Guide to Fiber Optic Measurements

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Guide to Fiber Optic Measurement

Reference: 901GFOM/ 00

Reprinted: September 2001

2001Acterna

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 Necker

42000 Saint-Etienne

Tel. +33 (0) 4 77 47 89 00

Fax +33 (0) 4 77 47 89 70

Web www.acterna.com
http://www.acterna.com/http://www.acterna.com/


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ii Guide to Fiber Optic Measurements

Acterna shall not be liable for errors contained herein.

This document must not be photocopied, reproduced, or translated intoanother language without the written consent of Wavetek.

Printed in France

Authors J. Laferrire

R. TawsS. Wolszczak


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Guide to Fiber Optic Measurements iii

1Table of contents

Fiber Principles ................................................................................1-1

Types of fibers.......................................................................................1-1

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

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

Transmission 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 ..................................................................... 3-1

Fiber Phenomena..................................................................................3-1

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

OTDR block diagram...........................................................................3-5

Laser diodes .................................................................................3-6Pulse generator with laser diode .................................................3-6

Photodiode ...................................................................................3-7Time base and control unit ......................................................... 3-7

OTDR specifications ............................................................................3-8

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

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

Acquisition.............................................................................................4-1


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iv Guide to Fiber Optic Measurements

Injection level .............................................................................. 4-2

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

Measurement ..................................................................................... 4-10

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

Measurement artifacts and anomalies ............................................... 4-19

Ghosts ......................................................................................... 4-19Splice "Gain" .............................................................................. 4-21

Getting the most out of your OTDR ................................................ 4-26

Using launch cables ................................................................... 4-26Verifying continuity to the fiber end ....................................... 4-28Fault location ............................................................................. 4-29Effective refractive index ......................................................... 4-30

Glossary ........................................................................................... A-1Notes ................................................................................................ N-1

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


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Guide to Fiber Optic Measurements 1-1

Chapter

11Fiber 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 coreand 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 the

core and cladding.

Principle of the transmission (simplified version):

a ray of light enters into the fiber at a small angle .

the capability (maximum acceptable value) of the fiber cable to

receive light on its core is determined by its numerical apertureNA:

where: 0: maximum angle of acceptance

(i.e limit between reflection and

refraction)n1: core refractive index

n2: cladding refractive index

Note : 2 0 is the full acceptance angle.

NA sin 0 n12

n22

= =

0 arc n12

n22

sin=

n2

n1

Cladding

Core0

Fullacceptanceangle


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1-2 Guide to Fiber Optic Measurements

Fiber Principles

Light propagation

If > 0: the ray is fully refracted and not captured by the core.

If < 0: 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 the

modes are attenuated.

n1

n2

n2

r

i

Refraction :n1 sin i = n2 sinr

0

i r n1

n2

n2

0

Reflection :i = r

nc

V----=


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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


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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

Heterogeneous

Structures

InjectionLoss

Absorption

Loss

Diffusion

Loss

Junction

Loss

Coupling

Loss

Output

Bending

Loss

Macro

or

micro

bending


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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 fiberwithin 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 m 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)

ScatteringOH-absorption peak

Wavelength (nm)

Infrared absorption loss


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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 per

kilometer: ps / (nm x km). This coefficient, at a given wavelength,

represents the difference after one kilometer between the propagation

time of two wavelengths which differ by a given number of nanometers.

Chromatic dispersion is the dominant dispersion mechanism in

singlemode fibers. In singlemode fibers there is a minimum or zero

(chromatic) dispersion wavelength determined by fiber design and

manufacture, and this wavelength is generally chosen to be near the

operating 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 dispersion

wavelength moved closer to 1550 nm to take advantage of the lower

fiber attenuation at that wavelength. In some systems, for example,

Dense WDM (Wavelength Division Multiplexing) applications, a slight

positive chromatic dispersion is desirable and fiber designs are available

to accommodate this.

This fiber is ideal for submarine cables because of the increased repeater

spacing and reduced cost. The maximum repeater spacing for high bit

rate transmission is found by measuring the ratio between the maximum

chromatic dispersion tolerated by the system (in ps/nm) and the fiber in

ps / (nm x km). The attenuation of the fiber must also be taken into

account.

