EN IEC 61757-2-1:2021

SLOVENSKI STANDARD
01-november-2021
Optični senzorji - 2-1. del: Merjenje temperature - Temperaturni senzorji na podlagi
optovlakenskih Braggovih rešetk (IEC 61757-2-1:2021)
Fibre Optic Sensors - Part 2-1: Temperature measurement - Temperature sensors based
on fibre Bragg gratings (IEC 61757-2-1:2021)
Lichtwellenleitersensoren - Teil 2-1: Temperaturmessung - Temperatursensoren auf der
Basis von Faser-Bragg-Gittern (IEC 61757-2-1:2021)
Capteurs fibroniques - Partie 2-1: Mesure de la température - Capteurs de température
basés sur des réseaux de Bragg à fibres (IEC 61757-2-1:2021)
Ta slovenski standard je istoveten z: EN IEC 61757-2-1:2021
ICS:
17.200.20 Instrumenti za merjenje Temperature-measuring
temperature instruments
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.

EUROPEAN STANDARD EN IEC 61757-2-1

NORME EUROPÉENNE
EUROPÄISCHE NORM
September 2021
ICS 33.180.99
English Version
Fibre optic sensors - Part 2-1: Temperature measurement -
Temperature sensors based on fibre Bragg gratings
(IEC 61757-2-1:2021)
Capteurs fibroniques - Partie 2-1: Mesure de la température Lichtwellenleitersensoren - Teil 2-1: Temperaturmessung -
- Capteurs de température basés sur des réseaux de Bragg Temperatursensoren auf der Basis von Faser-Bragg-Gittern
à fibres (IEC 61757-2-1:2021)
(IEC 61757-2-1:2021)
This European Standard was approved by CENELEC on 2021-09-01. 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, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, 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: Rue de la Science 23, B-1040 Brussels
© 2021 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 61757-2-1:2021 E

European foreword
The text of document 86C/1725/FDIS, future edition 1 of IEC 61757-2-1, prepared by SC 86C “Fibre
optic systems and active devices” of IEC/TC 86 “Fibre optics” was submitted to the IEC-CENELEC
parallel vote and approved by CENELEC as EN IEC 61757-2-1:2021.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2022–06–01
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2024–09–01
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 61757-2-1:2021 was approved by CENELEC as a
European Standard without any modification.
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
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.
NOTE 1 Where 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 - -
(IEV)
IEC 61757 - Fibre optic sensors - Generic specification EN IEC 61757 -
IEC 61757-1-1 2020 Fibre optic sensors - Part 1–1: Strain EN IEC 61757-1-1 2020
measurement - Strain sensors based on
fibre Bragg gratings
ISO/IEC Guide 98-3 - Uncertainty of measurement - Part 3: - -
Guide to the expression of uncertainty in
measurement (GUM:1995)
IEC 61757-2-1 ®
Edition 1.0 2021-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Fibre optic sensors –
Part 2-1: Temperature measurement – Temperature sensors based on fibre

Bragg gratings
Capteurs fibroniques –
Partie 2-1: Mesure de la température – Capteurs de température basés

sur des réseaux de Bragg à fibres

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 33.180.99 ISBN 978-2-8322-1009-5

– 2 – IEC 61757-2-1:2021 © IEC 2021
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms, definitions, abbreviated terms and symbols of quantities . 7
3.1 Terms and definitions . 8
3.2 Abbreviated terms . 11
3.3 Symbols of quantities . 12
4 Design and characteristics of an FBG temperature sensor . 13
4.1 Fibre Bragg grating (FBG) . 13
4.2 Dependence of Bragg wavelength on temperature . 13
4.3 Design features. 14
5 Performance parameters . 14
6 Test apparatuses for performance parameter determination . 15
6.1 Temperature calibration equipment . 15
6.2 Optical spectrum analyzer and interrogator . 16
6.3 Broadband light source . 16
7 Test procedures of performance parameters . 16
7.1 Sample preparation and test set-up . 16
7.2 Bragg wavelength λ . 17
Bref
7.2.1 Measuring procedure . 17
7.2.2 Evaluation . 18
7.2.3 Reporting . 18
7.3 FBG peak spectral width . 18
7.3.1 Measuring procedure . 18
7.3.2 Evaluation . 18
7.3.3 Reporting . 18
7.4 FBG reflectivity . 18
7.4.1 Measuring procedure . 18
7.4.2 Evaluation . 18
7.4.3 Reporting . 19
7.5 Side-lobe suppression ratio . 19
7.5.1 Measuring procedure . 19
7.5.2 Evaluation . 20
7.5.3 Reporting . 20
7.6 Signal-to-noise ratio . 21
7.6.1 Measuring procedure . 21
7.6.2 Evaluation . 21
7.6.3 Reporting . 21
7.7 Characteristic curve . 22
7.7.1 Measuring procedure . 22
7.7.2 Evaluation . 23
7.7.3 Reporting . 27
7.8 Thermal time constant. 28
7.8.1 Measuring procedure . 28
7.8.2 Evaluation . 28

