IEC 61757-6-1:2024

IEC 61757-6-1 ®
Edition 1.0 2024-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Fibre optic sensors –
Part 6-1: Displacement measurement – Displacement sensors based on fibre
Bragg gratings
Capteurs fibroniques –
Partie 6-1: Mesure de déplacement – Capteurs de déplacement basés sur des
réseaux de Bragg sur fibre
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IEC 61757-6-1 ®
Edition 1.0 2024-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Fibre optic sensors –
Part 6-1: Displacement measurement – Displacement sensors based on fibre

Bragg gratings
Capteurs fibroniques –
Partie 6-1: Mesure de déplacement – Capteurs de déplacement basés sur des

réseaux de Bragg sur fibre
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 33.180.99 ISBN 978-2-8322-8134-5

– 2 – IEC 61757-6-1:2024 © IEC 2024
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms, definitions, symbols and abbreviated terms . 8
3.1 Terms and definitions . 8
3.2 Symbols . 9
3.3 Abbreviated terms . 9
4 Structure and characteristics . 9
4.1 Fibre Bragg grating (FBG) . 9
4.2 FBG displacement sensor configuration . 10
4.3 Reference wavelength . 12
4.4 Stability behaviour . 13
4.4.1 Drift and creep . 13
4.4.2 Hysteresis . 13
4.5 Indication of the measured values . 13
4.6 Zero-point related measurement . 14
4.7 Non-zero-point related measurement . 14
4.8 Production set . 14
4.9 FBG displacement sensor standard type . 14
4.10 FBG displacement sensor series . 14
5 Features and characteristics to be measured . 14
5.1 Sampling and statistical evaluation . 14
5.1.1 Sampling . 14
5.1.2 Reporting the measuring result . 15
5.1.3 Sample conditioning . 15
5.1.4 Ambient test conditions . 16
5.1.5 Required types of tests for individual characteristics . 16
5.2 Bragg wavelength λ . 16
Β
5.2.1 General . 16
5.2.2 Measurement procedure . 16
5.2.3 Evaluation . 16
5.2.4 Reporting . 17
5.3 FBG spectral width. 17
5.3.1 Measurement procedure . 17
5.3.2 Evaluation . 17
5.3.3 Reporting . 17
5.4 FBG reflectivity . 17
5.4.1 Measurement procedure . 17
5.4.2 Evaluation . 17
5.4.3 Reporting . 18
5.5 Displacement measurement . 18
5.5.1 General . 18
5.5.2 Test setup . 18
5.5.3 Measurement procedure . 19
5.5.4 Calibration and evaluation . 20

5.6 Displacement conversion factor . 21
5.7 Temperature and humidity ranges . 21
5.7.1 General . 21
5.7.2 Measurement procedure . 22
5.7.3 Evaluation . 22
5.7.4 Reporting . 22
5.8 Durability . 22
5.8.1 General . 22
5.8.2 Measurement procedure . 22
5.8.3 Reporting . 22
6 Features and characteristics to be reported . 23
6.1 Construction details . 23
6.2 Configuration of the FBG displacement sensor. 23
6.3 Temperature and humidity range . 23
6.4 Connecting requirement . 23
7 Recommendations for use of FBG measuring instruments . 23

Figure 1 – Examples of sensor types for measuring displacement changes . 10
Figure 2 – Bragg wavelength change caused by displacement in a spring-type sensor . 11
Figure 3 – Bragg wavelength changes caused by displacement in a metal-plate-type
sensor . 11
Figure 4 – Schematic diagrams of displacement sensors using two FBGs . 12
Figure 5 – Schematic diagram of a displacement measurement test setup . 18
Figure 6 – Example of temperature dependence of the Bragg wavelengths of two FBGs . 19
Figure 7 – Example of displacement dependence of the Bragg wavelengths of FBG1

and FBG2 . 20

Table 1 – Required types of tests for individual characteristics . 16

– 4 – IEC 61757-6-1:2024 © IEC 2024
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIBRE OPTIC SENSORS –
Part 6-1: Displacement measurement –
Displacement 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
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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
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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
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC 61757-6-1 has been prepared by subcommittee 86C: Fibre optic systems and active
devices, of IEC technical committee 86: Fibre optics. It is an International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
86C/1874/CDV 86C/1891/RVC
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.

