IEC TR 62316:2017
(Main)Guidance for the interpretation of OTDR backscattering traces for single-mode fibres
Guidance for the interpretation of OTDR backscattering traces for single-mode fibres
IEC/TR 62316:2017 which is a Technical Report, aims to provide guidelines for the interpretation of backscattering traces, as obtained by traditional optical time domain reflectometers (OTDRs) – not including polarization OTDRs – for single-mode fibres. 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 60793-1-40:2001. This third edition cancels and replaces the second edition published in 2007. It constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:
a. the scope has been extended to include single-mode fibres;
b. backscattered power effects are discussed in case of unidirectional trace, including so-called losers and gainers.
c. example of apparent splice loss evaluation for unidirectional OTDR measurements has been added:
d. description of launch and tail cords have been added;
e. figures have been improved.
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IEC TR 62316 ®
Edition 3.0 2017-07
TECHNICAL
REPORT
colour
inside
Guidance for the interpretation of OTDR backscattering traces
for single-mode fibres
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IEC TR 62316 ®
Edition 3.0 2017-07
TECHNICAL
REPORT
colour
inside
Guidance for the interpretation of OTDR backscattering traces
for single-mode fibres
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 33.180.10 ISBN 978-2-8322-4553-8
– 2 – IEC TR 62316 © IEC 2017
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Backscattering phenomenon . 6
4.1 Rayleigh scattering . 6
4.2 Fresnel reflections and dead zone fibres . 6
5 Measurement of the backscattered power (OTDR) . 7
5.1 General . 7
5.2 Representation of the backscattered power. 7
5.3 Noise and perturbations . 8
6 Interpretation of a backscattering trace . 8
6.1 General . 8
6.2 Launch cord . 9
6.3 Tail cord . 9
6.4 Unidirectional trace . 9
6.4.1 General . 9
6.4.2 Slope as the attenuation coefficient of a fibre . 10
6.4.3 Impurity and discontinuity . 10
6.4.4 Pulse width . 10
6.4.5 Polarization effects . 10
6.5 Bi-directional trace . 11
6.5.1 General . 11
6.5.2 Attenuation uniformity . 11
6.5.3 MFD uniformity . 12
6.6 Splice loss evaluation . 12
6.6.1 General . 12
6.6.2 Event measurement methods . 13
6.6.3 Apparent losers and gainers . 14
6.6.4 Example of apparent splice loss evaluation for uni-directional OTDR
measurements . 17
7 Uncertainties, deviation and resolution . 18
7.1 General . 18
7.2 Attenuation coefficient measurements . 18
7.3 Fault locations . 19
Bibliography . 21
Figure 1 – Unidirectional OTDR trace showing splice and/or macro bend loss. 9
Figure 2 – Idealized unidirectional OTDR traces corresponding to a non-reflective
splice between two fibres . 13
Figure 3 – OTDR traces for similar or different fibre types with different MFD and/or
different backscatter properties . 14
Figure 4 – Loss in unidirectional OTDR measurements as function of the MFD
difference between two spliced fibres. 15
IEC TR 62316 © IEC 2017 – 3 –
Figure 5 – Theoretical power through splice loss due to MFD difference (with ω =
9µm) . 16
a) Mean spice loss measured from B6 to B1.3 fibre . 17
b) Mean spice loss measured from B1.3 to B6 fibre . 18
Figure 6 – Apparent cumulative unidirectional backscattering mismatch distribution for
six splice combinations of B1.3 and B6 reported in Table 1 . 18
Figure 7 – Schematic drawing of a fibre with two consecutive defects 1 and 2 . 19
Table 1 – Summary for six fibre splice combinations of B1.3 and B6 based on popular
1 310 nm MFD fibre distributions . 17
– 4 – IEC TR 62316 © IEC 2017
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
GUIDANCE FOR THE INTERPRETATION OF OTDR BACKSCATTERING
TRACES FOR SINGLE-MODE FIBRES
FOREWORD
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The main task of IEC technical committees is to prepare International Standards. However, a
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data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC TR 62316, which is a Technical Report, has been prepared by subcommittee 86A: Fibres
and cables, of IEC technical committee 86: Fibre optics.
This third edition cancels and replaces the second edition published in 2007. It constitutes a
technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) the scope has been extended to include single-mode fibres;
a) backscattered power effects are discussed in case of unidirectional trace, including
so-called losers and gainers.
b) example of apparent splice loss evaluation for unidirectional OTDR measurements has
been added:
IEC TR 62316 © IEC 2017 – 5 –
c) description of launch and tail cords have been added;
d) figures have been improved.
The text of this Technical Report is based on the following documents:
Enquiry draft Report on voting
86A/1754/DTR 86A/1768A/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 6 – IEC TR 62316 © IEC 2017
GUIDANCE FOR THE INTERPRETATION OF OTDR BACKSCATTERING
TRACES FOR SINGLE-MODE FIBRES
1 Scope
IEC 62316, which is a Technical Report, aims to provide guidelines for the interpretation of
backscattering traces, as obtained by traditional optical time domain reflectometers (OTDRs)
– not including polarization OTDRs – for single-mode fibres. 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 60793-1-40:2001.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
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.
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 should not be confused with the manufacturer’s
specification, 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.
IEC TR 62316 © IEC 2017 – 7 –
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 power back-reflected from the fibre at the same end. In OTDR traces, space
and time are completely equivalent through the relation:
(1)
z c
=
t n (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, to be supplied by the fibre manufacturer, 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
g
is related to the phase index n or n (which is measured on a fibre and its fundamental
p
attribute) by using the following expression:
dn
p
n = n − λ (2)
g p
dλ
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
(3)
λ
P(z) = C Pτ 10
i w
(ω(z))
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 at which the backscattered power is generated;
–1
α is the attenuation in m . Multiply α by 0,00023 to obtain α, and α is the
dB dB
attenuation in dB/km (assumed constant to simplify the equation);
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 (4)
s
C =
2 2
16π n n
eff g
where
c is the speed of light in vacuum;
–1
α is the Rayleigh scattering coefficient in m ;
s
– 8 – IEC TR 62316 © IEC 2017
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;
n is the group index of refraction.
g
Equation (3) shows the relation between the backscattered power, the pulse width, the
attenuation coefficient and the MFD. The optical reflected power, as given by Equation (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 below.
Note that Equation (3) is valid for short pulse width, i.e. τ α << 1, which applies in most
w
practical cases.
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 Equation (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.
Any event, such as a splice, connector, macro-bend, micro-bend, can be detected by the
OTDR and appear as a perturbation. Micro-bends 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.
LC C S TC
OTDR
L
F (dB)
L (m)
IEC
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 macro bend or splice loss
A
IEC TR 62316 © IEC 2017 – 9 –
TC tail cord S macro bend or splice
Figure 1 – Unidirectional OTDR trace showing splice and/or macro bend loss
6.2 Launch cord
The optical fibre within the launch cord at the connection to the cabling under test should be
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.
The length of the launch cord should be longer than the dead zone created by the pulse width
selected for a particular length of fibre to be measured. Suppliers of OTDR equipment should
recommend lengths. In addition, these lengths should 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 should be of the same type, nominal core
diameter and nominal numerical aperture as the optical fibre within the cabling under test.
The length of the tail cord should be longer than the dead zone created by the pulse width
selected for a particular length of fibre to 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 and
IEC 61280-4-2). 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 may 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, the following should be understood and taken care of.
– 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 should 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 should be 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.
___________
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