Fiber Optics Guide

Guide ui de to t o Fi Fi ber Opt Optii c Measureme Measurement nt Reference: efer ence:

901 90 1GFOM/ 00

Reprint epri nted: ed:

Sept Se ptember ember 200 2001 1



2001 Acterna

The information informati on contained contained in t his document document is the t he property propert y of Acterna Acterna.. It is i s only provided for th the e operati operation on and maintenanc maintenance e of the instrument. i nstrument. It must must not be duplic dupli cate ated d without wit hout the t he prior writt wri tten en permiss permission ion of Acterna. Acterna Saint-Etienne 34 rue Necker 42000 Saint-Etienne Tel. el . +33 + 33 (0) 4 77 47 89 00 Fax +33 (0) 4 77 47 89 70 Web www.acterna.com

Technical Specifications

11-8

OFI 2000

USER MANUAL

720000992/03

Acterna shall

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

Printed in France Authors

ii

J. Laferrière Laferrièr e R. Taws S. Wolszczak

Guide to Fiber Optic Measurements

1

Table of of contents contents

Fiber Principles .................................................................. ................................................................................ .............. 1-1 Types of fibers............................. fibers............................................................... .......................................................... ........................1-1 1-1 Multimode fiber ...........................................................................1-7 Singlemode fiber .......................................................................... ..........................................................................1-8 1-8 Fiber standards and recommendations ....................................1-10 Optical Testing .................................................................. ................................................................................ .............. 2-1 Families of optical fiber fiber tests tests ............................................................... ...............................................................2-1 2-1 Transmission tests................................................................................. tests.................................................................................2-2 2-2 Field tests .....................................................................................2-3 Different families of optical testers testers ..................................................... .....................................................2-7 2-7 Sources, Power meters and and Attenuators Attenuators .................................... .................................... 2-7 Mini-OTDR ...............................................................................2-11 Mainframe or full-featured OTDR OTDR .......................................... .......................................... 2-12 Monitoring systems ...................................................................2-13 Other general test equipment .................................................. .................................................. 2-16 Principles of an OTDR .................................................................. ..................................................................... ... 3-1 Fiber Phenomena..................................................................................3-1 Rayleigh scattering .....................................................................3-2 Fresnel reflection ......................................................................... .........................................................................3-4 3-4 OTDR block diagram...........................................................................3-5 Laser diodes .................................................................................3-6 Pulse generator with laser diode ................................................. ................................................. 3-6 Photodiode ...................................................................................3-7 Time base and control unit ......................................................... ......................................................... 3-7 OTDR specifications ............................................................... ............................................................................ ............. 3-8 Dynamic range .............................................................................3-8 Dead Zone .................................................................................. ..................................................................................3-11 3-11 Resolution ..................................................................................3-14 Accuracy .....................................................................................3-15 Wavelength ................................................................................3-16 Using an OTDR ............................................................................... ................................................................................. .. 4-1 Acquisition.............................................................................................4-1 Guide to Fiber Optic Measurements

iii

Injection level .............................................................................. .............................................................................. 4-2 OTDR wavelength .................................................................... ...................................................................... 4-3 Pulse width ..................................................................... .................................................................................. ............. 4-4 Range .............................................. ............................................................................... .............................................. ............. 4-6 Averaging ........................................................................ ..................................................................................... ............. 4-6 Smoothing ...................................... ....................................................................... .............................................. ............. 4-8 Fiber parameters ...................................... ........................................................................ .................................... 4-8 Measurement ............................................................... ..................................................................................... ...................... 4-10 Slope or fiber section loss ......................................................... ......................................................... 4-14 Event loss .............................................................................. ................................................................................... ..... 4-14 Reflectance and Optical Return Loss ...................................... ...................................... 4-17 Measurement artifacts and anomalies ............................................... ............................................... 4-19 Ghosts .................................................. .................................................................................... ....................................... ..... 4-19 Splice "Gain" .............................................................................. .............................................................................. 4-21 Getting the most out out of your OTDR OTDR ................................................ ................................................ 4-26 Using launch cables ................................................................... ................................................................... 4-26 Verifying continuity continuity to the fiber end ....................................... ....................................... 4-28 Fault location ............................................................................. ............................................................................. 4-29 Effective refractive index ......................................................... ......................................................... 4-30

Glossary ............................................................... ........................................................................................... ............................ A-1 Notes ........................................ ......................................................................... ........................................................ ....................... N-1 Index ............................................................... ................................................................................................... ....................................I-1 I-1

iv


Chapter 1

1.1

Fiber Principles

1

Types of fibers

An optical fiber is made of very thin glass rods composed of two parts: the inner portion of the rod or core and the surrounding layer or cladding. Light injected into the core of a glass fiber will follow the physical path of that fiber due to the total internal reflection of the light between b etween the core and the cladding. A plastic sheathing around the fiber provides the mechanical protection. Fibers are classified into different categories based on the way in which the light travels in them, which is closely related to the diameter of the core and cladding.

Principle of the transmission (simplified version): • a ray ray of light light ente enters rs into into the fiber fiber at at a smal smalll angle angle α. • the capa capabil bility ity (max (maximu imum m accept acceptabl able e value) value) of of the fibe fiberr cable cable to receive light on its core is determined by its numeric numerical al aper aper ture tur e NA: 2

N A = sin α 0 =

n1 – n 2 2

α 0 = a r c sin n 1 – n2

2

where:

2

α0: maxim maximum um angl angle e of accep accepta tanc nce e (i.e limit between reflection and refraction) n1: core refractive index n2: cladding refractive index

Note : 2 α0 is the full acceptance angle. Cladding Full acceptance angle

α0

n1

Core

n2


1-1

Fiber Principles

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

Refraction : n1 sin αi = n2 sinαr

n2

α0

n1

αi αr

n2

• If α < α0: the ray is reflected and remains in the core n2

α0

αi

αr

n1

Reflection : αi = αr

n2

Velocity The velocity at which light travels through a medium is determined by the refracti refractivv e index index of the medium. The refractive index (n) is a unitless number which represents the ratio of the velocity of light in a vacuum to the velocity of the light in the medium.

c n = ---V

where:n: Refractive Index c: Speed of light in a vacuum (approximately 3 x 10 8 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. Light entering the center of the fiber core at a very low angle will take a relatively direct path through the center of the fiber. Light injected at a high angle of incidence 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 the length of the fiber. Each path resulting from a given angle of incidence and entry point can give rise to a mode. As they travel along the fiber, all the modes are attenuated. 1-2


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 resonance) and extrinsic absorptions (due to impurities such as OH- ions at around 1240 nm and 1390 nm). In I n modern fibers, extrinsic factors are almost negligible. • Rayleigh scattering. Scattering, primarily Rayleigh scattering, also contributes to attenuation. Scattering causes the light energy to be dispersed di spersed in all directions, with some of the light escaping the fiber core. A small portion of this light energy is returned down the core and is termed « backscattering». Note

Forward light scattering (Raman Scattering) and backward scattering (Brillouin scattering) are two additional scattering phenomena that can be seen in optical materials under high-power conditions.

Backscattered light Scattered light Incident light

Backscattering effect

• bending losses which are caused by light escaping the core due to imperfections at the core/clad boundary (microbending), or the angle of incidence 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). (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. Minimum bend radius is dependent on fiber design and light wavelength. Guide to Fiber Optic Measurements

1-3

Fiber Principles

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

Output Heterogeneous Structures

Impurities Optical Fiber

Injection Loss

Absorption

Diffusion

Bending

Junction

Coupling

Loss

Loss

Loss Macro or micro bending

Loss

Loss

Loss mechanisms

The attenuation, for a given wavelength, is defined as the ratio between the input power and the output power of the fiber being measured. It is generally 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 the wavelength. If we look at the absorption spectrum of a fiber against the wavelength of the laser, we can notice some characteristics. The following graph illustrates the relationship between the wavelength of the injected light and the total fiber attenuation resulting from the contribution of all the loss mechanisms:

1-4


Types of fibers

Attenuation (dB)

Scattering

OH-absorption peak

Infrared absorption loss

850

1 3 00

1550

Wavelength Wavelength (nm)

Attenuation versus wavelength wavelength

The main telecommunication transmission wavelengths correspond to the points on the graph where the attenuation is a minimum. These wavelengths are known as the “telecom” windows and are typically as follows: • firs firstt win windo dow w fro from m 820 820 to to 880 880 nm • seco second nd wind window ow from from 12 1285 85 to 13 1330 30 nm • thir third d win windo dow w fro from m 152 1525 5 to to 157 1575 5 nm nm Another factor affecting the signal during transmission is dispersion. This reduces the effective bandwidth available for transmission. Two main types of dispersion are defined. • Modal dispersion : when a very short pulse is injected into the fiber within the numerical aperture, all of the energy does not reach the end of the fiber at the same time. Different modes of oscillation carry energy down the fiber down different paths and thus travel further. As an example, a 50 µm core multimode fiber may have several hundred modes. This pulse spreading by virtue of different light path lengths is called modal dispersion or more simply modal dispersion. • Chromatic dispersion : the pulse sent down the fiber is usually composed 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 equipGuide to Fiber Optic Measurements

1-5

Fiber Principles

ment which are at the same small spectrum of wavelengths as the wavelength of operation. Chromatic dispersion is expressed in picosecond per nanometer per kilome kilometer ter:: ps / (nm (nm x km). Thi Thiss coeffic coefficien ient, t, at a given given wave wavelen length gth,, 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 dispersion wav wa v eleng length th determined by fiber design and manufacture, and this wavelength is generally chosen to be near the operating wavelength of the system. Historically (in standard singlemode fiber), this was near 1310 nm, but for newer newer systems, socalled dispersion shifted fibers are used with the zero dispersion wavelength moved closer to 1550 nm to take advantage 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 attenu attenuati ation on of the fibe fiberr must must also also be taken taken into into account.

Bandwidth limitation

1-6


Types of fibers

The two major classes of fibers are those that exhibit modal dispersion ( multimode) and those that do not ( singlemode) : • Multimode Multimode fibers fibers have have much much larg larger er core core (> 50 50 µm) µm) than than singl singlemode emode fibers permitting many modes of light li ght to travel through the core. • The core core of of a single single mode fiber fiber is general generally ly 10 µm or less and will will allow allow only one mode of light (at 1310 or 1550 nm) to propagate, greatly reducing 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 quite sensitive to the modal dispersion. The primary advantages of multimode fiber are it’s it’s ease of coupling to light sources and to other fibers, reducing the cost of light sources (transmitters), connectorization and splicing. However H owever,, it’s relative higher attenuation and/ or low bandwidth limit it to short distance and low speed applications.

Core Diameter: from 50 µm to 100 µm

Cladding refractive index < core refractive index

Coating Diameter: 250 µm

Cladding Diameter: 125 µm µm and 140 µm

Multimode fiber

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


1-7

Fiber Principles

cladding diameter of 100/140 µm and numerical aperture between 0.2 and 0.5. Due to modal dispersion, the drawback to this design is its very low bandwidth, expressed as bandwidth-length product in MHz x km. This fiber’s bandwidth of approximately 20 MHz x km indicates that it is suitable for carrying a 20 MHz signal only a distance of 1 km, or a 10 MHz signal a distance 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 field is mostly in short distance links which can accommodate high attenuations.

