IEC 60793-1-22:2024

Optical fibres - Part 1-22: Measurement methods and test procedures - Length measurement

IEC 60793-1-22:2024 establishes uniform requirements for measuring the length and elongation of optical fibre (typically within cable). The length of an optical fibre is a fundamental value for the evaluation of transmission characteristics such as losses and bandwidths.

Fibres optiques - Partie 1-22: Méthodes de mesure et procédures d'essai - Mesure de la longueur

L’IEC 60793-1-22:2024 établit des exigences uniformes pour mesurer la longueur et l’allongement de la fibre optique (typiquement dans le câble). La longueur d’une fibre optique est une valeur fondamentale pour l’évaluation des caractéristiques de transmission, telles que les pertes et la bande passante.

General Information

• Status: Published
• Publication Date: 26-Jun-2024
• ICS: 33.180.10 - Fibres and cables
• Technical Committee: SC 86A - Fibres and cables
• Drafting Committee: WG 1 - TC 86/SC 86A/WG 1
• Current Stage: PPUB - Publication issued
• Start Date: 27-Jun-2024
• Completion Date: 19-Jul-2024

Relations

• Revises: IEC 60793-1-22:2001 - Optical fibres - Part 1-22: Measurement methods and test procedures - Length measurement
• Effective Date: 05-Sep-2023

Overview

IEC 60793-1-22:2024 - "Optical fibres - Part 1-22: Measurement methods and test procedures - Length measurement" defines uniform requirements and methods for determining the length and elongation of optical fibre, typically within cable constructions. Accurate fibre length is a fundamental parameter for evaluating transmission characteristics such as optical loss, bandwidth, and overall link performance. The standard describes multiple measurement techniques, required apparatus, sampling and specimen handling, calibration, calculations and reporting of results.

Key topics and requirements

• Measurement methods: The standard documents several principal methods:
  • Method A – Delay measuring (time-of-flight techniques)
  • Method B – Backscattering (OTDR-style techniques and variants)
  • Method C – Fibre elongation (optical measurement of elongation)
  • Method D – Mechanical length (physical/mechanical measurement)
  • Method E – Phase shift (frequency/phase based measurement)
  • Annex F (informative) - Brillouin frequency shift test method for distributed strain/length information
• Apparatus and calibration: Normative requirements for test equipment (sources, detectors, modulators, OTDR-like instruments), launch/receive conditions, and calibration procedures to ensure traceable, repeatable length measurements.
• Sampling and specimens: Guidance on specimen selection, sample preparation, and how to handle fibre within cables for representative measurement.
• Procedures, calculations and results: Defined test procedures, formulas for converting measured quantities to physical length (including use of group index), uncertainty considerations, and standardized reporting/specification information.

Practical applications and users

• Who uses it: fibre manufacturers, cable assemblers, test laboratories, network installers, quality assurance teams, R&D groups, and standards bodies.
• Typical applications:
  • Factory acceptance and quality control of fibre and cables (verify length and elongation under load)
  • Field verification of installed fibre links and splice/cable sections using OTDR/backscatter methods
  • R&D and component characterization where precise length or group-index-dependent measurements are required
  • Pulled-cable monitoring and strain assessment when combined with Brillouin techniques

Related standards

• IEC 60793 series (Optical fibres) - this part complements general optical fibre specifications and measurement guidance across the IEC 60793 family.
• Users should consult other IEC documents in the optical fibre series for linked requirements (terminology, optical loss, mechanical tests).

This standard is essential for anyone needing robust, repeatable optical fibre length measurement, ensuring consistent test methods, clear reporting and interoperability across manufacturing and field testing workflows.

Standards Content (Sample)

IEC 60793-1-22 ®
Edition 2.0 2024-06
COMMENTED VERSION
INTERNATIONAL
STANDARD
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Optical fibres –
Part 1-22: Measurement methods and test procedures – Length measurement
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IEC 60793-1-22 ®
Edition 2.0 2024-06
COMMENTED VERSION
INTERNATIONAL
STANDARD
colour
inside
Optical fibres –
Part 1-22: Measurement methods and test procedures – Length measurement
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 33.180.10 ISBN 978-2-8322-9346-1
– 2 – IEC 60793-1-22:2024 CMV © IEC 2024
CONTENTS
FOREWORD .5
INTRODUCTION .7
1 Scope .8
2 Normative references .8
3 Terms, definitions, and abbreviated terms .8
3.1 Terms and definitions .8
3.2 Abbreviated terms .8
4 Overview of method .9
4.1 General .9
4.2 Method A – Delay measuring .9
4.3 Method B – Backscattering .9
4.4 Method C – Fibre elongation .9
4.5 Method D – Mechanical length . 10
4.6 Method E – Phase shift . 10
4.7 Reference test method . 10
5 Apparatus . 10
6 Sampling and specimens . 10
7 Procedure . 10
8 Calculations . 10
9 Results . 10
10 Specification information . 11
Annex A (normative) Requirements specific to method A – Delay measuring . 12
A.1 General . 12
A.2 Principle . 12
A.3 Apparatus. 12
A.3.1 Two techniques . 12
A.3.2 Optical source . 13
A.3.3 Optical detector . 14
A.4 Procedure . 14
A.4.1 Calibration . 14
A.4.2 Average group index value . 14
A.4.3 Length measurement . 14
A.5 Calculations . 15
A.5.1 General . 15
A.5.2 Transmitted-pulse technique . 15
A.5.3 Reflected-pulse technique. 16
A.6 Results . 16
Annex B (normative) Requirements specific to method B – Backscattering . 17
B.1 General . 17
B.2 Apparatus. 17
B.2.1 General . 17
B.2.2 Optical transmitter . 17
B.2.3 Launch conditions . 18
B.2.4 Optical coupler or splitter . 18
B.2.5 Optical receiver . 18