This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 61757 series, published under the general title Fibre optic sensors,
can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
– 6 – IEC 61757-6-1:2024 © IEC 2024
INTRODUCTION
This document is part of the IEC 61757 series, which is dedicated to fibre optic sensors. Generic
specifications for fibre optic sensors are defined in IEC 61757.
The individual parts of the IEC 61757 series are numbered as IEC 61757-M-T, where M denotes
the measure and T the technology. The IEC 61757-6-T series is concerned with displacement
measurements.
FIBRE OPTIC SENSORS –
Part 6-1: Displacement measurement –
Displacement sensors based on fibre Bragg gratings

1 Scope
This part of IEC 61757 defines the terminology, structure, and measurement methods of optical
displacement sensors based on fibre Bragg gratings (FBGs) as the sensing element. This
document also specifies the most important features and characteristics of these fibre optic
displacement sensors and defines procedures for measuring these features and characteristics.
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 60068-2 (all parts), Environmental testing – Part 2-X: Tests
IEC 61300-2 (all parts), Fibre optic interconnecting devices and passive components – Basic
test and measurement procedures – Part 2-X: Tests
IEC 61754 (all parts), Fibre optic interconnecting devices and passive components – Fibre optic
connector interfaces
IEC 61757, Fibre optic sensors – Generic specification
IEC 61757-1-1:2020, Fibre optic sensors – Part 1-1: Strain measurement – Strain sensors
based on fibre Bragg gratings
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 62129-3, Calibration of wavelength/optical frequency measurement instruments – Part 3:
Optical frequency meters internally referenced to a frequency comb
ISO/IEC GUIDE 98-3, Uncertainty of measurement – Part 3: Guide to the expression of
uncertainty in measurement (GUM:1995)

– 8 – IEC 61757-6-1:2024 © IEC 2024
3 Terms, definitions, symbols and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61757, IEC 61757-1-1
and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1.1
displacement
D
distance change between two given points
Note 1 to entry: A displacement is designated as an absolute displacement if only one of the two given points
changes its position.
Note 2 to entry: A displacement is designated as a relative displacement if both of the two given points change their
position.
3.1.2
FBG displacement sensor
fibre optic sensor that uses one or more fibre Bragg gratings as a sensing element for
displacement measurements
3.1.3
displacement conversion factor
κ
D
ratio of the relative change in wavelength ∆λ / λ to a displacement change ∆D introduced to an
FBG displacement sensor
Note 1 to entry: The displacement conversion factor κ is calculated as
D
 
Δλ
 
 
λ
 
κ =
D
ΔD
Note 2 to entry: The displacement conversion factor κ is used by manufacturers to characterize the displacement
D
response of their products.
Note 3 to entry: The conversion factor κ for an FBG displacement sensor assumes a linear relation between
D
wavelength change and displacement. Considering the whole measurement system (sensor, device, and 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, the relation between wavelength change and displacement is considered to
be linear within a defined permissible measurement error.
Note 4 to entry: The term displacement sensitivity, expressed for example in pm/mm, is used by some
manufacturers to characterize the displacement response of their products.

