EN 61757-1-1:2017

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Fibre optic sensors - Part 2-1: Strain measurement - Strain sensors based on fibre Bragg gratings (IEC 61757-1-1:2016)33.180.99YODNQDOther fibre optic equipmentICS:Ta slovenski standard je istoveten z:EN 61757-1-1:2017SIST EN 61757-1-1:2017en01-maj-2017SIST EN 61757-1-1:2017SLOVENSKI
STANDARD
EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN 61757-1-1
February 2017 ICS 33.180.99
English Version
Fibre optic sensors - Part 1-1: Strain measurement - Strain sensors based on fibre Bragg gratings (IEC 61757-1-1:2016)
Capteurs à fibres optiques - Partie 1-1: Mesure de déformation - Capteurs de déformation basés sur des réseaux de Bragg à fibres (IEC 61757-1-1:2016)
LWL¬-Sensoren - Teil 1-1: Zugmessungen - Zugsensoren basierend auf Faser-Bragg-Gatter (IEC 61757-1-1:2016) This European Standard was approved by CENELEC on 2016-03-31. CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the same status as the official versions. CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom. European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung CEN-CENELEC Management Centre: Avenue Marnix 17,
B-1000 Brussels © 2017 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN 61757-1-1:2017 E SIST EN 61757-1-1:2017

The following dates are fixed: • latest date by which the document has to be implemented at national level by publication of an identical national standard or by endorsement (dop) 2017-08-17 • latest date by which the national standards conflicting with the document have to be withdrawn (dow) 2020-02-17
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such patent rights.
This publication is to be read in conjunction with EN 61757-1:2012.
Endorsement notice The text of the International Standard IEC 61757-1-1:2016 was approved by CENELEC as a European Standard without any modification. In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 60793-1-30 NOTE Harmonized as EN 60793-1-30. IEC 60793-1-31 NOTE Harmonized as EN 60793-1-31. IEC 60793-1-33 NOTE Harmonized as EN 60793-1-33. ISO 527-4 NOTE Harmonized as EN ISO 527-4. ISO 7500-1 NOTE Harmonized as EN ISO 7500-1. ISO 14125 NOTE Harmonized as EN ISO 14125.
Annex ZA (normative)
Normative references to international publications with their corresponding European publications
The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
NOTE 1 When an International Publication has been modified by common modifications, indicated by (mod), the relevant EN/HD applies.
NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is available here: www.cenelec.eu.
Publication Year Title EN/HD Year IEC 60050 series
International Electrotechnical Vocabulary_- Part_102: Mathematics_- General concepts and linear algebra - series
IEC 60068-2 series
Environmental testing -- Part 2: Tests EN 60068-2 series
IEC 60793-2 -
Optical fibres - Part 2: Product specifications - General EN 60793-2 -
IEC 60874-1 -
Fibre optic interconnecting devices and passive components - Connectors for optical fibres and cables -- Part 1: Generic specification EN 60874-1 -
IEC 61300-2 series
Fibre optic interconnecting devices and passive components - Basic test and measurement procedures -- Part 2-1: Tests - Vibration (sinusoidal) - series
IEC 61757-1 2012
Fibre optic sensors -- Part 1: Generic specification EN 61757-1 2012
IEC 62129-1 -
Calibration of wavelength/optical frequency measurement instruments - Part 1: Optical spectrum analyzers EN 62129-1 -
IEC 62129-2 -
Calibration of wavelength/optical frequency measurement instruments -- Part 2: Michelson interferometer single wavelength meters EN 62129-2 -
IEC/TR 61931 -
Fibre optic - Terminology - -
IEC/TS 62129-3 -
Calibration of wavelength/optical frequency measurement instruments - Part 3: Optical frequency meters using optical frequency combs - -
ISO/IEC Guide 99 -
International vocabulary of metrology - Basic and general concepts and associated terms (VIM) - -