B.2.6 Pulse duration and repetition rate . 18
B.2.7 Signal processor. 18
B.2.8 Display . 18
B.2.9 Data interface (optional) . 18
B.2.10 Reflection controller (optional) . 18
B.2.11 Splices and connectors . 19
B.3 Sampling and specimens . 19
B.4 Procedure . 19
B.4.1 Three techniques . 19
B.4.2 Procedure common to all three techniques . 19
B.4.3 Procedures specific to each technique . 20
B.4.4 Determination of group index . 22
B.5 Results . 23
Annex C (normative) Requirements specific to method C – Fibre elongation . 24
C.1 Principle . 24
C.2 Apparatus. 24
C.2.1 General requirements . 24
C.2.2 Optical measurement equipment . 25
C.2.3 Instrument resolution . 25
C.3 Procedure . 26
C.3.1 Calibration . 26
C.3.2 Specimen Sample measurement . 26
C.4 Results . 27
Annex D (normative) Requirements specific to method D – Mechanical length . 28
D.1 Principle . 28
D.2 Apparatus. 28
D.3 Procedure . 28
D.3.1 Calibration . 28
D.3.2 Operation . 28
Annex E (normative) Requirements specific to method E – Phase shift . 29
E.1 General . 29
E.2 Apparatus. 29
E.2.1 General . 29
E.2.2 Light source . 29
E.2.3 Modulator . 29
E.2.4 Launch optics . 30
E.2.5 Signal detector and signal detection electronics . 30
E.2.6 Reference signal . 30
E.2.7 Computation equipment . 31
E.3 Sampling and specimens . 31
E.4 Procedure . 31
E.4.1 Selection of starting frequency . 31
E.4.2 Selection of maximum frequency . 31
E.4.3 Phase measurement performance . 31
E.4.4 Measurement of length of test fibre . 32
E.5 Calculation and interpretation of results . 32
E.5.1 Determination of test fibre length .
E.6 Group index. 33
E.6.1 Introduction General . 33

– 4 – IEC 60793-1-22:2024 CMV © IEC 2024
E.6.2 Cut-back method . 33
E.6.3 Substitution method . 33
Annex F (informative) Brillouin frequency shift test method . 35
F.1 General . 35
F.2 Principle . 35
F.3 Apparatus. 36
F.3.1 General requirements . 36
F.3.2 Optical measurement equipment . 36
F.3.3 Instrument resolution . 37
F.4 Procedure . 38
F.4.1 Calibration . 38
F.4.2 Sample measurement . 38
F.5 Results . 40
Bibliography . 41
List of comments . 42

Figure A.1 – Time measurement of the transmitted pulse. 13
Figure A.2 – Time measurement of the reflected pulse . 13
Figure A.3 – Principle of fibre-length measurement . 15
Figure B.1 – Block diagram of an OTDR. 17
Figure B.2 – Schematic OTDR trace of a specimen sample (z to z ) with a section
1 0
(e.g. dead-zone fibre) of unknown length, z , preceding it and without a reflection
pulse from the fibre joint point (two-point technique (B.4.3.1)). 21
Figure B.3 – Schematic OTDR trace of specimen sample (z to z ) with a section (e.g.
1 2
dead-zone fibre) of unknown length, z , preceding it and with a reflection pulse from
the fibre joint point (two-point technique (B.4.3.1)) . 21
Figure B.4 – Schematic trace of a specimen sample (0 to z ) with no section preceding
it (single-point technique 0 (B.4.3.2)) . 22
Figure B.5 – Schematic OTDR trace of a specimen sample (z to z ) with a section
D 2
(e.g. dead-zone fibre) of known length, z , preceding it (single-point technique 1
D
(B.4.3.3)) . 22
Figure C.1 – Equipment set-up for phase-shift technique (C.2.2.2) . 25
Figure C.2 – Equipment set-up for differential pulse-delay technique (C.2.2.3) . 26
Figure E.1 – Apparatus for fibre length measurement . 34
Figure F.1 – Equipment setup for BOTDR technique . 37
Figure F.2 – Equipment setup for BOTDR technique . 37
Figure F.3 – Differential strain recorded during a pulling test over a 100 m of cable . 39
Figure F.4 – Absolute strain profile recorded during a pulling test over a 100 m of cable . 39

Table 1 – Measurement methods .9

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPTICAL FIBRES –
Part 1-22: Measurement methods and test procedures –
Length measurement
FOREWORD
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shall not be held responsible for identifying any or all such patent rights.
This commented version (CMV) of the official standard IEC 60793-1-22:2024 edition 2.0
allows the user to identify the changes made to the previous IEC 60793-1-22:2001
edition 1.0. Furthermore, comments from IEC SC 86A experts are provided to explain the
reasons of the most relevant changes, or to clarify any part of the content.
A vertical bar appears in the margin wherever a change has been made. Additions are in
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This publication contains the CMV and the official standard. The full list of comments is
available at the end of the CMV.

– 6 – IEC 60793-1-22:2024 CMV © IEC 2024
IEC 60793-1-22 has been prepared by subcommittee 86A: Fibres and cables, of IEC technical
committee 86: Fibre optics. It is an International Standard.
This second edition cancels and replaces the first edition published in 2001. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) Inclusion of category C single mode fibres in Table 1;
b) Inclusion of a new informative Annex F on Brillouin frequency shift test method to determine
the tensile strain applied to a fibre.
The text of this International Standard is based on the following documents:
Draft Report on voting
86A/2456/FDIS 86A/2474/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 60793 series, published under the general title Optical fibres, can
be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
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INTRODUCTION
Publications in the IEC 60793-1 series concern measurement methods and test procedures as
they apply to optical fibres.
Within the same series several different areas are grouped, as follows:
• parts 1-10 to 1-19: General
• IEC 60793-1-20 to IEC 60793-1-29: Measurement methods and test procedures for
dimensions
• IEC 60793-1-30 to IEC 60793-1-39: Measurement methods and test procedures for
mechanical characteristics
• IEC 60793-1-40 to IEC 60793-1-49: Measurement methods and test procedures for
transmission and optical characteristics
• IEC 60793-1-50 to IEC 60793-1-59: Measurement methods and test procedures for
environmental characteristics.
• IEC 60793-1-60 to IEC 60793-1-69: Measurement methods and test procedures for
polarization-maintaining fibres.
IEC 60793-1-2X consists of the following parts, under the general title: Optical fibres:
• Part 1-20: Measurement methods and test procedures – Fibre geometry
• Part 1-21: Measurement methods and test procedures – Coating geometry
• Part 1-22: Measurement methods and test procedures – Length measurement

