MULTIMODE FIBER LAN TESTING WITH · PDF file2 MULTIMODE FIBER LAN TESTING WITH OTDRS INTRODUCTION AND SCOPE The Optical Time Domain Reflectometer (OTDR) is a powerful tool for characterizing

MULTIMODE FIBERLAN TESTING WITH OTDRS

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T1 GUIDE—THE HIGH CAPACITY DIGITAL NETWORK 1

TABLE OF CONTENTS

Introduction and Scope . . . . . . . . . . . . . . . . . . . . .2

Fiber LAN Networks . . . . . . . . . . . . . . . . . . . . . . . . . .2

FAQs About OTDRs . . . . . . . . . . . . . . . . . . . . . . . . . .3

Optical Fiber Basics . . . . . . . . . . . . . . . . . . . . . . . .3

How Do Fibers Work? . . . . . . . . . . . . . . . . . . . . . . . .3

Fiber Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

Communication by Light . . . . . . . . . . . . . . . . . . . . . .5

Joining Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Principles of Optical Time Domain Reflectometry 5

The Physical Phenomenon . . . . . . . . . . . . . . . . . . . . .5

The OTDR Instrument . . . . . . . . . . . . . . . . . . . . . . . . .6

Main Components of the OTDR . . . . . . . . . . . . . . .6

OTDR Performance Parameters . . . . . . . . . . . . . . . . .8

Length Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Refractive Index . . . . . . . . . . . . . . . . . . . . . . . . . .8

Dead-zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

Receiver Linearity . . . . . . . . . . . . . . . . . . . . . . . .10

Dynamic Range . . . . . . . . . . . . . . . . . . . . . . . . . .11

Attenuation Dead-Zone . . . . . . . . . . . . . . . . . . .13

International Standards . . . . . . . . . . . . . . . . . . . . . .13

Mode Conditioning . . . . . . . . . . . . . . . . . . . . . . . .13

Wavelength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

OTDR Specific Standards . . . . . . . . . . . . . . . . . . . .13

Data Format Description . . . . . . . . . . . . . . . . . . . . .13

Laser Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

Patchcords and Modal Effects . . . . . . . . . . . . . . .14

Description of Modal Effects . . . . . . . . . . . . . . . . . .15

Comparison of Light Sources . . . . . . . . . . . . . . . . . .15

Mode Conditioning . . . . . . . . . . . . . . . . . . . . . . . . .15

Testing Optical Fiber Installations . . . . . . . . . . .16

Using an OTDR . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

Using the Correct Patchcords . . . . . . . . . . . . . . . .17

Tail Patchcords (Tail-cords) . . . . . . . . . . . . . . . . .17

Interpreting OTDR Traces . . . . . . . . . . . . . . . . . . . . .18

Measuring Link Loss and Link Length . . . . . . . . . .18

Measuring Fiber Loss . . . . . . . . . . . . . . . . . . . . . . .18

Measuring Connector Loss . . . . . . . . . . . . . . . . . .19

Measuring Splice Loss . . . . . . . . . . . . . . . . . . . . . .19

Bi-directional Measurements . . . . . . . . . . . . . . . .20

Measuring Bend Losses . . . . . . . . . . . . . . . . . . . . .20

Measuring Reflectance . . . . . . . . . . . . . . . . . . . . .20

Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . .22

Cleaning Connectors . . . . . . . . . . . . . . . . . . . . . . . .22

Patchcord Bends . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Saturation Effects . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Ghost Reflections . . . . . . . . . . . . . . . . . . . . . . . . . . .23

Wavelength Effects . . . . . . . . . . . . . . . . . . . . . . . . .23

Appendix A — Typical Values for Fibers andConnectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

Appendix B — Useful Information . . . . . . . . . . . .24

dB conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

Scaling factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

Appendix C — Glossary of Terms . . . . . . . . . . . . .25

Appendix D — Megger Fiber Testing Equipment 27

Crosschecking Reflectometer . . . . . . . . . . . . . . . . .27

Megger XC-850 . . . . . . . . . . . . . . . . . . . . . . . . . .27

Mode Conditioning Pactchcord Box . . . . . . . . . . . . .27

Visible Light Source . . . . . . . . . . . . . . . . . . . . . . . . .27

Megger MLS635 . . . . . . . . . . . . . . . . . . . . . . . . . .27

Multi Mode LED Light Source . . . . . . . . . . . . . . . . .28

Megger MLS1000 . . . . . . . . . . . . . . . . . . . . . . . . .28

Single Mode Laser Light Source . . . . . . . . . . . . . . .28

Megger MLS2000 . . . . . . . . . . . . . . . . . . . . . . . . .28

Optical Power Meter . . . . . . . . . . . . . . . . . . . . . . . .28

Megger MPM1000 . . . . . . . . . . . . . . . . . . . . . . . .28

Optical Power Meters . . . . . . . . . . . . . . . . . . . . . . . .28

Megger MPM2000 and Megger MPM2000H . . . .28

2 MULTIMODE FIBER LAN TESTING WITH OTDRS

INTRODUCTION AND SCOPE

The Optical Time Domain Reflectometer (OTDR) is a powerfultool for characterizing the performance of optical fibersystems. A typical optical fiber link may consist of severalsegments of optical fiber joined together by connectors orfusion splices and, while the overall insertion loss of the linkmay be measured using a simple light source and powermeter, the OTDR offers the unique ability to “see inside” thefiber and measure the losses of individual components andthe fiber itself.

The OTDR may be thought of as “optical radar,” sending outpulses of light along the fiber and monitoring the amplitudeof the returned echoes as a function of time. A clearadvantage of this technique is that access to only one end ofthe fiber is normally required in order to perform a fullcharacterization.

Historically, the use of this technology has been restricted dueto the high costs of OTDRs which have been designed forlong-haul applications in thetelecommunication business. This has led tothe design of OTDRs with features notappropriate to those installing short runs offiber across the plant, office, public buildingsor campus.

This is about to change with manufacturernow introducing a new generation of OTDRsbringing the technology down o anaffordable level with features moreappropriate to the data communicationsinstaller. This guide is intended to be impartialand informative, and should enable thereader to answer the frequently askedquestions featured on the following pages.

Fiber LAN Networks

This guide is intended primarily for engineersusing OTDRs for the testing andcommissioning of multimode Local AreaNetworks (LANs). A LAN, often known asPremise Wiring, typically consists of a site orbuilding-based communication systemproviding voice and/or data services to anetwork of computers and workstations.Typical point-to-point fiber lengths are in theorder of tens or hundreds of meters and arenormally connected by demountableconnectors at patch panels or outlet pointsfor connection to communication equipment.

An illustration of a LAN is shown in Figure 1 where featuressuch as optical fiber cross-connects, splice enclosures andbackbone distribution cables can be identified.

For definition purposes, the total fiber path from theworkstation to the main building network equipment isknown as a channel. The fiber path excluding the patchcordsat either end of a channel is known as a “link” and theindividual fibers between connection points within a link areknown as link segments. For testing fiber LANs, it is commonpractice to measure the insertion loss of links, or linksegments, excluding the patchcords that connect theworkstations and network equipment.

A fiber channel in most fiber LAN systems comprises a pair offibers. This is known as duplex operation where one fiber isused for transmitting data and the other fiber for receivingdata. The telecommunications outlet ina work area normallyconsists of a duplex socket which is connected to the workstation by a duplex plug and by either a duplex plug or twosimplex plugs on the network side.

Figure 1: Illustration of a fiber optic LAN

MULTIMODE FIBER LAN TESTING WITH OTDRS 3

FAQs About OTDRs

Having used this guide to further your understanding of theuse of OTDRs in fiber testing you will be able to resolve thisfrequently asked questions. For those that cannot wait, thenhere are the model answers.

Why should I use an OTDR on fiber links when I already owna light source (LS) and power meter (PM)?

You should continue using the LS and PM to certify opticallinks because they are the standard instruments to certify theloss and will probably be the contractual requirement.However, if you use an OTDR during installation you willquickly be able to test the link to ensure that it will pass theLSPM testing and it will show exactly the location of anyproblems. It will instantly show the quality of each connectorand also pinpoint any losses due to bends or kinds in thefiber run.

An OTDR is a useful tool to test the continuity, loss andlength of fiber on the cable drum before spending time andmoney installing it.

What wavelength should I test my fiber LAN system?

LSPM certification measurements are always required at thewavelength that the link will operate and normally at bothoperating wavelengths (850 nm and 1300 nm). There is noextra information to be gained in testing LAN fibers with anOTDR at any wavelength other than 850 nm.

Will OTDR measurements really enable me to accuratelylocate faults?

A good LAN OTDR will show where cable faults are to withinthe nearest meter.

Will my LSPM measurements tie up the OTDR results?

The losses shown by an OTDR will generally agree with LSPMmeasurements. However, both techniques are susceptible towavelength and optical launch conditions. It is generallyaccepted that the agreement will not be exact but closeenough for an OTDR to predict any problems with LSPMcertification.

Why do I need to use a launch patchcord box?

A launch patchcord box should always be used.This helps the results from the OTDR to agreewith LSPM results by defining the optical launchconditions, especially if the launch box contains amandrel wrap at the OTDR end. The launchpatchcord box also provides a long length offiber between the OTDR and the fiber beingtested that improves the trace quality. A launchpatchcord and a tailcord fiber on the far endallow the losses of the fiber segment connectorsto be evaluated as well as the fiber loss.

Can macro and micro bends be seen on OTDR traces?

Any form of bending mechanism that increases the loss ofthe fiber can be seen on an OTDR trace. Although bendlosses are higher at 1300 nm, the comparatively poorperformance of an OTDR at this wavelength means that theyare often easier to locate at 850 nm where the OTDR tracesare less noisy.

My reason for not using an OTDR is that they are expensiveand complicated. How has this changed?

Historically OTDRs have been designed for the requirementsof long-haul fiber telecommunication links. In general, thesehave been complex instruments, expensive and unsuited tothe specific requirements of LAN installation.