B. Graded-in Graded-index dex multim multimode ode fiber fibers s Graded-index (GI) fibers are obtained by giving to the core a non-uniform refractive 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 a sinusoidal manner. The highest order modes will have a longer travel, but outside o utside of the central axis, in areas of low index, their speeds will increase and the speed difference between the highest and lower order modes will be smaller than for step-index fibers. Typical attenuations attenuations are : 3 dB/km at 850 nm 1 dB/km at 1300 nm. The numerical aperture of graded-index fibers is typically about 0.2. The bandwidth-length product for Graded index fibers is approximately: 160 MHz x km at 850 nm 500 MHz x km at 1300 nm. Typical values of the group index : 1.49 for 62.5 µm at 850 nm 1.475 for 50 µm at 850 nm 1.465 at 1300 nm.

1.1.2 Singlemode fiber The advantage of singlemode fiber is its higher performance with respect to bandwidth and attenuation. The reduced core diameter limits the light to propagation of only one mode, eliminating modal dispersion completely. completely. With proper components, a singlemode fiber system can carry signals in excess of 10 GHz for over 100 km. The system carrying capacity may be fur1-8


Types of fibers

ther increased by injecting multiple signals of slightly differing wavelengths wavelengths (Wavelength Division Multiplexing) into one fiber. The small core size generally requires more expensive light sources and alignment systems to achieve efficient coupling and splicing and connectorization is also somewhat complicated. Nonetheless, for high performance system or systems over a few kilometers, singlemode fibers remain the best solution. The typical dimensions of single mode fibers range from 5 to 12 µm for the core and 125 µm for the cladding. A typical core-cladding angle is 8.5 degrees. The group index is typically 1.465 for the singlemode fiber. fiber. Core Diameter: 5 to 10 µm

Cladding refractive index < core refractive index

Cladding Diameter: 125 µm Coating Diameter: 250 µm

Singlemode fiber

The small core diameter decreases the number of propagation modes. In a single 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 as the section of the fiber where the majority of the light li ght energy passes. The MFD is larger than the physical core diameter i.e. an 8 µm physical core could yield a 9.5 µm MFD. This also shows that some of the light energy also transits through the cladding.


1-9

Fiber Principles

1.1.3 Fiber standards and recommendations recommendations There are many international and national standards governing optical cable characteristics 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 6079 60793-1 3-1 and -2 Optical Optical fibers fibers (contai (containing ning several several sections) sections) • IEC 607 60794 94-1, -1, -2, and -3 Optic Optical al fibe fiberr cable cabless • The ITU-T (formerly the CCITT) has more standards such as: • G650 Definition Definition and test methods methods for for the relevant relevant parame parameters ters of sinsingle-mode fibers, • G651 G651 Charact Characteri eristi stics cs of 50/12 50/125 5 µm multimo multimode de grade graded d index index optica opticall fiber • G652 G652 Charac Character terist istics ics of of single singlemod mode e optic optical al fiber fiber cable cable • G653 Characteri Characteristics stics of singlemo singlemode de dispersion dispersion shifted shifted optical optical fiber cable • G654 Characte Characteristi ristics cs of of 1550 1550 nm loss loss minimized minimized single singlemode mode optical optical fiber cable

National standards • The CEN is preparing the following recommendations for Europe: EN 186000 (Optical fibre connectors), EN 187000 (Optical fibres), and the EN 188000 (Optical fibre cables); • The ETSI provides additional recommendations for Europe; • The EIA/TIA provides additional recommendations for the USA (FOTP). Many other standards organizations exist in other countries.

Test equipment standards • IEC IEC 613 61350 50:: Pow Power er mete meterr cal calib ibra rati tion on • IEC IEC 617 61746 46:: OTD OTDR R cal calib ibra rati tion on

1-10


Chapter 2

2.1 2. 1

Optical Testing

2

Famil amiliies of opti optica call fibe fiberr test tests s

When analyzing a fiber optic cable over its product life, a series of measurements have to be performed: • mech echanic nical tests sts, • ge geom omet etrrica ical tests ests,, • optical tests • tran transm smiissio ssion n test testss. The three first measurements are only performed once, as there is minor variation of these parameters during the fiber's life. Several measurements are made on optical fibers or cables in order to characterize them before their use for transmission. Many of these measurements are described in the FOTP (Fiber Optic Test Procedure) propositions of the EIA (Electronic Industries Association) and are defined by the ITU-T G650 recommendations or the EN 188 000 document.

Different kinds of test M e c ha ni c a l

Ge ome t ri c a l

Opt i c a l

Tra nsmi ssi on

Tracti on

Concentri ci ty

Index Profi l e

Bandwi dth

Torsi on

Cyl i ndri ci ty

Numeri cal aperture

Optical Power

Bendi ng

Core di ameter

Spot si ze

Opti cal Loss

Temperature

Cl addi ng diameter

Reflectometry


2-1

Optical Testing

2.2

Transmission tests

The main measurements implemented on optical fibers and optical fiber systems in order to qualify their use for information transmission purposes are: • EndEnd-tto-En o-End d Opti Optica call Link Link Los Losss • Rate Rate of att atten enua uati tion on per per unit unit len lengt gth h • Attenuati Attenuation on contributio contribution n to splices, splices, connector connectors, s, couplers couplers (events) (events) • Leng Length th of of fibe fiberr or dist distan ance ce to to an ev even entt • Linearity Linearity of fiber fiber loss per unit length length (Attenu (Attenuation ation discontinu discontinuities ities)) • Refl Reflec ecta tanc nce e or or Opt Optic ical al Retu Return rn Loss Loss Other measurements such as bandwidth or polarization mode dispersion may also be done, but they are less important, except for some specific applications. 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 will reduce the time spent travelling from one end of the fiber cable system to the other. If we focus on field testing on optical cables, we can see that there are three main tasks - Installation, Maintenance and Restoration - where testing is required.

2-2


Transmission tests

2.2.1 Field tests Below is a non-exhaustive list of the various tests that can be performed during each task (Installation, Maintenance, Restoration). The exact nature of a testing program will depend on the system design, system criticality and contractual relationship between the cable and components suppliers, system owner, system installer and system user. Installation testing is performed to ensure that fiber cables received from the manufacturer are conform to specifications (length, attenuation, etc.) and have not been damaged in transit, and that they are not damaged during cable placement. Tests Tests also determine the quality of cables splices s plices and cable terminations (attenuation, location, reflectance) and that the completed cable subsystem is suitable for the intended transmission system (end-toend loss, system optical return loss) and provide complete documentation of the cable link for maintenance purposes. Maintenance testing involves periodic evaluation of the cable system to ensure 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 months and compared to historical test results to provide early warning of degradation. In very high capacity or critical systems, automated testing devices may be employed to test the integrity of the system every few minutes to give immediate warning of degradation or an outage. During cable restoration, testing is first performed to identify the cause of the outage (transmitter, receiver, receiver, cable, connector) and to locate the fault in the cable if the outage is caused by the cable. Testing Testing is then used to assess the quality of the repaired system (permanent splices), similar to the testing performed at the conclusion of cable installation.


2-3

Optical Testing

Pre-installation test on a drum

When installing a fiber network, network topology and equipment specifications have to be taken into consideration. One of the major parameters to measure is optical loss budget or end-to-end optical link loss. When calculating the budget of a fiber link, the following must be considered: the source, the detector and the optical transmission line. The transmission link includes the source-to-fiber coupling loss, the fiber attenuation loss, and the loss of all components along the line (connectors, splices, passive components, etc.).

Optical loss budget An optical loss budget lies within maximum and minimum values: • the maximum maximum value value is is defined defined as the ratio ratio of the the minimum minimum optical optical power power launched by the transmitter to the minimum which may be received by the receiver whist still maintaining communication; • the minimum minimum value value is defined defined as as the the ratio ratio of the maximum maximum optica opticall power power launched by the transmitter to the maximum which may be received by the receiver whist still maintaining communication.

2-4


Transmission tests

A typical example of a multimode system is described below. below. • Transmi ransmitte tterr output output power power (typic (typical) al) for for multimo multimode de fiber fiber (GI) = -12 dBm ±2 dB • Opti Optica call Rece Receiv iver er sens sensit itiv ivit ity y ≤ -27 dBm • Opti Optica call Rece Receiv iver er Dyna Dynamic mic Rang Range e ≥ 18 dB The transmitter specification provides the maximum (-10 dBm) and minimum (-14 dBm) power levels that will occur. occur. The receiver sensitivity gives us the minimum power level that will be detected. The receiver dynamic range provides the maximum power level that can be detected (-27 dBm + 18 dBm = -9 dBm). In this example, the maximum optical loss budget b udget is 13 dB : • Minimu Minimum m optic optical al powe powerr of the transm transmitt itter er (-14 (-14 dBm) dBm) • Mini Minimu mum m rece receiv iver er sen sensi siti tivi vity ty (-2 (-27 7 dBm) dBm)

Example of a typical budget loss

Tx

Optical network

Rx

Launched optical power (L) L max (dB) R max (dB)

Received optical power (R)

Minimum Optical loss budget (Bmin)

L min (dB) Maximum Optical loss budget (Bmax) R min (dB)

Opti Optical cal Budget B max = Lmin - Rmin B min = Lmax - Rmax

Optical loss budget


2-5

Optical Testing

Optical loss budgets should take into account the cable and equipment margins, which covers allowances for the effect of time and environmental factors (launched power, receiver sensitivity, connector or splice degradation...). In order to calculate this budget, typical values of attenuations of the different fiber components are given, for example: • 0.2 dB/km dB/km for for sing singlem lemode ode fiber fiber loss loss at 15 1550 50 nm; nm; • 0.35 0.35 dB/k dB/km m for for sing singlem lemode ode fiber fiber loss loss at at 1310 1310 nm; • 1 dB/k dB/km m for for mult multim imod ode e fibe fiberr loss loss at at 1300 1300 nm; nm; • 3 dB/k dB/km m for for mult multim imod ode e fibe fiberr los losss at 85 850 0 nm; nm; • 0.05 0.05 dB for for a fusi fusion on spli splice ce • 0.1 0.1 dB dB for for a mech mechan anic ical al spli splice ce;; • 0.2 0.2 - 0.5 0.5 dB dB for for a con conne nect ctor or pair pair;; • 3.5 dB dB for a 1 to to 2 splitt splitter er (3 dB splitti splitting ng loss loss plus plus 0.5 0.5 dB excess excess loss). loss). Once this analysis is performed, the cable installation can be made.

Example of a typical budget loss

NETWORK

SHORT HAUL

M EDI UM 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 c onnector l oss oss

0.5

0.5

0.5

Total connector connect or loss

1

1

1

TOTAL LOSS

10

22.6

40.25

2-6


Different families of optical testers

2.3 2. 3

Diffferen Di erentt fam famiilies lies of of opt optic ical al test tester ers s

2.3.1 Sources, Power meters and Attenuators Attenuat ors The most accurate way to measure overall attenuation in a fiber is to inject a known level of light in one end and measure the level when it comes out the other end. Light sources and power meters are the main instruments recommended by the ITU-T (G651) and the IEC 61350, to measure insertion loss. This method required access to both ends of the fiber which is not always possible.