IEC 61757-1-1 Edition 1.0 2016-02 INTERNATIONAL STANDARD
Fibre optic sensors –
Part 1-1: Strain measurement – Strain sensors based on fibre Bragg gratings
INTERNATIONAL ELECTROTECHNICAL COMMISSION
ICS 33.180.99
ISBN 978-2-8322-3188-3
– 2 – IEC 61757-1-1:2016 © IEC 2016 CONTENTS FOREWORD . 5 INTRODUCTION . 7 1 Scope . 8 2 Normative references. 8 3 Terms and definitions . 9 4 Symbols . 13 5 Structure and characteristics . 14 5.1 Fibre Bragg grating (FBG) . 14 5.2 FBG strain sensor configuration. 18 5.3 Measuring point and installation . 18 5.4 Gauge length . 19 5.5 Strain and reference strain . 19 5.6 Reference wavelength . 19 5.7 Stability behaviour . 20 5.7.1 Drift and creep . 20 5.7.2 Shape stability of the Bragg grating peak . 20 5.7.3 Hysteresis . 20 5.8 Test specimen . 20 5.9 Indication of the measured values . 21 5.10 Zero point related measurement . 21 5.11 Non-zero point related measurement . 21 5.12 Production set . 21 5.13 FBG strain sensor standard type . 21 5.14 FBG strain sensor series . 21 6 Features and characteristics to be reported . 21 6.1 Construction details and geometrical dimensions . 21 6.2 Configuration of the FBG strain sensor . 22 6.3 Temperature and humidity range . 22 6.4 Connecting requirement . 22 7 Features and characteristics to be measured . 22 7.1 Sampling and statistical evaluation . 22 7.1.1 Sampling . 22 7.1.2 Random sampling . 22 7.1.3 Type testing . 22 7.1.4 Series testing . 22 7.1.5 Individual sample testing . 23 7.1.6 Reporting the measuring result . 23 7.1.7 Sample conditioning . 23 7.1.8 Ambient test conditions . 23 7.1.9 Required type of test for individual characteristics . 23 7.2 Bragg wavelength B . 24 7.2.1 General . 24 7.2.2 Measuring procedure . 24 7.2.3 Evaluation . 25 7.2.4 Reporting . 25 SIST EN 61757-1-1:2017

IEC 61757-1-1:2016 © IEC 2016 – 3 – 7.3 FBG spectral width . 25 7.3.1 Measuring procedure . 25 7.3.2 Evaluation . 25 7.3.3 Reporting . 25 7.4 FBG reflectivity . 25 7.4.1 Measuring procedure . 25 7.4.2 Evaluation . 26 7.4.3 Reporting . 26 7.5 FBG Strain sensitivity . 26 7.5.1 General . 26 7.5.2 Tensile test set-up . 27 7.5.3 Measuring procedure tensile test . 27 7.5.4 Evaluation . 28 7.5.5 Reporting . 28 7.6 Gauge factor k . 28 7.6.1 General . 28 7.6.2 Bending test set-up . 29 7.6.3 Measurement procedure . 31 7.6.4 Evaluation . 32 7.6.5 Reporting . 32 7.7 Maximum strain range at room temperature . 32 7.7.1 General . 32 7.7.2 Test set-up . 32 7.7.3 Measuring procedure . 33 7.7.4 Evaluation . 33 7.7.5 Reporting . 33 7.8 Fatigue behaviour . 34 7.8.1 Test set-up . 34 7.8.2 Measuring procedure . 34 7.8.3 Evaluation . 34 7.8.4 Reporting . 35 7.9 Minimum operating radius of curvature . 35 7.9.1 Measuring procedure . 35 7.9.2 Evaluation . 35 7.9.3 Reporting . 35 7.10 Temperature and humidity ranges . 35 7.10.1 General . 35 7.10.2 Measuring procedure . 36 7.10.3 Evaluation . 36 7.10.4 Reporting . 36 7.11 Other environmental influences . 36 7.12 Temperature-induced strain response . 36 7.12.1 General . 36 7.12.2 Test set-up . 37 7.12.3 Measuring procedure . 38 7.12.4 Evaluation . 38 7.12.5 Reporting . 38 7.13 Proof test and lifetime considerations . 38 7.13.1 General . 38 SIST EN 61757-1-1:2017

– 4 – IEC 61757-1-1:2016 © IEC 2016 7.13.2 Measuring procedure . 39 7.13.3 Evaluation . 39 7.13.4 Reporting . 40 8 Recommendations for use of FBG measuring instruments . 40 Annex A (informative)
Further properties of FBG strain sensors . 41 A.1 General . 41 A.2 Extended explanation of FBG side-lobes for different conditions of use . 41 Annex B (informative)
Blank detail specification . 45 B.1 General . 45 B.2 Mechanical setup of the FBG strain sensor . 45 B.3 Operational characteristics of the FBG strain sensor . 45 B.4 Limiting parameters of the FBG strain sensor . 46 B.5 Temperature data of the FBG strain sensor . 46 B.6 Further information of the FBG strain sensor given upon request . 46 B.7 Key performance data of the FBG measuring instrument . 46 Annex C (informative)