Recently OTDRs have been designed for short-range, high-resolution use. These are easy to use and suited to thedemanding condition of the LAN installation environment.Some of the latest generation of LAN OTDRs are inexpensiveenough to be an everyday work tool of the LAN installer.

OPTICAL FIBER BASICS

How Do Fibers Work?

Optical fibers transmit light by the process of “Total InternalReflection” (TIR). This is where light traveling in a medium ofhigh refractive index; glass, for example, meets a secondmedium of lower index air, for example. If the direction of aray of light, with respect to the normal to the interfacebetween the two media exceed a certain angle, known as the“critical angle,” then the interface will act like a perfectmirror and the light ray will be reflected. Now, if the highindex medium has a second similar interface parallel to thefirst one, then the beam will bounce back and forth betweenthe interfaces as it travels along, as shown schematically inFigure 2.

This, then, is the principle of the optical fiber and the basis ofall optical fiber communications systems.

Figure 2: Schematic of optical waveguide principle


A particular drawback, however, with the process of internalreflection is that the reflection efficiency can be easilydegraded if the outside surface of the glass becomescontaminated with water or oil. This problem is solved byconstructing fibers with two glass layers, an inner “core,” tocarry interface between the core and cladding.

In order to protect the fiber further, a plastic buffer layer isapplied during the manufacturing process. A typicalcommunication fiber, which may be manufactured in lengthsof many kilometers, will have a glass cladding diameter of125 µm and a plastic buffer diameter of 250 µm.

Fiber Types

There are two main types of fiber: multimode fiber andsingle-mode fiber. In the former, the core region is typically50 µm, 62.5 µm or 100 µm in diameter, depending on theparticular application.

In single-mode fiber, the core is approximately 9 µm indiameter. In a multimode fiber, shown in Figure 3(a), it can beseen that rays of light may travel at many different anglesand, therefore, follow different routes through the fiber. Eachdifferent angle is known as a mode of the fiber. Theconsequence of this is that each ray arrives at the far end ofthe fiber at a slightly different point in time, depending onthe length of the fiber. The effect is to cause the transmittedpulse of light to spread out in time, a process known asmodal dispersion, and eventually causing neighboring pulsesto overlap. When this happens, the integrity of the opticalsignal is degraded and data may be lost. One method ofreducing the dispersion in multimode fibers is to constructthe core region such that its refractive index varies in anapproximately parabolic manner across its diameter. The

effect of this is to cause light that is traveling at steeperangles in the fiber, and hence taking a longer route, to catchup with light traveling at smaller angle,s as shown in Figure3(b). The result is a reduction in pulse spreading and asubsequent increase in the data-rate, or bandwidth, that maybe transmitted by the fiber.

In contrast, due to the small size of the core, single-modefibers do not behave in the same way as multimode fibers.Light propagation has to be described in terms ofelectromagnetic fields rather than individual rays of light.Effectively, this means that only light at a certain angle, ormode, is able to travel along the fiber. Thus the problem ofmodal dispersion is eliminated and so single-mode fibers canbe used at much higher data rates than multimode fibers.Typically, single-mode fiber systems may be used at data-ratesof more than 40 Gigabit per second (Gb/s) over distances ofmany tens of kilometers. The limiting factors in bandwidth insingle-mode fiber are Chromatic Dispersion (CD) andPolarization Mode Dispersion (PMD), both of which also occurin multimode fibers but are usually significantly less than theinherent modal dispersion limitations. The reach of modernsingle-mode systems, however, is normally limited byattenuation rather than by dispersion and the development ofin-line amplifier techniques is an ongoing field of research.

Recent improvements in controlling the exact form of therefractive index profile of graded-index fibers have led to anew generation of multimode fibers that are capable ofsupporting data-rates of up to 10 Gb/s at 850 nm overdistances of several hundred meters. As multimode systemsare generally cheaper to install than single-mode systems,due to the reduced costs of sources and connectors, they arefinding increasing application in high data-rate LANs.

Figure 3: Multimode fibers — (a) step index; (b) graded index

(a)

(b)


Communication by Light

A typical multimode communication system will consist of thefollowing components: a light source, which could be a light-emitting diode (LED) or a laser, the optical fiber itself, maybea few connectors and/or splices, and a photo detector at thefar end to convert the optical signal into an electrical signal.Information is normally encoded onto the optical beamdigitally by switching the laser on and off, or by using a“modulator” but some applications still transmit in analogform by varying the intensity of the light source.

Joining Fibers

A typical fiber link may consist of several pieces of fiberjoined together. The main method of joining fiber segments isusing de-mountable connectors but sometimes by-fusionsplicing is used. The quality of a joint is normally specified interms of (a) the insertion loss of the joint and, (b) the amountof back reflection that it generates. Back reflection, orreflectance, is normally undesirable as it can lead to instabilityin the optical source that is driving the fiber system.

There is a huge range of de-mountable connectors on themarket, some require the use of polishing, with or without anadhesive, and other employ a cleaving apparatus to preparethe fiber ends. The main advantage of de-mountableconnectors is that they do not require sophisticated fusionapparatus and they may be readily disconnected for re-routing or testing purposes. The main drawbacks with de-mountable connectors are the higher insertion loss andreflectance values compared to fusion splices, and also thesusceptibility to the ingress of dirt. Examples of connectorsused for multimode fiber are SMA, SC, FC, ST, DIN, and LC.

The loss and reflectance of de-mountable connectors canboth be improved by the use of the so-called Physical Contact(PC) connectors. Here, the fibers are polished so that theyhave a gentle curvature on their ends so that when matedthey physically touch in the center of the core region.Examples of this type of connector are SC/PC and FC/PC.Typical loss values for multimode PC connectors lie in therange of 0.15 dB to 0.5 dB and typicalreflectance values are -18 dB to -32 dB.

To make a fusion splice, the two fibersto be joined first have their buffercoating removed, their ends cleaved andare then inserted into a fusion splicer. Apre-programmed sequence is theninitiated which first cleans the fiber endswith a low energy electric discharge,aligns the fibers, softens the ends with ahigh energy discharge and then bringsthe ends into physical contact. By

correct choice of temperature, the viscosity of the glasscauses the two fibers to flow together forming an almostinvisible joint. The final task is to protect the fragile splicewith a splice protector sleeve. The insertion loss of a fusionsplice is typically less than 0.1 dB for a multimode fiber.

PRINCIPLES OF OPTICAL TIME DOMAIN REFLECTOMETRY

The Physical Phenomenon

As light travels along an optical fiber it is progressivelyattenuated by a combination of absorption and scatteringmechanisms. Absorption losses in the fiber are caused eitherby intrinsic material effects, such as electronic transitions ormolecular vibrations of the glass, or by extrinsic effects, suchas metallic impurities or hydroxyl (OH-) ions in the glass.

The other main cause of loss is due to scattering. Here, lightis scattered by microscopic variations in the uniformity of theglass, known as Rayleigh scattering, and also by variations inthe dimensions of the fiber core that are comparable to thewavelength of the light being transmitted, this is known asMie scattering. A further scattering mechanism results whenhigh optical power levels are present, causing changes to therefractive index profile of the fiber and the subsequentgeneration of new, unwanted, wavelengths, this is known asnon-linear scattering.

A third source of loss is known as bend loss and results fromthe physical environment of the fiber This may bemacrobending, which is due to the fiber being bent with aradius of less than a centimeter or so, or microbending, whichis caused by the fiber being distorted on the sub-millimeterscale by, for example, cabling stresses.

The main source of loss in modern fibers is, however, thatdue to Rayleigh scattering and it is this phenomenon onwhich rests the operating principle of the OTDR. Rayleighscattering is an omni-directional phenomenon, that is, light isscattered in all directions, including a small amount backalong the fiber towards the source, shown in Figure 4.

Figure 4: Rayleigh scattering


The OTDR sends out a high power optical pulse and aphotodetector measures the scattered light coming back as afunction of time. At any point in time, the light received bythe OTDR is the light scattered from the pulse as it passesthrough a region of the fiber. Effectively the OTDR pulse actslike a virtual source testing each part of the fiber as it travelsalong. Since the speed of the pulse in the fiber is known,the OTDR can correlate the backscattered light with theactual position along the fiber of the phenomena thatproduced it. The processor within the OTDR can thus create adisplay of the amount of backscattered light against positionalong the fiber.

Now, as the back scattered light is directly proportional to theamplitude of the light pulse traveling in the forwarddirection, the slope of the OTDR trace gives a direct measureof the attenuation of the fiber.

A typical OTDR trace is shown in Figure 5. The largereflections at the connectors and at the end of the fiber aredue to the abrupt change in refractive index between thefiber itself and an air interface. These events which areknown as Fresnel reflections and can reflect as much as 4%of the transmitted signal.

The OTDR Instrument

Main Components of the OTDR

An OTDR works by sending out a high power optical pulseinto the fiber under test and detecting the very small levels oflight that are backscattered by the microscopic variations inthe fiber material. The basic elements of the OTDR are showndiagrammatically in Figure 6. The main control unit of theOTDR first provides a drive signal to a semiconductor laserwhich then fires a pulse of light into the fiber under test via amultimode fiber coupler. As the pulse of light passes alongthe fiber some light is scattered by the microscopic variationsin the fiber material and by reflective interfaces such asconnectors. The returned light passes through the couplerand is detected by an Avalanche Photodiode (APD). Thedetected signal is then amplified, and digitally sampled usinga high speed analog to digital converter.

To improve the quality of the backscatter signal, in order torender visible small imperfections in the fiber, it is customaryto average the signal from many thousands of pulses.

Once the backscatter trace has been acquired there are somecalculations that are required to make the results useful. Forexample, the power loss (attenuation) along a fiber is an

exponentialfunction, so thebackscatter signal isnormally displayed indecibels (dB). Also,as the light has totravel out and comeback again it willexperience loss inboth directions andso the loss scale ofthe OTDR has to bedivided by two.Finally the time scaleof the OTDRmeasurement isconverted todistance along thefiber, from aknowledge of therefractive index ofthe fiber, enabling aplot of loss in dBversus distancealong the fiber to bedisplayed.