Light source, power meter and talk set


2-7

Optical Testing

Light sources A light source is a device used as a continuous and stable source (CW) for attenuation measurements. It includes a source - either an LED or a laser - that is stabilized through some type of Automatic gain Control: • LED’s LED’s are mainly used for multimod multimode e fibers. fibers. Lasers Lasers are are used used for for singlesinglemode applications. • The ligh lightt outpu outputt of eithe eitherr an LED LED or laser laser sourc source e may also also hav have e the option to be modulated (or "chopped") at a given frequency. The power meter can be set up to detect this frequency. 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 confirmation of continuity.

Power Power meter me ter The power meter is the standard tester in a typical fiber optic craftsman’s toolkit. It is an invaluable tool during installation and restoration. The power meter’s main function is to display the incident power on the photodiode. Features found on more sophisticated power meters may include temperature stabilization, ability to calibrate to different wavelengths, ability to display power relative to "reference" input, ability to introduce 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 transmitter being used (to verify operation) but be sensitive enough to measure the received power at the far (receive) end of the link. Long haul telephony systems and cable TV systems use transmitters with outputs as high as +16 dBm and amplifie amplifiers rs with outputs outputs as high high as +24 dBm. Receive Receive powers powers can be as low as -36 -36 dBm in systems that use an optical pre-amplifier. pre-amplifier. In local area networks, transmit powers are much lower, as are received power. the difference between the maximum input and the minimum sensitivity of the power meter is termed the Dynamic Range. While the dynamic range for a given meter has some limits, the useful power 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 2-8



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 for telep telephon hony y applic applicati ations ons 1, • +24 +24 dB dB to -50 -50 dB for for CATV CATV app appli lica cati tion onss 1, • -20 -20 dB dB to to -60 -60 dB for for LAN LAN appl applic icat atio ions ns..

Insertion Inserti on loss and cut cut back measurements • The cut cut back back techniq technique ue is the the most most accura accurate te measu measurem rement ent,, but is is also destructive, and cannot be applied in the field. This is the reason why it is not used during installation and maintenance. Testing with the cutback 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 substracting the two values: the result gives the attenuation of the cut fiber. fiber. • The insertion insertion loss technique technique is a non destructi destructive ve method method to measure measure the attenuation across a fiber, a passive component or an optical link. With the substitution method, the output from a source and a reference fiber is measured directly, directly, then a measurement is realized with the fiber to be 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 power meters meet this requirements requirements through two two modes of operation, a standard mode (-3 (-3 to -70 dBm) and a "high power" mode (+23 to - 50 dBm).


2-9

Optical Testing

Calibrated light source

Power meter

Reference pigtail

Calibrated light source

Measurement P1

Power meter

Reference fiber

Fiber under test Measurement P2

Total attenuation of the fiber :

AdB = P1dBm - P2dBm

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 optical power meter. For transmitted power, the power meter is connected directly to the optical transmitter’s transmitter’s output. In the case of received power, the optical transmitter is connected to the fiber system and then the power level l evel is read with the power meter m eter from the fiber cable at the point where the optical receiver should be. Power meter / light source combinations (also defined as loss test sets) measure cable continuity and cable attenuation. Link losses are sometimes measured in each direction and averaged to improve confidence in the measurements.

2-10



2.3.2 Mini-OTDR Using the same basic technology as the OTDR (see page 2-12), 2-12), a new class of instruments became available in the beginning of the 90’s. Known as "mini-OTDRs", these fiber test instruments are typically battery-powered, lightweight, and small enough to be carried in one hand. The simplest and earliest designs were capable of fault location as a minimum and some rudimentary analysis (attenuation, rate of attenuation, distance and reflectance) of fiber systems. Modern designs mimic the capabilities of mainframe OTDRs including sophisticated analysis (automatic event detection, 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 in mainframe OTDRs. A mini-OTDR has become the popular choice for pre-installation and restoration tests where ease-of-use and mobility mo bility are important.

Mini-OTDR


2-11

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 mainframe and different plug-in modules which can be implemented to suit the application. 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 source and optical detector and can be changed to allow testing at various wavelength and fiber type combinations. Mainframe OTDRs are being rapidly replaced by mini-OTDRs but remain the choice for laboratory and benchop applications where data acquisition functions are desired.

Mainframe OTDR

2-12


Optical Testing

Most network operators initially will use remote systems to look for and a nd sectionalize catastrophic failure of a link. In this case, the monitoring system is connected to only one or two fibers in a multifiber link, assuming that in the event of a catastrophic break all the strands will be cut. Optical Switch Output

Fiber not in use Dark fiber for transmission

Fiber under test

NTE

cable under test

NTE

Cable under test

Out-of-service

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

2-14



Optical Switch Output

WDM or Blocking Filter

lambda test Wavelength Division Multiplexer

Fiber under test lambda test

WDM

NTE

WDM

cable under test

NTE

Cable under test

In-service


2-15

Optical Testing

2.3.5 Other general test equipment Talk sets Talk sets transmit voice over installed fiber cable, allowing technicians splicing 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 telecommunications methods which may not be cost-effective or which may not operate at the distances common to fiber optic links.

OTS talk set

Visual Fault Locators Visual Fault Locators are red light lasers which visually locate faults, up to around 5 kilometers. By sending visual light, the operator can easily see breaks and important bends in the fiber, as the light escapes out. This function makes them useful for continuity testing of patch cords, jumpers, or short sections of fiber.

2-16



They can also be used in conjunction with: • splici splicing ng machi machines nes to ident identify ify fibers fibers to to be join jointe ted. d. • OTDR OTDR to anal analyze yze fail failure uress which which occur occur with within in the the dead dead zone. 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 VFL provides provides long distance distance fault fault location location and and correc correctt light light intenintensity • 635 nm VFL VFL provid provides es excell excellent ent visibility visibility by shorter shorter fault fault locatio location. n.

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 enable power measurements without disconnecting or damaging the fiber. The clip-on tester is performing measurement by putting a controlled bend in the fiber and measuring the level of light which escapes out of the fiber. The measurement can be performed non intrusively (low bend) or intrusively (tight bend).


2-17

Optical Testing

2-18


Chapter 3

Principles of an OTDR

3

n OTDR (Optical Time Domain Reflectometer) is a fiber optic tester characterizing fibers and optical networks. The aim of this instrument is to detect, locate and measure events at any location in the fiber link. One of the main benefits of the OTDR is that it can fully test a fiber from only one end, as it operates as a one dimensional radar system. The OTDR is similar to an accurate radar as its resolution can be between 6 cm and and 40 40 mete meters rs.. The OTDR technique produces geographic information with regard to localized loss and reflective events thereby providing a pictorial and permanent record which may be used as performance baseline.

3.1

Fiber Phenomena

The OTDR’s ability to characterize a fiber is based on detecting small signals returned back to the OTDR in response to injection of a large signal, much like a "radar". In this regard, the OTDR depends on two types of optical phenomena: Rayleigh Backscattering and Fresnel Reflections. The major difference between these two phenomena is as follows: • Rayleigh Rayleigh scatterin scattering g is intrinsic intrinsic to the the fiber material material itself itself and 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 transmission along the fiber length. • On the the other other hand, hand, Fresnel Fresnel reflection reflectionss are are "point" "point" events events and and occur occur Guide to Fiber Optic Measurements

3-1


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 are scattered in random directions from microscopic particles. This effect, referred to as Rayleigh scattering, provides amplitude and temporal information along the length of cable. Some of the light is scattered back in the opposite direction of the pulse and is called the backsc backscat attt er ed signa signall.

Backscattered light 1/1000 of scattered light Scattered light 5%/km at 1550 nm Incident light

The scattering loss is the main mechanism for fibers operating in the three telecom windows (850 / 1310 / 1550 nm). Typically, a singlemode fiber transmitting light at 1550 nm with a scattering coefficient coefficient ( αs) of 0.20 dB/km, will lose 5 % of the transmitted power over a 1 km section of fiber. The backscattering factor (S) describes the ratio between backscattered power and the scattered power. S is typically proportional to the square of the numerical aperture. Depending on the fiber scattering coefficient ( αs) and the fiber backscattering factor (S), the backscatter coefficient (K) is the ratio of the backscattered power to the t he energy launched into the fiber. The logarithmic value of the backscatter coefficient, normalized to a 1 ns pulse 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 backscatter power is - 80 dB d B below the incident pulse peak power.

3-2


Fiber Phenomena

Note that -80 dB at 1 ns n s is equivalent to -50 dB at 1 µs, i.e. : Kµs (dB) = K ns(dB) + 30 dB The Rayleigh scattering effect is like shining a flashlight in a fog at night: the light beam gets diffused -- or scattered -- by the particles of moisture. A thick fog will scatter more of the light because there are more particles to obstruct it. The Backscattering depends on the launched power Po (Watt), the pulsewidth used ∆t (seconds), the backscattering coefficient K (s-1), the distance d (meters) and the fiber attenuation ( α) in dB/km: Backscattering = Po . ∆t . K . 10 -α.d/5 OTDR parameters

A higher density of dopants in a fiber will also create more scattering and thus higher levels of attenuation per kilometer. An OTDR can measure the levels of backscattering very accurately, and uses it to measure small variations in the characteristics of fiber at any point along its length. While Rayleigh scattering is quite uniform down the length of any given fiber, the magnitude of Rayleigh scattering varies significantly at different wavelength as shown in the following diagram and with different manufacturer’s fiber. Attenuation (dB)

Scattering

OH-absorption peak

Infrare Infrared d absorpt absorption ion loss loss

850

1300

1550

Wavelength (nm)

Attenuation versus wavelength wavelength


3-3


3.1.2 Fresnel reflection Fresnel reflection is due to the light reflecting off a boundary of two optical o ptical transmissive materials, each having different index of refraction. This boundary can occur either at a joint (connector or mechanical splice), at an non-terminated fiber end, or at a break. The magnitude of the Fresnel reflection is dependent upon the incident power and the relative difference between the two indices of refraction. The amount of light reflected depends upon the boundary surface smoothness and the index difference.

Fiber

Reflection is:

Pi n1

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

n2

R=

Pr Pi

=

(n1 - n2) 2 (n1 + n2)2

Pr : reflected power Pi : injected power n1, n2 : index of refraction

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

3-4


OTDR block diagram

3.2

OTDR block diagram

Laser diode

Pulse Generator

Coupler Fiber

Photodiode

Time Base Control Unit

Amplifier

Sampling & ADC

Averaging Processing

Display unit

OTDR block diagram

The OTDR injects light energy into the fiber through a laser diode and pulse generator. The returning light energy is separated from the injected signal using a coupler and fed to the photodiode. The optical signal is converted to an electrical value, amplified, sampled and then displayed on a screen.