3.1.4
temperature compensation constant
C
constant for correcting the influence of temperature changes when the displacement is obtained
from wavelength changes
Note 1 to entry: The temperature compensation constant should be provided by the manufacturer.
Note 2 to entry: The term temperature sensitivity, expressed for example in (pm/°C), is used by some manufacturers
to characterize the influence of temperature changes of their products.
3.2 Symbols
For the purposes of this document, the following symbols apply:
R reflectivity of the FBG
FBG
n effective refractive index of the FBG
eff
∆D displacement change
∆T temperature change
Λ FBG period
λ Bragg wavelength
B
λ reference wavelength
3.3 Abbreviated terms
FBG fibre Bragg grating
FWHM full width at half maximum
SNR signal-to-noise ratio
UV ultraviolet
4 Structure and characteristics
4.1 Fibre Bragg grating (FBG)
Fibre Bragg gratings are phase diffraction gratings inscribed into optical waveguides. They are
frequently produced using ultraviolet (UV) light (e.g. from 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 this UV light.
The interference pattern is imaged onto the fibre core to permanently change the refractive
index of the fibre core, so that the refractive index varies periodically along the fibre. Incident
and transported light is reflected by these periodic refractive index changes along the fibre. At
a certain wavelength, the reflected light is additively superimposed (constructive interference);
this spectral part of the incident light is reflected back to the input port of the fibre. In the
transmitted light, this wavelength (denoted Bragg wavelength λ ) is attenuated accordingly, due
Β
to the FBG reflectivity.
The value of the reflected Bragg wavelength λ is determined by the Bragg condition shown in
Β
Formula (1).
λ = 2n Λ
B eff
(1)
– 10 – IEC 61757-6-1:2024 © IEC 2024
According to Formula (1), the Bragg wavelength λ of the FBG depends on the effective
Β
refractive index n of the FBG and on the FBG period Λ. The spectral width of the Bragg
eff
wavelength peak is determined by the number of grating periods and the magnitude of the
refractive index modulation (see IEC 61757-1-1:2020, 5.1).
4.2 FBG displacement sensor configuration
The FBG displacement sensor can be fabricated from various materials and in various forms
(using one or more FBGs as sensing elements). The FBG displacement sensor is typically used
to monitor the displacement occurring between two points of different objects, or between two
parts of the same object. One example is monitoring of displacement changes at expansion
joints that are installed in bridges to prevent damage to the structure; these joints contract and
expand due to temperature changes. Another example of detecting displacement changes is
monitoring of crack size changes in structures where cracks have occurred.
The method used to convert a displacement change into a change of the Bragg wavelength of
an FBG depends on the manufacturer of the displacement sensor. There are a variety of
methods, but a comprehensive description of these methods is outside the scope of this
document.
As an example, the FBG displacement sensor can be configured so that the movement of a
stylus (which is the displacement sensing part) by means of a mechanical transducer causes a
corresponding strain change in the FBG, as shown in Figure 1 for a spring-type sensor and for
a metal-plate-type sensor. This strain change in the FBG then changes the reflected Bragg
wavelength of the FBG, as shown in Figure 2 and Figure 3. Hence, the displacement can be
determined by measuring the reflected Bragg wavelength of the FBG.

a) Spring type b) Metal plate type

Figure 1 – Examples of sensor types for measuring displacement changes
In the spring-type sensor shown in Figure 1 a), a displacement change causes a pulley to rotate,
which changes the length of a tensile spring that is attached to an FBG. The resulting change
in the tensile force applied to the FBG causes a strain change in the FBG, which in turn changes
the reflective wavelength of the FBG, as illustrated in Figure 2 a) and Figure 2 b). When
installing the FBG displacement sensor shown in Figure 2 a), the stopper is often placed near
the centre of the expected displacement changes, so that positive as well as negative
displacement changes can be detected. If displacement changes are expected to occur in only
one direction, a larger change in displacement can be measured by placing the stopper more
towards the left or right side.

a) No displacement change b) With wavelength and displacement change

Figure 2 – Bragg wavelength change caused by displacement in a spring-type sensor
In the metal-plate-type sensor shown in Figure 1 b), a change in displacement causes an elastic
metal plate to bend. The change in the shape of the metal plate is detected by two FBGs,
denoted FBG1 and FBG2, which are attached to both sides of the metal plate, as shown in
Figure 3 a). When the displacement-measuring stylus moves from its original position to the left
direction, the elastic metal plate bends further to the left, which causes a contraction of FBG1
and an expansion of FBG2. Since the contraction of FBG1 results in increased stress and the
expansion of FBG2 in increased strain, the displacement changes the reflective Bragg
wavelengths of the two FBGs in opposite directions, as shown in Figure 3 b).

a) No displacement change b) With wavelength and displacement change

Figure 3 – Bragg wavelength changes caused by displacement
in a metal-plate-type sensor
A broadband light source and an optical spectrometer can be used to measure the change in
the Bragg wavelength of an FBG. The light source and the spectrometer are typically connected
to the displacement sensing FBG via an optical circulator, as shown schematically in Figure 4
for the spring-type and the metal-plate-type sensors.

