IEC 61757-3-2:2022

Fibre optic sensors - Part 3-2: Acoustic sensing and vibration measurement - Distributed sensing

IEC 61757-3-2:2022 specifies the terminology, characteristic performance parameters, related test and calculation methods, as well as specific test equipment for interrogation units used in distributed fibre optic acoustic sensing and vibration measurement systems. This document refers to the Rayleigh backscatter and phase detection method by phase-sensitive coherent optical time-domain reflectometry (ϕ-OTDR) only. Quasi-static and low frequency operation modes are not covered by this document. Generic specifications for fibre optic sensors are defined in IEC 61757.

Capteurs fibroniques – Partie 3-2: Détection acoustique et mesure des vibrations - Détections réparties

IEC 61757-3-2:2022 spécifie la terminologie, les paramètres de performance caractéristiques, les méthodes d’essai et de calcul associées, ainsi que le matériel d’essai spécifique pour les unités d’interrogation utilisées dans les systèmes fibroniques répartis de mesure de vibrations et de détection acoustique. Le présent document se réfère uniquement à la méthode de rétrodiffusion de Rayleigh et de détection de phase par réflectométrie optique cohérente dans le domaine temporel sensible à la phase (ϕ-OTDR). Les modes de fonctionnement quasi statiques et à basses fréquences ne sont pas traités dans le présent document. Les spécifications génériques applicables aux capteurs fibroniques sont définies dans l’IEC 61757.

General Information

• Status: Published
• Publication Date: 28-Mar-2022
• ICS: 33.180.99 - Other fibre optic equipment
• Technical Committee: SC 86C - Fibre optic systems, sensing and active devices
• Drafting Committee: WG 2 - TC 86/SC 86C/WG 2
• Current Stage: PPUB - Publication issued
• Start Date: 29-Mar-2022
• Completion Date: 26-Apr-2022

Overview

IEC 61757-3-2:2022 is an international standard issued by the International Electrotechnical Commission (IEC) that addresses fibre optic sensors specifically designed for acoustic sensing and vibration measurement using distributed sensing techniques. This part of the IEC 61757 series defines essential terminology, performance parameters, test methods, and specific test equipment for interrogation units used in distributed fibre optic acoustic sensing systems. The standard focuses on the Rayleigh backscatter and phase detection method via phase-sensitive coherent optical time-domain reflectometry (ϕ-OTDR) and excludes quasi-static and low-frequency operation modes.

This standard provides a comprehensive framework for manufacturers, researchers, and end-users to evaluate and verify the performance and reliability of distributed fibre optic acoustic and vibration sensing systems, which are critical for applications requiring high sensitivity and spatial resolution over long distances.

Key Topics

• Terminology and Definitions
  The standard establishes a common language for describing distributed fibre optic acoustic sensing technologies, including key concepts such as interrogation units, phase detection methods, and measurement parameters.

• Performance Parameters
  IEC 61757-3-2 defines crucial performance metrics, such as:

  • Dynamic range
  • Frequency response
  • Fidelity
  • Self-noise
  • Spatial resolution
  • Crosstalk
  • Loss budget
  • Sensor reflection robustness

• Test Apparatus and Methods
  The document outlines apparatus setups such as simulated fibre sensors, fibre stretchers, optical attenuators, and isolation chambers. It provides detailed test procedures to measure each performance parameter objectively and reproducibly. These standardized tests enable consistency in evaluation and facilitate quality assurance for fibre optic sensor interrogators.

• Focus on ϕ-OTDR Technique
  The standard narrowly addresses phase-sensitive coherent optical time-domain reflectometry, a core technology for distributed acoustic and vibration sensing using Rayleigh backscatter, ensuring precise phase measurement and enhanced acoustic event detection over optical fibres.

Applications

The scope and specifications of IEC 61757-3-2 make it highly relevant for industries and sectors where distributed acoustic sensing and vibration monitoring are increasingly vital:

• Infrastructure Monitoring
  Monitoring vibrations and acoustic signals in bridges, pipelines, railways, and large civil engineering structures for safety and maintenance planning.

• Oil and Gas Industry
  Leak detection, flow monitoring, and integrity assessment of pipelines using distributed fibre optic sensors to detect acoustic signals generated by anomalous events.

