Group index : The factor by which the speed of light in vacuum has to bedivided to yield the propagation velocity of light pulses in the fiber.

Hub : Houses the network software and directs communications within thenetwork.

Index of Refraction : see refractive index

n


Glossary


Insertion loss: The increase in the total optical attenuation caused by theinsertion of an optical component in the transmission path.

Joint : an assembly designed to connect 2 or more optical fibers.

Jumper :A cable unit or cable element without connectors used to make aconnection on a cross-connect.

LAN (Local Area Network) : A geographically limited communicationsnetwork intended for the local transport of data, video, and voice. It’s a highspeed transmission (Mbit/s) which facilitates information transfer.

Laser (Light Amplificated by Stimulated Emission of Radiation): Adevice that produces monochromatic, coherent light through stimulatedemission.

Spectral bandwidth of a laser

Launch fiber : A length of fiber used to create an equilibrium modaldistribution in multimode, and to measure the first connector of thenetwork in both multimode and singlemode systems.

Light Emitting Diode (LED) : A semiconductor device used to transmitlight into a fiber in response to an electrical signal. It typically has a broadspectral width. Its spectral width typically is 50 to 60 nm.

0.5

1

839 840 841

Laser : 0.6 nm

Wavelengths (nm)



Comparison of Laser and Led

Mechanical splice : A fiber splice accomplished by fixtures or materialsrather than thermal fusion.

Mechanical splice

Micro bend : Small distortion of a fiber caused by external factors such ascabling.

Mode field diameter (MFD) : A parameter which expresses for a singlemode fiber the section where the majority of the light energy passes. It canbe expressed as the diameter of optical energy in the fiber. Because theMFD is greater than the core diameter, MFD effectively replaces corediameter in practise.

Multimode fiber : An optical fiber in which light travels in multiple modes.

700 800 1000 Wavelength (nm)

LED : 65 nm

1

Laser : 0,6 nm


Glossary


Multimode fiber

Multiplex : Combining two or more signals into a single bit stream that canbe individually recovered.

Node : A point of flexibility and/or interconnection within the fiber opticcabling system.

Numerical aperture : The number that expresses light gathering capacityof a fiber related to the acceptance angle.

The sine of 5% optical power angle (corresponding to -13 dB) is used tomeasure the Numerical Aperture.

Optical Loss budget : The amount of signal loss that can be tolerated in asystem before errors occur.

ORL (Optical Return Loss): The ratio (expressed in dB) of the reflectedpower to the incident power from a fiber optic system or link ORL = -10 log (Pr/Pi) or ORL = 10 log (Pi/Pr)

CladdingSize: 125 µmIndex of refraction: 1.46

CoreSize: 62.5 µm or 50 µmIndex of refraction: 1.48

CoatingSize: 900 µm

Fiber under test

α0

-13 dB (5% of max power density)

-13 dB

0 dB Max. power density

NA = 2 α0



OTDR: Abbreviation for Optical Time Domain Reflectometer. Aninstrument used to characterize a fiber optic link. Useful in estimating fiberlink attenuation, attenuation coefficient, discrete reflections, splice/connector loss, and point defects, all as a function of fiber distance.

Patchcord : A cable assembly, permanently assembled at both ends withconnector components (principally for cross-connection within a patchingfacility).

Pigtail : A short length of optical fiber permanently attached to a connectorand intended to facilitate jointing between that connector and anotheroptical fiber or component.

Point to point : A connection established between two specific locations ordevices such as a hub and a workstation or between two buildings.

Reflectance : The ratio of reflected power to incident power of an event orconnector R = 10 log(Pr/Pi).

Refractive index : A property of light transmitting materials defined as theratio of the velocity of light in vacuum (c) to its velocity in a giventransmission medium (v).

n = c/vn = Refractive Index

c = 2.99792458 . 108 m/s

e.g.: n (air)= 1.0003 ; n (water) = 1.33 ; n (Glass)= 1.5

Repeater : A device used to regenerate an optical signal to allow an increasein the system length.

Scattering : A property that causes light to deflect out of the core area of thefiber, thereby contributing to attenuation.

Singlemode fiber : An optical wave guide (or fiber) in which the signaltravels in one mode.

SONET (Synchronous Optical NETwork): It is a transport interface thatenables the public network to carry various kind of services. SONET is the


Glossary


North American optical fiber standard that supports transmission rates thatstart at 51.84 Mb/s and reach to 2.488 Gb/s.

Telecommunications closet : An enclosed space for housingtelecommunications equipment, cable terminations, and cross-connectcabling. The telecommunications closet is a recognized cross-connectbetween the backbone and horizontal cabling subsystems.