Polarization effects . 48 Annex D (informative)
Applied FBG strain sensors . 49 D.1 General . 49 D.2 Recommended bonding process . 49 Bibliography . 50
Figure 1 – Characteristics of the Bragg grating reflectance spectrum . 10 Figure 2 – Operation principle of a fibre Bragg grating in an optical waveguide . 15 Figure 3 – Reflection spectrum of a fibre Bragg grating array . 17 Figure 4 – Gauge length between two attachment points . 18 Figure 5 – Reflection spectrum of a FBG [calculated (left)
and measured spectrum (right)] . 24 Figure 6 – Determination of RFBG from the FBG reflection spectrum (left, Equation (9))
and transmission spectrum (right, Equation (10)) . 26 Figure 7 – Example set-up of a tensile test facility . 27 Figure 8 – Test layout (left) for the 4-point bending test with scheme of
lateral force and bending moment curves (right) . 29 Figure 9 – Determination of the strain via displacement measurement . 30 Figure 10 – Whole-surface applied sensor on a bended flexural beam . 31 Figure 11 – Test specimen with applied FBG strain sensor . 34 Figure A.1 – Side-lobes in the case of a single FBG strain sensor . 42 Figure A.2 – Fundamental peaks and detected side-lobe peaks
in the case of serially multiplexed FBGs . 42 Figure A.3 – Spectral peaks in the case of serially multiplexed FBGs . 43 Figure A.4 – Parameters to identify fundamental peaks and side-lobes . 43 Figure A.5 – Identification of fundamental peaks and side-lobes . 44
Table 1 – Required type of test for individual characteristics . 23
IEC 61757-1-1:2016 © IEC 2016 – 5 – INTERNATIONAL ELECTROTECHNICAL COMMISSION ____________
FIBRE OPTIC SENSORS –
Part 1-1: Strain measurement –
Strain sensors based on fibre Bragg gratings
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 in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their 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 with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for Standardization (ISO) in accordance with conditions determined by agreement between the two organizations. 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international consensus of opinion on the relevant subjects since each technical committee has representation from all interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any misinterpretation by any end user. 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications transparently to the maximum extent possible in their national and regional publications. Any divergence between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter. 5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any services carried out by independent certification bodies. 6) All users should ensure that they have the latest edition of this publication. 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and members of its technical committees and IEC National Committees for any personal injury, property damage or other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publications.
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. International Standard IEC 61757-1-1 has been prepared by subcommittee SC 86C: Fibre optic systems and active devices, of IEC technical committee 86: Fibre optics. The text of this standard is based on the following documents: CDV Report on voting 86C/1322/CDV 86C/1353/RVC
Full information on the voting for the approval of this standard 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. 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.
– 6 – IEC 61757-1-1:2016 © IEC 2016 This International Standard is to be used in conjunction with IEC 61757-1:2012.
The committee has decided that the contents of this publication will remain unchanged until the stability date indicated on the IEC web site under "http://webstore.iec.ch" in 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.