Bandwidth limitation


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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 m) than singlemode

fibers permitting many modes of light to travel through the core.

The core of a single mode fiber is generally 10 m 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 its ease of coupling to lightsources and to other fibers, reducing the cost of light sources (transmitters),connectorization and splicing. However, its 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 m and 62.5 m,

CoreDiameter: from 50 m to 100 m

CoatingDiameter: 250 m

Cladding refractive index < core refractive index

CladdingDiameter: 125 m and 140 m


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Fiber Principles

cladding diameter of 100/140 m 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 fibersbandwidth 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 fibersGraded-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 nm1 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 m at 850 nm1.475 for 50 m at 850 nm1.465 at 1300 nm.

1.1.2 Singlemode fiber

The 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-


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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 m for thecore and 125 m 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 8m physical corecould yield a 9.5 m MFD. This also shows that some of the light energyalso transits through the cladding.

CoreDiameter: 5 to 10 m

CladdingDiameter: 125 m

CoatingDiameter: 250 m

Cladding refractive index < core refractive index


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Fiber Principles

1.1.3 Fiber standards and recommendations

There 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 m multimode graded index opticalfiber

G652 Characteristics of singlemode optical fiber cable

G653 Characteristics of singlemode dispersion shifted optical fibercable

G654 Characteristics of 1550 nm loss minimized singlemode opticalfiber 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);

TheETSI

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


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Chapter

22 Optical Testing

2.1 Families of optical fiber testsWhen 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 Claddingdiameter

Reflectometry


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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 access

to 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.


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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.


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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.


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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 = Lmin - RminB min = Lmax - Rmax

Tx Rx

L max (dB)

R max (dB)

L min (dB)

R min (dB)

Launchedopticalpower (L)

Receivedopticalpower (R)

Minimum Optical lossbudget (Bmin)

Maximum Optical lossbudget (Bmax)

Opticalnetwork


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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


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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 always

possible.

Light source, power meter and talk set


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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:

LEDs 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 craftsmanstoolkit. It is an invaluable tool during installation and restoration.

The power meters 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 measure

the 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 theDynamic Range.

While the dynamic range for a given meter has some limits, the usefulpower ranges can be extended beyond that by the of well characterized

attenuators in front of the power meter input; this does limit the low end


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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 alsodestructive, 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. Withthe 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 as

far as possible the losses caused by the various patch cables.

1. Most power meters meet this requirements through two modes of operation, a standard mode (-3to -70 dBm) and a "high power" mode (+23 to - 50 dBm).


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Optical Testing

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

Significant variations may occur in attenuation measurements if precautions

are 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 transmitters 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

MeasurementP1

Measurement P2

Total attenuation of the fiber :AdB = P1dBm - P2dBm


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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 90s. 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


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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


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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


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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.


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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).


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Optical Testing


33/108


Chapter

33Principles of an OTDR

n OTDR (Optical Time Domain Reflectometer) is a fiber optic

tester 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 to

localized loss and reflective events thereby providing a pictorial andpermanent record which may be used as performance baseline.

3.1 Fiber Phenomena

The OTDRs 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 two

types 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


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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 scattering

When 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 and

is 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 (s) 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 (s) 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 scatteredlight

Scattered light5%/km at 1550 nm

Incident light


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Fiber Phenomena

Note that -80 dB at 1 ns is equivalent to -50 dB at 1 s, i.e. :

Ks (dB) = Kns(dB) + 30 dBThe 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 () 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 different

wavelength as shown in the following diagram and with different manufac-turers fiber.

Attenuation versus wavelength

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

OTDR parameters

850 1300 1550

Attenuation (dB)Scattering OH-absorption peak

Wavelength (nm)

Infrared absorption loss


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3.1.2 Fresnel reflection

Fresnel 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)


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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

Averaging

Processing

Display

unit

Amplifier

Sampling& ADC

Photodiode

Laser diode

Pulse

GeneratorCoupler

Fiber


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3.2.1 Laser diodes

Laser 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 diode

A 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 s 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 convert

time 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.


