EN IEC 61757-1-4:2026

SLOVENSKI STANDARD
01-julij-2025
Optični senzorji - 1-4. del: Merjenje deformacij - Porazdeljeno zaznavanje na
podlagi Rayleighovega sipanja
Fibre optic sensors - Part 1-4: Strain measurement - Distributed sensing based on
Rayleigh scattering
Capteurs fibroniques - Partie 1-4: Mesure de déformation - Détection répartie basée sur
la diffusion de rayleigh
Ta slovenski standard je istoveten z: prEN IEC 61757-1-4:2025
ICS:
33.180.99 Druga oprema za optična Other fibre optic equipment
vlakna
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

86C/1972/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 61757-1-4 ED1
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2025-05-30 2025-08-22
SUPERSEDES DOCUMENTS:
86C/1961/CD, 86C/1969/CC
IEC SC 86C : FIBRE OPTIC SYSTEMS, SENSING AND ACTIVE DEVICES
SECRETARIAT: SECRETARY:
United States of America Mr Fred Heismann
OF INTEREST TO THE FOLLOWING COMMITTEES: HORIZONTAL FUNCTION(S):
TC 17,TC 18,TC 20,TC 38,TC 45,TC 65,TC 85
ASPECTS CONCERNED:
SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING
Attention IEC-CENELEC parallel voting
The attention of IEC National Committees, members of
CENELEC, is drawn to the fact that this Committee Draft
for Vote (CDV) is submitted for parallel voting.
The CENELEC members are invited to vote through the
CENELEC online voting system.
This document is still under study and subject to change. It should not be used for reference purposes.
Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of
which they are aware and to provide supporting documentation.
Recipients of this document are invited to submit, with their comments, notification of any relevant “In Some
Countries” clauses to be included should this proposal proceed. Recipients are reminded that the CDV stage is
the final stage for submitting ISC clauses. (SEE AC/22/2007 OR NEW GUIDANCE DOC).
TITLE:
Fibre optic sensors – Part 1-4: Strain measurement – Distributed sensing based on Rayleigh
scattering
PROPOSED STABILITY DATE: 2029
NOTE FROM TC/SC OFFICERS:
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2 IEC CDV 61757-1-4 ED1 © IEC 2025
1 CONTENTS
3 FOREWORD . 4
4 INTRODUCTION . 6
5 1 Scope . 7
6 2 Normative references . 7
7 3 Terms, definitions, abbreviated terms, and symbols . 8
8 3.1 Terms and definitions . 8
9 3.2 Abbreviated terms . 12
10 3.3 Symbols . 12
11 4 General test setup for measurement of performance parameters . 13
12 4.1 General and test setup requirements . 13
13 4.2 General documentation requirements . 16
14 5 Measurement procedures for performance parameters . 17
15 5.1 Strain measurement error . 17
16 5.1.1 Test procedure and conditions . 17
17 5.1.2 Parameter calculation and reporting . 17
18 5.2 Spatial resolution . 17
19 5.2.1 Test procedure and conditions . 17
20 5.2.2 Parameter calculation and reporting . 18
21 5.3 Strain repeatability . 18
22 5.3.1 Test procedure and conditions . 18
23 5.3.2 Parameter calculation and reporting . 19
24 5.4 Spatial strain uncertainty . 19
25 5.4.1 Test procedure and conditions . 19
26 5.4.2 Parameter calculation and reporting . 20
27 5.5 Warm-up time . 20
28 5.5.1 Test procedure and conditions . 20
29 5.5.2 Parameter calculation and reporting . 21
30 5.6 System performance with altered attenuation . 21
31 5.6.1 General . 21
32 5.6.2 At distance measurement range . 21
33 5.6.3 At short distance with high loss. 23
34 Annex A (informative) Application area of Rayleigh-based distributed strain
35 measurements . 25
36 Annex B (informative) Strain measurement using cross correlation of Rayleigh
37 scattering . 26
38 Bibliography . 28
40 Figure 1 – Optical fibre strain profile and related strain sample points. 10
41 Figure 2 – General test setup for a single-ended configuration. 13
42 Figure 3 – Measured versus applied strain (typical curve) . 15
43 Figure 4 – Rayleigh frequency shift as a function of elongation of a single-mode fibre . 15
44 Figure 5 – Illustration of spatial resolution test results . 18
45 Figure 6 – Performance evaluation at distance measurement range . 22
46 Figure 7 – Performance evaluation at short distance with high loss . 23

IEC CDV 61757-1-4 ED1 © IEC 2025 3
47 Figure B.1 – Strain measurement obtained from two Rayleigh scattering spectra
48 measured with the OTDR technique . 26
4 IEC CDV 61757-1-4 ED1 © IEC 2025
51 INTERNATIONAL ELECTROTECHNICAL COMMISSION
52 ____________
54 FIBRE OPTIC SENSORS –
56 Part 1-4: Strain measurement –
57 Distributed sensing based on Rayleigh scattering
59 FOREWORD
60 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
61 all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
62 co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
63 in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
