IEC 61757-1-2:2023

IEC 61757-1-2
®

Edition 1.0 2023-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside


Fibre optic sensors –
Part 1-2: Strain measurement – Distributed sensing based on Brillouin scattering

Capteurs fibroniques –
Partie 1-2: Mesure de déformation – Détection répartie basée sur la diffusion de
Brillouin
IEC 61757-1-2:2023-08(en-fr)

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IEC 61757-1-2

®


Edition 1.0 2023-08




INTERNATIONAL



STANDARD




NORME


INTERNATIONALE
colour

inside










Fibre optic sensors –

Part 1-2: Strain measurement – Distributed sensing based on Brillouin scattering



Capteurs fibroniques –

Partie 1-2: Mesure de déformation – Détection répartie basée sur la diffusion de

Brillouin














INTERNATIONAL

ELECTROTECHNICAL

COMMISSION


COMMISSION

ELECTROTECHNIQUE


INTERNATIONALE





ICS 33.180.99  ISBN 978-2-8322-7396-8




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– 2 – IEC 61757-1-2:2023 © IEC 2023
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Terms, definitions, abbreviated terms and symbols . 6
3.1 Terms and definitions . 6
3.2 Abbreviated terms . 10
3.3 Symbols . 11
4 General test setups for measurement of performance parameters . 11
4.1 General and test setup requirements . 11
4.2 General documentation requirements . 16
5 Measurement procedures for performance parameters . 16
5.1 Strain measurement error . 16
5.1.1 Test procedure and conditions . 16
5.1.2 Parameter calculation and reporting . 17
5.2 Spatial resolution . 17
5.2.1 Test procedure and conditions . 17
5.2.2 Parameter calculation and reporting . 18
5.3 Strain repeatability . 18
5.3.1 Test procedure and conditions . 18
5.3.2 Parameter calculation and reporting . 18
5.4 Spatial strain uncertainty . 19
5.4.1 Test procedure and conditions . 19
5.4.2 Parameter calculation and reporting . 19
5.5 Warm-up time . 20
5.5.1 Test procedure and conditions . 20
5.5.2 Parameter calculation and reporting . 20
5.6 System performance with altered attenuation . 20
5.6.1 General . 20
5.6.2 At distance measurement range . 21
5.6.3 At short distance with high loss. 22
Bibliography . 24

Figure 1 – Optical fibre strain profile and related strain sample points. 8
Figure 2 – General test setup for single-ended configuration . 12
Figure 3 – General test setup for loop configuration . 13
Figure 4 – Measured versus applied strain (typical curve) . 15
Figure 5 – Brillouin frequency shift as a function of elongation of a standard
telecommunication fibre . 15
Figure 6 – Performance evaluation at distance measurement range . 21
Figure 7 – Performance evaluation at short distance with high loss . 22

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IEC 61757-1-2:2023 © IEC 2023 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________

FIBRE OPTIC SENSORS –

Part 1-2: Strain measurement –
Distributed sensing based on Brillouin scattering

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
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
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preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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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
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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 61757-1-2 has been prepared by subcommittee 86C: Fibre optic systems and active
devices, of IEC technical committee 86: Fibre optics. It is an International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
86C/1857/CDV 86C/1872/RVC

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.

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– 4 – IEC 61757-1-2:2023 © IEC 2023
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 61757 series, published under the general title Fibre optic sensors,
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
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.

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IEC 61757-1-2:2023 © IEC 2023 – 5 –
INTRODUCTION
This document is part of the IEC 61757 series, which is dedicated to fibre optic sensors. Generic
specifications for fibre optic sensors are defined in IEC 61757.
The individual parts of the IEC 61757 series are numbered as IEC 61757-M-T, where M denotes
the measure and T the technology of the fibre optic sensor. The IEC 61757-1-T series is
concerned with strain measurements.

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– 6 – IEC 61757-1-2:2023 © IEC 2023
FIBRE OPTIC SENSORS –

Part 1-2: Strain measurement –
Distributed sensing based on Brillouin scattering



1 Scope
This part of IEC 61757 defines detailed specifications for distributed strain measurements with
a fibre optic sensor, also known as "fibre optic distributed strain sensing". It is applicable to
distributed strain sensing systems (DSS) based on spontaneous or stimulated Brillouin
scattering in the optical fibre sensor (strain sensitive element), that is, to sensors capable of
measuring absolute strain.
This document specifies the most important DSS performance parameters and defines the
procedures for their determination.
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 61757:2018, Fibre optic sensors – Generic specification
IEC 61757-2-2:2016, Fibre optic sensors – Part 2-2: Temperature measurement – Distributed
sensing
IEC 61757-3-2:2022, Fibre optic sensors – Part 3-2: Acoustic sensing and vibration
measurement – Distributed sensing
ISO/IEC Guide 98-3, Uncertainty of measurement – Part 3: Guide to the expression of
uncertainty in measurement (GUM:1995)
3 Terms, definitions, abbreviated terms and symbols
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61757,
IEC 61757‑2‑2, IEC 61757-3-2, and the following apply.
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
NOTE For the following definitions, the relevant test procedures and parameters are defined in Clause 4.