– 8 – IEC 60793-1-22:2024 CMV © IEC 2024
OPTICAL FIBRES –
Part 1-22: Measurement methods and test procedures –
Length measurement
1 Scope
This part of IEC 60793 establishes uniform requirements for measuring the length and
elongation of optical fibre (typically within cable).
The length of an optical fibre is one of the most a fundamental values and shall be known for
the evaluation of transmission characteristics such as losses and bandwidths.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60793-1-40, Optical fibres – Part 1-40: Attenuation measurement methods and test
procedures – Attenuation
IEC 60793-1-42, Optical fibres – Part 1-42: Measurement methods and test procedures –
Chromatic dispersion
IEC 60794-1-1, Optical fibre cables – Part 1-1: Generic specification – General
3 Terms, definitions, and abbreviated terms
3.1 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.2 Abbreviated terms
For the purposes of this document, the following abbreviated terms apply.
BOTDA Brillouin optical time domain analysis
BOTDR Brillouin optical time domain reflectometry
FWHM full-width half-maximum
OTDR optical time domain reflectometer
RMSW root-mean-squared width
RTM reference test method
4 Overview of method
4.1 General
This document gives five methods for measuring length, which are presented in Table 1.
Table 1 – Measurement methods
Characteristics Former designation
Method Fibre categories covered
covered
A Delay measuring Length IEC 60793-1-A6
All A1, B, and all B C 1
B Backscattering Length All A1, B, and all B C IEC 60793-1-C1C
a c b
IEC 60793-1-A7
C Fibre elongation Fibre elongation A1, B1 , and C
D Mechanical Length All IEC 60793-1-A5
E Phase shift Length All A1, B, and all B C IEC 60793-1-A8
a
The measurement of fibre elongation, method C, is part of several measurement methods for fibres and fibre
optic cables, such as those used in IEC 60794-1-1.
b
This measurement is applicable unreservedly to type B single-mode fibres. For type A1 multimode fibres, take
particular care when interpreting the results because the results of this measurement may can be influenced
by interfering modal effects, for example, due to the occurrence of non-longitudinal stresses on the fibre.
Application of the measurement to A2 to A4 multimode fibres is under consideration.
c
Informative Annex F has been added to determine the tensile strain applied to a fibre. It uses Brillouin
reflectometry (BOTDR) or so-called Brillouin analysis (BOTDA), which are single-sided and double-sided

methods respectively.
Information common to all measurements is contained in Clause 2 to Clause 8. Information on
specific application appears in Annex A, Annex B, Annex C, Annex D, and Annex E for methods
A, B, C, D and E, respectively.
4.2 Method A – Delay measuring
The delay measuring method applies to measurements of the fibre length by the measurement
of the propagation time of an optical pulse or a pulse train based on a known value of the group
index of the fibre.
Alternatively, this method is suitable for measuring the group index of a fibre of known length.
Therefore, in practice this fibre length measurement method is calibrated against a known
length of fibre of the same type.
4.3 Method B – Backscattering
The backscattering method, which is a single-sided measurement, uses an optical time domain
reflectometer (OTDR), and measures the optical power backscattered from different points in
the fibre to the beginning of the fibre.
4.4 Method C – Fibre elongation
This measurement method describes a procedure for determining the fibre elongation. It does
not measure absolute strain, but instead measures the changes in strain from one loading
condition to another.
– 10 – IEC 60793-1-22:2024 CMV © IEC 2024
4.5 Method D – Mechanical length
This measurement method describes a procedure for determining the fibre length by winding a
fibre around a fixed diameter calibrated wheel that rotates. The length is determined by the
number of revolutions of the wheel.
4.6 Method E – Phase shift
The phase shift method describes a procedure for determining the fibre length. The length is
determined from the phase shift that occurs when a predetermined modulation frequency f
max
is applied.
4.7 Reference test method
The reference test method (RTM), which shall be the one used to settle disputes, varies
depending on whether the fibre is cabled or not, such as
– uncabled fibre: method D;
– length of fibre within cable: method B;
– elongation of fibre within cable: method C;
– elongation of uncabled fibre: method C.
5 Apparatus
Annex A, Annex B, Annex C, Annex D, and Annex E include layout drawings and other
equipment requirements for each of the methods A, B, C, D and E, respectively.
6 Sampling and specimens 2
See the appropriate Annex A, Annex B, Annex C, Annex D or Annex E for specific
requirements. General requirements follow.
Prepare a flat end face, perpendicular to the fibre axis, at the input and output ends of each
specimen sample for measurements based on optical delay measurements.
7 Procedure
See the appropriate Annex A, Annex B, Annex C, Annex D or Annex E for specific
requirements.
8 Calculations
See the appropriate Annex A, Annex B, Annex C, Annex D or Annex E for specific
requirements.
9 Results
The following information shall be provided with each measurement:
– date and title of measurement;
– identification and description of specimen sample, including whether fibre or cable;
– specimen sample length, or elongation;
– measurement method used: A, B, C, D or E;

– other results, as required by the appropriate Annex A, Annex B, Annex C, Annex D or
Annex E.
The following information shall be available upon request:
– description of measurement apparatus arrangement;
– type and wavelength of measurement source;
– launch conditions;
– details of computation technique;
– date of latest calibration of equipment.
See Annex A, Annex B, Annex C, Annex D and Annex E for any additional information that shall
be available upon request.
10 Specification information
The detail specification shall specify the following information:
– type of fibre (or cable) to be measured;
– failure or acceptance criteria;
– information to be reported;
– deviations to the procedure that apply.