3-5


3.2.1 Laser diodes Laser diodes are selected according to the wavelength of the test. The current wavelengths wavelengths for OTDR are 850 nm, 1300 nm for multimode, and 1310 1310 nm, 1550 1550 nm for for single singlemod mode. e. 1625 nm laser diodes are sometimes also used, particularly in remote monitoring systems which are carrying live traffic. The purpose of using 1625 nm is 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) Watt) into the fiber. These pulses can have a width in the order of 2 ns up to 20 µs and a recurrence of some kHz. The duration of the pulse (pulse width) can be selected by the operator for different measuring conditions. The repetition rate of the pulses is limited to the rate at which the pulse return is completed, before another pulse is launched. The light goes through the coupler/splitter and into the fiber under test. The OTDR measures the time difference between the outgoing pulse and the incoming backscattered pulses hence the word "time domain". The power level of the backscattered signal and the reflected signal is sampled over time. Each measured sample is called an "acquisition point" and these points can be plotted on an amplitude scale with respect to time relative to timing of the launch pulse. It then converts this time domain information into distance based on the user entered index of refraction of the fiber. The index of refraction entered by the user is inversely proportional to the velocity 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 take the round trip (or two way) into account. If the user entered refractive index is incorrect or inaccurate, the resulting distances displayed by the OTDR can be in error.

3-6


OTDR block diagram

P (Injection)

PP(Reflection) (Reflection)

Propagation Propagation or group g roup delay in fiber :

V (Gp (Gp dela delay) y)

=

c/n c/n ~

3.10 3.108 / 1.5 1.5

=

2.10 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 conversion (round trip):

L (distance) = V(Gp delay). t/2 = c.t. c.t. / 2.n ~ 10 8 x time (seconds) E.g. for a 10 ns pulsewidth:

L = 10 8 x 10 ns = 1 m

3.2.3 Photodiode OTDR photodiodes are especially designed to measure the extremely low levels 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 relatively high power of reflected pulses of light. This causes some problems when analyzing the results of an OTDR (see "Dead Zone" on page 3-11). 3-11 ). The bandwidth, sensitivity, linearity and dynamic range of the photodiode and its amplification circuitry are carefully selected and designed to be compatible with the pulsewidths used and the levels backscattered from the fiber.

3.2.4 Time base and control unit The control unit is the brain of the OTDR. It takes all the acquisition points, performs the averaging, plots them as a log. function of time and then displays the resulting trace on the OTDR screen. The time base controls the pulsewidth, the spacing between subsequent pulses and the signal sampling. Multiple passes are used to improve the signal to noise ratio of the resulting trace. Since noise is random, by acquiring many data points at a given distance and averaging them, the noise will tend Guide to Fiber Optic Measurements

3-7


to average out toward zero and the remaining data will more accurately represent the backscatter or reflection level at that point. An OTDR may acquire up to 32,000 data points and fire thousands of pulses, so the OTDR processor 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 (or feet), and plots numerous acquisition points which represent the backscatter "signature" of the fibers under test. Connector pair

Fusion Splice

Connector pair

Fiber bend

Mechanical splice

Fiber end

OTDR Attenuation (dB)

Distance (km)

Typical OTDR trace

3.3

OTDR sp specifications

3.3.1 Dynamic range The dynamic range is one of the most important characteristics of an OTDR, since it determines the maximum observable length of a fiber and therefore the OTDR suitability for analyzing any particular network. The higher the dynamic range, the higher the signal to noise ratio and the better the trace will be, with a better event detection. This dynamic range is relatively difficult to determine since there is no standard computation method used by all the manufacturers. 3-8


OTDR specifications

Definitions of the dynamic range One method of determining dynamic range (approved and endorsed by the IEC 61746) is to take the difference between the extrapolated point of the backscatter trace at the near end of the fiber (taken at the interception between the extrapolated trace and the power axis) and the upper level of the noise floor at or after the fiber end. • The upper upper level level of of the the noise noise is defined defined as the upper upper limit of a range which contains at least 98% of all noise data points. • The The leve levell is exp expre ress ssed ed in in deci decibe bels ls (dB (dB). ). • This measuremen measurementt is performed performed with a 3 minute minute period period for the averagaveraging. • This value value of the dynamic dynamic range range was also recommended recommended by Bellc Bellcore. ore. Other definitions of the dynamic ranges are given by different manufacturers, which makes the values comparison very difficult: dB ) % 8 9 (

N = 0.1 dB

) S M R (

C E I c i m a n y D

~6.6 dB

Peak noise level

1.56 dB

SNR=1

e g n a r c i m a n y D

km

Dynamic range

• RMS. The RMS (Root Mean Square) also termed SNR=1 dynamic range is the difference between the extrapolated point of the backscatter trace at the near end of the fiber (taken at the intersection between the extrapolated trace and the power axis) and an d the RMS noise level. You You can compare this value to the IEC 61746 definition by substracting 1.56 dB from the RMS dynamic range if the noise is gaussian. • N=0.1 dB. This dynamic range definition gives an idea of the limit to


3-9


which the OTDR can measure when the noise level is 0.1 0 .1 dB on the trace. The difference between N=0.1 and SNR=1 RMS definition is approximately 6.6 dB. This means that an OTDR which has a dynamic range of 28 dB (SNR=1) can measure a fiber event of 0.1 dB up up to 21.5 dB. • End detection: The dynamic range end detection is the one way difference between the top of a 4% Fresnel reflection at the start of the fiber and the RMS noise noise level. This value is approximately 12 dB higher than the IEC value. • Bellcore measurement range : The Bellcore measurement range is defined as the maximum attenuation that can be placed between the OTDR and an event for which the instrument will still be able to meam easure 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 reflective splice splice (> 40 dB). • 4% Fresnel: This is more an echometric parameter than a reflectometric parameter. 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, detected, sometimes with a minimum of 0.3 dB higher than than the highest peak in the noise level; • Peak level plus 0.3 dB : the dynamic range is the difference between the front-end backscattered trace and 0.3 dB more than the peak noise level. The value of the dynamic range , for each definition can also be given according to different conditions: • typical value: this represents the average or mean value of the dynamic range of the OTDRs which come out of production. An increase of around 2 dB is usually shown in comparison with the specified value. • specified value: this is the minimum dynamic range specified by the manufacturer for its OTDR. • over a temperature range or at room temperature . At low and high temperature, the dynamic range decreases usually by 1 dB.

3-10


OTDR specifications

3.3.2 Dead Zone

Dead zone

OTDR dead zone example

Why do we have dead zone ? The OTDR is designed to detect the backscattering level all along the fiber link. It measures backscattered signals which are much smaller than the signal sent to the fiber. The component which receive those values is the photodiode. It is designed to receive a given level range. r ange. When there is a strong reflection, then the power received by the photodiode can be more than 4000 times higher than the backscattered power and can saturate the photodiode. The photodiode requires time to recover from the saturated condition; 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 that another car’s car’s headlights dazzle your vision momentarily. momentarily.

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


3-11


loss of 0.1 dB and gives different locations. In general, the higher the reflected power sent back to the OTDR, the longer the dead d ead zone. The attenuation dead zone depends on the pulsewidth, the reflectance, the loss, the displayed power level and the location. The attenuation dead zone usually indicates the minimum mi nimum distance after an event where the backscatter trace can be measured. ADZ Attenuation dead zone

∆F = 0.5 dB or 0.1 dB

Attenuation Dead Zone measurement

At short pulse widths, the recovery time of the photodiode is the primary determinant of the attenuation dead zone and can be 5 to 6 times larger than the pulse width itself. At long pulsewidths, the pulsewidth itself is the dominant factor, and the attenuation deadzone is, in effect, equal to the pulsewidth itself. The dead zone specified in the literature is generally measured at the shortest s hortest pulsewidth. Bellcore specifies objectives for two attenuation dead zone, the "front end" dead zone and the "network" dead zone. Historically, the connection between the OTDR was highly reflective; this an other factors often caused the dead zone seen at the front end of the OTDR, to be much longer than the dead zone resulting from a reflection in the network. Currently, the OTDR connection has been engineered to have very low reflectance and there is little difference between the front end dead zone and network dead zone. 3-12


OTDR specifications

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

Event dead zone Event dead zone is the minimum distance on the trace, where two separate events can still be distinguished. The distance to each event can be measured, but the separate loss of each events cannot be measured. This parameter usually gives an indication of the minimum distance in order to distinguish between reflective events which occur in close proximity. • For a reflectiv reflective e event, event, the event event dead zone definition definition is the the distanc distance e between the two opposite points which are 1.5 dB (or FWHM) FWHM ) down from the unsaturated peak. EDZ Event dead zone

1.5 dB ±0.1 dB ≤1 dB

± 0.1 dB Event dead zone Reflective event EDZ Definition

Non-reflective event EDZ example

Event Dead Zone measurement

• For an an non-re non-reflect flective ive event event,, the event dead zone can be be describe described d as the distance between the points where the beginning and ending levels at a splice or a given value ( ≤ 1 dB) are within ±0.1 dB of their initial and final values (this is not the definition). Event dead zones can also be reduced using smaller pulsewidths.


3-13


Front end event dead zone effects can also be minimized from a fiber under test 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 readout resol resolution ution is the the minimum minimum resolution resolution of the the displ displayed ayed value value (e.g. an attenuation of 0.031 dB will have a resolution of 0.001 dB). • The cursor cursor resolu resolution tion is the the minimu minimum m distance distance or attenu attenuation ation between 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 can be measured. This value is generally around 0.01 dB.

Sampling resolution The sampling (or data point) resolution is the minimum distance between two acquisition points. This data point resolution can go down to centimeters depending on pulsewidth and range. In general, the more datapoints that an OTDR can acquire and process, the better the sampling resolution. The number of datapoints an OTDR can acquire is therefore an important performance parameter. parameter. a typical value for a high resolution OTDR would be 1 cm sampling resolution.

Distance resolution Distance resolution is very similar to sampling resolution.

3-14


OTDR specifications

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

3.3.4 Accuracy The accuracy of a measurement is the capacity of the measurement to be compared with a reference value.

Linearity (Attenuation accuracy) The linearity of the acquisition circuit determines how close an optical level corresponds to an electrical level, across the whole range. Most OTDRs have an attenuation accuracy of 0.05 dB/dB. Some OTDRs can go down to 0.02 dB/dB. If an OTDR is non linear then with long fibers, the section loss values will change significantly.

Distance accuracy The distance measurement accuracy depends on the following parameters: • Group index : Whereas Whereas index index of refrac refraction tion refers refers to a single ray in in a fiber 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 base error error.. This is due to to the inaccuracy inaccuracy of the the quartz, quartz, which which can can -4 -5 vary from 10 to 10 . In order to have an idea of the distance error, one has to multiply this uncertainty by the measured distance. • Dist Distan ance ce error rror at the the ori origi gin. n. A typical value for the MTS 5100 mini-OTDR is : ± 5 x 10 -5 x distance ± 1m ± sampling resolution ± group index uncertainties


3-15


3.3.5 Wavelength OTDRs measure according to a wavelength. The major wavelengths are 850 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 given spectral width. The standard spectral width is ±30 nm, but that can be ±10 nm. Some OTDRs display the laser wavelengths used for the measurement. m easurement. The attenuation of optical fiber varies with the wavelength, and any measurement should be corrected to the transmission wavelength or to the central wavelength (850, 1310 or 1550 nm). Correction is most relevant in the first window at 850 nm.