• Security and Surveillance
  Perimeter security systems that monitor unauthorized intrusions or unusual vibrations along fences or borders via distributed fibre optic acoustic sensing.

• Seismic and Geophysical Studies
  Distributed fibre optic sensors provide high-resolution vibration measurements critical for earthquake monitoring, geological surveys, and structural health analysis of critical facilities.

• Industrial Equipment Monitoring
  Vibration analysis and early fault detection in machinery and rotating equipment across manufacturing plants and power generation facilities.

Related Standards

• IEC 61757 Series
  IEC 61757-3-2 is part of the broader IEC 61757 series on fibre optic sensors, which covers generic specifications and various sensing modalities.

• Optical Fibre Communication Standards
  Related IEC and ITU-T standards govern optical fibre characteristics and measurement techniques relevant for distributed sensing technologies.

• Instrumentation and Measurement Standards
  Standards detailing test and calibration procedures for acoustic sensors and vibration measurement instrumentation complement the IEC 61757-3-2 framework.

---

Keywords: IEC 61757-3-2, fibre optic sensors, acoustic sensing, vibration measurement, distributed sensing, ϕ-OTDR, Rayleigh backscatter, phase-sensitive interferometry, dynamic range, frequency response, spatial resolution, sensor testing, fibre optic interrogation unit, distributed acoustic sensing system, vibration monitoring, infrastructure monitoring, pipeline leak detection, seismic sensors, industrial vibration analysis.

Standards Content (Sample)

IEC 61757-3-2 ®
Edition 1.0 2022-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Fibre optic sensors –
Part 3-2: Acoustic sensing and vibration measurement – Distributed sensing

Capteurs fibroniques –
Partie 3-2: Détection acoustique et mesure des vibrations – Détections réparties

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IEC 61757-3-2 ®
Edition 1.0 2022-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Fibre optic sensors –
Part 3-2: Acoustic sensing and vibration measurement – Distributed sensing

Capteurs fibroniques –
Partie 3-2: Détection acoustique et mesure des vibrations – Détections réparties

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 33.180.99 ISBN 978-2-8322-1093-1

– 2 – IEC 61757-3-2:2022 © IEC 2022
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms, definitions, abbreviated terms and symbols . 8
3.1 Terms and definitions . 8
3.2 Abbreviated terms . 11
3.3 Symbols . 12
4 Performance parameters of a distributed acoustic sensing system . 13
5 Test apparatuses for performance parameter determination . 13
5.1 Simulated fibre sensor (SFS) . 13
5.2 Fibre stretcher . 14
5.3 Signal generation and amplification instrumentation . 15
5.4 Optical attenuator . 15
5.5 Isolation chamber . 15
6 Test procedures of performance parameters . 16
6.1 General . 16
6.2 Dynamic range . 16
6.2.1 General . 16
6.2.2 Set-up . 16
6.2.3 Stimulus . 16
6.2.4 Data collection and processing . 17
6.2.5 Data reporting. 19
6.3 Frequency response . 19
6.3.1 General . 19
6.3.2 Set-up . 19
6.3.3 Stimulus . 20
6.3.4 Data collection and processing . 20
6.3.5 Data reporting. 21
6.4 Fidelity . 23
6.4.1 General . 23
6.4.2 Set-up . 23
6.4.3 Stimulus . 23
6.4.4 Data collection and processing . 24
6.4.5 Data reporting. 24
6.5 Self-noise . 24
6.5.1 General . 24
6.5.2 Set-up . 24
6.5.3 Stimulus . 25
6.5.4 Data collection and processing . 25
6.5.5 Data reporting. 28
6.6 Spatial resolution . 28
6.6.1 General . 28
6.6.2 Set-up . 29
6.6.3 Stimulus . 29
6.6.4 Data collection and processing . 29