Figure 5: A typical OTDR trace


The final trace is then displayed bythe OTDR which normally providescursors and zoom facilities forcomplete analysis of the fiber. Data istypically stored on a flashcard, or afloppy disc, for filing and for furtheranalysis by computer.

For a particular OTDR the amount oflight scattered back to the OTDR isproportional to the energy in theoptical pulse that, is equal to thepeak power of the pulse multipliedby its width. Thus a largerbackscatter signal may be obtainedby increasing either the pulsewidth orthe peak power of the pulse, orboth. There is, however, a trade-offbecause as the pulsewidth isincreased the length of the fiber corethat is illuminated at a given timeincreases. The level of backscatteredlight is correspondingly larger but thespatial resolution of the instrumentdecreases. The OTDR cannot resolvefeatures of the fiber that are spacedcloser together than the distance offiber illuminated at any instance bythe transmitted pulse. This isdemonstrated in Figure 7 where twoconnectors are both within thespatial extent of the optical pulse andwill show on the OTDR trace as oneconnector, not two.

A particular subtlety in using anOTDR to measure the loss of aconnector or a splice is thatinterpretation of the trace requiressome knowledge of the relativebackscattering properties of the fibers before andafter the feature. For example, if the fiber after alow-loss splice has a higher backscatter coefficientthan the fiber preceding the splice, then thereturned signal may show a stepped increase inpower at this point. For this reason the lossesmeasured by an OTDR do not always agree withthose measured with a power meter. In order tocircumvent this problem, measurements of thistype are normally performed from both ends of thefiber and the mean value, which represents thetrue loss, is computed. This is discussed further insection titled “Interpreting OTDR Traces.”

Figure 6: Block diagram of an OTDR

Figure 7: Schematic of the optical pulse in a fiber


The sampling interval is the distance between each measureddata point along the fiber. To make use of the resolutiondefined by the pulse-width of the OTDR, the samplinginterval should be equal to or less than the pulse-width Theeffect of decreasing the sampling interval to be less than thepulsewidth is to give a smoother trace but with noimprovement in resolution.

Refractive Index

Perhaps the largest uncertainty in determining the position ofa feature in the fiber is caused by an incorrect knowledge ofthe refractive index of the fiber, which is used to convert timeinto distance. The refractive index, which is more correctlyknown as group index, is normally chosen by the user tomatch the type of fiber that is being measured and isnormally supplied by the fiber cable manufacturer. It shouldbe noted, however, that the length of fiber inside a cablemight be greater than the length of the cable itself if thefiber takes a spiral route inside the cable. Therefore, it issometimes more appropriate to use an effective group indexof the cable when measuring cable lengths. Again, some carehas to be taken when choosing this value.

Some typical values of group index for uncabled multimodefibers are given in Figure 8.

There is, however, a relatively straightforward procedure todetermine the group index for a particular fiber. Chooseseveral tens of meters of cable of the same type andconstruction as the cable under test and measure its lengthLref accurately using the distance marker figures printed onthe outside of the cable. (Check which units of distance areused!) If these are not available then use a steel rule or tapemeasure. To minimize errors caused by the finite samplinginterval of the OTDR it is recommended that the referencecable be made as long as possible.

Connect a patchcord, which is longer than the dead-zone ofthe OTDR, to the front panel and connect the other end tothe reference fiber. Now use the OTDR to measure the opticallength of the reference fiber Lopt using the method for linklength described in the section titled “Measuring Link Lossand Link Length.”

However, the additional spatial information gained from anOTDR usually makes it the preferred measurement forinstallers and operators of fiber optic transmission systems.

OTDR Performance Parameters

An OTDR provides essential information on fiber systems suchas splice losses, imperfections in the fiber and overall lossbudgets. It is essential, therefore, that the measured data isaccurate and of a high quality.

The OTDR measures backscattered power as a function oftime, which is converted to a distance scale through aknowledge of the refractive index of the fiber. The mainperformance parameters are, therefore, the accuracy ofpower measurement and the accuracy of lengthmeasurement. These parameters are now discussed in somedetail.

Length Accuracy

The ability to determine the exact distance along a fiber to afeature such as a connector, a break, or the end of the fiberitself depends on the following parameters:

■ Accuracy of the time-base clock in the OTDR

■ Length offset calibration at the front panel of the OTDR

■ Pulse-width and data sampling interval of the OTDR

■ Refractive index of the fiber

■ Recovery time of the OTDR detector amplifier (known asthe dead-zone)

The time-base is normally based on a quartz oscillator and, assuch, its error is normally considered to be negligiblecompared to the other factors listed above.

Length offset calibration is normally performed by the userand may be set so that the reflection occurring at the frontpanel of the OTDR is set to zero meters. In many cases,however, it is more convenient to zero the OTDR at the farend of a lead-in fiber, known as a patchcord, which ispositioned between the OTDR and the fiber under test. Inthis way, distances are measured relative to the front end ofthe fiber under test.

The choice of pulsewidth is normally a compromise betweenloss measurement range or dynamic range and the requiredspatial resolution. A wider pulse will give an increasedmeasurement range but for optimum spatial resolution ashorter pulse is required. In some circumstances a wide pulsemay be used for making a loss measurement but a narrowpulse could be used for accurately locating the position of anevent such as a break in the fiber. In measuring LAN fibers itis desirable to use the shortest possible pulsewidth.

Group index 50/125 µm 62.5/125 µm

850nm 1.482 1.496

1300nm 1.477 1.491

Figure 8: Typical group index values for graded-index multimodefibers


The new group index Nnew is then given by

Nnew = Lopt x NoldLref (1)

where Nold is the current group index used by the OTDRwhen making the measurement.

Dead-zone

Features such as connectors or breaks in the fiber show up asincreases in power because they reflect larger proportions ofthe transmitted light pulse than the normal Rayleigh

backscatter. For example, the reflectance from an air-glass canbe as much as 4% of the transmitted power. This is, in fact,about 46 dB (40,000 times) greater than the Rayleighbackscatter for a 3 ns pulse-width and can be a seriousproblem as it is likely to cause the OTDR receiver to saturateand will result in the OTDR being blinded for a period of timeuntil the receiver recovers.

The recovery time from an overload condition is normallyspecified as a particular length known as the Event Dead-Zone. It clearly places a limit on the ability of the OTDR todiscriminate between two closely spaced events. A second

definition of dead-zone,known as AttenuationDead-Zone, is the distancefollowing a reflective eventbefore the attenuationreading of the OTDRbecomes valid again. Thisis usually specified as thedistance for thebackscatter signal torecover to what it wouldhave been if the reflectionhad not been present.Therefore, it is notsurprising that theattenuation dead-zone issomewhat longer than theevent dead-zone.

One commonly useddefinition of event dead-zone is the distance takenfor the reflective event tofall by 1.5 dB from its

peak value. Clearly, in the case of a large reflectionthe top of the peak will not be resolved due toreceiver overload and some care has to be takenwhen interpreting such a figure. Similarly, acommonly used definition of attenuation dead-zoneis the distance taken for the backscatter signal toreturn to within 0.5dB of its expected value, basedon extrapolating backwards from the section of fiberfollowing the event. Dead-zone definitions areillustrated in Figure 9.

For a non-reflective event, such as a fusion splice, adifferent measure of event dead-zone is sometimesused. This is illustrated in Figure 10 and consists ofdescribing the distance between positions on eitherside of a splice that are within 0.1 dB of their initialand final values, respectively.

Figure 9: How Reflective Event Dead-Zone and Attenuation Dead-Zone are Defined

Figure 10: How non-reflectve dead zone and attenuation dead-zone are defined


A particularly well established set of definitions for dead-zoneis described in the Telcordia GR-196 specification. Here, dead-zone is defined in terms of the distance following a reflectiveevent that a second event can be detected, and is moreappropriately called resolution. Two sets of tests are specified,the first determines the resolution for events following thenormally large Fresnel reflection occurring at the front panelof the OTDR, which is known as front end resolution. Thesecond determines the resolution following a reflective eventwhich occurs at some distance from the front end, and whichis known as network resolution. In each case the resolutionsof both a reflective event and a non-reflective event aremeasured, as shown in Figure 11.

The precise definitions of event and attenuation dead-zoneshave been a longstanding source of dispute betweeninstrument manufacturers. Care should be exercised wheninterpreting specifications regarding the nature and size ofthe reflective events that are used in determining dead-zones.Poor practice includes the addition of an attenuator toimprove the dead-zone performance, but at the expense ofthe dynamic range.

When considering dead-zone specifications it should also beremembered that even for an idealized OTDR that has a zerorecovery time there still exists a dead-zone within whichevents cannot be resolved. This is due to the finite width ofthe optical pulse and is discussed in the section titled “ThePhysical Phenomenon.”

Attenuation

The accuracy of attenuation, or loss, measurement dependson the following parameters:

■ Receiver linearity

■ Dynamic range

■ Attenuation dead-zone

Receiver Linearity

Absolute calibration of the measured backscatter powerlevels is not necessary in an OTDR. Measurements arenormally displayed on a logarithmic (dB) scale with anarbitrary reference level that is usually the backscatter signalat the near end of the fiber, following the front-endreflection. Thus the loss of a fiber section, expressed in dB, issimply equal to the difference between the power values ateither end of the section. Of particular importance, however,is the linearity of power measurements. Instrument

manufacturers haveinvested much time andeffort in developing highquality detectors andamplifiers that are verylinear.