– 12 – IEC 61757-6-1:2024 © IEC 2024
In the spring-type sensor shown in Figure 4 a), an additional FBG (denoted FBG1) is inserted
near the displacement sensing FBG (denoted FBG2) to allow for compensation of the
temperature dependence of FBG2 (as described in 5.5.3). The additional FBG1 measures only
temperature changes, whereas FBG2 measures displacement and temperature changes. FBG1
and FBG2 can be connected in series, as shown in Figure 4 a), or in parallel, like in the
b).
arrangement shown in Figure 4
The metal-plate-type sensor shown in Figure 4 b) employs two FBGs for the displacement
measurement. An additional FBG for temperature measurement is not necessary in this
arrangement, because the displacement change is calculated from the differential change in
the reflected wavelengths of FBG1 and FBG2. If the Bragg wavelength variations due to
temperature changes are identical in both FBGs, the temperature dependence of the Bragg
wavelength is automatically compensated.
The metal-plate-type sensor of Figure 4 b) uses an optical coupler with 3 dB splitting ratio to
connect FBG1 and FBG2 to the broadband light source and spectrometer.

a) Spring type with additional FBG for temperature measurement

b) Metal plate type using two FBGs for displacement measurement
Figure 4 – Schematic diagrams of displacement sensors using two FBGs
4.3 Reference wavelength
The Bragg wavelength measured with a given FBG can depend on the evaluation method used
and, more importantly, on the specific installation of the FBG. In the context of this document,
the wavelength measured after installation of the FBG in the displacement sensor is denoted
as the reference wavelength λ
The reference wavelength is not necessarily the same as the Bragg wavelength specified by
the manufacturer of the FBG. If the FBG is pre-strained, for example, there is a difference
between the reference wavelength and the manufacturer's Bragg wavelength. If the FBG is not
pre-strained, the difference between the reference wavelength and the manufacturer's Bragg
wavelength is usually very small, so that both wavelength values can be used interchangeably
without significant error.
If the reference wavelength is measured when the measurement cycle is started, this
wavelength measurement may be considered as the zero-point measurement value.
4.4 Stability behaviour
4.4.1 Drift and creep
Stability, in general, is the ability of a measurement system to maintain its metrological
characteristics and to meet other specifications over the intended time of operation. In the
context of this document, stability describes the property of the applied FBG displacement
sensor to maintain its optical characteristics over the time period of use, which is determined
by the application, or to show only small permissible deviations.
Variations in the measured value can occur when:
– the materials concerned are subject to long-term stress (creep),
– no loading stress is applied (zero-point drift).
Creep and zero-point drift can result from slowly progressing chemical or physical degradation
of the materials used in the sensor (e.g. from ageing), or from changes of the initial
environmental conditions (e.g. either temperature or humidity, or both).
Creep is a quantity that depends on the materials used in the sensor, the set-up of the sensor,
and the type of operation. It can only be determined experimentally. Provided that the bonding
material prescribed by the manufacturer is used, the measurement errors resulting from creep
are usually insignificant compared to the measurement uncertainty of the displacement
conversion factor κ .
D
Drift is a slow change of the metrological characteristics of the measurement system. For state-
of-the-art FBG sensors, the measurement error resulting from drift is negligibly small. In this
case, no further specification of drift is required. However, if drifts are generated in a modified
production process, for example, or by applying inadequate recoating materials, the drift should
be stated.
4.4.2 Hysteresis
Hysteresis describes a particular material behaviour, whereby the material does not return to
its original state, or does so with a significant time delay, after the input load has been removed.
This means that the output value of a sensor with elasto-plastic deformation behaviour does
not only depend on the input value but also on rate-dependent processes.
When the displacement (or temperature) of a silica-based FBG changes, the Bragg wavelength
commonly shifts without showing a hysteresis effect. If hysteresis occurs for repeated or cyclic
displacement variations within the specified operation range of the sensor, the amount of
hysteresis should be described.
4.5 Indication of the measured values
The variations in Bragg wavelength induced in the FBG are measured by a connected
measuring device (yielding the measured values) and processed for metrological use (yielding
the result of the measurement). Customarily, the measuring device supplies the optical input
signal for the sensor and records the sensor response signal.