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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 Photodiode

OTDR 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 unit

The 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)


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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 range

The 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.

Connectorpair

Distance (km)

FusionSplice

Connectorpair

Fiberbend

Mechanicalsplice

OTDR

Attenuation(dB)

Fiberend


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42/108



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 isapproximately 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 isdefined as the maximum attenuation that can be placed between theOTDR 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 betweenthe 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 given

according to different conditions:

typical value: this represents the average or mean value of the dynamicrange 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 themanufacturer for its OTDR.

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


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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 recovery

period is termed the dead zone.This effect is similar to the one when you are driving a car at night, and thatanother cars 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 valueF (usually 0.5 dB or 0.1 dB). Bellcore specifies a reflectance of - 30 dB, a

Dead zone


44/108



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 and

there is little difference between the front end dead zone and network deadzone.

ADZAttenuationdead zone

F = 0.5 dB or 0.1 dB


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46/108



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 Resolution

There 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.


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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 Accuracy

The 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 canvary 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


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3.3.5 Wavelength

OTDRs 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.


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Chapter

44Using an OTDR

he OTDR is very versatile and has many applications. Firstly, itsimportant 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 :

t

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

t 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


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Using an OTDR

acquisition parameters selected by the OTDR.

A more experienced user might allow the unit to autoconfigure, analyzethe 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 are

used 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 level

Degrading 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 m in singlemode systems, thepresence of even a 4 m 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.


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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 wavelength

The 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 different

wavelengths.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 lostevery 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.


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Using an OTDR

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

1310 nm will generally measure splice and connector losses higherthan 1510 nm. These results come from a Corning study of over 250splices where the 1310 nm values were shown to be typically higherby 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 width

The 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 (nm)

0.5

0

1310

0.013

1550

0.042

1580

0.094

1620

0.048

Loss (dB)

1310

0.0051

1550

0.123

1580

0.489

1620

2.253

1300 1400 1600 17001500 (nm)


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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 10

8

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 1s 10 s 20 s

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

10ns

30ns

100ns

1s3s

10s

Dc T

2n------------=


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Using an OTDR

4.1.4 Range

The 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 Averaging

The 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 1.2

dB 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


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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.

IEC Helios @ 5ns PW

Theoretical


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Using an OTDR

4.1.6 Smoothing

Smoothing 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 parameters

Other parameters related to the fiber can affect the OTDR results as fol-lows:

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


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Acquisition

ments. When comparing distance results from two acquisitions, always

be sure that the appropriate index is being used. It should be noted thatusing 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 ofthe 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 will

cause 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 on

this.

Backscatter coefficient K: the backscatter coefficient K tells the

OTDR the relative backscatter level of a given fiber. This coefficient isentered 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


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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 location

lossreflectance

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

For the complete terminated system:link lengthlink loss in dB

ORL of the linkThe 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 andmeasure automatically all the events, sections and fiber end, using an

internal detection algorithm.


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Measurement

Fully automatic trace and table of events (Table mode)

2. semi automatic function: when this is selected, the OTDR willmeasure 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 notdetect 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.


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Using an OTDR

Measurement with markers

3. manual measurement function: For even more detailed analysis orspecial 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 isimportant to have this capability available for those fiber spans whose

design or construction are very unusual and difficult for automated

algorithms to analyze accurately.


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Measurement

Manual ORL measurement


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Using an OTDR

4.2.1 Slope or fiber section loss

The 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 loss

Using 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 users 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


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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 to

reduce 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


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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


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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 the

joints 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 selectingORL = 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


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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 Ghosts

False 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


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Measurement artifacts and anomalies

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 todays 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 OTDRs first pulse iscompleted at 20 km and the

second pulse is launchedinto the fiber.

As the fiber is longer thanthe distance range, theOTDRs first pulse is stillpresent on the fiber whilethe second pulse data isbeing acquired. The first

continues 2 km furtherdown the fiber until it hitsand reflects off the end.

20 km

22 km

First pulse Second pulse etc.

OTDR second pulsewaveform

OTDR first pulsewaveform

2 km


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4-22 Guide to Fiber Optic Me

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