IEC 61757-2-1:2021 © IEC 2021 – 3 –
7.8.3 Reporting . 28
7.9 Sensor stability . 29
7.9.1 Measuring procedure . 29
7.9.2 Evaluation . 29
7.9.3 Reporting . 29
Annex A (informative) Blank detail specification . 30
A.1 General . 30
A.2 Mechanical and optical set-up . 30
A.3 Operational characteristics . 30
A.4 Limiting parameters . 31
A.5 Further information given upon request . 31
Annex B (informative) Examples of specific temperature calibration equipment . 32
B.1 Simple liquid bath . 32
B.2 Liquid tube-thermostat . 33
B.3 Solid-state calibration equipment . 34
Annex C (informative) Contributions to measurement uncertainty . 37
Bibliography . 38

Figure 1 – Principal test set-up for FBG . 17
Figure 2 – Determination of the FBG reflectivity from the reflection spectrum (left) and
transmission spectrum (right) . 19
Figure 3 – Side-lobes in the case of a single FBG temperature sensor . 20
Figure 4 – Signal-to-noise ratio determination . 21
Figure 5 – Example of a polynomial fit of calibration points λ (T ) . 24
B,i N,i
Figure 6 – Example of a third-order polynomial fit . 25
Figure 7 – Example of a fourth-order polynomial fit . 26
Figure 8 – Example of a polynomial fit of the sensitivity . 27
Figure 9 – Typical response time curve . 28
Figure B.1 – Schematic representation of a simple liquid bath [3] . 32
Figure B.2 – Schematic representation of liquid calibration device for connection to
laboratory liquid thermostats [4] . 33
Figure B.3 – Schematic representation of a long-tube fluid calibration device [3] . 34
Figure B.4 – Schematic representation of a solid-state calibration device for higher
temperatures [4] . 35
Figure B.5 – Schematic representation of a dry-block calibrator for calibrating an FBG

temperature sensor at higher temperatures . 36

Table 1 – Calibration bath fluids. 15

– 4 – IEC 61757-2-1:2021 © IEC 2021
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIBRE OPTIC SENSORS –
Part 2-1: Temperature measurement –
Temperature 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.
IEC 61757-2-1 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:
FDIS Report on voting
86C/1725/FDIS 86C/1737/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.

IEC 61757-2-1:2021 © IEC 2021 – 5 –
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement,
available at www.iec.ch/members_experts/refdocs. The main document types developed by
IEC are described in greater detail at www.iec.ch/standardsdev/publications.
A list of all parts in the IEC 61757 series, published under the general title Fibre optic
sensors, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – IEC 61757-2-1:2021 © IEC 2021
INTRODUCTION
This document is based on the guideline VDI/VDE 2660 Blatt 2:2020-04, Technical
temperature measurement – Optical temperature sensor based on fibre Bragg gratings –
Recommendation on temperature measurement and statement of measurement uncertainty
[1] . It was prepared in cooperation with VDI/VDE-GMA Technical Committee 2.17 "Fibre optic
measurement techniques".
The IEC 61757 series is published with the following logic: the sub-parts are numbered as
IEC 61757-M-T, where M denotes the measure and T, the technology.