6.6.5 Data reporting. 30
6.7 Crosstalk . 32
6.7.1 General . 32
6.7.2 Set-up . 32
6.7.3 Stimulus . 32
6.7.4 Data collection and processing . 32
6.7.5 Data reporting. 33
6.8 Loss budget . 33
6.8.1 General . 33
6.8.2 Set-up . 34
6.8.3 Stimulus . 34
6.8.4 Data collection and processing . 34
6.8.5 Data reporting. 34
6.9 Sensor reflection robustness . 35
6.9.1 General . 35
6.9.2 Set-up . 35
6.9.3 Stimulus . 37
6.9.4 Data collection, processing, and reporting . 38
Annex A (informative) Conversion of optical phase measurement to strain . 39
Annex B (normative) Requirements for low uncertainty measurement . 42
B.1 Single tone stimulus testing . 42
B.2 Frequency response testing . 43
Annex C (informative) FFT window functions . 45
C.1 Flat top window used for frequency domain measurements of spectral peaks . 45
C.2 Window functions used for frequency domain noise measurements . 46
Bibliography . 49

Figure 1 – Distributed acoustic sensing system . 9
Figure 2 – Signal parameters relating to time series and their spatial point identification . 12
Figure 3 – Simulated fibre sensor . 14
Figure 4 – Configuration for SFS showing the locations TP , TP , and TP . 14
1 2 3
Figure 5 – Test set-up for dynamic range test . 16
Figure 6 – Example of a strain stimulus signal and recovered phase of IU response

with a limit at 17 s . 18
Figure 7 – Example of a zoom view of strain stimulus signal and recovered phase of IU
response showing a phase jump at 16,98 s . 19
Figure 8 – Test set-up for frequency response test . 20
Figure 9 – Magnitude response showing the 40 stimulus signals all with magnitude . 21
Figure 10 – Interrogator response to test stimulus, scaled in strain units, shown in the
frequency domain . 22
Figure 11 – Interrogator normalized frequency response . 23
Figure 12 – Test set-up for fidelity test . 23
Figure 13 – Test set-up for self-noise . 25
Figure 14 – 2D data field representing the time varying acoustic field as a function of
distance . 26
Figure 15 – System noise floor data processing schematic . 27
Figure 16 – Example plot of self-noise data . 28

– 4 – IEC 61757-3-2:2022 © IEC 2022
Figure 17 – Test set-up for spatial resolution test. 29
Figure 18 – Spatial sample points to be used for spatial resolution evaluation . 30
Figure 19 – Graphical plotting approach used to determine spatial resolution . 31
Figure 20 – Test set-up for crosstalk measurement . 32
Figure 21 – Highlighted points to be sampled for crosstalk test . 33
Figure 22 – Example plot for crosstalk test results . 33
Figure 23 – Test set-up for loss budget test . 34
Figure 24 – Test configurations for sensor reflection robustness . 36
Figure 25 – Fabrication examples for creating partial reflections . 37
Figure B.1 – Fibre stretcher spatial sample points . 42
Figure B.2 – Frequency domain plot of single tone stimulus . 43
Figure B.3 – Frequency response plot of single tone stimulus . 44
Figure C.1 – Flat-top window and its Fourier transform characteristics [4] . 45
Figure C.2 – Blackman-Harris window and its Fourier transform characteristics [5] . 46
Figure C.3 – Hamming window and its Fourier transform characteristics [6] . 47

Table A.1 – Optical phase and strain relationships . 41

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIBRE OPTIC SENSORS –
Part 3-2: Acoustic sensing and vibration measurement –
Distributed sensing
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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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) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC 61757-3-2 has been prepared by subcommittee SC 86C: Fibre optic systems and active
devices, of IEC technical committee TC 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/1700/CDV 86C/1719/RVD
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.

– 6 – IEC 61757-3-2:2022 © IEC 2022
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/standardsdev/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,
• replaced by a revised edition, or
• amended.
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.

INTRODUCTION
This document is based on SEAFOM Measuring Sensor Performance Document – 02 (SEAFOM
MSP-02) [1] . Within the framework of a type C liaison, SEAFOM proposed this document as a
new work item, which was approved by the participating members of IEC SC 86C.
NOTE Except for Figure 1, Figure C.1, Figure C.2, and Figure C.3, all figures in this document were adopted from
SEAFOM MSP-02 either in original or in modified form with permission from SEAFOM.
The IEC 61757 series is published with the following logic: the sub-parts are numbered as
IEC 61757-M-T, where M denotes the measure and T, the technology.