In order to quantify thelinearity of an OTDR, agolden fiber (referencefiber) may be used. Agolden fiber is a length offiber whose linearity hasbeen calibrated by anindependent laboratory. Thisis measured on the OTDRunder test and the resultscompared to the calibratedsignature. Another test fornon-linearity consists of along length of un-calibratedfiber preceded byprogrammable attenuator.Here, the loss between twogiven points in the fiber,some distance apart, is

Figure 11: Telcordia GR-196 resolution specifications

(c) Network reflective resolution (distance from connector to a reflective event)(d) Network nonreflective resolution (distance from connector to a nonreflective event)

(a) Front-end reflective resolution (distance from OTDR to a reflective event)(b) Front-end nonreflective resolution (distance from OTDR to a nonreflective event)


measured as the launch pulse is progressively attenuated. Anydeviation in the loss represents a measure of the non-linearityof the OTDR.

Linearity can be affected by the nature of the fiber link andthe components in it and therefore most manufacturers donot quote a linearity value.

When used, OTDR linearity is normally expressed in units ofdB/dB.

Dynamic Range

The dynamic range of an OTDR is a way of specifying themaximum fiber attenuation that can be tolerated before thebackscatter signal becomes indistinguishable from the noise.

There are several commonly used definitions of dynamicrange as follows:

■ RMS dynamic range

■ Peak dynamic range

■ Fresnel dynamic range

■ Measurement range

It is usually convenient to define the signal level at the nearend of the fiber, just following the front-end Fresnelreflection, as 0dB. In this case the RMS (Root Mean Square)noise level is the dynamic range of the OTDR correspondingto a Signal-to-Noise Ratio (SNR) of one. This definition isshown as RMS dynamic range in Figure 12 and is perhaps themost commonly used.

A second definition refersthe signal to the peaknoise level rather theRMS level and gives adynamic range that isapproximately dB lessthan for the RMSdefinition. This is shownin Figure 12 as the peakdynamic range.

A third definition is basedon the fiber loss that canbe tolerated before theFresnel reflection from acleaved or polished fiberend can no longer beresolved. A Fresnelreflection will emergefrom the noise long afterthe dynamic range of the

T1 GUIDE—THE HIGH CAPACITY DIGITAL NETWORK 11

Rayleigh backscatter has been exceeded. This is shown inFigure 12 as the Fresnel dynamic range. The height of theFresnel reflection peak will depend on the actual level ofRayleigh backscatter signal that is present at that point, andthis will in turn depend on the optical pulsewidth. (Seesection titled “Main Components of the OTDR.”) Thus theactual difference between the Fresnel dynamic range’ and theother dynamic range definitions will depend on the operatingconditions of the OTDR, but generally the Fresnel dynamicrange will be substantially greater than the other definitions.

Another way of presenting the dynamic range of an OTDR isin terms of its loss measurement range. This is described inTelcordia specification GR-196. Here the loss measurementrange, in dB, is defined by the amount of attenuation thatcan be placed before a given event such that the event canstill be resolved with accuracy within acceptable limits. Thereare four types of event specified in the Telcordia specificationas follows:

■ Measurement of a 0.5 dB splice loss

■ Measurement of fiber attenuation coefficient

■ Detection of a non-reflective fiber end

■ Detection of a reflective fiber end

The measurement range for each type of event is measured.As an example, the splice loss definition of measurementrange is illustrated in Figure 12.

Figure 12: Schematic showing various definitions of dynamic range


In order to increase the dynamic range of an OTDR thenumber of averages, or equivalently the data acquisition time,may be increased. Typically, increasing the number ofaverages form N1 to N2 will give an improvement in dynamicrange of

Thus a factor of 4 increase in the number of averages willgive a 1.5 dB increase in dynamic range. When using anOTDR a compromise has to be made between the totalmeasurement time that can be tolerated and the dynamicrange that is required. This is the subject of furtherdiscussion in section titled “Using an OTDR.”

Some care should be taken when comparing differentspecifications of dynamic range, taking account for thepulse-width used, the amount of averaging and, of course,the particular definition employed.

Figure 13: Standards for LAN Systems — (a) IEC, (b) TIA, and (c) EN

(a)

(b)

(c)

2

N1

N5 • log


OTDR Specific Standards

There are also some TIA/EIA standards written specifically forOTDR testing of fibers. These are shown in Figure 15.

Data Format Description

In order to compare measurements made on different OTDRsit has become necessary in the industry to develop a standardformat for storing OTDR traces. Furthermore, it is oftenexpedient to gather as much data whilst on-site and toanalyze the results data afterwards using a computer withOTDR emulation software. Thus a common format for datastorage is quintessential and one such format has beendeveloped by Telcordia (formerly Bellcore) known as the GR-196 specification.

It has become incumbent on manufacturers of OTDRs toensure that their methods of data storage are compliant withGR-196 or else to provide proprietary software to convertbetween their own standard and the Telcordia standard.

The GR-196 data format standard comprises nine blocks ofinformation, two of which are optional. The content of theseblocks is summarized below:

Map: Contains a catalog of the remaining blocks ofinformation.

General Parameters: Contains general information about thefile, such as fiber identification, cable code, wavelength,length offset, operator’s name and any other comments.

Supplier Parameters: Contains general information about theOTDR, such as supplier’s name, OTDR model and serialnumber, optical modules and serial numbers, software versionand any other comments.

Fixed Parameters: Contains specific information about theOTDR setup, such as date and time stamp, units of distance,actual wavelength, pulse-width, data point spacing, numberof data points, group index, backscatter coefficient, numberof averages, averaging time and front panel offset.

Attenuation Dead-Zone

A description of the dead-zone that occurs following areflective event was given in section titled “Principles ofOptical Time Domain Reflectometry — Dead Zone” where itwas shown that the attenuation dead-zone is a measure ofthe time taken for the receiver to recover sufficiently from anoverload to allow accurate loss measurements to be made.Clearly, the accuracy of, for example, a splice loss will dependon how far it occurs beyond the specified dead-zone. For ameasurement of fiber loss it is advisable to choose thestarting position for the section being measured to be wellbeyond the extent of the dead-zone.

International Standards

There is a variety of standards on test procedures for fiber-optic LAN systems. Some representative standards from theInternational Electrotechnical Commission (IEC), Europeanstandards (EN) and the Telecommunications IndustriesAssociation (TIA) in the United States are shown in Figure 13.

Mode Conditioning

Each standard describes the requirements for achieving thecorrect modal distribution in the patchcord connecting theOTDR (or light source) to the fiber under test. Thesespecifications are discussed in section titled “Optical FiberBasics.”

Wavelength

The wavelengths required for testing specified in thestandards are shown in Figure 14.

OTDR Measurement Standards

TIA/EIA-455-8: Measurement of splice or connector loss and reflectance using an OTDR

TIA/EIA-455-59A: Measurement of fiber point discontinuities using an OTDR

TIA/EIA-455-60A: Measurement of fiber or cable length using an OTDR

TIA/EIA-455-61A: Measurement of fiber or cable attenuation using an OTDR

Figure 15: Standards Specifically for OTDR Fiber Testing

Center Spectral WidthWavelength Range, nm (FWHM), nm

850nm ±30 30 to 60

1300nm ±20 100 to 140

Figure 14: Requirements on Testing Wavelengths

Coupled Power ratio

A parameter for characterizing the degree of modal filling ina multimode fiber, known as the Coupled Power Ratio (CPR),is described in IEC 61300-3-31: Examinations andmeasurements – coupled power ratio measurement. A briefdescription of the CPR method is given in the section titled“Optical Fiber Basics.”


Key Events: Contains a summary of the trace analysisperformed by the OTDR, such as the loss, reflectance andposition of events.

Link Parameters: Contains specific information about the firelink that relates geographical landmarks to the Key Events inthe fiber trace. This block is optional.

Data Points: Contains a list of power levels that represent thedata points from which the trace is made. The spacing of thedata points is provided in the Fixed Parameters block.

Special Proprietary: Contains unique information about themeasurement or supporting data that is not required toperform the basic analysis of the data file, but is used tosupport additional proprietary features of the manufacturer.This block is optional.

Checksum: Contains a checksum to enable the file to bechecked for errors.

Laser Safety

The operating wavelength of OTDRs for multimode LANtesting is normally in the near infrared, for example 850 nmor 1300 nm, and is therefore invisible to the human eye.Generally, the pulse widths from OTDRs are very short andthe spacing between pulses is quite large making the averagepower levels quite small. It is important, however, to ascertainthe particular hazard rating of an OTDR before use, byreferring to its hazard classification label which should beclearly displayed on the OTDR (or if Class 1, in the UserManual).

The following classes exist for devices used within opticalfiber systems (IEC 60825-1):

■ Class 1 — devices that are safe under reasonablyforeseeable conditions of operation, including the use ofoptical instruments for intrabeam viewing.

■ Class 1M — devices emitting in the wavelength range from302,5 nm to 4 000 nm which are safe under reasonablyforeseeable conditions of operation, but may be hazardousif the user employs optics within the beam. Because of therisk of an unskilled person employing such optics Class 1Mdevices should not be operated in uncontrolledenvironments such as the open office or engineering areas.

■ Class 2 — devices that emit visible radiation in thewavelength range from 400 nm to 700 nm where eyeprotection is normally afforded by aversion responses,including the blink reflex. This reaction may be expected toprovide adequate protection under reasonably foreseeableconditions of operation including the use of opticalinstruments for intrabeam viewing.

■ Class 2M — devices that emit visible radiation in thewavelength range from 400 nm to 700 nm where eyeprotection is normally afforded by aversion responsesincluding the blink reflex. However, viewing of the outputmay be more hazardous if the user employs optics tocollimate the beam or magnify the source. Because of therisk of an unskilled person employing such optics Class 1Mdevices should not be operated in uncontrolledenvironments such as the open office or engineering areas.

■ Class 3R — devices that emit in the wavelength range from302,5 nm to 1,000,000 nm where direct intrabeamviewing is potentially hazardous but the risk is lower thanfor Class 3B devices.

■ Class 3B — devices that are normally hazardous whendirect intrabeam exposure occurs but viewing diffusereflections is normally safe.

