IEC TR 62316:2026

IEC TR 62316 ®
Edition 4.0 2026-01
INTERNATIONAL
STANDARD
REDLINE VERSION
Guidance for the interpretation of OTDR backscattering traces for single-mode
fibres
ICS 33.180.10 ISBN 978-2-8327-0988-7
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IEC TR 62316:2025 RLV © 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 . 10
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 . 12
6.5 Bi-directional trace . 12
6.5.1 General . 12
6.5.2 Attenuation uniformity . 14
6.5.3 MFD uniformity . 16
6.6 Splice loss evaluation . 17
6.6.1 General . 17
6.6.2 Event measurement methods. 17
6.6.3 Apparent losers and gainers . 18
6.6.4 Example of apparent splice loss evaluation for uni-directional OTDR
measurements . 22
7 Uncertainties, deviation and resolution . 26
7.1 General . 26
7.2 Attenuation coefficient measurements . 26
7.3 Fault locations . 26
Bibliography . 29

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 . 17
Figure 3 – OTDR traces for similar or different fibre types with either different MFD
and/or different backscatter properties, or both . 19
IEC TR 62316:2025 RLV © IEC 2025
Figure 4 – Loss in unidirectional OTDR measurements as function of the MFD
difference between two spliced fibres . 20
Figure 5 – Theoretical power through splice loss due to MFD difference (with
ω Ω = 9 µm) . 21
1 1
Figure 6 – Apparent cumulative unidirectional backscattering mismatch distribution for
six splice combinations of B1.3B-652 and B6B-657 reported in Table 1 . 25
Figure 7 – Schematic drawing of a fibre with two consecutive defects 1 and 2 . 27

Table 1 – Summary for six fibre splice combinations of B1.3B-652 and B6
B-657 based on popular 1 310 nm MFD fibre distributions . 23

IEC TR 62316:2025 RLV © IEC 2025
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Guidance for the interpretation of OTDR backscattering traces for single-
mode fibres
FOREWORD
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IEC TR 62316:2025 RLV © IEC 2025
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.
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 RLV © 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 RLV © IEC 2025
1 Scope
This document provides guidelines guidance on the interpretation of backscattering traces, as
obtained by traditional optical time domain reflectometers (OTDRs) not including Polarization
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 60793-1-40:20012024 [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 RLV © 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 should is not be confused with the manufacturer’s specification, 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 back-reflected 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, to be 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
g p
and its fundamental attribute) by using formula (2):
dn
p
(2)
nn−λ
gp
dλ
=
IEC TR 62316:2025 RLV © 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;
–1
α is the attenuation in m . Multiply αdB by 0,00023 to obtain α, and α is the attenuation
dB
in dB/km dB per meter (assumed constant to simplify the formula);
ωΩ(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;
n is the group index of refraction.
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 RLV © 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 should be 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 should 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 should recommend can advise on the lengths. In addition, and
for practical reasons, these lengths should 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.
IEC TR 62316:2025 RLV © IEC 2025
6.3 Tail cord
The optical fibre within the receive or tail cord should be is usually of the same type, nominal
core diameter and nominal numerical aperture as the optical fibre within the cabling under test.
For practical reasons, the length of the tail cord should 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 may 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, the following should be understood and taken care of 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 should 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 should be 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)"):
(5)
 
λ
5lg[P(z)] = const. + 10lg  − α z

dB
 
ω(z)
 
λ
5⋅lg Pz( )= K+10⋅lg −αz
[ ] (5)
 dB
ωz()

___________
Further discussions on the same subject can be found in Annex C of IEC 60793-1-40:2001 [1].
IEC TR 62316:2025 RLV © IEC 2025
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
The constant in Formula (5) includes some numerical factors and the logarithm of the
parameter C:
(6)
const. = 5[lg(C) + lg(P) + lg(τ )]
i w
K=5⋅ lg CP++lg lgτ
( ) ( ) ( ) (6)
iw