___________
Numbers in square brackets refer to the Bibliography.

– 8 – IEC 61757-3-2:2022 © IEC 2022
FIBRE OPTIC SENSORS –
Part 3-2: Acoustic sensing and vibration measurement –
Distributed sensing
1 Scope
This part of IEC 61757 specifies the terminology, characteristic performance parameters,
related test and calculation methods, as well as specific test equipment for interrogation units
used in distributed fibre optic acoustic sensing and vibration measurement systems. This
document refers to the Rayleigh backscatter and phase detection method by phase-sensitive
coherent optical time-domain reflectometry (ϕ-OTDR) only. Quasi-static and low frequency
operation modes are not covered by this document.
Generic specifications for fibre optic sensors are defined in IEC 61757.
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
3 Terms, definitions, abbreviated terms and symbols
3.1 Terms and definitions
For the purposes of this document, terms and definitions given in IEC 61757, IEC 61757-2-
2:2016 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• ISO Online browsing platform: available at https://www.iso.org/obp
• IEC Electropedia: available at http://www.electropedia.org/
3.1.1
distributed fibre optic sensor
fibre optic sensor that provides a spatially resolved measurement of a measurand over an
extended region by means of a continuous sensing element
[SOURCE: IEC 61757:2018, 3.5]
3.1.2
distributed fibre optic acoustic sensing system
DAS
measurement set-up consisting of a distributed fibre optic sensor connected to an interrogation
unit including processor, data archive, and user interface, which can locally detect acoustic or
vibration induced disturbances (phase change of the backscattering centres) in the fibre
Note 1 to entry: The alternative term fibre optic distributed vibration sensing (DVS) is also used in the industry.
Note 2 to entry: See Figure 1 for a principal DAS set-up. Pulses from a coherent source are sent into the sensor
fibre through an optical circulator, which also taps the coherent Rayleigh backscattering signal to a detector for
subsequent digitization and fast real-time acquisition.
Note 3 to entry: Typically, a DAS detects acoustic or vibration induced disturbances at frequencies below 2 kHz.

Figure 1 – Distributed acoustic sensing system
3.1.3
distance measurement range
maximum distance (specified in length units) from the interrogation unit output connector along
the simulated fibre sensor within which the DAS measures an acoustic signal with specified
measurement performance under defined conditions
[SOURCE: IEC 61757-2-2:2016, 3.2, modified – adapted to acoustic sensing.]
3.1.4
fibre stretcher
device where an external stimulus causes a linearly proportional amount of fibre strain uniformly
across the fibre length attached
Note 1 to entry: Normally the fibre stretcher consists of a piezoelectric cylinder with an electrical stimulus.

– 10 – IEC 61757-3-2:2022 © IEC 2022
3.1.5
interrogation unit
IU
opto-electronic instrument that is connected to the distributed fibre optic sensor and measures
and records dynamic strain along the fibre
Note 1 to entry: The processor, data archive, and user interface could be combined within the IU or could be a
separate unit connected to the IU. The IU provides processing functions (standardized or customized), data archiving
(usually standardized), and provides an interface to control and "set" the interrogation unit, select processing options,
and define and implement the data collection options (triggered, timed, or other).
3.1.6
location
optical distance (expressed in length units) from the interrogation unit output connector to a
desired sample point in the simulated fibre sensor
[SOURCE: IEC 61757-2-2:2016, 3.7, modified – adapted to simulated fibre sensor.]
3.1.7
measurement time
time between independent acoustic signal measurements when making successive
measurements along the simulated fibre sensor
Note 1 to entry: Equivalently, it is the time interval between successive trace timestamps under these conditions
(see Figure 2).
Note 2 to entry: The interrogation rate (specified in Hz) is equivalent to the pulse rate for interrogation units that
provide optical pulse interrogation. It is equal to the inverse of the measurement time (see Figure 2).
[SOURCE: IEC 61757-2-2:2016, 3.8, modified – adapted to acoustic sensing.]
3.1.8
Nyquist frequency
frequency represented by the time duration or period of half of the interrogation or sample rate,
whichever is smaller
EXAMPLE For an interrogation rate of 20 kHz (20 000 samples per second), the Nyquist frequency is 10 kHz.
3.1.9
power spectrum density
PSD
square root of the power spectral density derived from time series data converted to the
frequency domain data
3.1.10
sample number
sequence number of a sample in a time series
3.1.11
sample rate
rate at which raw acoustic data is output from the interrogation unit
Note 1 to entry: The maximum sample rate is equal to the interrogation rate. Sample rate applies when the
interrogation rate is reduced (by decimation or otherwise) prior to being output by the interrogation unit.
3.1.12
sample point number
successively numbered spatial sample point that increases along the length of the sensor
Note 1 to entry: The first spatial sampling point starts at zero.