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Annex A
(normative)
Requirements specific to method A – Delay measuring
A.1 General
Use this method to measure the length of optical fibre by itself or installed in cable. If the
specimen sample is a fibre in a cable, determine the value of group index N under conditions
applicable to the specimen sample under measurement (for example, tension, temperature).
This is done by inverting Formula (A.1) and the measurements on a specimen sample with a
known length.
A.2 Principle
An optical pulse travelling through an optical fibre with length L and average group index N
experiences a travelling/delay time time delay, ∆t:
NL
∆=t
(A.1)
C
where
∆t is the time delay;
N is the average group index;
C is the velocity of light in vacuum.
If N is known, the measurement of ∆t gives L. On the other hand, the measurement of ∆t gives
the value of N when L is known.
A.3 Apparatus
A.3.1 Two techniques
There are two techniques for measuring the propagation time of an optical pulse:
– time measurement of the transmitted pulse (∆t measured);
– time measurement of the reflected pulse (2∆t measured).
See Figure A.1 and Figure A.2 for two different arrangements corresponding to the two
techniques applying a sampling oscilloscope.
Instead of the sampling oscilloscope, backscattering equipment, or a counter with separate
start-stop gate and averaging capability (e.g. at least 10 counts), can be used.

Figure A.1 – Time measurement of the transmitted pulse

Figure A.2 – Time measurement of the reflected pulse
A.3.2 Optical source
A.3.2.1 Measurement with the sampling oscilloscope
An optical pulse generator shall preferably be a high-power laser diode, excited by an electrical
pulse train generator, tunable in frequency and width. Record the wavelength and the spectral
width.
A.3.2.2 Measurement with a counter or a backscattering apparatus
An optical pulse generator shall preferably be a high-power laser diode, excited by an electrical
pulse train generator, tunable in width. The time between two pulses shall be longer than the
travelling time of the transmitted pulse (∆t, with counter) or the reflected pulse (2∆t, with
backscattering equipment). Record the wavelength and the spectral width of the laser diode.

– 14 – IEC 60793-1-22:2024 CMV © IEC 2024
A.3.3 Optical detector
The receiver shall preferably be a high-speed avalanche photodiode. The sensitivity of the
optical detector shall be sufficient at the measuring wavelength, and its bandwidth shall be large
enough so as not to not influence the shape of the pulse.
A.4 Procedure
A.4.1 Calibration
Measure the delay time of the optical source to the launching point (this is the delay time of the
measurement apparatus itself).
A.4.2 Average group index value
On a known length of mechanically measured fibre, the measurement of ∆t, gives the average
value, N, of the group index of the fibre.
A.4.3 Length measurement
The length measurement is a time-domain reading on the screen of an oscilloscope (or the
reading of the averaged travelling time on the display of an electronic counter to be corrected
for the calibration value).
NOTE See Figure A.3 for an illustration of an important practical improvement for achieving the accuracy of the
measurement, independent of the actual length of the fibre specimen sample. This uses a dual-channel approach.

a) Channel 1: emitted pulse
b) Channel 2: transmitted pulse

c) Emitted pulse after adjustment of the repetition rate in such a way that the second pulse of channel 1
coincides with the transmitted pulse of channel 2
Figure A.3 – Principle of fibre-length measurement
A.5 Calculations
A.5.1 General
Obtain the fibre length from one of the following formulae:
A.5.2 Transmitted-pulse technique
∆×t c
L = (A.2)
N
– 16 – IEC 60793-1-22:2024 CMV © IEC 2024
A.5.3 Reflected-pulse technique
∆×t c
L = (A.3)
2N
where
L is the fibre length, in m;
∆t is the transmission or reflection time, in ns;
c is the light velocity in vacuum, in m/ns;
N is the average group index.
A.6 Results
In addition to the results in Clause 9, the following information shall can be available upon
request:
– average group index;
– delay time of the measurement apparatus (optional);
– transmission or reflection time (optional).

Annex B
(normative)
Requirements specific to method B – Backscattering
B.1 General
This method uses an OTDR to measure the length of optical fibre by itself and installed in cable.
B.2 Apparatus
B.2.1 General
This method uses an optical time-domain reflectometer (OTDR), which shall normally consist
of the following minimum list of components. See Figure B.1 for a block diagram.

Figure B.1 – Block diagram of an OTDR
B.2.2 Optical transmitter
B.2.2.1 This usually includes one or more pulsed laser diode sources capable of one or more
pulse durations and pulse repetition rates. Unless otherwise specified in the detail specification,
the spectrum for each wavelength shall satisfy the following.
B.2.2.2 The central centroidal wavelength 3 shall lie within 15 nm of the specified value;
report the difference between the central centroidal wavelength and the specified value if it is
greater than 10 nm.
B.2.2.3 The root-mean-squared width (RMSW) shall not exceed 10 nm, or the full-width at half
maximum (FWHM) shall not exceed 25 nm.
B.2.2.4 If the data are to be used in a spectral attenuation model:
– the spectral width shall not exceed 15 nm (FWHM) or 6 nm (RMS) for wavelengths in the
water peak region (e.g. 1 360 nm to 1 430 nm);
– report the actual central centroidal wavelength to within 2 nm of the actual value.