where
C is a proportionality factor defined in formula (4);
P is the input OTDR pulse power into the fibre;
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 saturate).
It is possible to locate the position of the discontinuity by Formula (1). A peak can be detrimental
to link performance when its backscattered energy content is large enough to interfere with the
3)
source . The reflectance, as defined and used for the characterization of the connectors, can
be envisaged as the right parameter to evaluate the peak.
6.4.4 Pulse width
It is important to understand that the pulse width affects the returned energy and hence the
dynamic range. The wider the pulse, the higher the dynamic range (assuming signal averaging
4)
is constant). However, the wider the pulse, the wider the deadzone .
___________
Further discussions on the same subject can be found in Annex C of IEC 60793-1-40:2001 [1].
Further discussions on the same subject can be found in Annex C of IEC 60793-1-40:2001 [1].
IEC TR 62316:2025 RLV © IEC 2025
6.4.5 Polarization effects
The OTDR includes a splitter that can act as a polarizer on the output pulses and an analyser
on the receive side of the reflected pulses. Other elements can also polarize the light. As a
result of polarization mode dispersion (PMD) in the fibre, the Stokes vector of polarized light
rotates about the Poincaré's sphere as it propagates through the fibre in both the forward and
reflected directions. It also rotates with optical frequency or wavelength.
If the temporal OTDR pulse width and/or spectral width is sufficiently broad, the OTDR receiver
yields the average of many Stokes vectors and the effect of varying polarization states is not
seen. If these widths are reduced in comparison to the fibre PMD, or if the fibre PMD is low
enough compared to these widths, fewer Stokes vectors are averaged and the apparent pulse
magnitude can appear to vary with position along the fibre in a ripple pattern.
These apparent ripples can be reduced by rapidly varying the polarization of the source or by
using a polarization scrambler or other appropriate devices.
6.5 Bi-directional trace
6.5.1 General
Bi-directional traces are obtained by making measurements from each end of a fibre then
combining both traces by averaging the results. The OTDR shall be is physically moved to the
either end of the fibre to make the measurement. (See [6])
If using a launch and tail cord, commonly used when measuring a permanent link, only the
OTDR is moved during the measurement. The launch and tail cord do not move.
Bi-directional measurements are mostly used when there are connectors and different sections
of fibres in the link. The changes in backscatter coefficients between fibres are common and
necessitate bi-directional testing.
Starting from the expression of the backscattered optical trace (Formula (3)), it is
straightforward to calculate the bi-directional OTDR traces from unidirectional traces P and P
1 2
obtained from the two ends of the fibre, applying the coordinate's transformation z' = L – z on
P so that P (z) = P (L – z'), (where L is the total fibre length, and z and z' indicate the same
2 3 2
point taken on the two traces), thus obtaining:
(7)
 λ 
5lg[P (z)] = const. +10lg  −α z

1 dB
 
ω(z)
 

λ
5⋅lg Pz( )= K+10⋅lg −αz
[ ] (7)
1 1  dB
ωz()

where
(z) is the backscattered power by a point z along an optical fibre;
P
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
IEC TR 62316:2025 RLV © IEC 2025
(8)
 λ 
5lg[P (z')] = const. +10lg  −α z'
2 dB
 
ω(L − z')
 
λ
5⋅lg[P (z ')]= K+10⋅lg −αz⋅ '
(8)
2 2  dB
ωz'( ')

where
P (z') is the backscattered power by a point z along an optical fibre from the other fibre end;
K is a constant;
z' is the distance at which the backscattered power is generated;
L is the total fibre length;
λ is the wavelength;
ω'(z') is the fibre mode field diameter (MFD) at point z' from the other end of the fibre. As we
are considering a bidirectional trace, for the same fibre, we have ω'(z')= ω(L-z);
α is the attenuation in dB/km.
dB
 
λ
5lg[P (z)] = const. +10lg  −α (L − z)
3 dB
 
ω(z)
 
λ
5⋅lg P (z)= K+10⋅lg −α ⋅−(Lz)
[ ] (9)

3 3 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;
L is the total fibre length;
λ is the wavelength;
ω(z) is the fibre mode field diameter (MFD) at point z;
α is the attenuation in dB/km.
dB
Then, Formula (8) (7) and Formula (9) can be summed or subtracted side-by-side, and the
results, respectively S and D, are:
(10)
 
λ
S(z) = 2 × const. + 20lg  − α L

dB
 
ω(z)
 
λ
Sz( )=2⋅ K +⋅20 lg −αL⋅
(10)
s  dB
ωz()