3-16


Chapter

Using an OTDR

4

4

he OTDR is very versatile and has many applications. Firstly, it’s important to select an OTDR that has the proper specifications (see chapter 3) 3) for the task at hand. With recent breakthroughs in modularity, some OTDRs, like the MTS 5100, can be configured flexibly 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 the results 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 t he next fiber acquisition.

4.1

Acquisition

Most modern OTDRs now automatically select the optimal acquisition parameters for a particular fiber by sending out test pulses in a process known as auto-conf auto-conf iguration iguration. Using the Auto-configuration feature, the user would select the wavelength (or wavelengths) to test, the acquisition (or averaging) time, and the fiber parameters (e.g. refractive index if not already entered). There are about three major approaches to configuration of the OTDR: • A user user might might simply simply let let the the OTDR OTDR autoc autoconf onfigu igure re and and accept accept the the


4-1

Using an OTDR

acquisition parameters selected by the OTDR. • A more more experie experienced nced user might allow the unit to autoconfig autoconfigure, ure, analyze analyze the results briefly and change one or more acquisition parameters to optimize the configuration for the purposes of his test. • The experi experience enced d user user may choose choose not to use use the autoconfig autoconfigurati uration on feafeature altogether and enter acquisition parameters based on his experience and knowledge of the link under test. Typically, when testing multifiber cables, once appropriate acquisition parameters are selected, they are "locked in" and the same parameters are used for every fiber in the cable (this speeds the acquisition process and provides for consistency in the data which is helpful when analyzing or comparing fibers). Below, Below, various acquisition parameters and their effect on the resulting trace are discussed.

4.1.1 Injection level Degrading the quality of the OTDR front panel connector through noncleanliness will result in poor p oor measurements. The injection level is defined as the power level which OTDR injects into the fiber under test. The higher this level, the higher the dynamic range. If the injection level is low, low, traces will be noisy and measurement m easurement accuracy will be degraded. Poor launch conditions resulting in low injection levels are the primary reason for reductions in precision. The presence of dirt on connector faces and damaged or low quality pigtails or patchcords are the primary cause of low injection levels. It is important that all physical connection points are free of dust and dirt in an optical system. With core diameters of less than 10 µm in singlemode systems, the presence of even a 4 µm speck of dirt or dust (approximately the size of the particulate matter in cigarette smoke) can severely degrade injections levels. Cleaning kits are available for optical systems from basic tools including isopropynol cleaning solution, joseph paper, compressed-air spays, and readyto-use impregnated wipes, to more advanced methods with cassette cleaners. Mating of dirty connectors to the OTDR connector, may scratch the OTDR connector, connector, permanently degrading d egrading launch conditions. 4-2


Acquisition

Some OTDRs, like the MTS 5100, will display the measured injection level during real time acquisition or just prior to averaging. The result is displayed 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 launch pulse and compares this to an expected value. It is sometimes possible for the injection level to show "bad" when it is in fact acceptable. This will happen if there is an attenuator in the system, near the OTDR or if there is a splitter near the OTDR; in this case, the backscatter level will be lower than "expected" by the injection level meter. Even though the injection level increases as pulsewidth increases, the scale displayed is calibrated separately for each pulsewidth so the scale is meaningful at any pulsewidth and increasing 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 of transmission. Not only optical fiber will exhibit different loss characteristics at 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 used for transmission. This means 850 nm and/or 1300 nm for multimode systems, and 1310 nm and/or 1550 nm for singlemode systems. If testing is only to be performed at one wavelength, the following parameters need to be considered: 1. For a given given Dynamic Dynamic range, range, 1550 1550 nm will will see longer longer distan distances ces down down the the same fiber than 1310 nm due to the lower attenuation in the fiber: • 0.35 0.35 dB/km dB/km at 1310 1310 nm means means that that appr approxi oximat mately ely 1 dB of sign signal al is lost every 3 km. • 0.2 dB/k dB/km m at 1550 1550 nm mean meanss that that appro approxim ximate ately ly 1 dB of of signa signall is lost lost every 5 km. 2. Single Single mode fiber has larger larger mode mode field field diamete diameterr (see (see MFD page 1-9) 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. Guide to Fiber Optic Measurements

4-3

Using an OTDR

• 1550 1550 nm is more more sensi sensitiv tive e to bends bends in in the fiber fiber than than 1310 1310 nm. nm. This This is shown diagrammatically below. below. This can also be termed as macrobending. • 1310 nm will will general generally ly measure measure splice splice and and connec connector tor losses losses higher higher than 1510 nm. These results come from a Corning study of over 250 splices where the 1310 nm values were shown to be typically t ypically higher by 0.02dB over the 1550nm values for dispersion-shifted fiber. Loss (dB) 1310 0.013

1550 1580 1620 0.042 0.094 0.048

0.5

0 1300

1400

1500

1600

1700

λ (nm)

Sensitivity to bending radius = 37,5 mm

Loss (dB) 1310 0.0051

1300

1550 1580 1620 0.123 0.489 2.253

1400

1500

1600

1700

λ (nm)

Sensitivity to bending radius = 30 mm

4.1.3 Pulse width The OTDR pulsewidth duration controls the amount of light that will be injected into the fiber. The longer the pulsewidth means the more the light energy injected. The more light injected means the more light backscattered or reflected back from the fiber to the OTDR. Long pulsewidths are used to see long distances down a cable. Long pulsewidths will also produce longer zones in the OTDR trace waveform where 4-4


Acquisition

measurements are impossible. We call this the dead zone of an OTDR (see page 3-11). 3-11). Short pulsewidths inject lower “levels” of light but reduce this dead zone.

10ns

10 s

30ns

100ns 3 s 1 s

Different pulsewidths

The pulse width duration is usually given in ns but can also be estimated in meters according to the following formula:

c×T D = ------------ . 2n

where c represents the speed of light in vacuum (3 x 10 8 m/s), T the pulse duration in ns, and n the refractive index. As an example, a 100 ns pulse could be interpreted as a "10 m" pulse. Time or Pulse width

5 ns

10 ns

100 ns

1µs

10 µs

20 µs

Distance or fiber length

0 .5 m

1m

10 m

10 0 m

1 km

2 km


4-5

Using an OTDR

4.1.4 Range The range on an OTDR is the maximum distance that the OTDR will acquire data samples. The longer this parameter, the more distance the OTDR 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 some measurement artifacts (see "Ghosts" on page 4-19 4-19). ).

4.1.5 Averaging The OTDR detector works with extremely low optical power levels (as low as 100 photons per meter of fiber). Averaging is the process by which each acquisition point is sampled repeatedly and the results averaged to improve the signal-to-noise ratio. By selecting the time of acquisition or the number of averages, the user controls this process within an OTDR. The longer the time or the higher the number of average, the more signal the trace waveform will display, display, in random noise conditions. The relationship between the acquisition time (number of averages) and the amount of improvement of the signal-to-noise ratio is expressed by the equation below: 5 log 10 N

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. Averaging will improve the signal to noise ratio by increasing the number of acquisitions, but the time taken to average the trace is increased. However, However, according to the equation, beyond a certain time, there is no advantage to be gained as only the signal remains. In theory, theory, four times more averaging equals + 1.5 dB gain in dynamic range.

4-6


Acquisition

13,5 13 12,5 C E12 I . n y D11,5

Helios @ 5ns PW Theoretical

11 10,5 10

0 0 2 4

0 6

0 8

0 0 1

0 2 1

0 4 1

0 6 1

0 8 1

Averaging Time (s)

Dynamic range versus averaging averaging


4-7

Using an OTDR

4.1.6 Smoothing Smoothing is a technique whereby the signal-to-noise ratio is improved by digitally filtering the acquisition points. To improve accuracy at lower light levels an OTDR can use filters and averaging 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 is performed by using specific coefficients. A given true point value is modified to another value which combines previous and subsequent acquisitions with relevant coefficients. coefficients.

4.1.7 Fiber parameters Other parameters related to the fiber can affect the OTDR results as follows: • Refractive Index n : this index is directly related to distance measure-

4-8


Acquisition

ments. When comparing distance results from two acquisitions, always be sure that the appropriate index is being used. It should be noted that using the refractive index reported by the fiber manufacturer will cause the OTDR to report fiber length accurately. accurately. However, often, particularly during fault location, the user wishes to determine the cable length. l ength. 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. manufacturer. While it is possible to have this value (typically termed the "helix factor") reported by the manufacturer, the precision of the value still allows for large uncertainty in fault location. It is often recommended to measure a known length of similarly constructed cable and determine an "effective refractive index" that will cause the OTDR to report cable length instead of fiber length. See "Getting the most out of your OTDR" on page 4-26 for more information on this. • Backscatter Backscatter coefficient K: the backscatter coefficient K tells the OTDR the relative backscatter level of a given fiber. This coefficient is entered at the factory and generally the user will not change this parameter. Changing it will affect the reported value of reflectance and optical return loss. While the assumption is made that the backscattered coefficient for the entire span is consistent, it is possible that there will be very slight variations from one fiber span s pan to the other. other. This variation can cause measurement anomalies such as splices with negative loss values (or gainers). See section Measurement artifacts and anomalies on page 419 for measurement techniques that minimize the impact of these. Typical backscatter coefficients at 1 ns are: - for stan standar dard d single single mode mode fiber fiber:: - 79 dB at13 at1310 10 nm - 81 dB at 1550 nm - for for sta stand ndar ard d mult multim imod ode e fibe fiber: r:

- 70 70 dB dB at at 850 850 nm nm - 75 dB at 1300 nm


4-9

Using an OTDR

4.2

Measurement

Most modern OTDRs will perform fully automatic measurements with very little user intervention. In general, there are two types of events: reflective and non reflective. • Reflective Reflective events events where where a discontinu discontinuity ity in the the fiber fiber causes causes an abrupt abrupt change in the refractive index are either caused by breaks, connectors junctions, mechanical splices or the undeterminated end of fiber. Connector loss can be around 0.5dB and mechanical splices can range from 0.1dB up to 0.2dB • Non reflective reflective events events occur occur where there are no discon discontinui tinuities ties in the 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. operator. The following measurements can be performed by an OTDR. For each event:

distance location loss reflectance

For each section of fiber:

section length section loss in dB section loss rate in dB/km ORL (Optical Return Loss) of the section

For the complete terminated system: link length link loss in dB ORL of the link The OTDR allows the user, at his discretion to perform measurements on the fiber span in at least three different ways. The user can also use a combination of these methods: 1. full automatic function : in this case, the OTDR will detect and measure automatically all the events, sections and fiber end, using an internal detection algorithm.

4-10


Measurement

Fully automatic trace and table of events (Table mode)

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


4-11

Using an OTDR

Measurement with markers

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

4-12


Measurement

Manual ORL measurement


4-13

Using an OTDR

4.2.1 Slope or fiber section loss The slope of section of fiber, fiber, given in dB/km, can be measured either using a 2-point method (described on page 4-14) 4-14) or by using a least-squares approximation (LSA). The least-squares approximation method tries to determine the measurement line that has the closest fit to the set of acquisition points. It is the most precise means to measure fiber loss but requires a continuous section of fiber, a minimum number of OTDR acquisition points, and a relatively clean backscatter signal free of noise.