– 14 – IEC 61757-6-1:2024 © IEC 2024
4.6 Zero-point related measurement
The concepts of "zero-point measurement" and "static or quasi-static measurement",
respectively, are used to denote all measurements where the measured value refers to an initial
value (the zero point).
The following influencing factors shall also be considered:
– drift in the measuring instrument;
– method of evaluation:
different evaluation methods (using different measuring devices) can result in different offset
quantities with respect to the zero point. In case of replacement of the measuring device,
the zero-point offset between the old and the new instrument should be determined
correspondingly;
– creep of the applied sensor.
The scanning procedure of the FBG displacement sensors shall take place in a route-neutral
manner, so that the characteristics of the connecting leads and of the optical connectors or
splices do not affect the zero point. Nevertheless, intermittent zero-point checking is
recommended.
4.7 Non-zero-point related measurement
For non-zero-point related or periodic dynamic measurements, the measured values are not
referred to a fixed initial value. This applies, for example, to the amplitude measurement of a
periodic oscillation.
4.8 Production set
An FBG set is a batch of FBG produced in the same manufacturing process.
4.9 FBG displacement sensor standard type
An FBG displacement sensor standard type is a batch of FBG displacement sensors with
identical physical properties (geometrical dimensions, manufacturing process, materials used,
post-processing, and Bragg wavelength).
4.10 FBG displacement sensor series
A series is a batch of FBG displacement sensors for which the materials used, and the
manufacturing processes are identical, but which can show differences in their Bragg
wavelength or dimensions.
5 Features and characteristics to be measured
5.1 Sampling and statistical evaluation
5.1.1 Sampling
5.1.1.1 General
The following sampling methods shall be used according to the intended scope of testing:
– random sampling;
– type testing;
– series testing;
– individual sample testing.
Many of the FBG displacement sensor properties can only be determined on an installed sensor.
A statistical evaluation shall be performed in this case. The number of sample sensors and the
date of evaluation should be noted.
5.1.1.2 Random sampling
The requirement for performing random sampling is the assumption that the variations of the
characteristic parameter follow a Gaussian distribution. All sensors chosen for characteristic
testing shall belong to the same production set. At least five samples shall be selected. The
result of a random sampling test is valid for one production set.
5.1.1.3 Type testing
The type test is a random sampling test as described in 5.1.1.2, whereby the result of testing
of at least five specimens of this type is declared valid for all production sets.
5.1.1.4 Series testing
The series test is a random sampling test as described in 5.1.1.2, whereby the result is
determined for a single specimen out of a sensor series and declared valid for the whole series.
5.1.1.5 Individual sample testing
Each specimen of a sensor series or just a prototype of a unique FBG displacement sensor
shall be tested.
5.1.2 Reporting the measuring result
The results of the series tests, type tests and random sampling tests are expressed as the
arithmetic mean value and the corresponding standard deviation. The form of the statement of
the standard deviation shall be specified. Estimation of measurement uncertainty shall be
carried out according to ISO/IEC GUIDE 98-3.
If sensors X to X are tested, then the characteristic is quoted as the mean value x of the n
1 n
values x to x measured with the sensors, as shown in Formula (2).
1 n
n
xx=
(2)
∑
i
n
i=1
The standard deviation of the n measured values is given by Formula (3).
n
(3)
s ( x− x)
∑ i
n−1
i=1
5.1.3 Sample conditioning
The sensors selected for testing shall be allowed to reach equilibrium with the environment in
which the test shall be performed; exposure of at least 2 h to such an environment should be
adopted.
=
– 16 – IEC 61757-6-1:2024 © IEC 2024
5.1.4 Ambient test conditions
All tests shall be performed at specified temperature and relative humidity conditions; the values
of the parameters and their tolerance shall be reported. The recommended ambient temperature
is 15 °C to 35 °C, the recommended humidity is 25 % RH to 75 % RH, and the recommended
atmospheric pressure is 86 kPa to 106 kPa.
5.1.5 Required types of tests for individual characteristics
The required types of tests for individual characteristics are specified in Table 1.
Table 1 – Required types of tests for individual characteristics
Design-specific features and characteristics Type of test
Bragg wavelength Individual sample test
FBG spectral width Series test
Reflectivity Type test
Displacement conversion factor Series test
Temperature compensation constant Series test
Operating temperature and humidity ranges Series test
Durability Series test
5.2 Bragg wavelength λ
Β
5.2.1 General
The following characteristics of an FBG spectrum shall be measured as required by this
document or upon customer request:
– peak of Bragg wavelength in nm;
– FBG spectral width in nm;
– FBG reflectivity in %;
– relative side-lobe level (also called side-lobe suppression ratio) in dB;
– FBG signal-to-noise ratio (SNR) in dB;
– first poles (minima) at the peak of the reflected Bragg wavelength (denoted Λ and Λ ,).
+0 −0
5.2.2 Measurement procedure
For FBGs with relatively low reflectivity (R < 50 %), the Bragg wavelength shall be measured
FBG
in reflection. For FBGs with higher reflectivity (R > 90 %), on the other hand, the Bragg
FBG
wavelength shall be measured in transmission, because the maximum of the reflected Bragg
wavelength becomes progressively more difficult to determine accurately. Therefore, the
transmission minimum shall be used for measuring the Bragg wavelength. For intermediate
values of reflectivity (R between 50 % and 90 %), either configuration can be used.
FBG
The Bragg wavelength of the FBG shall be measured with sufficient spectral resolution and
reported. The spectral resolution shall be less than one-tenth of the measurement uncertainty
of the wavelength to be measured. The measurement method used, and the corresponding
uncertainty (spectral resolution) should be reported. In case of polarization effects, special
measurements shall be carried out.
5.2.3 Evaluation
No evaluation is necessary for the Bragg wavelength measurement.