IEC TR 62316:2025 RLV © IEC 2025
where
S(z) is the sum of formula (7) and (9);
K is a constant;
S
z is the distance at which the backscattered power is generated;
L is the total fibre length;
λ is the wavelength;
ω(z) is the fibre mode field diameter (MFD) at point z;
α is the attenuation in dB/km.
dB
Dz()=−2α zα+ L (11)
dB dB
where
D(z) is the difference of formula (7) and (9);
z is the distance at which the backscattered power is generated;
L is the total fibre length;
α is the attenuation in dB/km.
dB
Formula (10) and Formula (11) show that the bi-directional approach allows, in principle, the
separation of contributions due to variations of geometrical parameters of the single-mode fibre
(the MFD ωΩ) from contributions due to changes in the attenuation coefficients (α) of the fibre.
Further details for the computation of S and D can be found in 6.5.2 and 6.5.3.
6.5.2 Attenuation uniformity
6.5.2.1 General
The attenuation coefficient of optical fibre and cable is measured over the entire length. It can
vary locally within an overall length. When an overall length is cut into subsections, the
attenuation coefficient of each subsection can be larger or smaller than the attenuation
coefficient of the overall length from which it was cut.
The attenuation uniformity is the change of the attenuation coefficient within an overall length
of an optical fibre or cable.
For a cabled fibre, which is often cut to the length of use then measured, there is little value in
attenuation uniformity measurements. For an uncabled fibre, there is a number of strategies
engaged to produce the maximum volume of a low-attenuation cabled fibre. These strategies
depend on many factors including:
– mix of lengths and attenuation coefficient requirements on a finished cable;
– mix of lengths and attenuation coefficients on an uncabled fibre;
– inventory methods and available data;
– guard banding strategies that can be used to control additional attenuation induced by the
cabling process.
).
Attenuation uniformity is based on the bi-directional backscattering technique (see [7] and [8]
The bi-directional back-scattering trace can be represented as a function, y(z), with y being the
trace (in dB) and z being the position (in km). It is computed by reversing the position of each
___________
The text of this paragraph is an extract from the more detailed IEC TS 62033.
IEC TR 62316:2025 RLV © IEC 2025
location of one of the uni-directional traces and computing the difference between the two
uni-directional traces, divided by two, for each position. The bi-directional trace can be derived
from multiple measurements or from appropriately filtered data having the same effect.
6.5.2.2 Sliding window
The uniformity parameter, X , is defined in terms of the sliding window (SW) algorithm, in which
A
the attenuation coefficient is evaluated across a fixed sub-length, S (sliding window width in
L
km) of the fibre, ideally sliding along the fibre starting from each of a set of positions: z , z , etc.
1 2
The attenuation coefficient values at those positions can be represented as:
(12)
y(z ) − y(z + SL)
i i
A(z ;SL) =
i
SL
yz( ) −+yz( S )
i iL
A(;z S ) =
(12)
iL
S
L
where
A(z ; S ) is the uniformity parameter calculated between the distance z and z + S (in dB/km);

i L i i L
z is the distance at which the backscattered power is generated;
i
y(z ) is the value of the bidirectional backscattering trace evaluated at distance z ; (in dB)
i i
y(z +S ) is the value of the bidirectional backscattering trace evaluated at distance z + S
i L i L
(in dB);
S is the sliding window width in km.
L
Alternatively, the fitted slope of the trace at the defined positions can be substituted for the
values of A(z ; S ). The uniformity parameter is the difference between the maximum of the
i L
A(z , S ) values and the average attenuation coefficient of the whole fibre, given by its
i L
end-to-end attenuation coefficient, α, as determined by any method in IEC 60793-1-40 [1]:
X max[A(z ;SL)]−α
(13)
Ai
where
X is the uniformity parameter (in dB/km);
A
S is the sliding window width in km.
L
max[A(z ;S )] is the maximum value of the uniformity parameter calculated over all the points
i L
z of the fibre length, using a sliding window S ;
i L
α is the end-to-end attenuation coefficient of the fibre, in dB/km.
NOTE the attenuation uniformity depends on the length of the sliding window. It is important for the sliding window
length to be reported in each measurement.
=
IEC TR 62316:2025 RLV © IEC 2025
6.5.2.3 Generalised sliding window
The generalised sliding window (GSW) algorithm will provide coefficients, α , and ε such that
r r
:
for a defined range of SL S
L
(14)
ε
r
Max{A(z ,SL)} = α +
i r
SL
ε
r
max{A(z ,S )} α+
(14)
iL r
S
L
where
max{A(z ; S )} is the maximum value of the uniformity parameter calculated over all the
i L
points z of the fibre length, using a sliding window S (in dB/km);
i L
S is the sliding window width (in km);
L
α is a baseline attenuation coefficient (in dB/km);
r
is a loss penalty parameter that allows scaling non-uniformity with variable
ε
r
SW lengths
The GSW parameters may can be used to compute the sliding window maximum in Formula (14)
for a variety of sub-lengths, SL.
6.5.3 MFD uniformity
Combining the two traces P and P in the half-sum S(z)
1 3
(15)
10lg[P (z)]+ 10lg[P (z)]
1 3
S(z) =
10 ⋅lg Pz( ) +10 ⋅lg P (z)
[ ] [ ]
(15)
Sz() =
the MFD as a function of the position, ωΩ(z), can be easily calculated, inserting a reference
fibre before the fibre under test with a known MFD, ωΩ(z ), at the position z , and neglecting
0 0
longitudinal variations of α:
S'( z,λ)
−
ω(z )
(16)
S'(z) = S(z) − S(z ) = 20lg  ω(z,λ) = ω(z ,λ)10
0 0
 