___________
Numbers in square brackets refer to the Bibliography.

IEC 61757-2-1:2021 © IEC 2021 – 7 –
FIBRE OPTIC SENSORS –
Part 2-1: Temperature measurement –
Temperature sensors based on fibre Bragg gratings

1 Scope
This part of IEC 61757 specifies the terminology, characteristic performance parameters and
related test methods of optical temperature sensors based on fibre Bragg gratings (FBG) that
carry out temperature measurements in the temperature range between –260 °C and 600 °C.
Generic specifications for fibre optic sensors are defined in IEC 61757.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60050 (all parts), International Electrotechnical Vocabulary (IEV) (available at
www.electropedia.org)
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
ISO/IEC GUIDE 98-3, Uncertainty of measurement – Part 3: Guide to the expression of
uncertainty in measurement (GUM:1995)
3 Terms, definitions, abbreviated terms and symbols of quantities
For the purposes of this document, terms and definitions given in IEC 60050 (all parts),
IEC 61757, IEC 61757-1-1, and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp

– 8 – IEC 61757-2-1:2021 © IEC 2021
3.1 Terms and definitions
3.1.1
Bragg wavelength under reference conditions
λ
Bref
wavelength of maximal reflectance or minimal transmittance of a mechanically stress-free
fibre Bragg grating at reference or standard temperature conditions, without the effect of a
temperature change
Note 1 to entry: The term Bragg wavelength λ specified in the data sheet of the manufacturer of a fibre Bragg
B
grating normally describes the Bragg wavelength without further details on reference or standard temperature
conditions.
[SOURCE: IEC 61757-1-1:2020, 3.3, modified – transformed to temperature sensing.]
3.1.2
birefringence
optical property of the directional dependence of the index of refraction of an optically
anisotropic material having orientation-dependent refractive indices that lead to different
propagation velocities of light in different propagation and polarization directions
Note 1 to entry: In fibre optic sensors terminology, the term "birefringence" is mainly applied when optical
waveguides with birefringence properties, such as PANDA or bow-tie fibres, are used. Birefringence in fibre Bragg
gratings becomes important only when polarized light is used for the measurement. Because of the properties of
fibre Bragg gratings, this can lead to an additional systematic increase in the measurement uncertainty.
[SOURCE: IEC 61757-1-1:2020,3.22, modified – clarified and Note modified.]
3.1.3
drift
shift of the characteristic curve (or, rarely, change of the characteristic curve parameters)
under the same measuring and operating conditions or reference conditions due to material
ageing or/and mechanical and/or thermal continuous or alternating stress
3.1.4
dynamic measurement deviation
ΔT(t)
time-dependent thermal measurement deviation resulting from time-varying differences
between the sensor temperature T (t) and the temperature of the medium T (t) with
S M
ΔTt T t− T t
() () ()
SM (1)
3.1.5
FBG peak
reflection peak or transmission minimum in the Bragg grating reflectance or transmittance
spectrum
Note 1 to entry: Maximum peak value is typically at the Bragg wavelength λ .
B
3.1.6
FBG peak spectral width
full width at half maximum (FWHM) of the FBG peak
Note 1 to entry: Full width at half maximum of the FBG peak is the wavelength range of the spectrum for which
the amplitude is greater than 50 % (3 dB).
[SOURCE: IEC 61757-1-1:2020, 3.7, modified – transformed for temperature sensor.]
=
IEC 61757-2-1:2021 © IEC 2021 – 9 –
3.1.7
FBG period
Λ
distance between the periodically changing refractive index zones (grating planes) in an
optical waveguide having an effective refractive index n
eff
Note 1 to entry: The FBG period defines the Bragg wavelength λ through the formula
B
k⋅λ
B
Λ=
for k = positive integer
(2)
2 n
eff
[SOURCE: IEC 61757-1-1:2020, 3.18, modified – definition extended.]
3.1.8
FBG temperature sensor
fibre optic sensor that uses a fibre Bragg grating as a sensitive element for temperature
measurements
Note 1 to entry: FBG temperature sensors can be used in a series configuration with multiple gratings allowing to
perform distributed measurements.
3.1.9
FBG temperature sensing system
measurement set-up consisting of one or more serial arranged FBG temperature sensors
connected to an interrogation unit consisting of a light source, detector module, processor,
data archive, and user interface
Note 1 to entry: An FBG temperature sensing system normally works as follows: After light from the light source is
sent into the fibre and partially reflected or transmitted by the FBG temperature sensor, it is guided to the detector
module of the interrogator, which determines the FBG peak wavelength. From a shift of the FBG peak wavelength
caused by temperature change, the temperature change can be quantitatively determined in the measuring units.
3.1.10
maximum operating temperature
highest value of temperature at which the FBG temperature sensor meets the specified
performance
3.1.11
minimum operating temperature
lowest value of temperature at which the FBG temperature sensor meets the specified
performance
3.1.12
minimum bending radius
minimum radius at which an FBG temperature sensor is bent without change of the specified
performance parameters
Note 1 to entry: This value can differ from the minimum bending radius given for transport and storage.
[SOURCE: IEC 61757-1-1:2020, 3.16, modified – transformed for temperature sensor.]
3.1.13
parasitic strain effect
non-thermally induced deformation of the fibre Bragg grating during temperature
measurement resulting in a change in the wavelength response of the FBG
Note 1 to entry: A non-rateable deformation of the FBG during temperature measurement occurs as an apparent
temperature change in the response signal and shall therefore be excluded or be assessable.