Telecommunications outlet : A fixed connecting device where thehorizontal cable terminates. The telecommunications outlet provides theinterface to work area cabling.

Transmitter : An electronic package used to convert a signal carryingelectronic information to a corresponding optical signal for transmission byfiber. The transmitter can be a light emitting diode (LED), laser diode, orvertical cavity surface emitting laser (VCSEL).

SNR (Signal to Noise Ratio): The ratio of the received optical signal powerdivided by the RMS noise floor for the detector.

Splice : A permanent junction between optical fibers

Splitter : A passive device which devises optical power among severaloutput fibers from a common input.

Step-index fiber : A fiber whose index of refraction (n) changes sharply atthe interfaces of its core and cladding.

Visual Fault Locator : The visual fault locator is a visual light source usedto locate breaks or point of excess loss in fiber cable. The commonwavelengths are 635 nm, 650 nm and 670 nm.

n



WDMs: Abbreviation for Wavelength Division Multiplexers. Passive fiberoptic components which combine optical channels on differentwavelengths.

WAN (Wide Area Network): A network used for the transport of informationover many miles.


Glossary



Otdr.bk : Notes.fm Page 1 Mercredi, Mars 4, 1998 3:31 PM

1Notes

Guide to Fiber Optic Measurements N-1

Notes


N-2 Guide to Fiber Optic Measurements



Notes





Notes



Otdr.bk : OtdrIX.fm Page 1 Mercredi, Mars 4, 1998 3:31 PM

1Index

AAccuracy

distance accuracy 3-15level accuracy 3-15

Acquisition 4-1Attenuation 1-3, 1-4, 1-8Attenuator 2-7Auto-configuration 4-1Averaging 4-6

BBackscatter coefficient 4-9Backscatter profile 4-24Backscattering 3-3Backscattering factor 3-2Bending losses 1-3Bidirectional Analysis 4-23

CCladding 1-1, 1-9Clip-on tester 2-17Continuity check 4-28Core 1-1, 1-9Cut back measurement 2-9

DDead zone

attenuation dead zone 3-11event dead zone 3-13

front end dead zone 3-12using launch cable 4-26why a dead zone? 3-11

Dispersionchromatic dispersion 1-5dispersion shifted 1-6modal dispersion 1-5

Distance error 3-15Dynamic range 2-8, 3-8

EElectronic Industries Association

2-1Event loss measurement

2-point method 4-145-point method 4-15

FFault location 4-29Fiber Identifier 2-17Fiber Optic Test Procedure 2-1Fresnel reflection 3-4

GGain 4-21Ghosts 4-19Graded-index multimode 1-8Group delay 3-7Group index 1-8, 1-9


Index


IIEC 1-10Injection level 4-2Insertion loss 2-9, 4-26ITU-T 1-10, 2-1

LLaser diode 3-6Launch cable 4-26Least-squares approx. (LSA) 4-14Light absorption 1-3

MMeasurement 4-10

artifacts 4-19event loss 4-14full automatic function 4-10ghosts 4-19manual measurement function

4-12reflectance 4-17section loss 4-14semi automatic function 4-11slope 4-14

Mini-OTDR 2-11Modal equilibrium 4-26Mode 1-2

multimode 1-7singlemode 1-8

Mode field diameter (MFD) 1-9, 4-3

Monitoring system 2-13Multimode fiber 1-7

NNumerical aperture 1-1, 1-8

OOptical loss budget 2-4

Optical Return Loss (ORL) 4-13, 4-18

OTDRblock diagram 3-5definition 3-1description 2-12measurement 4-10specifications 3-8use 4-1

PPhotodiode 3-7Power meter 2-8Pulse generator 3-6Pulse width 4-4

RRange 4-6Rayleigh scattering 1-3, 3-1Reflectance 4-17Refractive index 1-2, 4-8Resolution

display resolution 3-14distance resolution 3-14loss resolution 3-14sampling resolution 3-14

SSection los 4-14Section loss 4-14Singlemode fiber 1-8Slope 4-14Smoothing 4-8Source 2-8Standards 1-10Step index multimode 1-7

TTalk set 2-16



Testinginstallation testing 2-3maintenance testing 2-3

Time basedescription 3-7

Time base error 3-15Transmission tests 2-2

VVelocity 1-2Visual Fault Locator 2-16

WWavelength 3-16, 4-3


Index



Fiber Optics Guide

Documents

fiber standards

multimode fiber

singlemode fiber

fiber phenomena

fiber optic measurements

acterna saintetienne

otdr specifications

property of acterna