IMPORTANT – The 'colour inside' logo on the cover page of this publication 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.
IEC 61757-1-1:2016 © IEC 2016 – 7 – INTRODUCTION It has been decided to restructure the IEC 61757 series, with the following logic. From now on, the sub-parts will be renumbered as IEC 61757-M-T, where M denotes the measure and T, the technology.
The existing part IEC 61757-1:2012 will be renumbered as IEC 61757 when it will be revised as edition 2.0 and will serve as an umbrella document over the entire series.
– 8 – IEC 61757-1-1:2016 © IEC 2016 FIBRE OPTIC SENSORS –
Part 1-1: Strain measurement –
Strain sensors based on fibre Bragg gratings
1 Scope This part of IEC 61757 defines detail specifications for fibre optic sensors using one or more fibre Bragg gratings (FBG) as the sensitive element for strain measurements. Generic specifications for fibre optic sensors are defined in IEC 61757-1:2012. This standard specifies the most important features and characteristics of a fibre optic sensor for strain measurements based on use of an FBG as the sensitive element, and defines the procedures for their determination. Furthermore, it specifies basic performance parameters and characteristics of the corresponding measuring instrument to read out the optical signal from the FBG. This standard refers to the measurement of static and dynamic strain values in a range of frequencies. A blank detail specification is provided in Annex B. 2 Normative references The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC 60050 (all parts), International Electrotechnical Vocabulary (available at http://www.electropedia.org) IEC 60068-2 (all parts), Environmental testing – Part 2: Tests IEC 60793-2, Optical fibres – Part 2: Product specifications – General IEC 60874-1, Fibre optic interconnecting devices and passive components – Connectors for optical fibres and cables – Part 1: Generic specification IEC 61300-2 (all parts), Fibre optic interconnecting devices and passive components – Basic test and measurement procedures – Part 2: Tests IEC 61757-1:2012, Fibre optic sensors – Part 1: Generic specification IEC 62129-1, Calibration of wavelength/optical frequency measurement instruments – Part 1: Optical spectrum analyzers IEC 62129-2, Calibration of wavelength/optical frequency measurement instruments – Part 2: Michelson interferometer single wavelength meters IEC TS 62129-3, Calibration of wavelength/optical frequency measurement instruments – Part 3: Optical frequency meters using optical frequency combs IEC TR 61931, Fibre optic – Terminology SIST EN 61757-1-1:2017

IEC 61757-1-1:2016 © IEC 2016 – 9 – ISO/IEC Guide 99, International vocabulary of metrology — Basic and general concepts and associated terms (VIM) Terms and definitions 3 Terms and definitions For the purposes of this document, the definitions given in IEC 61757-1:2012, the IEC 60050 series, IEC TR 61931, ISO/IEC Guide 99 (VIM), as well as the following apply.
NOTE Long period gratings, non-uniform gratings, angled gratings, and FBG in polarization maintaining fibre are not considered. 3.1
FBG fibre Bragg grating phase diffraction grating integrated in optical single-mode silica-based fibres, according to category B of IEC 60793-2, to selectively reflect a very narrow range of wavelengths while transmitting others Note 1 to entry: To achieve this characteristic, periodically spaced zones in the fibre core are altered to have different refractive indexes slightly higher than the core.
3.2
FBG strain sensor device that uses one or more fibre Bragg gratings (3.1) as a sensitive element for strain measurements
Note 1 to entry: Different configurations are possible (see 5.2). 3.3
B Bragg wavelength
wavelength of the FBG (3.1), generally corresponding to the Bragg reflection peak or transmission minimum, without applied strain under reference ambient conditions Note 1 to entry: If referred to as an FBG strain sensor (see 3.2), it refers to the configuration prior to its installation. 3.4
0 reference wavelength wavelength response of an FBG after installation or at the beginning of measurement to the affecting loading and ambient conditions 3.5
RFBG FBG reflectivity ratio of the incident optical power P0 to the reflected optical power PB at Bragg wavelength B Note 1 to entry: The power transmitted to the FBG strain sensor is less than the incident (input) optical power due to losses in the fibre at the connector and even in the grating. The definition of the FBG reflectivity should therefore use the incident optical power P0 (see formulas in 7.4.2,) that represents the measurable part at the connector of a fibre optic sensor.
