IEC 61757-1-1:2016

IEC 61757-1-1 ®
Edition 1.0 2016-02
INTERNATIONAL
STANDARD
colour
inside
Fibre optic sensors –
Part 1-1: Strain measurement – Strain sensors based on fibre Bragg gratings

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IEC 61757-1-1 ®
Edition 1.0 2016-02
INTERNATIONAL
STANDARD
colour
inside
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 λ . 24
B
7.2.1 General . 24
7.2.2 Measuring procedure . 24
7.2.3 Evaluation . 25
7.2.4 Reporting . 25

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

– 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 R from the FBG reflection spectrum (left, Equation (9))
FBG
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

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
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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8) Attention is drawn to the normative references cited in this publication. Use of the referenced publications is
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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.
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

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
λ
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
λ
reference wavelength
wavelength response of an FBG after installation or at the beginning of measurement to the
affecting loading and ambient conditions
3.5
R
FBG
FBG reflectivity
ratio of the incident optical power P to the reflected optical power P at Bragg wavelength λ
0 B
λ
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 P (see formulas in 7.4.2,) that represents the measurable part at the connector of a
fibre optic sensor.
Note 2 to entry: P 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

– 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 λ (see Figure 1).
B
–30
3 dB
(1)
(2)
–40
(3)
–50
FWHM
Bragg wavelength
–60
1 543,75 1 545,00 1 546,25
Wavelength (nm)
IEC
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 SNR for (2)
FBG
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 λ ; peak power of the
B
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.
Intensity (dBm)
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
SNR
FBG
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: SNR shall not be confused with the side-lobes of an FBG caused by the inscription process
FBG
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 ∆λ/λ for a given strain change ∆ε defined by the
equation
Δλ
= (1− p)Δε
λ
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 (∆ε in µm/m) related to a specified reference wavelength λ
0.
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.

– 12 – IEC 61757-1-1:2016 © IEC 2016

3.14
k
gauge factor
ratio of the relative change in wavelength ∆λ/λ to a mechanical strain ∆ε introduced to an
FBG strain sensor and expressed by the dimensionless gauge factor k measured by the

manufacturer
Δλ
λ
k=
Δε
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
Λ
FBG period
distance between the periodically varying refractive index zones (grating planes) in the fibre
and expressed by Λ
Note 1 to entry: The FBG period defines the Bragg wavelength (see 3.3) by the equation
λ
B
Λ=
2⋅ n
eff
3.19
fatigue behaviour
change in sensor properties as a result of sinusoidal load alternation under reference ambient
conditions
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
I  optical power intensity of the reference fibre
ref
k  gauge factor k
l, L  length
L  original length of the object of measurement
o
L  length of the object of measurement after deformation
L  length of the free fibre inside a strain transducer
F
L  length between the anchoring points of the FBG strain sensor to the object of
G
measurement (gauge length)
n  refractive index of the waveguide
n  effective refractive index of the Bragg grating (see 5.1)
eff
– 14 – IEC 61757-1-1:2016 © IEC 2016

p  effective photo-elastic constant
ε
p  photo-elastic constant
P  incident optical power
P  optical power of the FBG
λ
B
R  reflectivity of the FBG
FBG
R  reflectivity of the FBG reference fibre
ref
s  distance of the fibre sensor from the surface of the object of measurement

SNR signal-to-noise ratio of the FBG
FBG
T  temperature
x  mean value
th
x  i measured value
i
X  physical parameter (e.g. temperature, strain or pressure)
α  thermal expansion coefficient of the fibre material
α  thermal expansion coefficient of the load-carrying material of the strain gauge
gm
α  thermal expansion coefficient of the test sample
sp
∆λ  ∆λ = λ-λ , FBG peak wavelength shift under the given strain ∆ε
ε  strain (here always observed in the direction of the fibre axis)
ε  strain applied to the test sample
a
ε temperature-induced strain (thermal output)
n
eff
ε  flexural strain at the surface of the object of measurement
OF
ε strain measured by an applied FBG strain sensor (for bent objects of
OSS
measurement, see 7.6.2)
ε  strain at the surface of a flexural beam
p
ε  strain of a flexural beam measured with an attached sensor of finite thickness
p'
ε  apparent strain
s
λ  reference wavelength
λ  Bragg wavelength
B
Λ FBG period
ξ  thermo-optical coefficient
φ  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 λ ) is attenuated
B
according to FBG reflectivity. Figure 2 shows the principle of a fibre Bragg grating in an
optical waveguide.
The value of the reflected Bragg wavelength λ is determined from the Bragg condition:
B
λ = 2⋅ n ⋅Λ
(1)
B eff
According to Equation (1), the Bragg wavelength λ of the FBG depends on the effective
B
refractive index of the FBG and the FBG period Λ. The spectral width of the Bragg wavelength

peak is essentially determined by the number of grating periods and the magnitude of the
refractive index modulation.
According to Equation (1), the FBG is susceptible to changes in the FBG period and in the
effective refractive index, which may essentially be affected by changes in strain and
temperature. The Bragg wavelength λ changes (is "shifted") with changes in the FBG period
B
Λ, or with changes in the effective refractive index n .
eff
The wavelength is shifted to higher values when the glass fibre grating is placed in tension or
the temperature increases. The opposite process occurs for compression and a temperature
decrease. These effec
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