– 18 – IEC 60793-1-22:2024 CMV © IEC 2024
B.2.3 Launch conditions
Provide a means for connecting the test fibre (or the optional dead-zone fibre of B.2.10) to the
instrument panel, or to a fibre pigtail from the source.
For type A fibre, optical sources may not can produce launch conditions that are neither well
controlled nor appropriate for this measurement method. Therefore, unless otherwise specified
in the detail specification, launch conditions for attenuation measurements shall be those used
in cut-back attenuation measurements (IEC 60793-1-40 method A).
B.2.4 Optical coupler or splitter
A coupler/ or splitter within the instrument directs the power from the transmitter into the fibre.
It also directs light returning in the fibre from the opposite direction to the receiver.
B.2.5 Optical receiver
This usually includes a photodiode detector having a bandwidth, sensitivity, linearity and
dynamic range compatible with the pulse durations used and signal levels received.
B.2.6 Pulse duration and repetition rate
The OTDR may can be provided a choice of several pulse durations and repetition rates
(sometimes coupled to the distance control) to optimize the trade-off between resolution and
range. With a high amplitude reflection, it may can be necessary to set the rate or range to a
value exceeding twice the distance of the reflection in order to prevent spurious ‘ghost’ images.
Pulse coding techniques may can also be used.
NOTE Care should be taken when selecting the pulse duration, repetition rate and source power. For shorter
distance measurements, short pulse durations are necessary to provide adequate resolution. This in turn will limit
dynamic range and maximum measurable length. For long length measurements, the dynamic range can be increased
by increasing the peak optical power up to a level below which non-linear effects are insignificant. Alternatively,
pulse width can be increased, which will reduce the resolution of the measurements.
B.2.7 Signal processor
If required, the signal-to-noise level may can be increased using signal averaging over a longer
measurement time.
B.2.8 Display
This is incorporated into the OTDR and is part of the equipment controlling the OTDR. The
OTDR signal is displayed in a graphical form with the vertical scale as decibels and the
horizontal scale as distance. The vertical decibel scale shall correspond to half the round-trip
of the backscatter loss. The horizontal scale shall correspond to half the associated (round-trip)
optical group delay, converted to distance. Tools such as cursors may can be used to manually
or automatically measure all or part of the OTDR trace on the display.
B.2.9 Data interface (optional)
The instrument may can be capable of interfacing with a computer for automatic analysis of the
signal or for providing a hard copy of the display trace.
B.2.10 Reflection controller (optional)
Means of minimizing transient saturation of the receiver due to high Fresnel reflections may can
be required to reduce the length of fibre "dead zone" following each reflector. This can be
incorporated into the coupler/ or splitter or may can be done by electronic masking. To
overcome the initial reflection at the OTDR connector, a dead-zone fibre (with a length in metres
numerically exceeding one-tenth the displayed pulse duration in nanoseconds) may can be
used between the OTDR connector and the specimen sample.

B.2.11 Splices and connectors
Unless otherwise indicated in this procedure, any splices or connectors required by the OTDR
(e.g. to join the OTDR or the dead-zone fibre to the test fibre) shall have low insertion loss and
reflectance (high return loss). This is to minimize extraneous effects upon the OTDR trace of
interest.
B.3 Sampling and specimens
The sample comprises a fibre on a reel or within a cable, under conditions specified in the detail
specification. The measurement may can be performed in the factory or in the field, upon either
single or concatenated sections.
NOTE Care should be taken to Ensure that winding does not introduce substantial elongation for length
measurements.
B.4 Procedure
B.4.1 Three techniques
There are three techniques:
• two-point technique (B.4.3.1), to use when a fibre or cable section of unknown length
precedes the test fibre or cable;
• single-point technique 0 (B.4.3.2), to use with no preceding section of fibre or cable;
• single-point technique 1 (B.4.3.3), to use with a preceding section of fibre of known length
and similar group index as the fibre to be m
...

IEC 60793-1-22 ®
Edition 2.0 2024-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Optical fibres –
Part 1-22: Measurement methods and test procedures – Length measurement
Fibres optiques –
Partie 1-22: Méthodes de mesure et procédures d'essai – Mesure de la longueur
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IEC 60793-1-22 ®
Edition 2.0 2024-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Optical fibres –
Part 1-22: Measurement methods and test procedures – Length measurement
Fibres optiques –
Partie 1-22: Méthodes de mesure et procédures d'essai – Mesure de la longueur
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 33.180.10 ISBN 978-2-8322-9195-5
– 2 – IEC 60793-1-22:2024 © IEC 2024
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms, definitions, and abbreviated terms . 8
3.1 Terms and definitions . 8
3.2 Abbreviated terms . 8
4 Overview of method . 9
4.1 General . 9
4.2 Method A – Delay measuring . 9
4.3 Method B – Backscattering . 9
4.4 Method C – Fibre elongation . 9
4.5 Method D – Mechanical length . 9
4.6 Method E – Phase shift . 10
4.7 Reference test method . 10
5 Apparatus . 10
6 Sampling . 10
7 Procedure . 10
8 Calculations . 10
9 Results . 10
10 Specification information . 11
Annex A (normative) Requirements specific to method A – Delay measuring . 12
A.1 General . 12
A.2 Principle . 12
A.3 Apparatus . 12
A.3.1 Two techniques . 12
A.3.2 Optical source . 13
A.3.3 Optical detector . 14
A.4 Procedure . 14
A.4.1 Calibration . 14
A.4.2 Average group index value . 14
A.4.3 Length measurement . 14
A.5 Calculations . 15
A.5.1 General . 15
A.5.2 Transmitted-pulse technique . 15
A.5.3 Reflected-pulse technique . 16
A.6 Results . 16
Annex B (normative) Requirements specific to method B – Backscattering . 17
B.1 General . 17
B.2 Apparatus . 17
B.2.1 General . 17
B.2.2 Optical transmitter . 17
B.2.3 Launch conditions . 18
B.2.4 Optical coupler or splitter . 18
B.2.5 Optical receiver . 18