64 Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their
65 preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
66 may participate in this preparatory work. International, governmental and non-governmental organizations liaising
67 with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
68 Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
69 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
70 consensus of opinion on the relevant subjects since each technical committee has representation from all
71 interested IEC National Committees.
72 3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
73 Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
74 Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
75 misinterpretation by any end user.
76 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
77 transparently to the maximum extent possible in their national and regional publications. Any divergence between
78 any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
79 5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
80 assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
81 services carried out by independent certification bodies.
82 6) All users should ensure that they have the latest edition of this publication.
83 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
84 members of its technical committees and IEC National Committees for any personal injury, property damage or
85 other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
86 expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
87 Publications.
88 8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
89 indispensable for the correct application of this publication.
90 9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
91 patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
92 respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
93 may be required to implement this document. However, implementers are cautioned that this may not represent
94 the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
95 shall not be held responsible for identifying any or all such patent rights.
97 IEC 61757-1-4 has been prepared by subcommittee 86C: Fibre optic systems and active
98 devices, of IEC technical committee 86: Fibre optics. It is an International Standard.
99 The text of this International Standard is based on the following documents:
Draft Report on voting
86C/XX/FDIS 86C/XX/RVD
101 Full information on the voting for its approval can be found in the report on voting indicated in
102 the above table.
103 The language used for the development of this International Standard is English.

IEC CDV 61757-1-4 ED1 © IEC 2025 5
104 This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
105 accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
106 at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
107 described in greater detail at www.iec.ch/publications.
108 A list of all parts in the IEC 61757 series, published under the general title Fibre optic sensors,
109 can be found on the IEC website.
110 The committee has decided that the contents of this document will remain unchanged until the
111 stability date indicated on the IEC website under webstore.iec.ch in the data related to the
112 specific document. At this date, the document will be
113 • reconfirmed,
114 • withdrawn,
115 • replaced by a revised edition, or
116 • amended.
6 IEC CDV 61757-1-4 ED1 © IEC 2025
119 INTRODUCTION
120 This document is part of the IEC 61757 series, which is dedicated to fibre optic sensors. Generic
121 specifications for fibre optic sensors are defined in IEC 61757.
122 The individual parts of the IEC 61757 series are numbered as IEC 61757-M-T, where M denotes
123 the measurand and T the technology of the fibre optic sensor. The IEC 61757-1-T series is
124 concerned with strain measurements.
IEC CDV 61757-1-4 ED1 © IEC 2025 7
126 FIBRE OPTIC SENSORS –
128 Part 1-4: Strain measurement –
129 Distributed sensing based on Rayleigh scattering
131 1 Scope
132 This part of IEC 61757 defines the terminology, structure, and measurement methods of
133 distributed fibre optic sensors for absolute strain measurements based on spectral correlation
134 analysis of Rayleigh backscattering signatures in single-mode fibres, where the fibre is the
135 distributed strain measurement element in a measurement range from about 10 m to tens of
136 km. The document is also applicable to hybrid sensor systems that combine the advantages of
137 Brillouin and Rayleigh backscattering effects to obtain optimal measurement quality.
138 This document also specifies the most important features and performance parameters of these
139 distributed fibre optic strain sensors and defines procedures for measuring these features and
140 parameters.