– 10 – IEC 61757-2-1:2021 © IEC 2021
3.1.14
reference wavelength
λ
ref
wavelength response of a fibre Bragg grating to which a specific temperature T value is
ref
referred
Note 1 to entry: Depending on the evaluation method, the interrogator devices of the sensor often emit different
wavelengths to determine the filter function of the Bragg grating. The reference wavelength is not necessarily equal
to the Bragg wavelength λ . However, because of the small difference between the reference wavelength and
B
Bragg wavelength, both wavelengths can be used in Formulae (6) to (8) and in Formulae (16) to (20) without
relevant errors occurring.
[SOURCE: IEC 61757-1-1:2020, 3.4, modified – transformed to temperature sensing]
3.1.15
reference wavelength at reference temperature
λ
Tref
not
Bragg wavelength of the fibre Bragg grating for a specified reference temperature T
ref
equal to 0 °C
3.1.16
reference wavelength at 0 °C
λ
Tref,0
Bragg wavelength of the fibre Bragg grating at reference temperature T = 0 °C
3.1.17
response time
t
R
time after which the difference between the sensor temperature T (t) and the temperature of
S
the medium T is smaller than a meaningful defined fraction δ of the initial temperature
M
difference T (0) – T , as described by Formula (3)
S M
Tt−= T δT 0−T 
( ) ( )
sR M  S M (3)
3.1.18
sensitive area
length of the fibre Bragg grating that is sensitive to the
temperature to be measured and thereby changes the wavelength response of the fibre Bragg
grating
3.1.19
side-lobe suppression ratio
R
SLS
ratio of the intensity of the FBG peak to the nearest largest side-lobe
Note 1 to entry: It is expressed in dB.
3.1.20
sensor stability
ability of the FBG temperature sensor to keep its performance characteristics under specified
limits within a specified time interval, all other conditions being the same
3.1.21
static thermal measurement deviation
ΔT
th
time-invariant temperature difference in the steady state between the sensor temperature
T (t) and the temperature of the medium T
S M
IEC 61757-2-1:2021 © IEC 2021 – 11 –
ΔT Tt− T for t→∞
()
th S M (4)
3.1.22
temperature sensitivity
S(T)
ratio of the wavelength change Δλ of an FBG temperature sensor caused by a temperature
T
change ΔT in steady state, which is the temperature-dependent slope of the characteristic
curve λ (T) expressed in nm/K
T
∆λ
T
S T = (5)
( )
∆T
Note 1 to entry: The unit can also be pm/K.
3.1.23
operating temperature range
interval for which the FBG temperature sensor under specified conditions is able to perform
temperature measurements in accordance with the specified performance
3.1.24
thermal time constant
τ
t
time it takes an FBG temperature sensor to reach 63,2 % of the total difference between the
initial and the final medium or body temperature when subjected to a step-like change in
temperature under defined conditions
Note 1 to entry: In many industrial applications, the thermal time constant is also provided for 50 % of the total
temperature difference and for 90 % of the total temperature difference, so as to characterize the dynamic
behaviour of the temperature sensor. The dynamic response of some surface temperature sensors, such as
jacketed thermocouples with a measuring point at the temperature sensor bottom and similar approaches, does not
follow an exponential function but is rather characterized by an initial rapid rise in temperature and a subsequent
slow creep to the final value. For these temperature sensors, the thermal time constant is also specified for 95 %
and 99 % of the total temperature difference.
3.1.25
thermal measurement deviation
(t) and the temperature of
time-dependent difference between the sensor temperature T
S
origin to be measured, of the measuring medium or body T (t)
M
Note 1 to entry: For t → ∞, the thermal measurement deviation is equal to static thermal measurement deviation.
3.2 Abbreviated terms
ASE amplified spontaneous emission
FBG fibre Bragg grating
ITS-90 international temperature scale of 1990
OSA optical spectrum analyzer
=
– 12 – IEC 61757-2-1:2021 © IEC 2021
3.3 Symbols of quantities
k number of order (mathematical description)
ΔL length change of the Bragg grating due to external load in the fibre direction
n index of refraction
n effective index of refraction (of the Bragg grating)
eff
P incident optical power
P optical power of the nearest side-lobe
SL
P optical power of the FBG at λ
λB B
R FBG reflectivity
FBG
R side-lobe suppression ratio expressed in dB
SLS
SNR FBG signal-to-noise ratio
FBG
S(T) temperature sensitivity in nm/K
T temperature in °C
T reference temperature for T = 0 °C, at which the FBG has the reference
wavelength λ in °C
T reference temperature (freely selectable) in °C
B
T actual temperature of the medium/measuring object to be measured in °C
M
T actual temperature provided by the standard thermometer
N
T defined reference temperature at which the FBG has the reference