3.1.13
spatial resolution
mean value of the spatial sample intervals which are within the range of the increasing and/or
decreasing slope of a step response generated by a locally abrupt change of the stimulus signal,
expressed in length units
Note 1 to entry: The manufacturer of the interrogation unit designs and/or implements by hardware and/or software
the spatial resolution, which may be controlled by the user.
Note 2 to entry: Figure 19 shows the graphical plotting approach used to determine spatial resolution.
3.1.14
spatial sample spacing
distance (expressed in length units) between two consecutive sample points in a single acoustic
signal
[SOURCE: IEC 61757-2-2:2016, 3.11, modified – adapted to acoustic sensing.]
3.1.15
spatial sample point
SSP
one of the points along the simulated fibre sensor that are defined through the interrogation unit
configuration or set-up and represented spatially along the fibre as uniformly spaced points
from which the interrogation unit samples the backscattered light
3.1.16
strain sensitivity
ε
sens
noise floor of the DAS defined as a strain:
λφd
ε =
sens
(1)
4π nGξ
where
λ is the operational optical (vacuum) wavelength of the DAS;
n is the refractive index of the sensing fibre (group index);
G is the spatial resolution employed by the DAS;
ξ is the photo-elastic scaling factor for longitudinal strain in isotropic material (= 0,78);
dϕ is the noise floor of the system in radians.
3.1.17
time series
data set for a particular sample point, which is represented as sampled at the interrogation rate
Note 1 to entry: Time series can also be represented at a sub-sample rate of the interrogation rate. Whichever rate
is used, it shall be used for all tests described in this document.
Note 2 to entry: Figure 2 illustrates the signal parameters relating to time series and their spatial point identification.
3.2 Abbreviated terms
CL crosstalk level
DAS distributed fibre optic acoustic sensing system
FBG fibre Bragg grating
FFT fast Fourier transform
IU interrogation unit
– 12 – IEC 61757-3-2:2022 © IEC 2022
OTDR optical time-domain reflectometry
PR partial reflector
PSD power spectral density
RL reference level
SEC section
SFS simulated fibre sensor
SNR signal-to-noise ratio
SR signal response
SSP spatial sample point
TP test point
SOURCE: SEAFOM MSP-02 [1], reproduced with the permission of SEAFOM.
Figure 2 – Signal parameters relating to time series and their spatial point identification
3.3 Symbols
L fibre stretcher length
s
total fibre length
L
F,tot
L , L length of left respectively right downward sloping line
L R
M, M , M magnitude, compensated magnitude, compensated and normalized magnitude
C CN
P normalization factor
THD total harmonic distortion
FT
µε technical unit for linear strain with a ratio of increase in length ∆l to the length l
−6
in the order of 10
4 Performance parameters of a distributed acoustic sensing system
The technical performance of a DAS shall be characterized by the following performance
parameters. Clause 6 describes the related test procedures to determine the parameters:
a) dynamic range (see 6.2);
b) frequency response (see 6.3);
c) fidelity [optional] (see 6.4);
d) self-noise (see 6.5);
e) spatial resolution (see 6.6);
f) crosstalk (see 6.7);
g) loss budget (see 6.8);
h) sensor reflection robustness (see 6.9).
5 Test apparatus for performance parameter determination
5.1 Simulated fibre sensor (SFS)
Each of the performance parameters shall be evaluated using a simulated fibre sensor (SFS)
of an appropriate total fibre length (L ) connected to each item of IU equipment under test.
F,tot
The SFS shall be arranged as shown in Figure 3. It functionally shall comprise four delay coils
and three fibre stretchers that are spliced together to represent a contiguous length which is
L . In this depiction, they are all shown with optical fibre pigtails that are angle terminated
F,tot
with a connector type of choice at the start and end of L . The end of L should have a
F,tot F,tot
return loss of > 50 dB.
All elements of the SFS shall be housed in an isolated chamber that provides immunity to
acoustics and vibration (see 5.5).
Each of the three fibre stretchers consists of a fibre of length L . The fibre stretchers are located
s
between the four delay coils and represent locations for many of the performance parameter
tests. These are identified by TP , TP , and TP .
1 2 3
The lengths of delay coils 1 to 4 shall be determined by using the following relationships:
a) the lengths of delay coils 1 and 4 are set to be equal to 250 m;
b) the lengths of delay coils 2 and 3 are set to be equal to L ′ / 2,
F,tot
where
L ′ = L − 500 m − 3 L ;
F,tot F,tot s
L is the total fibre length;
F,tot
L is the fibre stretcher length.
s
– 14 – IEC 61757-3-2:2022 © IEC 2022