141 This part of IEC 61757 does not apply to point measurements or to dynamic strain
142 measurements. Distributed strain measurements using Brillouin scattering in single-mode fibres
143 are covered in IEC 61757-1-2.
144 The most relevant applications of this strain measurement technique are listed in informative
145 Annex A, while informative Annex B provides a short description of the underlying measurement
146 principle.
147 2 Normative references
148 The following documents are referred to in the text in such a way that some or all of their content
149 constitutes requirements of this document. For dated references, only the edition cited applies.
150 For undated references, the latest edition of the referenced document (including any
151 amendments) applies.
152 IEC 60793-2-50, Optical fibres - Part 2-50: Product specifications - Sectional specification for
153 class B single-mode fibres
154 IEC 61757, Fibre optic sensors – Generic specification
155 IEC 61757-1-2:2023, Fibre optic sensors – Part1-2: Strain measurement- Distributed sensing
156 based on Brillouin scattering.
157 IEC 61757-2-2, Fibre optic sensors – Part 2-2: Temperature measurement – Distributed sensing
158 IEC 61757-3-2, Fibre optic sensors – Part 3-2: Acoustic sensing and vibration measurement –
159 Distributed sensing
160 ISO/IEC Guide 98-3, Uncertainty of measurement – Part 3: Guide to the expression of
161 uncertainty in measurement (GUM:1995)

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162 3 Terms, definitions, abbreviated terms, and symbols
163 3.1 Terms and definitions
164 For the purposes of this document, the terms and definitions given in IEC 61757, IEC
165 61757‑1‑2, IEC 61757‑2‑2, IEC 61757-3-2, and the following apply.
166 ISO and IEC maintain terminology databases for use in standardization at the following
167 addresses:
168 • IEC Electropedia: available at https://www.electropedia.org/
169 • ISO Online browsing platform: available at https://www.iso.org/obp
170 NOTE For the following definitions, the relevant test procedures and parameters are defined in Clause 4.
171 3.1.1
172 distributed fibre optic strain sensing system
173 DSS
174 measurement set-up consisting of a distributed fibre optic sensor connected to an interrogation
175 unit, including processor, data archive, and user interface, which provides a spatially resolved
176 strain measurement
177 [SOURCE: IEC 61757-1-2:2023, 3.1.1]
178 3.1.2
179 distance measurement range
180 maximum distance from the DSS interrogation unit output connector along the fibre optic sensor
181 within which the DSS measures strain with specified measurement performance under defined
182 conditions
183 Note 1 to entry: Defined conditions are spatial resolution (3.1.8), spatial strain uncertainty (3.1.9), and
184 measurement time (3.1.5).
185 Note 2 to entry: This supporting parameter is closely related to the total accumulated optical loss (one way)
186 tolerated by the interrogation unit without affecting specified measurement performance. In test cases used to prove
187 or verify the reported specifications, the total fibre length is equal to or greater than the specified distance
188 measurement range, for the tolerated total accumulated optical loss.
189 Note 3 to entry: The distance measurement range is usually expressed in m or km.
190 [SOURCE: IEC 61757-1-2:2023, 3.1.2, modified – note 4 to entry deleted]
191 3.1.3
192 strained spot
193 ΔL
194 length of fibre optic sensor that experiences a small elongation (δL), which causes strain that
195 is significantly bigger than the strain repeatability of the interrogation unit and which is
196 confirmed by a reference strain measurement
197 Note 1 to entry: The applied strain ε is equal to (δL/ΔL).
198 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.
199 [SOURCE: IEC 61757-1-2:2023, 3.1.3]
200 3.1.4
201 location
202 L
203 optical distance from the DSS interrogation unit output connector to a desired strain sample
204 point along the fibre optic sensor

IEC CDV 61757-1-4 ED1 © IEC 2025 9
205 Note 1 to entry: The farthest location from the DSS interrogation unit output connector for the particular test is
206 quantified as L and is often chosen to be the same as the distance measurement range for purposes of
F,long
207 comparing the measurement results with quoted specifications.