Note 2 to entry: P0 depends on the measurement device and has no absolute characteristic value. From the user’s point of view, the reflectivity is important if operational or installation conditions exist that influence the reflective characteristic. 3.6
transmission loss of an FBG sensor loss of power of the transmitted optical signal passing along the optical fibre, the fibre Bragg grating and the components to connect an FBG strain sensor outside the FBG spectrum SIST EN 61757-1-1:2017

– 10 – IEC 61757-1-1:2016 © IEC 2016 Note 1 to entry: When considering transmission loss in an FBG sensor configuration, all parts that contribute to the reduction of power, for example transmission losses due to joining and connecting techniques, have to be considered. The transmission spectra of the grating can show a reduction of the grating transmissivity due to influences on grating performance. Such propagation losses in the grating should be considered separately. The entry only applies to wavelength multiplexed FBG strain sensors double-ended for in-series connection.
3.7
FBG spectral width
FWHM of the reflection peak or transmission minimum at Bragg wavelength Note 1 to entry: FWHM of an FBG spectrum is the wavelength range of the spectrum over which the amplitude is greater than 50 % (3 dB) of its reflectance maximum value at B (see Figure 1).
Key (1) Difference in intensity between Bragg peak and largest side-lobe (called relative side-lobe level) (2) Recorded spectral distance (see 3.12) from the maximum value of one or both sides of the Bragg wavelength (3) FBG signal-to-noise ratio SNRFBG for (2) Figure 1 – Characteristics of the Bragg grating reflectance spectrum
3.8
side-lobes reflection peaks aside the Bragg wavelength peak of an FBG spectrum Note 1 to entry: Side-lobes are also called side modes.
Note 2 to entry: Side-lobes shall be considered according to conditions of use (see Figure 1 and Clause A.2).
3.9
relative side-lobe level ratio of the maximum value of the amplitude of the specified field component in a side-lobe to the maximum value in a reference lobe Note 1 to entry: The reference lobe of an FBG is the peak power at the Bragg wavelength B; peak power of the largest side-lobe in the FBG spectrum is the related field component (see Figure 1).
Note 2 to entry: Relative side-lobe level is usually expressed in decibels. IEC 1 546,25 1 545,00 1 543,75 Wavelength
(nm) Bragg wavelength FWHM (3) (2) (1) 3 dB –60 –50 –40 –30 Intensity
(dBm)
IEC 61757-1-1:2016 © IEC 2016 – 11 – 3.10
width level relative amplitude difference between a local maximum and a specified amplitude, at which a spectral feature is evaluated for a two sided threshold crossing for purposes of defining that local maximum as either a fundamental peak or as a side-lobe Note 1 to entry: The width level is applied as an evaluative relative threshold to a local maximum. Note 2 to entry: Width level is expressed in decibels. 3.11
peak width
width over which a local maximum exhibits a two-sided spectrum crossing over a threshold defined by the width level parameter Note 1 to entry: The quantity FBG spectral width is defined as the spectral width of the FBG fundamental mode and will be equal to or greater than the peak detection algorithm’s peak width requirement when the width level is defined as 3dB. Note 2 to entry: The peak width requirement is applied in conjunction with the width level parameter to distinguish fundamental peaks from side-lobes in an array spectrum where side-modes may be at an absolute amplitude higher than adjacent fundamental peaks. Note 3 to entry: Peak width is expressed in nanometres. 3.12
SNRFBG FBG signal-to-noise ratio ratio of the maximum amplitude of the Bragg wavelength peak to that of the coexistent side-lobe amplitude at a wavelength distance of 1 nm under unloaded conditions
Note 1 to entry: SNRFBG shall not be confused with the side-lobes of an FBG caused by the inscription process and depending on the grid number, grid distance Λ and the change in the refractive index of the FBG. Noise is generated by the measurement device; side-lobes are generated during inscription of the grating and have great importance for the use of an FBG as strain sensor (see Figure 1 and 3.7).