B.2.6 Pulse duration and repetition rate . 18
B.2.7 Signal processor . 18
B.2.8 Display . 18
B.2.9 Data interface (optional) . 18
B.2.10 Reflection controller (optional) . 18
B.2.11 Splices and connectors . 19
B.3 Sampling. 19
B.4 Procedure . 19
B.4.1 Three techniques . 19
B.4.2 Procedure common to all three techniques . 19
B.4.3 Procedures specific to each technique . 20
B.4.4 Determination of group index . 22
B.5 Results . 23
Annex C (normative) Requirements specific to method C – Fibre elongation . 24
C.1 Principle . 24
C.2 Apparatus . 24
C.2.1 General requirements . 24
C.2.2 Optical measurement equipment . 25
C.2.3 Instrument resolution . 25
C.3 Procedure . 26
C.3.1 Calibration . 26
C.3.2 Sample measurement . 26
C.4 Results . 27
Annex D (normative) Requirements specific to method D – Mechanical length . 28
D.1 Principle . 28
D.2 Apparatus . 28
D.3 Procedure . 28
D.3.1 Calibration . 28
D.3.2 Operation . 28
Annex E (normative) Requirements specific to method E – Phase shift . 29
E.1 General . 29
E.2 Apparatus . 29
E.2.1 General . 29
E.2.2 Light source . 29
E.2.3 Modulator . 29
E.2.4 Launch optics . 30
E.2.5 Signal detector and signal detection electronics . 30
E.2.6 Reference signal. 30
E.2.7 Computation equipment . 31
E.3 Sampling. 31
E.4 Procedure . 31
E.4.1 Selection of starting frequency . 31
E.4.2 Selection of maximum frequency . 31
E.4.3 Phase measurement performance . 31
E.4.4 Measurement of length of test fibre . 32
E.5 Calculation and interpretation of results . 32
E.6 Group index . 33
E.6.1 General . 33
E.6.2 Cut-back method . 33

– 4 – IEC 60793-1-22:2024 © IEC 2024
E.6.3 Substitution method . 33
Annex F (informative) Brillouin frequency shift test method . 35
F.1 General . 35
F.2 Principle . 35
F.3 Apparatus . 36
F.3.1 General requirements . 36
F.3.2 Optical measurement equipment . 36
F.3.3 Instrument resolution . 37
F.4 Procedure . 38
F.4.1 Calibration . 38
F.4.2 Sample measurement . 38
F.5 Results . 40
Bibliography . 41
Figure A.1 – Time measurement of the transmitted pulse . 13
Figure A.2 – Time measurement of the reflected pulse . 13
Figure A.3 – Principle of fibre-length measurement . 15
Figure B.1 – Block diagram of an OTDR . 17
Figure B.2 – Schematic OTDR trace of a sample (z to z ) with a section of unknown
1 0
length, z , preceding it and without a reflection pulse from the fibre joint point (two-
point technique (B.4.3.1)). 21
Figure B.3 – Schematic OTDR trace of sample (z to z ) with a section of unknown
1 2
length, z , preceding it and with a reflection pulse from the fibre joint point (two-point
technique (B.4.3.1)) . 21
Figure B.4 – Schematic trace of a sample (0 to z ) with no section preceding it (single-
point technique 0 (B.4.3.2)) . 22
Figure B.5 – Schematic OTDR trace of a sample (z to z ) with a section of known
D 2
length, z , preceding it (single-point technique 1 (B.4.3.3)) . 22
D
Figure C.1 – Equipment set-up for phase-shift technique (C.2.2.2) . 25
Figure C.2 – Equipment set-up for differential pulse-delay technique (C.2.2.3) . 26
Figure E.1 – Apparatus for fibre length measurement . 34
Figure F.1 – Equipment setup for BOTDR technique . 37
Figure F.2 – Equipment setup for BOTDR technique . 37
Figure F.3 – Differential strain recorded during a pulling test over a 100 m of cable . 39
Figure F.4 – Absolute strain profile recorded during a pulling test over a 100 m of cable . 39
Table 1 – Measurement methods . 9

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPTICAL FIBRES –
Part 1-22: Measurement methods and test procedures –
Length measurement
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
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Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC 60793-1-22 has been prepared by subcommittee 86A: Fibres and cables, of IEC technical
committee 86: Fibre optics. It is an International Standard.
This second edition cancels and replaces the first edition published in 2001. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) Inclusion of category C single mode fibres in Table 1;
b) Inclusion of a new informative Annex F on Brillouin frequency shift test method to determine
the tensile strain applied to a fibre.

– 6 – IEC 60793-1-22:2024 © IEC 2024
The text of this International Standard is based on the following documents:
Draft Report on voting
86A/2456/FDIS 86A/2474/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 60793 series, published under the general title Optical fibres, can
be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
INTRODUCTION
Publications in the IEC 60793-1 series concern measurement methods and test procedures as
they apply to optical fibres.
Within the same series several different areas are grouped, as follows:
• IEC 60793-1-20 to IEC 60793-1-29: Measurement methods and test procedures for
dimensions
• IEC 60793-1-30 to IEC 60793-1-39: Measurement methods and test procedures for
mechanical characteristics
• IEC 60793-1-40 to IEC 60793-1-49: Measurement methods and test procedures for
transmission and optical characteristics
• IEC 60793-1-50 to IEC 60793-1-59: Measurement methods and test procedures for
environmental characteristics.
• IEC 60793-1-60 to IEC 60793-1-69: Measurement methods and test procedures for
polarization-maintaining fibres.
IEC 60793-1-2X consists of the following parts, under the general title: Optical fibres:
• Part 1-20: Measurement methods and test procedures – Fibre geometry
• Part 1-21: Measurement methods and test procedures – Coating geometry
• Part 1-22: Measurement methods and test procedures – Length measurement