208 Note 2 to entry: The location is usually expressed in m or km.
209 [SOURCE: IEC 61757-1-2:2023, 3.1.4]
210 3.1.5
211 measurement time
212 time between independent strain measurements when making successive measurements on a
213 single fibre optic sensor
214 Note 1 to entry: Equivalently, it is the time interval between successive strain trace timestamps under these
215 conditions.
216 Note 2 to entry: This parameter includes acquisition time and processing time for the measured data. This
217 parameter is typically selectable by the user in some limited fashion. Multiple independent strain measurements can
218 be averaged together to provide an overall measurement time.
219 [SOURCE: IEC 61757-1-2:2023, 3.1.5]
220 3.1.6
221 point defect
222 local deviation of a fibre optic sensor from its nominal optical and mechanical properties
223 occurring at a single location, or over a length substantially less than the DSS spatial resolution
224 Note 1 to entry: The definition of a point defect encompasses a wide range of situations, which can produce similar
225 effects on the strain trace. Examples include:
226 – a point loss, like a bad fibre splice,
227 – a reflectance (or return loss), as can be introduced by a fibre connector,
228 – a localized region of high loss, like a bend or kink in the fibre, and
229 – a physical discontinuity in the fibre, like a splice between two fibres of different core diameters.
230 [SOURCE: IEC 61757-1-2:2023, 3.1.6]
231 3.1.7
232 sample spacing
233 distance between two consecutive strain sample points in a single strain trace
234 Note 1 to entry: Sample spacing can be a user-selectable parameter in the interrogation unit.
235 Note 2 to entry: The sample spacing is usually expressed in mm.
236 Note 3 to entry: See Figure 1.
237 Note 4: This parameter is also called “gage pitch”.

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239 Figure 1 – Optical fibre strain profile and related strain sample points
240 [SOURCE: IEC 61757-1-2:2023, 3.1.7]
241 3.1.8
242 spatial resolution
243 smallest length of strain-affected fibre optic sensor for which a DSS can measure and confirm
244 the reference strain of a defined strained spot within the specified strain measurement error of
245 the DSS
246 Note 1 to entry: The spatial resolution is usually expressed in mm.
247 [SOURCE: IEC 61757-1-2:2023, 3.1.8]
248 3.1.9
249 spatial strain uncertainty
250 uncertainty of the location of strain data in a single strain trace, expressed by twice the standard
251 deviation of a specified number of adjacent strain sample points, with the fibre optic sensor held
252 at constant strain and temperature
253 Note 1 to entry: Due to a potential cross-sensitivity of DSS to temperature, it can be necessary to stabilize the
254 temperature of the fibre optic sensor.
255 Note 2 to entry: The spatial strain uncertainty is usually expressed in units of με and noted as a tolerance
256 (e.g. ±xx με), where 1 με corresponds to a δL of 1 μm over a ΔL of 1 m.
257 [SOURCE: IEC 61757-1-2:2023, 3.1.9]
258 3.1.10
259 strain dead zone
260 limited zone of a strain trace, where the strain sample points deviate from the undisturbed parts
261 of the trace by a specified limit due to a point defect
262 Note 1 to entry: The strain dead zone is usually expressed in mm.
263 [SOURCE: IEC 61757-1-2:2023, 3.1.10]
264 3.1.11
265 strain measurement error
266 maximum difference between a centred and uniformly weighted moving average of the
267 measured strain and a reference strain for all data points of the fibre optic sensor over the full
268 operating temperature range and all acquisition times

IEC CDV 61757-1-4 ED1 © IEC 2025 11
269 Note 1 to entry: Single value (worst case) is expressed like a tolerance in units of με (e.g. ±xx με).
270 Note 2 to entry: The number of elements used for the moving average is defined in this document. In practical
271 applications, other methods of smoothing might be applicable.
