IEC TR 62316:2026

IEC TR 62316 ®
Edition 4.0 2026-01
TECHNICAL
REPORT
Guidance for the interpretation of OTDR backscattering traces for single-mode
fibres
ICS 33.180.10  ISBN 978-2-8327-0942-9

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IEC TR 62316:2025 © IEC 2025
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Terms, definitions and abbreviated terms . 6
3.1 Terms and definitions. 6
3.2 Abbreviated terms . 6
4 Backscattering phenomenon . 6
4.1 Rayleigh scattering . 6
4.2 Fresnel reflections and dead zone fibres . 7
5 Measurement of the backscattered power (OTDR) . 7
5.1 General . 7
5.2 Representation of the backscattered power . 8
5.3 Noise and perturbations . 8
6 Interpretation of a backscattering trace . 9
6.1 General . 9
6.2 Launch cord . 9
6.3 Tail cord . 9
6.4 Unidirectional trace . 10
6.4.1 General . 10
6.4.2 Slope as the attenuation coefficient of a fibre . 10
6.4.3 Impurity and discontinuity . 11
6.4.4 Pulse width . 11
6.4.5 Polarization effects . 11
6.5 Bi-directional trace . 12
6.5.1 General . 12
6.5.2 Attenuation uniformity . 14
6.5.3 MFD uniformity . 15
6.6 Splice loss evaluation . 16
6.6.1 General . 16
6.6.2 Event measurement methods. 16
6.6.3 Apparent losers and gainers . 18
6.6.4 Example of apparent splice loss evaluation for uni-directional OTDR
measurements . 21
7 Uncertainties, deviation and resolution . 23
7.1 General . 23
7.2 Attenuation coefficient measurements . 23
7.3 Fault locations . 23
Bibliography . 26

Figure 1 – Unidirectional OTDR trace showing splice and/or macrobend loss . 9
Figure 2 – Idealized unidirectional OTDR traces corresponding to a non-reflective
splice between two fibres . 16
Figure 3 – OTDR traces for similar or different fibre types with either different MFD or
different backscatter properties, or both . 18
IEC TR 62316:2025 © IEC 2025
Figure 4 – Loss in unidirectional OTDR measurements as function of the MFD
difference between two spliced fibres . 19
Figure 5 – Theoretical power through splice loss due to MFD difference (with
Ω = 9 µm) . 20
Figure 6 – Apparent cumulative unidirectional backscattering mismatch distribution for
six splice combinations of B-652 and B-657 reported in Table 1 . 22
Figure 7 – Schematic drawing of a fibre with two consecutive defects 1 and 2 . 24

Table 1 – Summary for six fibre splice combinations of B-652 and B-657 based on
popular 1 310 nm MFD fibre distributions . 21

IEC TR 62316:2025 © IEC 2025
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Guidance for the interpretation of OTDR backscattering traces for single-
mode fibres
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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IEC TR 62316 has been prepared by subcommittee 86A: Fibres and cables, of IEC technical
committee 86: Fibre optics. It is a Technical Report.
This fourth edition cancels and replaces the third edition published in 2017. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) update of the fibre types;
b) addition of information as regards attenuation uniformity.
IEC TR 62316:2025 © IEC 2025
The text of this Technical Report is based on the following documents:
Draft Report on voting
86A/2621/DTR 86A/2643/RVDTR
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 Technical Report 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.
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.
IEC TR 62316:2025 © IEC 2025
INTRODUCTION
This document is a guide for the interpretation of OTDR backscattering traces for single mode
fibres. It does not provide any normative reference, but adequate measurement and test
procedures for attenuation (see IEC 60793-1-40 [1] and IEC 61280-4-2 [2]), connector
inspection (see IEC 61300-3-35 [3]), as well as for calibration of single mode fibre OTDRs (see
IEC 61746-1 [4]) that are applied to ensure the validity of such measurements.