– 8 – IEC 60793-1-22:2024 © IEC 2024
OPTICAL FIBRES –
Part 1-22: Measurement methods and test procedures –
Length measurement
1 Scope
This part of IEC 60793 establishes uniform requirements for measuring the length and
elongation of optical fibre (typically within cable).
The length of an optical fibre is a fundamental value for the evaluation of transmission
characteristics such as losses and bandwidths.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60793-1-40, Optical fibres – Part 1-40: Attenuation measurement methods
IEC 60793-1-42, Optical fibres – Part 1-42: Measurement methods and test procedures –
Chromatic dispersion
3 Terms, definitions, and abbreviated terms
3.1 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.2 Abbreviated terms
For the purposes of this document, the following abbreviated terms apply.
BOTDA Brillouin optical time domain analysis
BOTDR Brillouin optical time domain reflectometry
FWHM full-width half-maximum
OTDR optical time domain reflectometer
RMSW root-mean-squared width
RTM reference test method
4 Overview of method
4.1 General
This document gives five methods for measuring length, which are presented in Table 1.
Table 1 – Measurement methods
Method Characteristics covered Fibre categories covered
A Delay measuring Length All A1, B, and C
B Backscattering Length All A1, B, and C
a c b
C Fibre elongation Fibre elongation A1, B1 , and C
D Mechanical Length All
E Phase shift Length All A1, B, and C
a
The measurement of fibre elongation is used in IEC 60794-1-1.
b
This measurement is applicable unreservedly to type B single-mode fibres. For type A1 multimode fibres, take
particular care when interpreting the results because the results of this measurement can be influenced by
interfering modal effects, for example, due to the occurrence of non-longitudinal stresses on the fibre.
Application of the measurement to A2 to A4 multimode fibres is under consideration.
c
Informative Annex F has been added to determine the tensile strain applied to a fibre. It uses Brillouin
reflectometry (BOTDR) or so-called Brillouin analysis (BOTDA), which are single-sided and double-sided
methods respectively.
Information common to all measurements is contained in Clause 2 to Clause 8. Information on
specific application appears in Annex A, Annex B, Annex C, Annex D, and Annex E for methods
A, B, C, D and E, respectively.
4.2 Method A – Delay measuring
The delay measuring method applies to measurements of the fibre length by the measurement
of the propagation time of an optical pulse or a pulse train based on a known value of the group
index of the fibre.
Alternatively, this method is suitable for measuring the group index of a fibre of known length.
Therefore, in practice this fibre length measurement method is calibrated against a known
length of fibre of the same type.
4.3 Method B – Backscattering
The backscattering method, which is a single-sided measurement, uses an optical time domain
reflectometer (OTDR), and measures the optical power backscattered from different points in
the fibre to the beginning of the fibre.
4.4 Method C – Fibre elongation
This measurement method describes a procedure for determining the fibre elongation. It does
not measure absolute strain, but instead measures the changes in strain from one loading
condition to another.
4.5 Method D – Mechanical length
This measurement method describes a procedure for determining the fibre length by winding a
fibre around a fixed diameter calibrated wheel that rotates. The length is determined by the
number of revolutions of the wheel.

– 10 – IEC 60793-1-22:2024 © IEC 2024
4.6 Method E – Phase shift
The phase shift method describes a procedure for determining the fibre length. The length is
determined from the phase shift that occurs when a predetermined modulation frequency f
max
is applied.
4.7 Reference test method
The reference test method (RTM), which shall be the one used to settle disputes, varies
depending on whether the fibre is cabled or not, such as
– uncabled fibre: method D;
– length of fibre within cable: method B;
– elongation of fibre within cable: method C;
– elongation of uncabled fibre: method C.
5 Apparatus
Annex A, Annex B, Annex C, Annex D, and Annex E include layout drawings and other
equipment requirements for each of the methods A, B, C, D and E, respectively.
6 Sampling
See the appropriate Annex A, Annex B, Annex C, Annex D or Annex E for specific
requirements. General requirements follow.
Prepare a flat end face, perpendicular to the fibre axis, at the input and output ends of each
sample for measurements based on optical delay measurements.
7 Procedure
See the appropriate Annex A, Annex B, Annex C, Annex D or Annex E for specific
requirements.
8 Calculations
See the appropriate Annex A, Annex B, Annex C, Annex D or Annex E for specific
requirements.
9 Results
The following information shall be provided with each measurement:
– date and title of measurement;
– identification and description of sample, including whether fibre or cable;
– sample length, or elongation;
– measurement method used: A, B, C, D or E;
– other results, as required by the appropriate Annex A, Annex B, Annex C, Annex D or
Annex E.
The following information shall be available upon request:
– description of measurement apparatus arrangement;
– type and wavelength of measurement source;
– launch conditions;
– details of computation technique;
– date of latest calibration of equipment.
See Annex A, Annex B, Annex C, Annex D and Annex E for any additional information that shall
be available upon request.
10 Specification information
The detail specification shall specify the following information:
– type of fibre (or cable) to be measured;
– failure or acceptance criteria;
– information to be reported;
– deviations to the procedure that apply.

– 12 – IEC 60793-1-22:2024 © IEC 2024
Annex A
(normative)
Requirements specific to method A – Delay measuring
A.1 General
Use this method to measure the length of optical fibre by itself or installed in cable. If the sample
is a fibre in a cable, determine the value of group index N under conditions applicable to the
sample under measurement (for example, tension, temperature). This is done by inverting
Formula (A.1) and the measurements on a sample with a known length.
A.2 Principle
An optical pulse travelling through an optical fibre with length L and average group index N
experiences a time delay, ∆t:
NL
∆=t
(A.1)
C
where
∆t is the time delay;
N is the average group index;
C is the velocity of light in vacuum.
If N is known, the measurement of ∆t gives L. On the other hand, the measurement of ∆t gives
the value of N when L is known.
A.3 Apparatus
A.3.1 Two techniques
There are two techniques for measuring the propagation time of an optical pulse:
– time measurement of the transmitted pulse (∆t measured);
– time measurement of the reflected pulse (2∆t measured).
See Figure A.1 and Figure A.2 for two different arrangements corresponding to the two
techniques applying a sampling oscilloscope.
Instead of the sampling oscilloscope, backscattering equipment, or a counter with separate
start-stop gate and averaging capability (e.g. at least 10 counts), can be used.

Figure A.1 – Time measurement of the transmitted pulse

Figure A.2 – Time measurement of the reflected pulse
A.3.2 Optical source
A.3.2.1 Measurement with the sampling oscilloscope
An optical pulse generator shall be a high-power laser diode, excited by an electrical pulse train
generator, tunable in frequency and width. Record the wavelength and the spectral width.
A.3.2.2 Measurement with a counter or a backscattering apparatus
An optical pulse generator shall be a high-power laser diode, excited by an electrical pulse train
generator, tunable in width. The time between two pulses shall be longer than the travelling
time of the transmitted pulse (∆t, with counter) or the reflected pulse (2∆t, with backscattering
equipment). Record the wavelength and the spectral width of the laser diode.