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IEC 61757-1-2:2023 © IEC 2023 – 7 –
3.1.1
distributed fibre optic strain sensing system
DSS
measurement set-up consisting of a distributed fibre optic sensor connected to an interrogation
unit, including processor, data archive, and user interface, which provides a spatially resolved
strain measurement
[SOURCE: IEC 61757-3-2:2022, 3.1.2, modified − adapted to distributed strain measurement]
3.1.2
distance measurement range
maximum distance from the DSS interrogation unit output connector along the fibre optic sensor
within which the DSS measures strain with specified measurement performance under defined
conditions
Note 1 to entry: Defined conditions are spatial resolution (3.1.8), spatial strain uncertainty (3.1.9) and measurement
time (3.1.5).
Note 2 to entry: This supporting parameter is closely related to the total accumulated optical loss (one way)
tolerated by the interrogation unit without affecting specified measurement performance. In test cases used to prove
or verify the reported specifications, the total fibre length shall be equal to or greater than the specified distance
measurement range, for the tolerated total accumulated optical loss.
Note 3 to entry: The distance measurement range is usually expressed in km.
Note 4 to entry: For fibre loop configurations, the distance measurement range is given by half the fibre length
between the output and input connector of the interrogation unit.
[SOURCE: IEC 61757-2-2:2016, 3.2, and ISO/IEC Guide 99:2007, 4.7, modified − adapted to
distributed strain measurement]
3.1.3
strained spot
ΔL
length of fibre optic sensor that experiences a small elongation (δL), which causes strain that
is significantly bigger than the strain repeatability of the interrogation unit and which is
confirmed by a reference strain measurement
Note 1 to entry: The applied strain ε is equal to (δL/ΔL).
Note 2 to entry: It is useful to define strain in με, where 1 με corresponds to a δL of 1 μm over a ΔL of 1 m.
[SOURCE: IEC 61757-2-2:2016, 3.6, modified − adapted to distributed strain measurement]
3.1.4
location
L
optical distance from the DSS interrogation unit output connector to a desired strain sample
point along the fibre optic sensor
Note 1 to entry: The farthest location from the DSS interrogation unit output connector for the particular test is
quantified as L km and is often chosen to be the same as the distance measurement range for purposes of
F,long
comparing the measurement results with quoted specifications.
Note 2 to entry: The location is usually expressed in km.
[SOURCE: IEC 61757-2-2:2016, 3.7, modified − adapted to distributed strain measurement]

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– 8 – IEC 61757-1-2:2023 © IEC 2023
3.1.5
measurement time
time between independent strain measurements when making successive measurements on a
single fibre optic sensor
Note 1 to entry: Equivalently, it is the time interval between successive strain trace timestamps under these
conditions.
Note 2 to entry: This parameter includes acquisition time and processing time for the measured data. This
parameter is typically selectable by the user in some limited fashion. Multiple independent strain measurements may
be averaged together to provide an overall measurement time.
[SOURCE: IEC 61757-2-2:2016, 3.8, modified − adapted to distributed strain measurement]
3.1.6
point defect
local deviation of a fibre optic sensor from its nominal optical and mechanical properties
occurring at a single location, or over a length substantially less than the DSS spatial resolution
Note 1 to entry: The definition of a point defect encompasses a wide range of situations, which can produce similar
effects on the strain trace. Examples include
– a point loss, like a bad fibre splice,
– a back reflection (or return loss), as can be introduced by a fibre connector,
– a localized region of high loss, such as a bend or kink in the fibre, and
– a physical discontinuity in the fibre, like a splice between two fibres of different core diameters.
[SOURCE: IEC 61757-2-2:2016, 3.9, modified − adapted to distributed strain measurement]
3.1.7
sample spacing
distance between two consecutive strain sample points in a single strain trace
Note 1 to entry: Sample spacing can be a user-selectable parameter in the interrogation unit.
Note 2 to entry: The sample spacing is usually expressed in m.
Note 3 to entry: See Figure 1.

Figure 1 – Optical fibre strain profile and related strain sample points
[SOURCE: IEC 61757-2-2:2016, 3.11, modified − adapted to distributed strain measurement]