___________
Numbers in square brackets refer to the Bibliography
IEC TR 62316:2025 © IEC 2025
1 Scope
This document provides guidance on the interpretation of backscattering traces, as obtained by
traditional optical time domain reflectometers (OTDRs) for single-mode fibres.
This document does not cover Polarization OTDRs.
Also, backscattered power effects are discussed in case of unidirectional trace.
Full description of the test measurement procedure can be found in Annex C of
IEC 60793-1-40:2024 [1].
2 Normative references
There are no normative references in this document.
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
FTTH fibre to the home
GSW generalised sliding window to calculate attenuation uniformity
LSPM light source power meter
LSA least-square approximation
MFD mode field diameter
OTDR optical time domain reflectometer
PMD polarization mode dispersion
SNR signal to noise ratio
SW sliding window algorithm to calculate attenuation uniformity
4 Backscattering phenomenon
4.1 Rayleigh scattering
Rayleigh scattering or backscattering originates from fluctuations in the density, and hence in
the index of refraction, of the material constituting the wave-guide; optical fibres are made of
amorphous silica, and density fluctuations are a consequence of the manufacturing process.
IEC TR 62316:2025 © IEC 2025
4.2 Fresnel reflections and dead zone fibres
When a light ray reaches a surface at an angle of incidence from the normal to that surface and
that surface separates two media of different index of refraction, part of this light ray is refracted
in the second medium and part of it is reflected backward into the first medium. This is the
Fresnel reflection, which can be very high, depending on the difference in the index of refraction
of the two media, on the aspect of the surface, the surface roughness, the angle of incidence
and the surface defects. In most situations, strong Fresnel reflections cause non-linearities at
the receiver. These non-linearities can overload the receiver resulting in signal clipping, pulse
widening, tailing, and ghosts. The corresponding section of the optical time domain
reflectometer (OTDR) trace following the intense Fresnel reflection defines the deadzone. This
particular deadzone is not the same as the manufacturer's specification which is always defined
with a narrow pulse and small Fresnel reflection. The effect of the strong reflection on the
deadzone is usually resolved by cleaning the connector responsible for the reflection. The so-
called deadzone eliminator (adding a length of fibre after a strong reflection) does not reduce
the deadzone nor the strong reflection. It artificially moves the virtual bulkhead connector to
another location and assumes the following connector has a low reflection. Depending on the
type of photodetector used in the receiver, the tailing due to a strong reflection can be greater
than the fibre length inserted between the OTDR and the fibre under test.
5 Measurement of the backscattered power (OTDR)
5.1 General
The power backscattered by an optical fibre is measured by means of OTDRs. They are based
on the principle of sending one pulse or typically a train of pulses from one fibre end and
measure the returned power from the fibre at the same end. In OTDR traces, space and time
are completely equivalent through the relation:
zc
=
(1)
tn ()λ
g
where
z is the distance (in meters);
t is the time (in seconds);
c is the speed of light in vacuum (299 792 458 meters/second);
n is the group index of refraction (as a function of the wavelength).