■ Class 4 — devices that are also capable of producinghazardous diffuse reflections. They may cause skin injuriesand could also constitute a fire hazard. Their use requiresextreme caution.

Therefore, it is important to be aware of the classification ofan OTDR, or any other device containing a laser source, andobserve safe working practices. For example, BS 7718: 1996states:

“Under no circumstances should a connector end-face,prepared optical fiber or fractured optical fiber be vieweddirectly unless the power emitted from the optical fiber isknown to be safe (as defined within IEC 60825) and underlocal control. This allows inspection of components usinglocally injected visible light but prevents the inspection ofcomponents using light injection from a remote non-controlled location.”

Note: while a particular OTDR may carry a Class 1 hazardrating there may be an additional light source built into theOTDR, such as a Visual Fault Locator (VFL), which could be ofa higher hazard rating.

PATCHCORDS AND MODAL EFFECTS

It can be demonstrated that the exact shape of thebackscatter trace obtained with an OTDR will depend on theparticular characteristics of the laser source in the OTDR. Forexample, the manner in which light is coupled from the lasersource into the fiber under test can have a strong influenceon the measurements of both fiber attenuation and spliceloss. This section considers in some detail these so-calledmodal effects.


Description of Modal Effects

In the section titled “Optical Fiber Basics — Fiber Types” itwas shown that light may take many different paths througha multimode fiber corresponding to reflections at differentangles within the fiber. These different paths are known asmodes’ of the fiber, hence the term multimode’ fiber. Inmultimode fiber light traveling at steeper angles is generallysaid to have high-order modes and light traveling at lowangles corresponds to low order modes.

It is normally found that at bends in a multimode fiber thehigh-order modes are more likely to suffer greater losses thanlower-order modes, a process known as macrobending loss.Similarly, the presence of microscopic distortions in the fibercaused by cabling stresses also selectively attenuates high-order modes, a process known as microbending loss. Thusthe modal distribution in the fiber is a convolution of theactual distribution that is launched into it and theenvironment through which the light passes. Furthermore,the presence of connectors and other components, such asfiber couplers, will also cause modally-dependent loss. Thusthe loss of a particular fiber link is very dependent on theprecise distribution of power amongst the modes, knowngenerally as the Mode Power Distribution’ (MPD) of the fiber.

Therefore,t is important that the MPD of the light sourceused to test a fiber span is similar to that of the source that isto be used in the operational system. For example, if the testsource launches light preferentially into low-order modesthen the measured fiber loss is likely to be underestimated.

Comparison of Light Sources

Fiber loss measurements made with the Light Source andPower Meter method (LSPM) normally employ an LED sourceof the appropriate wavelength. There are, however, differenttypes of LED having different emission characteristics thataffect the way they couple light into a fiber. For example, theSurface-emitting LED (SLED) emits over a large area and atfairly large angles, while the edge-emitting LED (ELED) emitsover a small area and at very small angles, similar to a laserdiode. Thus the SLED would be expected to excite a greaterproportion of high-order modes in the fiber than an ELEDand consequently the SLED launch would be expected tosuffer higher transmission losses.

It has become necessary, therefore, to establish a means ofquantifying the amount of mode-filling that different sourcesproduce. One approach, known as the Coupled Power Ratio(CPR) method, consists of comparing the total amount oflight exiting the patchcord with the amount that is coupledinto a single-mode fiber that is butt-coupled to the end ofthe patchcord. A large value of CPR indicates a greateramount of high-order mode-filling while a small valueindicates that predominantly low-order modes are present.Any measurement of link loss is therefore only meaningfulwhen accompanied by the CPR value of the source that isused.

A table showing the CPR values in dB, at 850 nm for severaldifferent fiber sizes is given in Figure 16 where it can be seenthat a very small CPR corresponds to the fiber being greatlyunder-filled and a large CPR value corresponds to the fiberbeing over-filled, i.e. all of the modes in the fiber are excited.

A particular type of laser source known as the VCSEL isfinding increasing application in 1 Gb/s and 10 Gb/s Ethernetsystems. The VCSEL (Vertical Cavity Surface Emitting Laser)has a circularly symmetric emitting surface and a fairly smallemitting angle. It is therefore well suited for efficient couplingto a multimode fiber. It does, however, under fill the fiber andis typically a CPR category 4 device.

Some care must be exercised, however, in making use of CPRvalues. It has been shown that a patchcord with a dip in thecenter of the refractive index profile can give an erroneouslylarge CPR value, indicating an overfilled launch when, in fact,the fiber may be underfilled.

Mode Conditioning

It was shown in “Description of Modal Effects” that high-order modes are more readily attenuated than low-ordermodes in a fiber. In the case of a fully filled fiber the result isthat the highest-order modes will gradually be lost and themodal distribution in the fiber will settle down to a steady-state known as the Equilibrium Mode Distribution (EMD). Thedistance required for this to happen may be anything from afew hundred meters to several kilometers, depending on thetype of fiber and the cabling environment.

Category 1 Category 5(Over filled) Category 2 Category 3 Category 4 (Very under filled)

50/125 µm 20 to 24 16 to 19.9 11 to 15.9 6 to 10.9 0 to 5.9

62.5/125 µm 25 to 29 21 to 24.9 14 to 20.9 7 to 13.9 0 to 6.9

100/140 µm 30 to 34 26 to 29.9 18 to 25.9 10 to 17.9 0 to 9.9

Figure 16: Table of CPR values at 850 nm (IEC 61280-4-1)


In order to simulate an EMD in the launch patchcord it hasbeen found that, in the case of an overfilled launch, themodal distribution may be modified by winding the fiberaround a mandrel of a particular radius. This process isknown as mode-filtering. The international standardsdescribed previously are a little ambiguous on the precisemandrel sizes to be used as the actual bend radius assumedby the fiber depends on the diameter of the patchcord cableas well as on the mandrel diameter. Summaries of thespecified mandrel sizes for TIA/EIA and IEC/EN standards aregiven in Figure 17 and Figure 18 respectively.

TESTING OPTICAL FIBER INSTALLATIONS

Using an OTDR

To obtain meaningful measurements with an OTDR, it isnecessary to ensure the instrument is set up correctly. For anideal measurement the spatial resolution would be as small aspossible, the dynamic range would be as large as possibleand the measurement time would be short. . Unfortunatelythese requirements are somewhat in opposition to each otherand so a compromise has to be made.

Depending on the type of OTDR there are typically severaloperating parameters that can be set by the user. These mayinclude the following:

■ Source wavelength

■ Pulsewidth

■ Pulse peak power

■ Number of averages (or integration time)

■ Sampling interval

■ Refractive index

The source wavelength is normally chosen to suit theparticular testing requirements. Typically the backscattercoefficient is greater at 850 nm and so this wavelengthnormally gives the best dynamic range and resolution and isto be preferred for diagnostic work. In some cases,particularly for contractual requirements, measurements arealso required at 1300 nm although no extra knowledge willbe gained beyond that obtainable at 850 nm. Care should betaken, however, when comparing fiber loss measurementsmade with different sources due the dependence of fiberattenuation on wavelength. A summary of the sourceparameters required by international standards is given in .

The pulsewidth puts a lower limit on the spatial resolutionthat may be achieved in the fiber and is normally chosen togive sufficient resolution between expected events in thefiber. In a typical fiber, light travels at about 0.2 m/ns and,allowing for the fact that the time axis is normally divided bytwo to allow for the return trip, the spatial resolution isapproximately 0.1 m/ns. So for a pulse width of 5 ns thespatial resolution is about 0.5 m.

The dynamic range and the measurement range both dependon the quality of the backscatter signal and may be improvedby increasing either the peak power of the pulse, thepulsewidth or by increasing the number of averages taken forthe measurement. See section titled “Dynamic Range.”

Figure 18: Summary of mandrel diameters (in mm) in IEC/EN standards

Cable diameter 50/125µm 62.5/125µm

250µm 18.0 20

3.0mm 15.0 17

Cable diameter 50/125 µm 62.5/125 µm

250 µm 25.0 20

900 µm 24.1 19.1

2.0 mm 23.0 18.0

2.4 mm 22.6 17.6

3.0 mm 22.0 17.0

Figure 17. Summary of mandrel diameters (in mm) in TIA/EIA standards

Generally it is easier to reduce the proportion of high-ordermodes than it is to increase it. So in the case of a lasersource, some care has to be exercised in interpretingmeasurements of link loss. It would be expected that a lasersource would under-estimate the link loss compared to anLED source. One method of producing a pseudo-EMDdistribution from a laser source is to use a long launchpatchcord of, say 500 meters, so that power can becomedistributed between the modes, a process known as mode-scrambling. An alternative method is to use a relatively shortpatchcord, but longer than the dead-zone of the OTDR,which is treated to achieve an EMD distribution. In general,this requires the use of some mechanism to encourage thecoupling of power from low-order modes into high-ordermodes.

The CPR values for an EMD source at 850nm is considered tobe 13.5dB and 17.5dB for 50µm and 62.5µm respectively(IEC 61300-1). From Figure 16, this corresponds to a category3 launch in both cases.


The spatial resolution and dynamic range are interdependent.A longer pulse has a higher energy content and thereforemore backscatter signal but at the cost of reduced resolution.A typical measurement approach might be to set thepulsewidth to resolve expected events in the fiber span, suchas connections, and set the number of averages so that thefar end of the fiber can be clearly seen. Then, if a feature ofsome kind is observed at a position along the fiber the OTDRdisplay can be zoomed in’ on the feature and the pulsewidthreduced so as to see the feature in more detail. The numberof averages may be increased to improve the measurement ofloss and position of the feature.

The distance between measured data points, known as thesampling interval, may be adjusted on some OTDRs andshould be chosen so as to be commensurate with theresolution determined by the pulsewidth.