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IEC 61757-1-2:2023 © IEC 2023 – 9 –
3.1.8
spatial resolution
smallest length of strain-affected fibre optic sensor for which a DSS can measure and confirm
the reference strain of a defined strained spot within the specified strain measurement error of
the DSS
Note 1 to entry: The spatial resolution is usually expressed in m.
[SOURCE: IEC 61757-2-2:2016, 3.12, modified − adapted to distributed strain measurement]
3.1.9
spatial strain uncertainty
uncertainty of the location of strain data in a single strain trace, expressed by twice the standard
deviation of a specified number of adjacent strain sample points, with the fibre optic sensor held
at constant strain and temperature
Note 1 to entry: Due to a potential cross-sensitivity of DSS to temperature, it can be necessary to stabilize the
temperature of the fibre optic sensor.
Note 2 to entry: The spatial strain uncertainty is usually expressed in units of με and noted as a tolerance
(e.g. ±xx με), where 1 με corresponds to a δL of 1 μm over a ΔL of 1 m.
[SOURCE: IEC 61757-2-2:2016, 3.13, modified − adapted to distributed strain measurement]
3.1.10
strain dead zone
limited zone of a strain trace, where the strain sample points deviate from the undisturbed parts
of the trace by a specified limit due to a point defect
Note 1 to entry: The strain dead zone is usually expressed in m.
[SOURCE: IEC 61757-2-2:2016, 3.14, modified − adapted to distributed strain measurement]
3.1.11
strain measurement error
maximum difference between a centred and uniformly weighted moving average of the
measured strain and a reference strain for all data points of the fibre optic sensor over the full
operating temperature range and all acquisition times
Note 1 to entry: Single value (worst case) is expressed like a tolerance in units of με (e.g. ±xx με).
Note 2 to entry: The number of elements used for the moving average is defined later in the document. In practical
applications, other methods of smoothing might be applicable.
[SOURCE: IEC 61757-2-2:2016, 3.15, modified − adapted to distributed strain measurement]
3.1.12
strain repeatability
precision of strain data based on repeated strain traces at a given location expressed by twice
the standard deviation of corresponding strain sample points in each strain trace, with the fibre
optic sensor held at constant strain and temperature
Note 1 to entry: The strain repeatability is expressed like a tolerance in units of με (e.g. ±xx με).
[SOURCE: IEC 61757-2-2:2016, 3.16, modified − adapted to distributed strain measurement]

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– 10 – IEC 61757-1-2:2023 © IEC 2023
3.1.13
strain sample point
measured strain value associated with a single point at a known location along a fibre optic
sensor
Note 1 to entry: Due to signal averaging effects, the measured value represents the strain along a very small section
of the fibre optic sensor that includes the strain sample point.
Note 2 to entry: See Figure 1.
[SOURCE: IEC 61757-2-2:2016, 3.17, modified − adapted to distributed strain measurement]
3.1.14
strain trace
set of strain sample points distributed along a fibre optic sensor and spaced by the sample
spacing
Note 1 to entry: All sample points are associated with a common time of measurement, often called "trace
timestamp". The measured values represent the strain during a time period that includes the timestamp.
Note 2 to entry: All sample points in a strain trace are measured values produced by the DSS, and not interpolated
or smoothed values produced by subsequent processing outside the interrogation unit.
[SOURCE: IEC 61757-2-2:2016, 3.18, modified − adapted to distributed strain measurement]
3.1.15
total fibre length
L
F,tot
distance from the DSS interrogation unit output connector to the final end of the fibre optic
sensor
Note 1 to entry: The final end of the fibre optic sensor can be either a purposely cut or terminated end of the fibre,
physically located far from the interrogation unit (in a single-ended configuration), or the end of a loop consisting of
a connector that is connected to the same interrogation unit (in a loop configuration).
Note 2 to entry: This parameter is either equal to or greater than the distance measurement range and usually
expressed in km.
[SOURCE: IEC 61757-2-2:2016, 3.19, modified − adapted to distributed strain measurement]
3.1.16
warm-up time
duration between the instant after which the power supply of the DSS interrogation unit is
energized and the instant when the interrogation unit may be used as specified by the
manufacturer
Note 1 to entry: Warm-up time is usually expressed in seconds or minutes.
Note 2 to entry: The warm-up time helps to upload software and to stabilize operating temperatures of optical and
electronic components.
[SOURCE: IEC 60050-311:2001, 311-03-18, modified − adapted to distributed strain
measurement]
3.2 Abbreviated terms
DSS distributed fibre optic strain sensing system
FAT factory acceptance test
LVDT linear variable differential transformer
VOA variable optical attenuator

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IEC 61757-1-2:2023 © IEC 2023 – 11 –
3.3 Symbols
A cross-sectional area
E Young’s modulus
F force
L optical distance from the output connector to a desired strain sample point
L , L fibre lengths
F,opt F,short
L , L fibre lengths
F,loop F,long
L total fibre length
F,tot
ΔL length of fibre optic sensor to be strained (strained spot)
δL small change in length of ΔL
N, n number of traces, number of data points
S standard deviation
T temperature
T , T , T minimum, typical, and maximum DSS operating temperature
low op high
T ambient operating temperature of the strain test section
STC
ε strain
ε strain repeatability
rep
ε spatial strain uncertainty
unc
σ stress
4 General test setups for measurement of performance parameters
4.1 General and test setup requirements
General test setups for single-ended and loop configurations are schematically shown in
Figure 2 and Figure 3, respectively. The aim of these setups is to provide a common base for
determining the measurement specifications while at the same time minimizing complexity, cost,
re
...