IEC TR 62316:2007

TECHNICAL IEC
REPORT TR 62316
Second edition
2007-01
Guidance for the interpretation
of OTDR backscattering traces
Reference number
IEC/TR 62316:2007(E)
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TECHNICAL IEC
REPORT TR 62316
Second edition
2007-01
Guidance for the interpretation
of OTDR backscattering traces
© IEC 2007 ⎯ Copyright - all rights reserved
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– 2 – TR 62316 © IEC:2007(E)
CONTENTS
FOREWORD.3

1 Scope.5

2 Normative references .5

3 Backscattering phenomenon.5

3.1 Rayleigh scattering.5

3.2 Fresnel reflections and dead zone fibres .5

4 Measurement of the backscattered power (OTDR).6
4.1 Representation of the backscattered power .6
4.2 Noise and perturbations .7
5 Interpretation of a backscattering trace.7
5.1 Unidirectional trace .7
5.2 Bi-directional trace .9
6 Uncertainties, deviation and resolution .12
6.1 Attenuation coefficient measurements .12
6.2 Fault locations.12

Bibliography.14

Figure 1 – Unidirectional OTDR trace .7
Figure 2 – OTDR traces for fibres with different MFD.11
Figure 3 – Schematic drawing of a fibre with two consecutive defects 1 and 2 .13

TR 62316 © IEC:2007(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
GUIDANCE FOR THE INTERPRETATION OF

OTDR BACKSCATTERING TRACES
FOREWORD
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example "state of the art".
IEC 62316, which is a technical report, has been prepared by subcommittee 86A: Fibres and
Cables, of IEC technical committee 86: Fibre optics.
This second edition cancels and replaces the first edition published in 2003. It constitutes a
technical revision. In this edition,
– polarization effects are discussed in case of unidirectional trace;
– a new clause dealing with uncertainties, deviation and resolution has been introduced.

– 4 – TR 62316 © IEC:2007(E)
The text of this technical report is based on the following documents:

Enquiry draft Report on voting

86A/1090/DTR 86A/1114/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 publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

The committee has decided that the contents of this publication will remain unchanged until

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the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

TR 62316 © IEC:2007(E) – 5 –
GUIDANCE FOR THE INTERPRETATION OF

OTDR BACKSCATTERING TRACES
1 Scope
This Technical Report provides guidelines for the interpretation of backscattering traces, as
obtained by an optical time domain reflectometer (OTDR).

A full description of the test measurement procedure can be found in Annex C of
IEC 60793-1-40.
2 Normative References
The following referenced documents are indispensable for the application 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: Measurement methods and test procedures –
Attenuation
3 Backscattering phenomenon
3.1 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.
3.2 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 some situations, the detected Fresnel reflection is very
intense, which can bring the receiver to saturation (flat response vs. time). The corresponding

section of the OTDR trace defines the so-called dead zone, i.e. a length of the fibre where the
backscattered signal is lower than the reflector’s saturated signal, hence not detectable and
visible on the OTDR trace. The effect of the dead-zone connected to the Fresnel reflection at
the input end of the fibre is often reduced by adopting a short length of a fibre positioned
between the OTDR output and the fibre under test. This dead-zone or buffer fibre should
preferably be at least one tenth (1/10, in m) of the OTDR pulse-width (in ns), e.g. for a 1
000 ns pulse the dead zone fibre should be ≥100 m long.

– 6 – TR 62316 © IEC:2007(E)
4 Measurement of the backscattered power (OTDR)

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 (the optical echo) at the same end. In

OTDR traces, space and time are completely equivalent through the relation

z c
= (1)
t n (λ)
g
where z is the distance (in m), t is the time (in s), c is the speed of light in vacuum
(299 792 458 m/s) and n (as a function of the wavelength) is the group index of refraction.
g
This parameter, 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 is related to
g
the phase index n or n (which is measured on a fibre and its fundamental attribute) by using
p
the following expression
dn
p
n = n – λ (2)
g p
dλ
4.1 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
P(z) = C Pτ 10 (3)
i w
()ω(z)
where
P is the input OTDR pulse power;
i
τ is the input OTDR pulse width;
w
z is the distance from the origin;
α is the attenuation coefficient of the fibre (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.
Equation 3 shows the relation between the backscattered power, the pulse-width, the
attenuation coefficient and the MFD. The optical echo, as given by Equation 3, is
conventionally represented on a logarithmic graph: it therefore appears as a (theoretically)
straig
...