The refractive index usually contributes the largest componentof uncertainty when making distance measurements on anOTDR. To accurately locate a feature, such as break, in thefiber span it is important to use the correct refractive indexfor the type of fiber being tested. For example, an error of0.5% in the refractive index will cause a positional error of10m in a 2km fiber span. It should be noted, however, thatthe length of a fiber in a cable my be longer than the cableitself due to a spiral winding of the fiber in the cable and insome cases an effective cable index’ should be used ratherthan the fiber index. In the absence of other informationtypical index values are given in Figure 8.

Using the Correct Patchcords

A patchcord is a piece of fiber cable used to connect theOTDR to the fiber under test. In a typical installation thefibers inside an incoming cable will be broken out’ andterminated at a patch panel’, which consists of rows of fiberconnectors. One end of the OTDR patch cord is normally leftconnected to the OTDR and the other end is progressivelymoved along the patch panel as measurements are made oneach fiber.

The patchcord therefore plays a very important part in fibermeasurements and some care should be taken to ensure thecorrect patchcord is being used. Factors affecting the choiceof patchcord are as follows:

■ The type of fiber

■ The type of connector

■ The OTDR dead-zone

■ The required modal distribution

The type of fiber is probably the most important factor toconsider when choosing a patchcord. There are two maintypes of fiber found in multimode Local Area Network (LAN)installations, designated by their core diameters, either 50 µmor 62.5 µm. If the patchcord is not of the correct size thenthe dynamic range is likely to suffer and loss measurementsof the fiber and connectors may not be representative of thesystem after commissioning.

The patchcord will need to have a connector on one end thatis compatible with the bulkhead connector on the front panelof the OTDR. This might be, for example, an FC, SC or STconnector. The connector on the other end of the patchcordis, of course, chosen to be compatible with the fiber undertest.

The length of the patchcord is chosen so that the effects ofreceiver overload caused by the Fresnel reflection at the OTDRend of the patchcord have sufficiently died away before thefiber under test is measured. Depending on the OTDR, thislength may be a few meters or up to hundreds of meters.This patchcord is sometimes known as a dead-zone cable or alead-in cable.

Another, often overlooked, factor affecting the length of thepatchcord is the amount of mode-mixing that occurs withinit. It is generally assumed that the modal volume of the fiberis quite well filled when making loss measurements using theLight Source and Power Meter (LSPM) and that a mode-filteris effective in creating a steady state distribution or EMD. Theexact state of mode-filling is, in fact dependent on the lengthof the patchcord and the manner in which the patchcord isdeployed. For example, a long patchcord (100s of meters)that is initially fully filled may eventually settle down to theEMD without the need for mandrel-wrap mode-filters.Conversely, an under filled launch may tend to approach theEMD if some mode-coupling mechanism, such as bends, ispresent. It is recommended, therefore, by IEC11801, that theCPR, for a particular source and patchcord combination, isalways reported. This facilitates comparisons betweenmeasurements made under differing conditions.

Tail Patchcords (Tail-cords)

The use of a patchcord at the far (output) end of a fiberunder test is often found to be useful when the loss of aconnector at the far end is to be measured. If the tailpatchcord is of similar type to the fiber under test then theconnector loss can be measured in the usual way bycomparing the backscatter signal just before and after theconnector. The length of the tail patchcord must be longerthan the attenuation dead-zone of the OTDR.

18 MULTIMODE FIBER LAN TESTING WITH OTDRS18 MULTIMODE FIBER LAN TESTING WITH OTDRS

In the case of a duplex cable (a twin fiber cable) the use oftwo tail patchcords of different lengths can be effective inidentifying which fiber is which when connection to apatchpanel is to be made, a process known as polaritychecking’. For example, if one fiber has a 20 m tail patchcordand the other has a 15m patchcord then the positions of thefar-end Fresnel reflections will differ by 5 m, thus allowingthe two fibers to be identified.

Interpreting OTDR Traces

A typical fiber link may consist of severallengths of fiber, some reflective events, suchas connectors or breaks and some fusionsplices. Each of these components has itsown characteristic backscatter signature thatcan be readily identified and measured by theOTDR. The following sections describe howto identify these features and to characterizethem.

Measuring Link Loss and Link Length

Link loss is the total loss between two givenpoints in a fiber channel. Typically this willinclude at least one section of fiber and aconnector pair at either end, as shown in Figure 19.

Link loss is defined as the difference between the backscatter signal P1 at aposition just prior to the reflection at the first connector, and the backscatter signal P3 at some position in the tail-cord justbeyond the extent of the attenuation dead-zone of the OTDR. In this manner the linkloss includes the insertion loss of the fiberitself and a connector pair at each end of the fiber. It can be seen that the use of a tail-cord is necessary in this procedure toobtain an accurate measurement of the loss of the second connector. If a tail-cordwas not present then the link loss wouldhave to be measured to a position Z2 justprior to the reflection at the secondconnector and would be reported as the loss of the fiber plus one connector pair.

Link length is measured as the distancebetween the position Z1 just prior to theconnector at the front end to the equivalentposition Z2 just prior to the connector at the far end and corresponds to the length of the fiber section and does not include the patchcords.

Measuring Fiber Loss

Fiber loss is the loss due to the fiber section of the link anddoes not include any additional losses due to connectors. Tomake an accurate measurement of fiber loss it is important toavoid the effects of the dead-zone region following the firstconnector. Therefore, fiber loss is measured as the differencebetween the backscatter signal P1 at a position well clear of

Figure 19: Measurement of link loss and link length

Figure 20: Measurement of fiber loss


the dead zone and the backscatter signal P2 just prior to thereflection from the second connector (or the fiber end), asshown in Figure 20. If a normalized fiber loss is required thenthe measured loss is simply divided by the distance betweenthe two measured points (Z2-Z1) and expressed as dB/km.

Measuring Connector Loss

A connector is normally identified by a reflective peakfollowed by a reduction in backscatter power. To measureconnector loss accurately it is necessary to reduce the effectof noise by fitting a straight line to the backscatter signal inthe fiber section preceding the connector and to the sectionfollowing the dead-zoneregion, as shown in Figure21(a). Connector loss isdefined as the difference, indB, between the two fittedlines. Typically, a fit to twopoints on either side of theconnector is normally foundto be sufficient.

If the dead-zone of the OTDR is quite short then asufficiently accurate lossmeasurement can beobtained without the needfor line fitting, as shown inFigure 21(b). As an example,if the OTDR dead-zone is 3meters then the error in lossmeasurement without fittingis less than 0.01 dB for a fiber with a typical loss of 3 dB/km. A furtherimprovement can, however,be readily made by averaginga few points on either side of the connector.

Note: If the backscattercoefficient of the fiberfollowing the connector isdifferent to that in the fiberpreceding the connector, then the measurement ofconnector loss may differ tothat made using the lightsource and power metermethod, see the section titled “Bi-directionalMeasurements.”

Measuring Splice Loss

A fusion splice is normally characterized by a reduction inbackscatter signal, but with no reflective peak, as shown inFigure 22. Splice loss is defined as the difference, in dB,between the backscatter signal just prior to the splice and theextrapolated signal from the fiber following the splice.

Note: If the backscatter coefficient of the fiber following thesplice is different to that in the fiber preceding the splice thenthe measurement of splice loss may differ to that made usingthe light source and power meter method, see the followingsection titled “Bi-directional Measurements.”

Figure 21: Measurement of connector loss: (a) long dead-zone; (b) short dead-zone


Bi-directional Measurements

In certain circumstances a splice may show apower gain rather than the characteristicreduction in power that is shown in Figure 22.The reason for this is that if the fiber followingthe splice has a larger backscatter coefficient dueto, for example a larger numerical aperture, thenthe larger backscatter signal may completely maskthe loss of the splice.

One method of compensating for thisphenomenon is to measure the fiber from bothends and average the two backscatter traces. Thisis achieved by reversing and inverting one of thetraces. In this manner, what appears to be apower gain in one direction will show as a largepower loss in the other direction and the averagevalue will represent the true splice loss. This is illustrated in Figure 23.

In a typical fiber LAN where each fiber channel consists of aduplex pair of fibers it is often convenient to connect eachpair of fibers together at the far end of the link using shortjumper’ patchcords. In this manner bi-directionalmeasurements of both fibers in a duplex pair can be obtainedwithout having to transport the OTDR to the other end of thelink.

Measuring Bend Losses

When a fiber is bent around a radius of less than acentimeter, it experiences an effect known as macrobendingwhere light leaks out of the core causing a subsequentreduction in the backscatter signal. This results in an eventthat looks very similar to a splice loss.

The actual value of bend loss can be determined in a similarmanner to that shown in for splice loss. In the case of bendloss there is no need to make a bi-directional measurement,as clearly the fiber is the same type both before and after theevent.

Macrobending can be used to good effect in identifying theexact position at which an event, such as a fiber break,occurs. This is done by deliberately introducing a bend in thefiber near to where the break is suspected. If a bend-inducedloss is visible on the OTDR trace then the break must occurafter that point. If the loss is not visible then the break mustoccur before that point.

Measuring Reflectance

The Reflectance of a component, such as a connector, isdefined as the ratio of the amount of reflected light to theincident light and is expressed in dB. A more negative valueof reflectance indicates a smaller amount of reflected light.

Figure 22: Measurement of splice loss

Figure 23: Bi-directional measurement of splice loss


The term Optical Return Loss (ORL) is often encountered andnormally refers to the total reflected optical power from thewhole of the fiber link including connectors and fibersegments. ORL can be derived from an OTDR by integratingunder the backscatter trace, but it is usually measured using acontinuous source, such as an LED, and a power meter. ORLis expressed as a positive quantity; thus a more positive valueindicates a smaller amount of reflected light.

There is seldom any requirement to measure ORL whentesting multimode fiber LANs and it is normally not possibleto do so at the short pulsewidths used. Nearly all reflectiveevents cause the OTDR to go into saturation and render themeasurement invalid.