272 [SOURCE: IEC 61757-1-2:2023, 3.1.11]
273 3.1.12
274 strain repeatability
275 precision of strain data based on repeated strain traces at a given location expressed by twice
276 the standard deviation of corresponding strain sample points in each strain trace, with the fibre
277 optic sensor held at constant strain and temperature
278 Note 1 to entry: The strain repeatability is expressed like a tolerance in units of με (e.g. ±xx με).
279 [SOURCE: IEC 61757-1-2:2023, 3.1.12]
280 3.1.13
281 strain sample point
282 single point at a known location along a fibre optic sensor, where a strain value is to be
283 measured
284 Note 1 to entry: Due to signal averaging effects, the measured value represents the strain along a very small section
285 of the fibre optic sensor that includes the strain sample point.
286 Note 2 to entry: See Figure 1.
287 [SOURCE: IEC 61757-1-2:2023, 3.1.13]
288 3.1.14
289 strain trace
290 set of strain sample points distributed along a fibre optic sensor and spaced by the sample
291 spacing
292 Note 1 to entry: All sample points are associated with a common time of measurement, often called "trace
293 timestamp". The measured values represent the strain during a time period that includes the timestamp.
294 Note 2 to entry: All sample points in a strain trace are measured values produced by the DSS, and not interpolated
295 or smoothed values produced by subsequent processing outside the interrogation unit.
296 [SOURCE: IEC 61757-1-2:2023, 3.1.14]
297 3.1.15
298 total fibre length
299 L
F,tot
300 distance from the DSS interrogation unit output connector to the far end of the fibre optic sensor
301 Note 1 to entry: The far end of the fibre optic sensor can be either a purposely cut or a terminated end of the fibre,
302 physically located far from the interrogation unit (in a single-ended configuration).
303 Note 2 to entry: This parameter is either equal to or greater than the distance measurement range and usually
304 expressed in m or km.
305 [SOURCE: IEC 61757-1-2:2023, 3.1.15]
306 3.1.16
307 warm-up time
308 duration between the instant after which the power supply of the DSS interrogation unit is
309 energized and the instant when the interrogation unit may be used as specified by the
310 manufacturer
311 Note 1 to entry: Warm-up time is usually expressed in seconds or minutes.

12 IEC CDV 61757-1-4 ED1 © IEC 2025
312 Note 2 to entry: The warm-up time helps to upload software and to stabilize operating temperatures of optical and
313 electronic components.
314 [SOURCE: IEC 61757-1-2:2023, 3.1.16]
315 3.1.17
316 cross correlation coefficient
317 value showing similarity between two Rayleigh scattering spectra at the same optical fibre
318 section
319 Note 1 to entry: The cross correlation coefficient indicates the quality of the measurement. The possible range of the
320 cross correlation coefficient is generally from 0 to +1,0, where larger values indicate better correlation.
321 Note 2 to entry: In the optical time domain reflectometry method (OTDR method), the Rayleigh scattering spectrum
322 is observed by the intensity as a function of wavelength, while changing the wavelength of the launched signals. In
323 the optical frequency domain reflectometry method (OFDR method), Rayleigh scattering is observed by phase shifts
324 of Fourier transformed waveforms.
325 3.2 Abbreviated terms
326 FAT factory acceptance test
327 LVDT linear variable differential transformer
328 OFDR optical frequency domain reflectometry
329 OTDR optical time domain reflectometry
330 TW-COTDR tuneable-wavelength coherent optical time domain reflectometry
331 VOA variable optical attenuator
332 3.3 Symbols
333 A  cross-sectional area
334 E  Young’s modulus
335 F  force
336 L  optical distance from the output connector to a desired strain sample point
337 L , L optional and short fibre lengths
F,opt F,short
338 L  long fibre lengths
F,long
339 L  total fibre length
F,tot
340 ΔL  length of fibre optic sensor to be strained (strained spot)
341 δL  small change in length of ΔL
342 N, n number of traces, number of data points
343 S  standard deviation
344 T  temperature
345 T , T , T minimal, typical, and maximal DSS operating temperature
low op high
346 T ambient operating temperature of the strain test section
STC
347 ε  strain
348 ε strain repeatability
rep
349 ε spatial strain uncertainty
unc
350 σ  stress
IEC CDV 61757-1-4 ED1 © IEC 2025 13
351 4 General test setup for measurement of performance parameters
352 4.1 General and test setup requirements
353 A general test setup for single-ended configurations is schematically shown in Figure 2. The
354 aim of this setup is to provide a common base for determining the measurement specifications
355 while minimizing complexity, cost, reconfiguration requirements, and test execution time.