5.2.4 Reporting
The measured or calculated Bragg wavelength and the measurement procedure shall be
reported. On customer request, the typical FBG spectrum shall also be reported.
5.3 FBG spectral width
5.3.1 Measurement procedure
The FBG spectrum of the FBG displacement sensor shall be measured with a spectral resolution
that is less than one-tenth of the measurement uncertainty of the wavelength to be measured.
The constancy of the spectral width impacts the measurement uncertainty when using
mathematical evaluation principles for determining the Bragg wavelength. The spectral width
can be affected by different influencing quantities, for example by temperature, maximum
possible displacement, and continuous oscillation behaviour.
5.3.2 Evaluation
The measured FBG spectrum shall be evaluated according to the definition. The spectral width
shall be determined from a reflection spectrum, whereby the difference of the two wavelength
values at the 3 dB drop-off points is taken from both sides of the reflection maximum.
Alternatively, the transmission spectrum shall be used to determine the Bragg wavelength using
an appropriate spectrum evaluation technique.
5.3.3 Reporting
The typical spectral width shall be reported. On customer request, the FBG spectrum shall also
be reported.
5.4 FBG reflectivity
5.4.1 Measurement procedure
The FBG spectrum of the FBG displacement sensor shall be measured with sufficient spectral
resolution.
5.4.2 Evaluation
The measured FBG spectrum shall be evaluated according to the defi
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