In Figure 24, the height of the reflected peak H of an eventgives some information about the reflectance, but it is notthe whole story. This is because the power reflected by, forexample, a connector is proportional to the amplitude of theoptical pulse but the backscattered power just prior to thereflection is proportional to both the amplitude and the widthof the optical pulse, so H will depend on the particularpulsewidth used.

The reflectance may be calculated from the followingexpression:

R = Bs + 10logD + 10log (10H⁄5 - 1)

where Bs is the backscatter coefficient of the fiber and D isthe pulsewidth in ns.

If H is greater than about 8dB the following simplifiedexpression may be used:

R = Bs + 0logD + 2H

The backscatter coefficient Bs will depend on the type offiber and the wavelength used, some typical values forgraded-index multimode fibers are given in Figure 25.

Figure 24: Measurement of reflectance

Backscatter coefficient 50/125 µm 62.5/125 µm

850 nm, dB -67 -64

1300 nm, dB -73 -70

Figure 25: Typical values of backscatter coefficient for multimode fibers

Thus, if H is measured from the OTDR trace and thepulsewidth and fiber type are known, then reflectance canbe calculated. As an example, the reflectance from a polishedfiber end is about –14.4dB which would correspond to avalue of H=25dB for a 62.5µm fiber at 850nm with a 1nspulsewidth.

Note: Care should be taken to ensure that the top of thereflective peak does not cause the OTDR receiver to saturateas this will lead to errors in the determination of reflectance.


TROUBLESHOOTING

To obtain good, meaningful, measurements,it is necessary to follows good workingpractices with regard to cleanliness and fiberhandling. The following sections give someadvice on potential sources of problems.

Cleaning Connectors

It is vital to ensure that the end faces of theconnectors used to terminate patchcords arefree from contamination and damage.Clearly, the reflectance of a contact styleconnector would be greatly increased if thetwo fibers surfaces were prevented from intimate contact by the presence ofcontamination.

The specific cleaning procedures forconnectors vary dependent upon the designbut typical methods include the use of mildlyabrasive tape, such as in a reel cleaners, orwiping with a lint-free cloth soaked in a solvent such asisopropyl alcohol (isopropanol).

It is good practice, therefore, to minimize the number oftimes the connector on the OTDR front panel is re-mated. Ifpossible, leave the launch patchcord in place for as manymeasurements as possible.

Patchcord Bends

The presence of a sharp bend in the OTDR patchcord, such asat front panel connector, may cause a non-reflective loss inthe OTDR trace, leading to a subsequent reduction indynamic range. Care should be taken to avoid undue stresson any patchcords used.

Also, the presence of sharp bends in the fiber under test maycause similar non-reflective events, which may reduce themeasurement range that can be achieved.

Saturation Effects

The presence of a large reflective event arising from, forexample, a connector or a fiber break, may cause the OTDRreceiver to saturate, as shown in Figue 26. It is important thatwhen this is suspected care should be exercised when makingReflectance measurements which require the top of thereflected peak to be resolved.

Furthermore, the presence of saturation may cause the eventand attenuation dead-zones to be longer than expected andso care should be taken when making loss measurements atpositions immediately following such an event.

Figure 26: Receiver saturation


Ghost Reflections

A particular problem with characterizingmultimode optical fiber links with an OTDR isthe occurrence of a phenomenon known asghosting. In contrast to fusion splicesconnectors usually reflect a proportion of theincident light back towards the source. Whenthe reflection from a patchcord, for example,reaches the front panel of the OTDR it isagain reflected and acts like a secondlaunched pulse that produces its ownbackscatter trace. The result is that there aretwo overlapping backscatter traces that aredisplaced in space by the length of thepatchcord. The reflected trace is normallymuch weaker than the primary signal butmakes itself visible by the occurrence of ghostreflections following each connector in thelink.

In the example detailed in Figure 27, peaks 1,2 & 4 represent the normal reflections fromthe connector and the end of the fiberwhereas the peaks 3 & 5 represent the ghostreflections of these features. It can be seenthat the ghosts occur at distances exactlyequal to the patchcord length. Also, ghostsreflection can occur after the physical end ofthe fiber. Therefore, some care has to betaken when interpreting traces whereghosting is present. A particular feature ofghost reflections is that they are notaccompanied by the loss in power that is seenwith a normal reflection and this feature cansometimes be used to identify them. In thecase of a fiber link with many connectors thetrace can be very confusing and so it isrecommended that testing be carefullyplanned to avoid having to test multiplesections of fiber if possible.

Wavelength Effects

When comparing attenuation measurementsmade by different test instrumentation someattention must be paid to the operatingwavelength of the particular test equipment.A typical plot of multimode fiber attenuation againstwavelength is shown in Figure 28. The attenuation changesby approximately -0.014 dB/km/nm in the 850 nm region. Forexample, a difference in wavelengths of 50 nm could causean error of 0.14 dB on a 200 m length of fiber.

Figure 27: Illustration of ghosting

Figure 28: Typical attenuation curve for 62.5 µm multimode fiber


APPENDIX A — TYPICAL VALUES FOR FIBERS ANDCONNECTORS

The following tables give typical values for multimode fibersand connectors:

Fiber attenuation 50/125 µm 62.5/125 µm

850 nm, dB/km 2.5 3.0

1300 nm, dB/km 0.7 0.7

Group index 50/125µm 62.5/125µm

850 nm 1.482 1.496

1300 nm 1.477 1.491

Numerical aperture 50/125 µm 62.5/125 µm

0.200 0.275

Backscatter coefficient 50/125 µm 62.5/125 µm

850 nm, dB -67 -64

1300 nm, dB -73 -70

Insertion Loss

Fusion splice, dB <0.1

Connector (physical contact, PC), dB <0.5

Reflectance

Polished fiber end, dB -14

PC connector, dB -22 to -18

APPENDIX B — USEFUL INFORMATION

dB conversions

dB value Multiplier dB value Multiplier

0 dB 1

+3 dB 2 -3 dB 0.5

+10 dB 10 -10 dB 0.1

+20 dB 100 -20 dB 0.01

+30 dB 1 000 -30 dB 0.001

+40 dB 10 000 -40 dB 0.000 1

+50 dB 100 000 -50 dB 0.000 01

+60 dB 1 000 000 -60 dB 0.000 001

Scaling factors

Name Symbol Multiplier

Tera T 1012

Giga G 109

Mega M 106

kilo k 103

milli m 10-3

micro m 10-6

nano n 10-9

pico p 10-12

femto f 10-15


APPENDIX C — GLOSSARY OF TERMS

Attenuation: The decrease in the magnitude of the opticalsignal between two points, expressed in dB. Also known asLoss.

Attenuation Coefficient: The attenuation per unit length of afiber, expressed in dB/km.

Averaging Time: All Optical Time–Domain Reflectometersproduce a trace by averaging the signal received back fromthe fiber being tested. The higher the number of averagesused, the less noise remains on the trace.

Bi-directional Measurement: A measurement result obtainedby averaging the OTDR trace taken from both ends of thefiber. Useful to correct for loss errors caused by jointsbetween dissimilar fibers, see Gainers.

Campus Backbone: The portion of LAN cabling that connectsMain Building Cross-Connects together.

Channel: See Fiber Channel.

Consolidation Point: An enclosure where several differentfiber links are combined into single cable.

Dead-Zone: The distance following a reflective event, forexample, for the backscatter trace to return to its nominalvalue within a given uncertainty.

Dead-Zone Fiber: A patchcord connected to the OTDR whichis of length greater than the Dead-Zone of the OTDR. TheDead-Zone Fiber may also function as a Mode ConditioningPatchcord. Sometimes known as a lead-in fiber.

Decibel (dB): The ratio between two optical powers, definedas

Power = 10 • log (P1/P2) dB

The 0 dB level on an OTDR display is normally definedrelative to the backscatter power P2 at a point near to thefront panel connector of the OTDR, and within the launchPatchcord.

Duplex Operation (Full): Operating method in whichtransmission is possible simultaneously, in both directions of aFiber Channel. See Half Duplex Operation.

Dynamic Range: The ratio of the backscatter signal from thenear end of the fiber to the noise level, expressed in dB. Seealso Measurement Range.

Event: Any discontinuity on an OTDR trace caused by aconnector, break, or region of different loss. Sometimes theseare referred to as features in the fiber.

Fiber Channel: Any end-to-end fiber connection betweenterminal equipment including terminal patchcords and one ormore fiber segments. (Note that the term Fiber Channel isalso used to refer to a compute communications protocol).

Fiber Link: Any fiber connection between two interfacessuch as connectors but not including patchcords to terminalequipment. It may contain one or more fiber segments withtheir associated connectors.

Fiber Segment: The single length of fiber that, together withother segments, makes up a fiber link. It will beconnectorised at each end but will not contain anyconnections along its length.

Fresnel Event: The reflection of light from a glass-airinterface such as in a connector or a fiber break.

Gainer: An apparent negative loss at a splice caused by thefiber following the splice having a greater backscattercoefficient.

Group (Refractive) Index: The ratio of the speed of light in avacuum fiber to the speed of light in a fiber.

Half Duplex Operation: Operating method in whichtransmission is possible simultaneously, in both directions of aFiber Channel, but not simultaneously. See Duplex Operation.

Horizontal Cabling (or Backbone): The portion of LANcabling that connects a Horizontal Cross Connect to theWork Station Telecommunication Outlets.

Horizontal Cross-Connect: An enclosure containing a PatchPanel for connecting Horizontal Cabling to, for example, aVertical Backbone.

Insertion Loss: The attenuation caused by a component suchas a connector or a coupler.

LAN: See Local Area Network

Link: See Fiber Link

Local Area Network (LAN): A LAN typically consists of a siteor building-based communication system providing voiceand/or data services to a network of computers andworkstations. Typical-point to-point fiber lengths are in theorder of hundreds of meters, and are connected by eitherpermanent fusion splices or demountable connectors, such asat Telecommunication Outlets for connection tocommunication equipment. Also known as Premises Wiring,

Macrobending Loss: Loss in a multimode fiber caused by thefiber being bent with a radius of less than a centimeter or so.