ω(z)
 
Sz',( λ)
−
ωz( )

20 (16)
S '(z)=Sz( )−=Sz( ) 20⋅lg
→ ωz,λ =ωz ,λ 10
( ) ( )
 0
ωz()

=
IEC TR 62316:2025 RLV © IEC 2025
6.6 Splice loss evaluation
6.6.1 General
It is difficult to evaluate a system loss budget by measuring splice losses, since there exist
several different approaches and some of them can sometimes lead to misunderstandings.
One traditional method for measuring link loss is by an end-to-end light source power meter
(LSPM) measurement, but note that this method does not provide fault location. For that type
of problem or troubleshooting, the OTDR plays an important role in evaluation of systems.
The basic backscattering principle makes the OTDR very sensitive to many parameters of
optical fibre which can influence the light coupling properties (see Formula (3)). Different fibres
will intrinsically capture more or less backscattered light resulting in varying signal levels back
to the OTDR.
"Different" here means different in terms of either geometrical and/or transmission properties,
or both, and does not refer to the manufacturing method. This means that the same fibre type
can be manufactured by different technologies and are easy to splice to each other, provided
the geometry is well controlled.
6.6.2 Event measurement methods
The unidirectional splice loss is usually provided by the OTDR equipment itself thanks to some
signal processing and events detection algorithms. In Figure 2, an idealized OTDR trace is
illustrated at the vicinity of a splice joint made between two fibres A and B. The splice location
is at point O.
Figure 2 – Idealized unidirectional OTDR traces corresponding to a non-reflective splice
between two fibres
Two methods can be employed to analyse the traces and estimate the event loss: least-square
method or two point methods.
– When using the least-square approximation (LSA) method, the apparent splice loss α
LSA
which is reported on the OTDR equipment correspond to the vertical separation between
the linear attenuation curve fittings for Fibre A and Fibre B at the splice location. On the
figure, this correspond to height OS.
– The two-point method consists in measuring the vertical height of the event between two
points located just before and just after the event. Doing so, the two-point method is usually
known to be less accurate and more dependent on the accuracy of the positioning of the
points and the OTDR settings (as the OTDR curve could exhibits more or less noise and
IEC TR 62316:2025 RLV © IEC 2025
therefore influencing the result). In Figure 2 above, the splice loss α as per the two-point
2Pt
method will report a splice loss corresponding to the height of segment OE.
In that case, it is obvious that OS ≤ OE so that:
αα≤
(17)
LSA 2Pt
where
α is the splice loss measured while using the LSA method;
LSA
α is the splice loss measured while using the two-point method.
2Pt
While the least-square method is almost independent on the OTDR pulse width, so that α
LSA
would be of the same magnitude whatever the OTDR settings, the two-point method is on the
contrary highly dependent on the OTDR settings (and position of the markers) and the
attenuations of the second fibre. The longer the OTDR pulse width, the higher α .
2pt
Assuming short pulses (< 1 µs), it is possible to relate α with α and τ

2Pt LSA w
αα− ~α ×τ
(18)
2Pt LSA fibreB w
where
~ means "is proportional to"
α is the splice loss measured while using the LSA method;
LSA
α is the splice loss measured while using the two-point method
2Pt
is the attenuation of the fibre B
α
fibreB
τ is the input OTDR pulse width.
w
Decreasing the OTDR pulse width will reduce the difference between the bias for the two
methods. Nevertheless, to avoid possible misinterpretation in the results, it is highly
recommended to use the LSA method will actually be used when evaluating unidirectional splice
loss.
In the rest of this document, the apparent loss will be referred to as being the splice loss
reported as per the LSA method at the splice position.
6.6.3 Apparent losers and gainers
When two fibres with different backscatter properties, such as with different MFD values, are
joined and measured with an OTDR, either an "apparent loss" or "apparent gain" appears at
the interface as shown in Figure 3. This is a result of the backscatter coupled power detected
by the OTDR. In the case of similar fibre types being spliced together, this change in coupled
power can be a function of the mode field diameters of the joined fibres. The apparent splice
loss for a gainer is negative and positive for a loser.
IEC TR 62316:2025 RLV © IEC 2025

Figure 3 – OTDR traces for similar or different
...