356 Temperature stabilisation is used to avoid possible crosstalk from temperature variations.
358 Key
359 1 temperature-controlled encasement (e.g. temperature chamber)
360 2 DSS interrogation unit
361 3 DSS interrogation unit output connector
362 4 fibre fusion splice
363 5 optional variable optical attenuator
364 6 optional long fibre length L (normal spool)
F,opt
365 7 temperature-controlled environment for stable ambient conditions
366 8 long fibre length L (loose and strain-free wound)
F,long
367 9 strain test section with temperature-controlled environment for stable ambient conditions
368 10 fixed fibre clamping unit
369 11 movable fibre clamping unit
370 12 short fibre length L (loose wound), longer than 5 times the spatial resolution
F,short
371 13 fibre termination
372 Figure 2 – General test setup for a single-ended configuration
373 The temperature-controlled encasement containing the DSS interrogation unit shall provide a
374 steady temperature, for an extended period of time, within the temperature operating range
375 (T ≤ T ≤ T ) of the device under test. It is recommended to use commercial off-the-shelf
low op high
376 temperature chambers for determining the performance parameters. Minimum requirements for
377 such a device are:
378 – minimal and maximal temperature settings shall exceed the minimal and maximal operating
379 temperatures of the interrogation unit under test;

14 IEC CDV 61757-1-4 ED1 © IEC 2025
380 – temperature variation in time (steady state) shall be less than (±0,5 °C);
381 – temperature homogeneity in encasement volume shall be less than (±1,5 °C).
382 For the optical power adjustment, a calibrated optical attenuator or an optical attenuator that
383 can be self-calibrated shall be used. Recommendations for a variable optical attenuator are:
384 – calibrated for the wavelength of operation (or self-calibrated with a power meter);
385 – variable attenuation range from 2 dB to 6 dB;
386 – resolution of attenuation setting as needed, assumed to be accurate within 0,1 dB.
387 It is also acceptable to use calibrated fixed attenuators. More information on fibre optic passive
388 power control devices can be found in IEC 60869-1.
389 The strain test section shall provide reproducibly different levels of constant strain of the optical
390 sensor fibre length ΔL. A commercially available single-mode fibre (IEC 60793-2-50 B.652) shall
391 be used as the optical sensor fibre. The optical fibre strain shall be monitored and measured
392 with suitable measuring instruments. Suitable measuring instruments to determine the fibre
393 elongation with approximately less than 1 µm uncertainty are, for example, a laser
394 interferometric displacement measuring system or a linear variable differential transformer
395 (LVDT). If Young’s modulus of the fibre is known, a force transducer may be used to measure
396 the pulling force, from which the strain can be calculated using Formula (1). No interfering
397 influences (e.g. due to high compressive load or slipping) on the strain measurement shall be
398 caused by fibre clamping. Since slipping depends on the coating of the fibre, a thin and hard
399 coating (e.g. polyimide coating) should be used. Minimum requirements for the strain test
400 section are:
401 – minimal length of fibre optic sensor ΔL to be strained shall be ΔL > (3 x spatial resolution);
402 – optical fibre strain ε to be obtained shall be 5 με ≤ ε ≤ 20.000 με, which is appropriate for
403 many applications. If necessary, the strain range can be adapted to the intended application.
404 NOTE When determining the performance of the DSS interrogation unit with a dedicated sensor cable, one can first
405 determine the DSS performance with a reg
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