Least square approximation : fitting a straight line li ne

The section loss can be reported either in dB or in dB/km. Typical section losses range between 0.15 to 0.25dB/km for 1550nm systems, 0.25 to 0.35 dB/km for 1310 nm nm singlemode, 0.5 to to 1.5 dB/km for 1300 nm multimode, 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 the event, 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. However, the precision of this method depends on the user’s ability to place the cursors at the correct positions and can be compromised if the trace has a large amount of residual noise. If the trace is very noisy or ‘spiky’, then the user should try to place the cursor on a data point on the trace that is not the top of a spike or bottom of a

4-14


Measurement

trough: this is a sort of visual “averaging” of the trace. If the user is using the two point method to measure a ‘point’ event (like a splice as opposed to a length of fiber), then the user should be aware that the result will also include the effects of any fiber losses between the cursors, because the distance 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 by performing a least squares analysis on the fiber spans, and to minimize the additional fiber loss that is reported as event loss due to the non-zero distance between the cursors. In order to do this, the software uses the position of the 5 cursors to extrapolate the fiber data before and after the event and take 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 reflective events. To accomplish this, first the operator must make a slope measurement before and after the event on the linear backscattered level of the trace. The Guide to Fiber Optic Measurements

4-15

Using an OTDR

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

5-point method

5-point measurement

4-16


Measurement

4.2.3 Reflectance and Optical Return Loss The reflectance of an event represents the ratio of the reflected power to the incident power at that discrete location in a fiber fib er span. It is expressed in decibels (-dB). The small negative value indicates a larger reflection than a large negative value. That is, a reflectance of -33 dB is larger than a reflectance of -60 dB. The larger reflectance will show up as a higher peak on the trace waveform.

Reflectance measurement

The amount of reflection at a connector, break or mechanical splice depends on the difference in the refractive index in the fiber and the material at the fiber fib er interface (another fiber, air, index matching gel), and geometry of the break or connector (flat, angled, crushed, each of which will permit a different amount of reflection to be captured by in the fiber core). Most mechanical splices use an index-matching gel or fluid to reduce the amount of change. Smaller changes in the refractive index produce smaller reflections. Some OTDRs can measure the amount of reflecting light automatically by placing one cursor just in front of the reflection, an another cur-


4-17

Using an OTDR

sor at the top of the reflection and by pressing the appropriate button on the control panel. The Optical Return Loss (ORL) represents the total optical power returning to the source from the complete fiber span. This includes the backscattered light from the fiber itself, as well as the reflected light from all the joints and terminations.

ORL = -10 log (Pr/Pi) with:

in dB

Pr = reflected power Pi = incident power

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

ORL of a link

4-18


Measurement artifacts and anomalies

4.3 4. 3

Meas Measur urem emen entt art artif ifac acts ts and and ano anoma mali lies es

From time to time, unexpected results and events can be seen on the backscattered trace.

4.3.1 Ghosts False Fresnel reflections on the trace waveform can be observed from time to time. They can be a result of either: • strong strong reflecti reflective ve event event on the fiber fiber,, causing causing a large large amount amount of reflected reflected light to be sent back to the OTDR • or incorr incorrect ect range range settin setting g during during acquis acquisiti ition on

OTDR

Ghost

OTDR

Ghosts principle

In both cases, the ghost can be identified as no loss is incurred at the signal passes through this event. In the first case, the distance that the ghost occurs along the trace is a multiple of the distance of that strong reflective event from the OTDR.


4-19

Using an OTDR

Example of ghost in the noise

In order to reduce the reflection, you can use index matching gel at the reflection, or reduce the injected power by selecting a shorter pulse width, or reducing the power (some OTDRs provide this option) or adding attenuation in the fiber before the reflection. If the event causing the ghost is situated at the end of the fiber, a few short turns around a suitable tool (pen, pencil, mandrel etc.) will sufficiently attenuate the amount of light being reflected back to the source and eliminate the ghost. This is known as a mandrel wrap. Caution: be sure to select a mandrel of the appropriate diameter for the type of cable, jacketed fiber, or coated fiber used, so as not cause permanent damage to the span! It is never recommended to bend a fiber or cable to introduce i ntroduce attenuation without without the use of a suitable mandrel to prevent excess bending.

Ghosts can also be introduced on the OTDR trace waveform if we incorrectly set the distance range. 4-20



OTDR laser distance range Fiber length OTDR laser pulses

20 km 22 km First pulse

The OTDR’s first pulse is completed at 20 km and the second pulse is launched into the fiber. As the fiber is longer than the distance range, the OTDR’s first pulse is still present on the fiber while the second pulse data is being acquired. The first continues 2 km further down the fiber until it hits and reflects off the end. 2 km

Second pulse

etc.

OTDR first pulse waveform

OTDR second pulse waveform

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

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


4-21

Using an OTDR

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

B, Kb

OTDR S

Ka = Kb = Backscatter Backscatter coefficient

S: Splice attenuation

Normal splice

This phenomenon can occur when jointing different types of fiber in multimode or 2 fibers with different backscattering coefficients. coefficients. A , Ka

B, Kb

OTDR

S1 Ka < Kb

Kb-Ba= k

S1= S+ K

Positive Positive splice from A to B

B, Kb

A , Ka

OTDR

S2

Ka < Kb

Kb-Ba= k

S2 = S- K

Negative splice from B to A S1 + S2 2

The sum gives the bidirectional or average splice loss value : S = -------------------

4-22



Bidirectional Analysis We all know that there is no such thing as a passive amplifier, and that we can’t get a “gain” in optical power from a fusion splice, but the OTDR will sometimes report a gain caused by differences in backscatter coefficient. Note that while these backscatter differences will not always cause a gain to be reported, they can cause erroneous splice loss readings even if the reading is still a loss. Bidirectional analysis is a technique used to minimize the effect of backscatter coefficient differences along a span causing these erroneous splice readings. It is used where very accurate baseline data on a span is desired or during acceptance testing, where accurate measurement of splicing, often performed by subcontractors, is desired. The concept of bidirectional analysis is as follows: If there is a backscatter coefficient mismatch between two spliced fibers, the sense (algebraically) of that difference will change depending on the direction of measurement. That is, if measured in one direction, the difference will appear as a gain, if measured in the opposing direction, it will appear as a loss. This difference will combine with the actual splice loss during measurement. However, if the splice loss reading taken in the two directions is averaged, then the backscatter effect will subtract out, yielding the actual splice loss. While the concept is presented here in detail and the manual calculations presented, in actuality, this analysis is usually performed using programs such as WinTrace ® which will automatically perform this analysis on much more complex spans than that shown here. Example of Bidirectional analysis on a hypothetical span

Fusion splice A

Connector West

F iber 1

Fusion splice B

Fiber 2

Connector East

F iber 3

Span Architecture

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


4-23

Using an OTDR

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

F iber 1

Fiber 2

F iber 3

Backscatter Profile of span

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

0.05 dB

Apparent loss/gain at junction due to backscatter coefficient difference

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

-0.03 dB

-0.07 dB

Actual Fusion Splice Loss

4-24


Connector East


What the OTDR sees……

OTDR

Connector West

Connector East

+0.02 dB (West) (East)

A

-0.12 dB

B

Measurement one (W -> E)

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

Connector East

OTDR

-0.02 dB (East)

B

(West)

-0.08 dB

A

Measurement two (E -> W)


4-25

Using an OTDR

When measuring from East to West, remember that SPLICE B is now on the left of the OTDR screen and SPLICE S PLICE A is on the right, then: • SPLICE SPLICE A appea appears rs to to be a loss loss of 0.08 0.08 dB (the (the actu actual al -0.03 -0.03 dB dB plus plus the apparent -0.05 dB loss due d ue to backscatter). • SPLICE SPLICE B appears appears to be a -0.02 -0.02 loss (the actual actual -0.07 -0.07 loss loss plus plus the apparapparent 0.05 “gain” due to backscatter). After taking the two measurements, we can now make a simple chart showing the loss/"gain" of Splices A and B taken in each direction. We can sum the two readings and then divide by two to take the average. Note that the result now accurately represents the actual splice losses of the two events.

OTDR W→E

E→W

Sum

Average

Actual loss

Splice A

+0.02

-0.08

-0.06

-0.03

-0.03

Splice B

-0.12

-0.02

-0.14

-0.07

-0.07

Bidirectional analysis

4.4 4. 4

Getti ettin ng the the mos mostt out out of your our OT OTDR

4.4.1 Using launch cables The use of a launch cables in an OTDR measurement enable a number of effective tasks: • correct correct measur measurement ement of the the insert insertion ion loss of the the system system end connectors connectors • moves moves the the dead dead zone zone caused caused by by the OTDR OTDR fro front nt panel panel conne connecto ctorr outoutside of the system under test trace waveform • impro improves ves modal modal equili equilibri brium um charac character terist istics ics in multi multimod mode e systems systems so that measurement are more precise • allows allows the the user user to contr control ol the the OTDR inje injecti ction on level level into into the the syste system m under test.

4-26


Getting the most out of your OTDR

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

Trace without launch cable

Note

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


4-27

Using an OTDR

System System end connectors

Trace with launch cable and receive cable

4.4.2 Verifying continuity to the fiber end Sometimes a multifiber cable is installed and you wish to verify that the cable is continuous between the two exposed ends. You can make an OTDR measurement on the cable in each direction and that will confirm that it is continuous. You can also make an OTDR measurement in one direction and observe the length of the cable as represented on the trace, however, the length of each fiber in the cable will often vary by a few meters due to slightly different buffer tube overlength or helix geometry within the cable. It is difficult, if not impossible, to distinguish a fiber with a much lower overlength, from a fiber that is broken inside the cable, 1 meter from the far end. A simpler way to verify continuity, without having to do a complete OTDR test from both ends can be accomplished as follows. In this case, you need access to both ends of the cable, or a helper and communication to the helper. Simply hook the OTDR to one of the fibers in the cable, say Fiber 1. Turn the OTDR on Real Time mode and observe the end of the trace. If the 4-28



length looks approximately right then do the following (if the length is grossly short, you know it’s broken). If you can’t see an end spike (reflective event at the unterminated glass/air interface at the end of the cable), then have the helper cleave the fiber end squarely with a hand cleaver. The end spike or end reflection should become apparent, if it does not, then the helper is not holding the end of Fiber 1, Fiber 1 is broken somewhere inside the cable near the end. If at first you do see a large end spike, have the helper dip the end of the fiber in index matching gel, or alcohol, or wrap the fiber around a small mandrel near the end. Doing any of these will attenuate the end spike. If it does not, then the fiber is broken somewhere else near the end of the cable.