[SOURCE: SEAFOM MSP-02 [1]. Reproduced with permission.]
Figure 3 – Simulated fibre sensor
The three test points in Figure 3 coincide with the centre locations of the fibre stretchers and
are designated as follows:
TP : "start" location: fibre stretcher between delay coils 1 and 2
TP : "midpoint" location: fibre stretcher between delay coils 2 and 3
TP : "end" location: fibre stretcher between delay coils 3 and 4
These three locations are depicted in Figure 4 and identified as TP , TP , and TP .
1 2 3
SOURCE: SEAFOM MSP-02 [1], reproduced with the permission of SEAFOM.
Figure 4 – Configuration for SFS showing the locations TP , TP , and TP
1 2 3
Individual evaluation procedures may be performed with a modified type of set-up providing the
required measurement conditions. In this case, a detailed set-up description and documentation
is required.
5.2 Fibre stretcher
The fibre stretcher is used for most tests of the performance parameters that require a
"stimulus" in the form of a variable vibrational stretching of the fibre. Typically, it is comprised
of a sensor fibre wrapped around a piezoelectric cylinder or tube actor which is radially polarized.
For the fibre stretchers to be effective in conducting performance parameter testing, they should
meet the following conditions:
Cylinder diameter (typical): > 5 cm (e.g. 2 in) (for low macrobend loss)
Fibre type: Same as the SFS fibre