g
The group index of refraction, that is supplied by the fibre manufacturer (see IEC 60793-2 [5]),
takes into account the wave-guiding properties of the fibre and the different materials used for
the cladding and the core. It also adjusts the speed of light in the studied material. The group
index of refraction n is related to the phase index n or n (which is measured on a fibre and its
g p
fundamental attribute) by using formula (2):
dn
p
nn−λ (2)
gp
dλ
=
IEC TR 62316:2025 © IEC 2025
5.2 Representation of the backscattered power
A possible schematic representation of the OTDR power P(z) at wavelength λ backscattered by
a point z along an optical fibre is:
− αz
λ
Pz = C Pτ 10
( ) (3)
iw
ωz
( ( ))
where
P is the input OTDR pulse power into the fibre;
i
τ is the input OTDR pulse width (in seconds);
w
z is the distance in meter at which the backscattered power is generated;
α is the attenuation in dB per meter (assumed constant to simplify the formula);
Ω(z) is the fibre mode field diameter (MFD) at point z;
C is a proportionality factor, which depends on several parameters such as the fibre
material or the refractive index value. For step-index single-mode fibre, this factor is
expressed by:
3cα
s
C=
(4)
16πn n
eff g
where
c is the speed of light in vacuum;
–1
α is the Rayleigh scattering coefficient in m ;
s
n is the effective refractive index of the fundamental mode, which is a number
eff
quantifying the phase delay per unit length in a wave guide, relative to the phase
delay per unit length in vacuum;
is the group index of refraction.
n
g
Formula (3) shows the relation between the backscattered power, the pulse width, the
attenuation coefficient and the MFD. The optical reflected power, as given by Formula (3), is
conventionally represented on a logarithmic graph: it therefore appears as a (theoretically)
straight line, whose slope is the attenuation coefficient of the fibre, α, as better explained in
Clause 6.
NOTE Formula (3) is valid for short pulse width, i.e. τ α << 1, which applies in most practical cases.
w
5.3 Noise and perturbations
Normally, the fluctuations of fibre parameter and receiver linearity affect the backscatter traces;
the trace can therefore appear as a perturbed line. The linear signal decreases exponentially –
as from Formula (3); over long distance, the signal to noise ratio (SNR) decreases as a function
of distance. As the backscatter signal approaches the noise floor, non-linearities can appear. A
practical way to improve the SNR, also known as dynamic range, is to increase averaging time
or increase the pulse width.
IEC TR 62316:2025 © IEC 2025
Any event, such as a splice, connector, macrobend, microbend, can be detected by the OTDR
and appear as a perturbation. Microbends are more evident at long wavelengths such as 1625
nm, far from the cut-off wavelength where the MFD is larger and the confinement of light in the
fibre is reduced.
6 Interpretation of a backscattering trace
6.1 General
Figure 1 shows a typical unidirectional OTDR trace of an optical fibre showing a loss A dB,
which can be a macrobend loss or splice loss. The reflection at the input face is exaggerated
for clarity; normally it is reduced by means of a launch cord with clean connector meeting
IEC 61300-3-35 [3].
Key
OTDR optical time domain reflectometer F reflected power level
LC launch cord L distance from OTDR launch cord output port
C cabling under test A macrobend or splice loss
TC tail cord S macrobend or splice
Figure 1 – Unidirectional OTDR trace showing splice and/or macrobend loss
6.2 Launch cord
The optical fibre within the launch cord at the connection to the cabling under test is usually of
the same type, in terms of core diameter and numerical aperture, but not necessarily bandwidth,
as the optical fibre within the cabling under test.
For practical reasons, the length of the launch cord is chosen to be longer than the dead zone
created by the pulse width selected for the particular fibre length that will be measured.
Suppliers of OTDR equipment can advise on the lengths. In addition, and for practical reasons,
these lengths are chosen to be long enough for a reliable straight line fit of the backscatter
trace that follows the attenuation dead zone with standard connector reflectance.
6.3 Tail cord
The optical fibre within the receive or tail cord is usually of the same type, nominal core diameter
and nominal numerical aperture as the optical fibre within the cabling under test.
IEC TR 62316:2025 © IEC 2025
For practical reasons, the length of the tail cord is chosen to be longer than the dead zone
created by the pulse width selected for a particular length of fibre that will be measured.
6.4 Unidirectional trace
6.4.1 General
The accepted method of determining the attenuation of installed links by OTDR is performing
bi-directional OTDR measurements and average both these traces (see IEC 60793-1-40 [1]
and IEC 61280-4-2 [2]). However, in some situations, it is difficult in practice to perform such
bi-directional OTDR measurements, in particular fibre-to-the-home (FTTH) applications. In
those cases, OTDR traces obtained by the processing of the optical backscattered light
collected from one end only of the fibre can be used, called unidirectional traces. Such
unidirectional OTDR traces can be useful to quickly evaluate the optical continuity of a fibre
and to estimate the link attenuation coefficient, which reliability, however, can be affected by
several effects (such as perturbation changes in the fibre, backscatter coefficient changes, non-
linearities, and ghosts).
For unidirectional measurement, it is usually important to understand and consider what follows.
– The main requirement for total single-mode unidirectional attenuation measurements using
an OTDR is that the launch and tail cords used for the set-up have the same backscatter
coefficient. In order to verify this hypothesis, the following test will be performed before
using an OTDR for single direction measurement every time when it is not sure the launch
and tail cords have the same backscatter coefficient.
– Launch cable test procedure: Connect the launch and tail cords together. Adjust the OTDR
pulse width, so that a sufficiently large number of data points and an appropriate
signal-to-noise ratio are obtained. Determine the backscatter traces from both fibre ends
with averaging OTDR measurements from both directions.
– For each direction A and B, calculate the average loss between launch and receive cords
LA and LB. The difference between the losses from both directions is usually equal to zero,
given the device and measurement uncertainties. This step ensures that the backscatter
coefficient of the launch and receive cords are the same, allowing to proceed with total
attenuation measurements for single-mode links.
– For conformance testing of links and channels, an optical light source and power meter are
required.
6.4.2 Slope as the attenuation coefficient of a fibre
Starting from Formula (3) for the backscattered power, taking logarithms on both sides, one
obtains (with decimal logarithm written as "lg(x)"):
λ
5⋅lg Pz( )= K+10⋅lg −αz
[ ] (5)

dB
ωz()

where
P(z) is the backscattered power by a point z along an optical fibre;
K is a constant;
z is the distance at which the backscattered power is generated;
λ is the wavelength;
ω(z) is the fibre mode field diameter (MFD) at point z;
α is the attenuation in dB/km.
dB
___________
Further discussions on the same subject can be found in Annex C of IEC 60793-1-40 [1].
IEC TR 62316:2025 © IEC 2025
The constant in Formula (5) includes some numerical factors and the logarithm of the
parameter C:

K=5⋅ lg(CP)++lg( ) lg(τ ) (6)
iw

where
C is a proportionality factor defined in formula (4);
is the input OTDR pulse power into the fibre;
P
i
τ is the input OTDR pulse width expressed in seconds (s).
w
Formula (5), plotted on a logarithmic scale as a function of z, will appear as a straight line with
slope α (taking properly into account the factor 2).
Due to the SNR characteristics described in 5.3, the evaluation of the attenuation coefficient is
better undertaken with the best-fit straight line. Recall that a factor of 5 is used instead of 10 to
report measurement traces on OTDR equipment, as the light travels through the fibres and
events under test twice (round-trip).
6.4.3 Impurity and discontinuity
If an impurity, or any discontinuity, is present within the fibre (in the MFD region), the light can
suffer a Fresnel reflection (see 4.2) and will appear on the OTDR trace as a peak, the amplitude
of which depends on the size of the discontinuity (in some situations, the receiver can sat
...