Note 2 to entry: The value "1 nm" is still valid even if the central wavelength of an FBG is extended to the visible range. Note 3 to entry: FBG signal-to-noise ratio is expressed in decibels. 3.13
FBG strain sensitivity ratio of the relative change in wavelength ∆/0 for a given strain change ∆0 defined by the equation
ελλû)1(û0p−= Note 1 to entry: FBG strain sensitivity describes the response of an FBG to uniaxial strain deformation ∆ε of the grating area. The strain response is represented by the photo-elastic coefficient p. For practical use, the gauge factor k is introduced as a linear approximate for (1 – p). In this case, the sensitivity can be considered as a linear function for a uniformly non-integrated stretched grating area (see 7.6), i.e. only the optical fibre and coating are deformed.
Note 2 to entry: Frequently, this term is defined, for practical reasons, as the peak shift (∆λ in nm) over the introduced strain change (∆0 in µm/m) related to a specified reference wavelength λ0. Note 3 to entry: Strain sensitivity can be superimposed by temperature-induced deformation of the optical fibre. Note 4 to entry: If the strain sensitivity gets a non-linear characteristic because of the set-up of for example a strain transducer, higher order terms may be used. The calibration function and the parameters have to be defined. SIST EN 61757-1-1:2017

– 12 – IEC 61757-1-1:2016 © IEC 2016 3.14
k gauge factor
ratio of the relative change in wavelength ∆/0 to a mechanical strain ∆0 introduced to an FBG strain sensor and expressed by the dimensionless gauge factor k measured by the manufacturer ελλûû0=k Note 1 to entry: The gauge factor k is used by manufacturers to express the strain response of their products.
Note 2 to entry: The gauge factor k considers all influences of the FBG strain sensor on the strain sensitivity. It can vary with the selected structural form of the strain sensor (e.g. Bragg grating fibre with special protecting layer or FBG strain gauge) and therefore has to be distinguished from the strain sensitivity of the Bragg grating in the optical fibre (see 3.13).
Note 3 to entry: The gauge factor k for an FBG strain sensor assumes a linear characteristic. Considering the whole measurement system (sensor, device, cabling), it can be separately defined for the components of the measurement system. It is only valid for defined conditions. In the case of a non-linear characteristic (e.g. by creeping effect in the strain transfer), the gauge factor k is considered as linear within a defined permissible error. 3.15
gauge length length within which a strain will cause a change in the measured value of the FBG strain sensor Note 1 to entry: The gauge length depends on the FBG strain sensor configuration, see 5.2. 3.16
minimum operating radius of curvature minimum radius that an FBG may be bent without change of the specified performance parameters 3.17
FBG sensor strain range
maximum strain range that the FBG can measure being excited according to the stated mechanical conditions without change of the specified performance parameters Note 1 to entry: This could include axial tensile strain and compression. Note 2 to entry: Outside the range, the FBG strain sensor may not be physically damaged, but the specified measurement performance may be affected. 3.18
FBG period
distance between the periodically varying refractive index zones (grating planes) in the fibre and expressed by
Note 1 to entry: The FBG period defines the Bragg wavelength (see 3.3) by the equation
effB2n⋅=λΛ 3.19
fatigue behaviour change in sensor properties as a result of sinusoidal load alternation under reference ambient conditions SIST EN 61757-1-1:2017

IEC 61757-1-1:2016 © IEC 2016 – 13 – Note 1 to entry: The relevant sensor properties specifying fatigue behaviour are the zero point displacement (see 3.20) and the change in the reflection spectrum of the FBG strain sensor as a function of the number of load cycles. 3.20
zero point initial value of a measurement cycle to which all following measurement values are referred
Note 1 to entry: The zero point is also called null set.