Fibre stretcher length: The length of fibre to be strained shall be wound on the fibre
stretcher. The length (not including leads) shall be greater than
twice the spatial resolution of the interrogation unit being
evaluated. However, if this length does not cover at least 10
SSPs, length shall be added for such coverage. Longer lengths
are acceptable.
Fibre stretch: The stretch shall be uniform over the entire length of fibre to be
strained and constant over the specified frequency range. If the
frequency response is not constant, especially at higher
frequencies, data compensation may be used to achieve sufficient
uniformity.
Frequency range: 1 % to 80 % of Nyquist frequency
Strain levels (dynamic): Up to (14 µε peak)/(spatial resolution) depending on application
NOTE 1 The high strain level indicated is able to satisfy the lowest frequency for the dynamic range test. It is
possible (for this particular test) that a voltage amplifier will be needed between the signal generator and the fibre
stretcher. All other tests in this document involve much lower dynamic strain levels and do not need amplification.
NOTE 2 An independent fibre Bragg grating measuring system can be used to control the strain level provided by
the fibre stretcher.
If an alternative construction is used instead of the piezoelectric cylinder, it shall have
comparable performance parameters.
5.3 Signal generation and amplification instrumentation
A signal generator is needed to produce the drive signals for the fibre stretcher. The signal
generator shall be capable of operating over the frequency response range specified in 6.3.
The signal generator should produce low distortion sine waves (typically a total harmonic
distortion < −54 dB) and should have low spurious outputs (typically < −60 dB) within the
frequency response range.
Most fibre stretcher designs can be driven directly with commercial off-the-shelf signal
generators. If amplification is needed, the amplifier shall produce sufficiently large drive voltage
amplitudes to attain fibre strain levels commensurate with the stimulus requirements specified
in Clause 6.
5.4 Optical attenuator
For the optical budget performance parameter testing (see Figure 22), a calibrated optical
attenuator or an optical attenuator approach or attenuator that can be self-calibrated is required
(see IEC 60869-1 [2] for more information).
Recommendations for a tuneable optical attenuator:
Calibrated for the operation wavelength: Yes (or self-calibrated with a power meter)
Variable attenuation range: 2 dB to 6 dB
Attenuation setting resolution: As needed; assumed accurate to 0,1 dB
It is also acceptable to use calibrated fixed attenuators.
5.5 Isolation chamber
The SFS uses fibre coils that are sensitive to environmental disturbances, such as room
acoustics and room/benchtop vibrations. Therefore, measures shall be taken to ensure that
such environmental disturbances do not degrade test data for the measurements of
performance parameters.
– 16 – IEC 61757-3-2:2022 © IEC 2022
The isolation chamber shall consist of an acoustic enclosure to provide a thermally stable and
acoustically shielded experimental area. Within this enclosure, the experiment shall further be
decoupled from the environment with a negative stiffness vibration isolation platform, to provide
isolation from low frequency vibrations of the order of 1 Hz.
6 Test procedures of performance parameters
6.1 General
All test procedures have been developed such that a single test bed is implemented. This is
defined as a simulated fibre sensor SFS (see 5.1), which shall be used for all tests. The SFS
shall be isolated from room acoustics and vibration by means of an isolation chamber described
in 5.5 for all tests.
Depending on the intended DAS application, appropriate stimulus test parameter values shall
be used. The stimulus test parameter values listed in 6.2 to 6.9 are intended as
recommendations. They should be used as much as possible to facilitate easier system
comparisons.
The personnel carrying out the measurements and evaluations shall be sufficiently familiar with
the fibre optic sensor technology and measurement technique.
6.2 Dynamic range
6.2.1 General
The aim of a DAS is to yield a signal that is directly proportional to the amplitude of applied
time-varying acoustic strain acting on the sensing fibre. The dynamic range of the system is a
measure of the range of amplitudes over which the system can accurately represent the applied
acoustic stimulus.
6.2.2 Set-up
The simulated fibre sensor shall be stimulated by fibre stretchers up to the maximum level of
stimulation, at which the result signal is not corrupted when compared to the input stimulus.
Stimulus at three different locations, TP , TP , and TP , along the SFS shall be applied, as
1 2 3
shown in Figure 5.
SOURCE: SEAFOM MSP-02 [1], reproduced with the permission of SEAFOM.
Figure 5 – Test set-up for dynamic range test
6.2.3 Stimulus
To measure the dynamic range, the following stimulus should be used:
Signal type: Sinusoidal
Frequencies: 1 %, 5 %, and 20 % and 80 % Nyquist frequency
Signal level (peak): Noise floor to (π/spatial resolution/measurement time) (within the limit
given by the fibre stretcher)
EXAMPLE If frequency is 1 % of Nyquist frequency, the maximum signal level range is 200 π/spatial resolution.
The stimulus signal shall start at a lower signal level than the maximum expected limit and
increase the level linearly with time to the past maximum expected limit. Stimulation shall be
applied independently at each test position location, TP , TP , and TP .
1 2 3
6.2.4 Data collection and processing
The following data shall be collected at each location TP , TP , and TP :
1 2 3
Samples per location: As needed to cover stimulus range
Time duration at each location: As needed to ensure low uncertainty measurement
(see Annex B)
The collected data shall be processed as follows:
a) Signal: Examination of time domain stimulus and response signal.
b) Dynamic range limit: This limit is reached when the response signal becomes distorted or
experiences its first discontinuity.
c) The test shall be repeated 5 times. The mean value of the determined limits shall be
calculated.
The dynamic range is equal to the amplitude of the mean stimulus signal (optical phase units
converted to strain units according to Formula (A.7) in Annex A).
Figure 6 and Figure 7 illustrate signal processing. They show a simulation of an IU
measurement of a 100 Hz strain stimulus signal that starts at zero amplitude and is gradually
increased over a 30 s time frame. Figure 6 shows both the stimulus signal and the optical phase
signal (or dynamic strain signal) measured by the IU. After approximately 17 s (corresp
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