4.4.3 Fault location The OTDR can be an invaluable tool for fault location. Accurate fault location depends on careful measurement technique with the OTDR and on complete and accurate system (cable) documentation. While entire courses are often taught on the subject of fault location, following the few recommendations below may make the process more accurate and efficient should the need ever arise. Cable breaks can be partial or complete (catastrophic). The most common cause of cable breaks is «dig-ups» (over 40 % of all breaks are dig ups). In the case of a dig up, fault location does not need to be extremely precise as the damage can usually be easily located once one is in the vicinity. Other types of breaks including ballistic (from hunting weapons) or rodent damage are difficult to find and accurate location with an OTDR can save a great deal of time and money. When a cable is damaged the resulting break may be highly reflective or non reflective. It is generally much easier to determine an accurate distance to a reflective event. Therefore, it is sometimes helpful to measure several broken fibers until a reflective break is found. If the break is non-reflective, it is usually best to let the OTDR software determine the distance to events using automated analysis. Placing a marker or cursor visually can be inaccurate. The operator may wish to calibrate the OTDR to display distance in Cable or Sheath distance by using an «effective refractive index».This is important, while the OTDR can accurately determine distances to 5 meters in 10,000, the helix factor of the cable will contribute up to 600 meters of inaccuracy over a 10,000 meter span. An alternate method of determining actual Guide to Fiber Optic Measurements

4-29

Using an OTDR

distance from optical distance is to measure the break from both endpoints and determining the position of the break relative to the total span length. This ratio of the optical distance to the break to the total optical length of the span will be the same as the ratio of the sheath distance to the break to the total sheath length. It is important to remember any locations that cable slack is stored. If the OTDR reads 1800 meters to the break but there are 200 meters of slack stored at an intermediate handhole, manhole or pole, then the distance to the break will be similarly shorter. shorter. It is important to remember sag in aerial plant Sheath distance will differ somewhat from pole distance. After the location of the break is determined, it should be correlated to a cable sequential marking. Then, when excavating the cable or examining the aerial plant with binoculars, the correct section of cable can be quickly confirmed. It is always best to measure the distance to the break from the last event whose physical location is known on the OTDR signature using the cursors. In this manner, the shortest possible measurement is made on the OTDR reducing the OTDR contribution to measurement inaccuracy. inaccuracy. During initial cable documentation, take advantage of some of the OTDR features that permit the addition of notes to events or files. Geographic or GPS data can be entered here that will be very useful during fault location. Again, there is absolutely no substitute for complete, detailed, accurate cable documentation records during fault location.

4.4.4 Effective refractive index The user is reminded that the OTDR determines the distance to the event based on time. The refractive index serves as a correlation factor between time and distance allowing the OTDR to display distance. If the user knows the refractive index provided by the fiber manufacturer, he can enter this value on the OTDR thus improving the accuracy of the optical distance displayed. In most cable designs, the length of the fiber is greater than the length of the cable. This can be caused by fiber overlength in the buffer tubes (in “ loose ” buffe bufferr design designs) s) and/or and/or “ helixing helixing ” of the buffer buffer tubes or ribbon ribbonss inside the cable. The cable length or physical distance can therefore vary significantly from the fiber length or optical distance.

4-30



In some cases, notably fault location, the users wishes the OTDR to display cable or physical distance instead of optical distance. This can be accomplished by entering a different value of refractive index, sometimes termed the “effective refractive index” that is adjusted for fiber overlength. There are two ways ways to determine effective effective refractive index : 1. Using cable recor records ds or knowing knowing the the cable cable or physica physicall distance distance (Leff) (Leff) between two known events on the OTDR trace, the user must obtain from the OTDR OTDR the followi following ng data : Op tical tical distance betw betw een 2 know n events (Lop t ) Reffractive Re ractive ind ex u sed by the instru m ent (RI (RI op t ) The effective refractive index (RI eff ) can then be calculated using the form formul ulaa : RIeff = (Lopt* RIopt) / Leff 2. On some some OTDR’ OTDR’ss like like the MTS MTS 5100 5100,, the RI eff can be calculated automatically by delimiting the two known events with two cursors and changing the refractive index until the OTDR reports cable or physical distance instead of optical distance.


4-31

Using an OTDR

4-32


Chapter

Glossary

5

5

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

Absorption: in an optical fiber, loss of optical power resulting from conversion of power into heat. Adaptor: female part of a connector in which one or two connector plugs are inserted and aligned. APD (Avalanche Photodiode): photodiode which operates in the avalanche mode, providing internal gain that is advantageous in reception. Architecture : The protocol that defines computer communication networks. With respect to optical fiber cabling, this term refers to the layout of the cabling in star or ring configuration, for example. Armored cable: A fiber cable that includes a layer of corrugated steel to prevent rodent ingress. Primarily for direct buried applications, occasionally used in aerial applications where squirrels are a severe problem Attenuation dead zone : for a reflective or attenuating event it is the region after the event where the displayed trace deviates from the undisturbed backscatter trace by more than a given vertical value ∆F (usually 0.5 dB or 0.1 dB). Bellcore specifies a reflectance of - 30 dB, a loss of 0.1 dB and gives different locations. In general, the higher the reflected power sent back to the OTDR, the longer the dead zone. The attenuation dead zone depends on the pulsewidth, the reflectance, the loss, the displayed power level and the location. It usually indicates Guide to Fiber Optic Measurements

A-1

Glossary

the minimum distance after an event where the backscatter trace can be measured. ADZ Attenuation dead zone

∆F = 0.5 dB or 0.1 dB

Attenuation Dead Zone measurement

ATM (Asynchronous Transfer Mode): A network standard that specifies fixed length cells to transmit data, voice and video information. ATM is scalable in that it can operate at different transmission speeds such as 51, 100, 155, 622 Mb/s and beyond. Attenuation: in optical fibers, loss of average optical power due to absorption, scattering and other radiation losses. It is generally expressed in dB without a negative sign.

A-2


(µm)

Attenuation coefficient : The rate of optical power loss with respect to distance along the fiber, usually measured in decibels per kilometer (dB/ km) at a specific wavelength. The lower the number, the better the fiber’s attenuation. Attenuation is specified at 850 and 1300 nm for multimode fiber. and 1310 and 1550 nm for singlemode fiber, over a temperature range of -60°C to +85°C. Backscattering: portion of scattered light which returns in a direction generally reverse to the direction of propagation.

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


A-3

Glossary

Attenuation 20

Twisted screened pair TG22U

Coaxial RG217U

15

Coaxial RG220U

Graded Index

λ = 1.3 µm

10 Graded Index λ = 0.85 µm Monomode

5

λ = 1.3 µm

Frequency

0 0.1

1

10

100MHz

1

10

100 GHz

Attenuation versus frequency

Bend Radius (minimum) : The radius a fiber can bend before increased loss or mechanical damage occurs. Broadband : A signal technique that involves modulating the signal on a carrier before transmission. This allows multiple information signals to be transmitted simultaneously on different carrier frequencies. Buffer tube : A thermoplastic tube which is a component of fiber optic cables serving to segregate the fibers into “groups” and to mechanically decouple mechanical forces on the cable from the fibers by permitting the fibers to “float” in the tube. The tubes can be filled (in outdoor cables) or unfilled (in indoor cables). Building backbone cable : A cable that connects the building distributor to a floor distributor. Building backbone cables may also connect floor distributors in the same building. Building Distributor : A distributor in which the building backbone cable(s) terminate(s) and at which connections to the campus backbone cable(s) may be made. A-4


Cable: A structure carrying multiple fibers, usually more than 4 (less than 4 fibers in a structure is usually referred to as “CORD”) and providing mechanical and environmental protection, tensile strength and fire resistance. Two different design concepts exist : L oose tube tu be cable: This design allows primary coated optical fiber or bundles of primary coated optical fibers to lie loosely inside a polymer tube or "former" thus taking advantage of the minimum strain configuration within the tube or former whist protecting them from abrasion and other external forces.

The tube or former may be filled with compounds to prevent ingress and propagation of moisture which may affect the optical fibers. These cables are designed to withstand the mechanical stresses involved when cables are pulled through extensive duct systems and are particularly suitable for external use. Thermoplastic tube

Outer jacket polyethylene

Bloisture blocking gel

Central FRP strength member

Flooded core

Multiple 250 micron fibers

Aramid strength element

Loose tube cable

T igh ight buf fered fered tube t ube cable: This design features secondary coated (buffered) optical fibers within a flexible and durable construction. The cables are of generally low fiber count with aramid strength element protection layers and a polymer outer sheath. This design is particularly suited to internal applications.


A-5

Glossary

Thermoplastic jacket

Outer jacket polyethylene

Aramid strength element

Central member

900 micron Tight Buffered fibers Overall polyester tape barrier

Tight buffered tube cable

Cabling system : Campus distributor

Building backbone

Campus backbone cabling subsystem

Floor distributor

Building backbone cabling subsystem

Telecommunication Outlet

Horizontal cabling subsystem

Terminal Equipment

Work area cabling

Generic cabling system

Campus backbone cable : A cable that connects the campus distributor to the building distributor(s). campus backbone may also connect building distributors directly. Campus Distributor: The distributor from which the campus backbone cabling emanates.

A-6


Chromatic dispersion : A type of dispersion that causes broadening of input pulses along the length of the fiber. Chromatic dispersion is due to the different wavelengths of light traveling at different speeds through the fiber. It is at a minimum value at the fiber zero dispersion wavelength. Cladding : The glass layer surrounding the core of an optical fiber. The lower index of refraction of the cladding as compared to the core causes the light within the core to be totally internally reflected and remain in the core. Coating : An acrylate polymer material put on a fiber during the draw process to protect it from the environment and rough handling. Connector: A junction which allows an optical fiber or cable to be repeatedly connected or disconnected to a device such as a source or a detector. Coupling ratio/loss (Cr, Cl) : ratio /loss of optical power from one output port to the total output power, expressed expressed as a percent. Core: The central region of an optical fiber through which light is transmitted. CPE: Customer Premises Equipment. Cutoff wavelength : In singlemode fiber, the shortest wavelength at which a single mode can be transmitted. Beyond this wavelength, several modes transmit simultaneously, simultaneously, and the fiber becomes multimode. CW: Abbreviation for continuous wave. Dense WDM (Wavelength division multiplexing) : Technique used to multiplex several signals on the same fiber within a narrow wavelength band. Dead Zone: Distance sections in trace, which are associated with every reflective event and represent the distance between the beginning of the event and the point where a consecutive event can be detected.