Main Building Cross-Connect: An enclosure containing aPatch Panel for connecting Vertical Cabling to, for example, aCampus Backbone.

Measurement Range: The maximum attenuation that can betolerated while still allowing an event to be measured withina given uncertainty, expressed in dB. See also Dynamic Range.

Microbending Loss: Loss in a multimode fiber caused by thefiber being distorted on the sub-millimeter scale by, forexample, cabling stresses.

Mode: An electromagnetic field distribution that satisfiesMaxwell’s equations and the boundary conditions given bythe fiber. It can be considered as a light path in the fibertraveling at a discrete propagation angle. A multimode fibermay carry several hundred individual modes.

Mode Group: A group of Modes having the same GroupIndex. A multimode fiber may carry twenty or so modegroups.

Mode Conditioning Patchcord: A fiber patchcord thatprovides some modification to the distribution of modeslaunched into it. See Mode Filtering and Mode Scrambling.

Mode Filtering: A means of preferentially attenuating certainMode Groups in order to achieve, for example a Steady StateDistribution. Often performed by wrapping the fiber around amandrel of a given diameter.

Mode Scrambling: A means of re-distributing power betweenmodes in a fiber. Often performed by combining differenttypes of fiber in a single Patchcord and by mechanicaldistortion.

Optical Distance: An OTDR measures fiber length by timingthe light pulse traveling down the fiber. Therefore alldistances are referred to as "optical distances" because theydepend on the velocity of light in the particular fiber beingknown. See Group Index.

Optical Return Loss: The total amount of light reflected by aFiber Channel, expressed in dB.

Optical Time Domain Reflectometer (OTDR): A testinstrument that injects a pulse of light into a fiber andmeasures the amount of light scattered by the fiber as afunction of Optical Distance. Used for measuring theattenuation of the fiber and of connectors and splices, andfor diagnosing faults.

Patchcord: A length of fiber cable with a connector on eachend used to connect a piece of terminal equipment, such as aworkstation, to a Local Area Network. A patchcord is alsoused to connect an OTDR to a fiber being tested.

Patch Panel: An array of connectorised fiber terminationsused for connecting Fiber Segments together or to terminalequipment.

Rayleigh Scattering: The scattering of light inherent in alltransparent mediums. In a fiber some of this scattered light isguided backwards by the fiber to the light source. It is thisscattered light that is measured by an OTDR and gives thecharacteristic backscatter trace for a fiber.

Reflectance: A measure of the light reflected by a singleevent in a fiber, expressed in dB.

Segment: See Fiber Segment.

Simplex Operation: Operation in which transmission occurs inone and only one pre-assigned direction.

Splice (Fusion Splice): A permanent joint between two fibersmade by fusing the fiber ends together using an electricdischarge.

Tailcord or Tailcord Fiber: A Patchcord attached to the TailEnd of the fiber being measured with an OTDR. A Tailcord isrequired to measure connector loss on the end of the FiberLink.

Telecommunications Outlet A fixed connecting device wherethe horizontal cables terminates, providing the interface toWork Area Cabling.

Vertical Cabling (or Backbone): The portion of LAN cablingthat connects Horizontal Cross-Connects to the Main BuildingCross-Connect.

Work Area Cabling: Cabling that connects theTelecommunications Outlet to the Work Stations.

Work Station: A piece of user equipment such as a computerthat is connected to a wall outlet via a Network interfaceCard (NIC) and a Patchcord.


APPENDIX D — MEGGER FIBER TESTING EQUIPMENT

The Megger optical fiber optic product range has beendesigned to simplify testing and fault location in LAN cabling.

The instruments, used independently or combined, perform arange of functions including optical loss testing of fibercables, fiber continuity testing, connector testing, acceptancetesting and patch lead testing as well as visual and opticaltesting of optical fiber cables, splices and connectors on bothmulti- and single-mode systems.

Crosschecking Reflectometer

Megger XC-850

The XC-850 is a new concept in the evolution of OpticalTime-Domain Reflectometers (OTDRs). It has been designedspecifically with the requirements of the premise datacominstaller in mind. Inthis environmentmany of thefeatures associatedwith complextelecommunicationOTDRs are notappropriate andmany new featuresare desirable.

Datacom fiber linksare short lengths offiber installed as part of a Local Area Network (LAN) within abuilding. Many topologies are used but all require theindividual fiber segments to be tested to measure the loss ofthe fiber and the connectors.

The XC-850 is the OTDR to measure fiber loss and connectorloss of the datacom links. It does it quickly and automaticallywhile being easy to use. It stores the results for use later in areport and will establish the cause and precise location of anyfaults, this is due to an extremely short dead zone andresolution enabling fault location to within half a meter. Thiscannot be achieved using other techniques such as a lightsource and power meter.

There are many individual fibers in a LAN installationespecially where fiber to the desktop is used. The XC-850 hasmany features to aid the measurement and documentation ofthese installations: Automated link analysis, a full keyboard toannotate cable and fiber identities, a barcode reader option,an automatic duplex cable option and a large capacitystorage card for all the results.

Mode Conditioning Pactchcord Box

The importance of using the correct patchcord for OTDRmeasurements on optical fiber links cannot be overstated forthe following reasons:

The patchcord connector mustbe representative of the typeand quality of those used toconnect the link during itsnormal operation. This is theonly way to accurately assessthe quality of the near end linkconnector.

The fiber used in the patchcord must be of the same corediameter and refractive index profile (Graded or step index.)as that used in the link under test. Different fibercharacteristics will have different backscatter coefficients andresult in confusing

Patchcord lengths must be long enough to enable equilibriummode distribution (EMD) to be set up prior to the lightreaching the fiber link under test. If high order leaky modesare present at the point of launch into this fiber the loss ofthe link will be nonlinear and overestimated. EMD is bestachieved using mandrel wrapping at the near end of thepatchcord followed by a long (>20m) length of fiber.

Ergonomically the patchcord needs to be compact and itsconnectors well protected. It is a vital part of the test systemand needs to be maintained in good condition as part of aoverall quality plan. The Megger range of patch cords areavailable in all common cable sizes with appropriateconnectors and mandrels to either IEC or TIA format.

Visible Light Source

Megger MLS635

The MLS635 is a hand-held, stable, visible light source thatcan be used to visually test optical fiber cables, splices andconnectors on both multimode and single mode systems.

Any sharp bends and breaks in jacketed or bare fibers, orpoorly mated connectors, will cause a proportion of the

guided visible light to escape makingthe MLS635 ideal for identifyingfaults in fiber optic jumper cables,distribution frames, patch panels andsplice trays. It can also be used forend-to-end continuity tests,connector identification in patchpanels, fiber tracing, and fiberidentification during splicingoperations.


Multi Mode LED Light Source

Megger MLS1000

The MLS1000 is a stable optical dual LED source that can beused in conjunction with an optical power meter for optical

loss testing of fiber optic cables. It hasbeen designed to output 850nm or1300nm at a level of -20 dBm into a62.5/125 multi-mode fiber.

It is particularly suitable for the testingof LANs, FDDI, and other multimodelinks whether inside or outside abuilding.

Although the MLS1000's main use is inmultimode fiber optic cable loss testing, other applicationsinclude fiber continuity testing, connector testing, and patchlead testing. Both CW (constant wave) and modulatedoutputs are available with LED mode indication.

Single Mode Laser Light Source

Megger MLS2000

The MLS2000 is a stable optical duallaser source that can be used inconjunction with an optical powermeter for optical loss testing of fiberoptic cables. It has been designed tooutput 1310 nm or 1550 nm at a levelof -6 dBm into a 9/125 single-modefiber.

It is particularly suitable for the testing of SDH, CATV, Telecomand other single mode links. Although the MLS2000's mainuse is in single mode fiber optic cable loss testing, otherapplications include optical loss testing, and acceptancetesting of optical receivers.

Optical Power Meter

Megger MPM1000

The MPM1000 is an accurate optical power meter that canbe used for optical loss testing of fiber optic cables. It hasbeen pre-calibrated for absolute power levels with referenceto 1mW (dBm) for 850 nm, 1300 nm and 1550 nm laser

frequencies using multi-modecables. However, it can also be usedin relative power mode and cantherefore also be used on single-mode cables.

The MPM1000 is accurate to 5% @-23 dBm (±0.22 dB’s) and has awide dynamic range of +5 dBm to -60 dBm with a resolution of 0.1dBm.

The MPM1000 is particularly suitable for the testing of LAN’s,FDDI, and other multimode links whether inside or outside abuilding. Although the MPM1000’s main use is in fiber opticcable attenuation testing, other applications include fibercontinuity testing, connector testing, and patch lead testing.

Optical Power Meters

Megger MPM2000 and Megger MPM2000H

The MPM2000 and MPM2000H areadvanced optical power meters thatcan be used for optical loss testingof fiber optic cables. These havebeen pre-calibrated for absolutepower levels with reference to 1mW(dBm) for 1310nm and 1550nmlaser frequencies using single mode cables. However, they canalso be used in relative power mode and can therefore alsobe used on multi-mode cables.

Custom instruments can be produced with a further 12 laserfrequencies programmed in at the factory. The MPM2000 isaccurate to ±5% (±0.22dB’s) and has a wide dynamic rangeof +10dBs to -70dB’s making it particularly suitable for thetesting of SDH, Telecom and other Single Mode links.

The MPM2000H is a high powered version with the samelevel of accuracy but a dynamic range of +20dB’s to -60dB’smaking it ideal for CATV and other long distance transmissionapplications.


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For more information about Megger and its diversified line oftest and measurement instruments:

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MULTIMODE FIBER LAN TESTING WITH · PDF file2 MULTIMODE FIBER LAN TESTING WITH OTDRS INTRODUCTION AND SCOPE The Optical Time Domain Reflectometer (OTDR) is a powerful tool for characterizing