– 14 – IEC 60793-1-22:2024 © IEC 2024
A.3.3 Optical detector
The receiver shall be a high-speed avalanche photodiode. The sensitivity of the optical detector
at the measuring wavelength, and its bandwidth shall not influence the shape of the pulse.
A.4 Procedure
A.4.1 Calibration
Measure the delay time of the optical source to the launching point (this is the delay time of the
measurement apparatus itself).
A.4.2 Average group index value
On a known length of mechanically measured fibre, the measurement of ∆t, gives the average
value, N, of the group index of the fibre.
A.4.3 Length measurement
The length measurement is a time-domain reading on the screen of an oscilloscope (or the
reading of the averaged travelling time on the display of an electronic counter to be corrected
for the calibration value).
NOTE See Figure A.3 for an illustration of an important practical improvement for achieving the accuracy of the
measurement, independent of the actual length of the fibre sample. This uses a dual-channel approach.

a) Channel 1: emitted pulse
b) Channel 2: transmitted pulse

c) Emitted pulse after adjustment of the repetition rate in such a way that the second pulse of channel 1
coincides with the transmitted pulse of channel 2
Figure A.3 – Principle of fibre-length measurement
A.5 Calculations
A.5.1 General
Obtain the fibre length from one of the following formulae:
A.5.2 Transmitted-pulse technique
∆×t c
L =
(A.2)
N
– 16 – IEC 60793-1-22:2024 © IEC 2024
A.5.3 Reflected-pulse technique
∆×t c
L = (A.3)
2N
where
L is the fibre length, in m;
∆t is the transmission or reflection time, in ns;
c is the light velocity in vacuum, in m/ns;
N is the average group index.
A.6 Results
In addition to the results in Clause 9, the following information can be available upon request:
– average group index;
– delay time of the measurement apparatus;
– transmission or reflection time.

Annex B
(normative)
Requirements specific to method B – Backscattering
B.1 General
This method uses an OTDR to measure the length of optical fibre by itself and installed in cable.
B.2 Apparatus
B.2.1 General
This method uses an optical time-domain reflectometer (OTDR), which shall normally consist
of the following minimum list of components. See Figure B.1 for a block diagram.

Figure B.1 – Block diagram of an OTDR
B.2.2 Optical transmitter
B.2.2.1 This usually includes one or more pulsed laser diode sources capable of one or more
pulse durations and pulse repetition rates. Unless otherwise specified in the detail specification,
the spectrum for each wavelength shall satisfy the following.
B.2.2.2 The centroidal wavelength shall lie within 15 nm of the specified value; report the
difference between the centroidal wavelength and the specified value if it is greater than 10 nm.
B.2.2.3 The root-mean-squared width (RMSW) shall not exceed 10 nm, or the full-width at half
maximum (FWHM) shall not exceed 25 nm.
B.2.2.4 If the data are to be used in a spectral attenuation model:
– the spectral width shall not exceed 15 nm (FWHM) or 6 nm (RMS) for wavelengths in the
water peak region (e.g. 1 360 nm to 1 430 nm);
– report the actual centroidal wavelength to within 2 nm of the actual value.

– 18 – IEC 60793-1-22:2024 © IEC 2024
B.2.3 Launch conditions
Provide a means for connecting the test fibre (or the optional dead-zone fibre of B.2.10) to the
instrument panel, or to a fibre pigtail from the source.
For type A fibre, optical sources can produce launch conditions that are neither well controlled
nor appropriate for this measurement method. Therefore, unless otherwise specified in the
detail specification, launch conditions for attenuation measurements shall be those used in cut-
back attenuation measurements (IEC 60793-1-40 method A).
B.2.4 Optical coupler or splitter
A coupler or splitter within the instrument directs the power from the transmitter into the fibre.
It also directs light returning in the fibre from the opposite direction to the receiver.
B.2.5 Optical receiver
This usually includes a photodiode detector having a bandwidth, sensitivity, linearity and
dynamic range compatible with the pulse durations used and signal levels received.
B.2.6 Pulse duration and repetition rate
The OTDR can be provided a choice of several pulse durations and repetition rates (sometimes
coupled to the distance control) to optimize the trade-off between resolution and range. With a
high amplitude reflection, it can be necessary to set the rate or range to a value exceeding twice
the distance of the reflection in order to prevent spurious ‘ghost’ images. Pulse coding
techniques can also be used.
NOTE Care should be taken when selecting the pulse duration, repetition rate and source power. For shorter
distance measurements, short pulse durations are necessary to provide adequate resolution. This in turn will limit
dynamic range and maximum measurable length. For long length measurements, the dynamic range can be increased
by increasing the peak optical power up to a level below which non-linear effects are insignificant. Alternatively,
pulse width can be increased, which will reduce the resolution of the measurements.
B.2.7 Signal processor
If required, the signal-to-noise level can be increased using signal averaging over a longer
measurement time.
B.2.8 Display
This is incorporated into the OTDR and is part of the equipment controlling the OTDR. The
OTDR signal is displayed in a graphical form with the vertical scale as decibels and the
horizontal scale as distance. The vertical decibel scale shall correspond to half the round-trip
of the backscatter loss. The horizontal scale shall correspond to half the associated (round-trip)
optical group delay, converted to distance. Tools such as cursors can be used to manually or
automatically measure all or part of the OTDR trace on the display.
B.2.9 Data interface (optional)
The instrument can be capable of interfacing with a computer for automatic analysis of the
signal or for providing a hard copy of the display trace.
B.2.10 Reflection controller (optional)
Means of minimizing transient saturation of the receiver due to high Fresnel reflections can be
required to reduce the length of fibre "dead zone" following each reflector. This can be incor-
porated into the coupler or splitter or can be done by electronic masking. To overcome the initial
reflection at the OTDR connector, a dead-zone fibre (with a length in metres numerically
exceeding one-tenth the displayed pulse duration in nanoseconds) can be used between the
OTDR connector and the sample.

B.2.11 Splices and connectors
Unless otherwise indicated in this procedure, any splices or connectors required by the OTDR
(e.g. to join the OTDR or the dead-zone fibre to the test fibre) shall have low insertion loss and
reflec
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