Note 2 to entry: The zero point shall be recorded for all types of measurements (static, dynamic). In case of off-line measurements, where recording devices are switched-off or disconnected, continued measurement shall be referable to the zero point.
3.21
temperature influence to an FBG strain sensor change in Bragg wavelength (3.3) of an FBG strain sensor subject to thermal excitation only Note 1 to entry: The temperature-induced strain is observed as an apparent strain. Note 2 to entry: The term "temperature sensitivity" is not used because it refers to temperature measurement, whereas the characteristic here considered is related to “temperature compensation” of the signal.
3.22
birefringence optical property of an optically anisotropic material having orientation-dependent refractive indices that leads to different propagation velocities of light in different propagation directions Note 1 to entry: Birefringence is a property of optical materials. Note 2 to entry: For fibre optic sensors, the term "birefringence" is correctly used when optical fibres with birefringent property are used, for example panda or bow-tie fibres. 3.23
polarization dependence dependence which occurs when transverse loading causes a fibre's nominally circular cross section to become elliptical with the result of splitting the back-reflected Bragg spectra into two unequally reflected or transmitted waves which produces a double peak in the spectra
Note 1 to entry: Polarization dependence of Bragg wavelength can also occur during writing of the fibre Bragg grating if the writing laser is not correctly focused in the centre of the core but is instead focused on one side in the cladding. In this case, asymmetry in the refractive index of the glass due to asymmetry of the expose is created. Note 2 to entry: Polarization dependence of Bragg wavelength can also lead to measurement uncertainty of Bragg wavelength, spectral width and FBG reflectivity. 4 Symbols
For the purposes of this document, the following symbols apply. h
thickness of the deformed object of measurement Iref
optical power intensity of the reference fibre k
gauge factor k l, L
length Lo
original length of the object of measurement L1
length of the object of measurement after deformation LF
length of the free fibre inside a strain transducer LG
length between the anchoring points of the FBG strain sensor to the object of
measurement (gauge length)
n
refractive index of the waveguide neff
effective refractive index of the Bragg grating (see 5.1) SIST EN 61757-1-1:2017

– 14 – IEC 61757-1-1:2016 © IEC 2016 p0
effective photo-elastic constant p
photo-elastic constant P0
incident optical power
BλP
optical power of the FBG RFBG
reflectivity of the FBG Rref
reflectivity of the FBG reference fibre s
distance of the fibre sensor from the surface of the object of measurement SNRFBG signal-to-noise ratio of the FBG T
temperature x
mean value xi
ith measured value X
physical parameter (e.g. temperature, strain or pressure) .
thermal expansion coefficient of the fibre material .gm
thermal expansion coefficient of the load-carrying material of the strain gauge .sp
thermal expansion coefficient of the test sample ∆
∆ = λ-λ0, FBG peak wavelength shift under the given strain ∆0 0
strain (here always observed in the direction of the fibre axis) 0a
strain applied to the test sample 0neff
temperature-induced strain (thermal output) 0OF
flexural strain at the surface of the object of measurement 0OSS
strain measured by an applied FBG strain sensor (for bent objects of measurement, see 7.6.2) 0p
strain at the surface of a flexural beam 0p'
strain of a flexural beam measured with an attached sensor of finite thickness 0s
apparent strain 0
reference wavelength B
Bragg wavelength
FBG period
thermo-optical coefficient 3
logarithmic strain 5 Structure and characteristics 5.1 Fibre Bragg grating (FBG) Fibre Bragg gratings are phase diffraction gratings inscribed into optical waveguides. They are frequently produced using UV-light (e.g. by an excimer laser at 248 nm). The fibre is exposed to an interference pattern of this UV radiation. UV photosensitive processes then produce changes in the refractive index of the fibre core which is susceptible to these. The interference pattern is an image in the fibre core of a periodically changing refractive index. Incident and transported light along the fibre is additively superposed for a certain wavelength at these points (constructive interference); this spectral part of the incident light is reflected. In the transmitted light, this wavelength (denoted Bragg wavelength B) is attenuated according to FB
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