A-7

Glossary

DFB: Abbreviation for distributed feedback laser. This laser has a Bragg reflection grating in the active region in order to suppress multiple longitudinal modes and enhance a single-longitudina s ingle-longitudinall mode. Dispersion : The cause of bandwidth limitation in a fiber. The spreading (or broadening) of a light pulse as it spreads along a fiber. Major types are : • modal dispersion dispersion cause by differential optical path lengths in a multimode fiber, fiber, • chromatic dispersion caused by a differential delay of various wavelengths of light passing through a fiber. Distributor: The term used for the functions of a collection of components (e.g. : patch panels, patch cords) used to connect cables. DTE: Data Terminal Equipment, generally. Dynamic range: IE I E C Dynam Dynamic ic Rang Ra ngee (int (i ntrroduce oduced by Bellc Bellcore ore)

The difference between the extrapolated point of the backscatter trace at the near end of the fiber (taken at the interception between the extrapolated trace and the power axis) and the upper level of the noise floor at or after the fiber end. The upper level of the noise is defined as the upper limit of a range which contains at least 98% of all noise data points. The dynamic range is expressed in decibels (dB). This measurement is performed for 180 seconds usually with largest pulsewidth of the OTDR. RMS Dynamic Range

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

A-8


dB ) % 8 9 (

N = 0.1 dB

) S M R (

C E I c i m a n y D

~6.6 dB

Peak noise level

1.56 dB

SNR=1

e g n a r c i m a n y D

km

RMS dynamic range

EDFA : Abbreviation for Erbium Doped Fiber Amplifier . Device which amplifies an optical signal without employing O/E and E/O conversions. Electromagnetic Electromagnetic Spectrum: It is a term used to describe the entire range of light radiation, from gamma rays to radio. E lec lectromagne tr omagneti ticc spec spectrum tr um Type of Radiation

Frequency Range

Wavelength Range

Gamma- rays

1020 - 10 1 024

<10-12 m

X- rays

1017 - 10 1 020

1 nm - 1 pm

Ul travi ol et

1015 - 10 1 017

400 nm - 1 nm

Vi si bl e

4.1014 - 7.5x10 7.5x1014

750 nm - 400 nm

Near i nfrared

1012 - 4.10 14

300 µm - 750 nm

Infrared

1011 - 10 1 012

300 µm

M i crowaves

108 - 1012

3m - 300 µm

Radi o waves

100 - 10 1 08

>1 mm

Ethernet: A network protocol specified for operation to 10 Mbit/s. Standards are being developed for 100 Mbit/s and beyond. Event dead zone : minimum distance on the trace, where two separate events can still be distinguished. The distance to each event can be measured, but the separate loss of each events cannot be measured. This param-


A-9

Glossary

eter usually gives an indication of the minimum distance in order to distinguish between reflective events which occur in close proximity. proximity. • For a reflectiv reflective e event, event, the event event dead dead zone zone definition definition is the the distance distance between the two opposite points which are 1.5 dB (or FWHM) down from the peak. The reflectance of the event shall be specified: as an example Bellcore gives a reflectance of -30 dB. • For an an non-re non-reflectiv flective e event, event, the the event event dead dead zone zone definit definition ion is is the distance between the points where the beginning and ending levels at a splice or a given value ( ≤ 1 dB) are within ±0.1 dB of their initial and final values. Usually this dead zone is a fixed value and depends only on the pulsewidth and the fiber. This definition is not n ot often used. EDZ Event dead zone

1.5 dB

≤1 dB

Event dead zone Reflective event

Non reflective event

Event Dead Zone measurement

Ferrule: A mechanical fixture, generally a rigid tube, used to confine and align the polished or cleaved end of the fiber in a connector. Generally associated with fiber-optic fiber-optic connectors. Fiber Distributed Data Interface (FDDI) : A standard for 100 Mbit/s M bit/s fiberoptic local area network. Fiber optic span: A series of one or more terminated optical fiber elements which may contain complex passive components.

A-10


Floor distributor : The distributor used to make connections between the horizontal cabling, other cabling subsystems and active equipment. FTTB: abbreviation for Fiber-To-The-Building FTTC / FTTK: abbreviation for Fiber-To-The-Curb / Kerb FTTH: abbreviation for Fiber-To-The-Home FTTO: abbreviation for Fiber-To-The-Office Fusion Splice : A permanent joint accomplished by the application of localized heat sufficient to fuse or melt the ends of the optical fiber together, together, forming a continuous single fiber.

Fusion splice by electrical arcing

Graded-index Graded-index fiber: Fiber design in which the refractive index of the core is lower toward the outside of the core and increases toward the center with the peak at the centerline. This multimode fiber design reduces the time difference between the arrival of different modes, minimizing modal dispersion and maximizing bandwidth.

n

Group index : The factor by which the speed of light in vacuum has to be divided to yield the propagation velocity of light pulses in the fiber. Hub : Houses the network software and directs communications within the network. Index of Refraction : see refractive index


A-11

Glossary

Insertion loss: The increase in the total optical attenuation caused by the insertion of an optical component in the transmission path. Joint: an assembly designed to connect 2 or more optical fibers. Jumper :A cable unit or cable element without connectors used to make a connection on a cross-connect. LAN (Local Area Network) : A geographically limited communications network intended for the local transport of data, video, vid eo, and voice. It’s It’s a high speed transmission (Mbit/s) which facilitates information transfer. Laser (Light Amplificated by Stimulated Emission of Radiation): A device that produces monochromatic, coherent light through stimulated emission.

1

0.5 Laser : 0.6 nm

839

840

841

Wavelengths (nm)

Spectral bandwidth of a laser

Launch fiber : A length of fiber used to create an equilibrium modal distribution in multimode, and to measure the first connector of the network in both multimode and singlemode systems. Light Emitting Diode (LED) : A semiconductor device used to transmit light into a fiber in response to an electrical signal. It typically has a broad spectral width. Its spectral width typically is 50 to 60 nm. nm .

A-12


1

LED : 65 nm Laser : 0,6 nm

700

80 0

1000 Wavelength (nm)

Comparison of Laser and Led

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

Mechanical splice

Micro bend: Small distortion of a fiber caused by external factors such as cabling. Mode field diameter (MFD) : A parameter which expresses for a single mode fiber the section where the majority of the light energy passes. It can be expressed as the diameter of optical energy in the fiber. Because the MFD is greater than the core diameter, MFD effectively replaces core diameter in practise. Multimode fiber: An optical fiber in which light travels in multiple modes.


A-13

Glossary

Cladding Size: 125 µm Index of refraction: 1.46

Core Size: 62.5 µm or 50 µm Index of refraction: 1.48

Coating Size: 900 µm

Multimode fiber

Multiplex: Combining two or more signals into a single bit stream that can be individually recovered. Node: A point of flexibility and/or interconnection within the fiber optic cabling system. Numerical aperture : The number that expresses light gathering capacity of a fiber related to the acceptance angle. The sine of 5% optical power angle (corresponding to -13 dB) is used to measure the Numerical Aperture. -13 dB (5% of max power density)

α0

0 dB

Max. power density

Fiber under test NA = 2 α0

-13 dB

Optical Loss budget : The amount of signal loss that can be tolerated in a system before errors occur. ORL (Optical Return Loss): The ratio (expressed in dB) of the reflected power to the incident power from a fiber optic system or link ORL ORL = -10 -10 log log (Pr/ (Pr/Pi Pi)) or ORL ORL = 10 log log (Pi/ (Pi/Pr Pr))

A-14


OTDR: Abbreviation for Optical Time Domain Reflectometer. An instrument used to characterize a fiber optic link. l ink. Useful in estimating fiber link attenuation, attenuation coefficient, discrete reflections, splice/ connector loss, and point defects, all as a function of fiber distance. Patchcord: A cable assembly, permanently assembled at both ends with connector components (principally for cross-connection within a patching facility). Pigtail: A short length of optical fiber permanently attached to a connector and intended to facilitate jointing between that connector and another optical fiber or component. Point to point: A connection established between two specific locations or devices such as a hub and a workstation or between two buildings. Reflectance: The ratio of reflected power to incident power of an event or connector R = 10 log(Pr/Pi). Refractive index : A property of light transmitting materials defined as the ratio of the velocity of light in vacuum (c) to its velocity in a given transmission medium (v). n = c/v n = Refractive Index c = 2.9 2.99792458 . 10 8 m/s

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

Repeater: A device used to regenerate an optical signal to allow an increase in the system length. Scattering: A property that causes light to deflect out of the core area of the fiber, thereby contributing to attenuation. Singlemode fiber : An optical wave guide (or fiber) in which the signal travels in one mode. SONET (Synchronous Optical NETwork): It is a transport interface that enables the public network to carry various kind of services. SONET is the


A-15

Glossary

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

Tele elecom comm municat unication ions s close closett : An enclosed space for housing telecommunications equipment, cable terminations, and cross-connect cabling. The telecommunications closet is a recognized cross-connect between the backbone and horizontal cabling subsystems. Tele elecom comm municat unication ions s outlet outlet: A fixed connecting device where the horizontal cable terminates. The telecommunications outlet provides the interface to work area cabling. Transmitter: An electronic package used to convert a signal carrying electronic information to a corresponding optical signal for transmission by fiber. The transmitter can be a light emitting diode (LED), laser diode, or vertical cavity surface emitting laser (VCSEL). SNR (Signal to Noise Ratio): The ratio of the received optical signal power divided by the RMS noise floor for the detector. Splice : A permanent junction between optical fibers Splitter: A passive device which devises optical power among several output fibers from a common input. Step-index fiber: A fiber whose index of refraction (n) changes sharply at the interfaces of its core and cladding.

n

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

A-16


WDMs: Abbreviation for Wavelength Division Multiplexers. Passive fiber optic components which combine optical channels on different wavelengths. WAN (Wide Area Network): A network used for the transport of information over many miles.


A-17

Glossary

A-18


Notes

1


N-1

Notes

N-2



N-3

Notes

N-4



N-5

Notes

N-6


1Index

A Accuracy distance accuracy 3-15 level accuracy 3-15 Acquisition 4-1 Attenuation 1-3, 1-4, 1-8 Attenuator 2-7 Auto-configuration 4-1 Averaging 4-6

front end dead zone 3-12 using launch cable 4-26 why a dead zone? 3-11 Dispersion chromatic dispersion 1-5 dispersion shifted 1-6 modal dispersion 1-5 Distance error 3-15 Dynamic range 2-8, 3-8

B

E

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

Electronic Industries Association 2-1 Event loss measurement 2-point method 4-14 5-point method 4-15

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

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

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

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


1-1

Index

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

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

M Measurement 4-10 artifacts 4-19 event loss 4-14 full automatic function 4-10 ghosts 4-19 manual measurement function 4-12 reflectance 4-17 section loss 4-14 semi automatic function 4-11 slope 4-14 Mini-OTDR 2-11 Modal equilibrium 4-26 Mode 1-2 multimode 1-7 singlemode 1-8 Mode field diameter (MFD) 1-9, 43 Monitoring system 2-13 Multimode fiber 1-7

Optical Return Loss (ORL) 4-13, 418 OTDR block diagram 3-5 definition 3-1 description 2-12 measurement 4-10 specifications 3-8 use 4-1

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

R Range 4-6 Rayleigh scattering 1-3, 3-1 Reflectance 4-17 Refractive index 1-2, 4-8 Resolution display resolution 3-14 distance resolution 3-14 loss resolution 3-14 sampling resolution 3-14

S

Numerical aperture 1-1, 1-8

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

O

T

Optical loss budget 2-4

Talk set 2-16

N

1-2


Testing installation testing 2-3 maintenance testing 2-3 Time base description 3-7 Time base error 3-15 Transmission tests 2-2

V Velocity 1-2 Visual Fault Locator 2-16

W Wavelength 3-16, 4-3


1-3

Index

1-4


Fiber Optics Guide

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