ref
wavelength λ in °C
ref
T output signal of the temperature sensor (measured temperature) in °C
S
T (0) output signal of the temperature sensor at an abrupt change of the
S
temperature of the measuring body or medium in °C
ΔT static thermal measurement deviation in K
th
x i-th measured value
i
α coefficient of thermal expansion in 1/K
δ part of the initial temperature difference, which has appropriately to be
determined according to the uncertainty of measurement in K
σ mechanical stress
ε strain applied to the temperature sensor (always considered in the direction
of the fibre axis)
p effective photo-elastic (stress-optical) constant
ε
λ reference wavelength for T = 0 °C in m
λ Bragg wavelength in m
B
λ reference wavelength of the FBG of a temperature sensor at T = 0 °C in m
FBG0
λ Bragg wavelength under reference conditions, mechanically stress-free
Bref
operated at a specified reference temperature in m
λ reference wavelength in m
ref
λ Bragg wavelength of the FBG at reference temperature T = 0 °C
Tref,0 0
λ
Bragg wavelength of the FBG at reference specified temperature T ≠ 0 °C
Tref
ref
Λ FBG period
τ, t thermal time constant
ξ thermo-optic coefficient
IEC 61757-2-1:2021 © IEC 2021 – 13 –
4 Design and characteristics of an FBG temperature sensor
4.1 Fibre Bragg grating (FBG)
A detailed description of the structure, function, principle characteristics and manufacturing
process of an FBG is provided in IEC 61757-1-1.
4.2 Dependence of Bragg wavelength on temperature
As explained in IEC 61757-1-1, the change of the Bragg wavelength caused by a change in
the grating temperature can be described by Formula (6).
∂∂nn∂∂ΛΛ
eff eff
Δλ =22⋅+ΛΔnL⋅ +⋅+ΛΔnT⋅
B eff eff (6)
∂∂LL ∂∂TT
 
where
Δλ is the Bragg wavelength shift;
B
T is the temperature;
ΔT is the temperature difference;
n is the effective index of refraction (of the Bragg grating);
eff
ΔL is the change of length of the Bragg grating due to external load in fibre direction;
Λ is the FBG period.
The first two terms on the right side of Formula (6) describe the effects resulting from the
mechanical deformation (∂Λ/∂L) and the elasto-optical response (∂n /∂L) of the optical fibre;
eff
these effects are to be considered parasitic in temperature measurements. The last two terms
in Formula (6) describe the effects of temperature on the quantities n and Λ.
eff
The term (∂Λ/∂T) describes the effect of the thermal expansion of the Bragg grating with
regard to the grating period Λ. The thermal effect on the refractive index of the optical fibre,
on the other hand, is expressed by the term (∂n /∂T).
eff
In practice, the effects of strain and temperature are approximately described by the linear
relationship displayed in Formula (7).
Δλ εT,
( )
B
= 1− p ⋅+εα+ξ⋅ΔT
( )
( )
ε (7)
λ
B
where
ε is the strain of the fibre in axial direction;
is the effective photo-elastic (stress-optical) constant;
p
ε
α is the coefficient of thermal expansion;
ξ is the thermo-optic coefficient.
When a temperature change is applied to a mechanically unstressed FBG, a corresponding
shift of the Bragg wavelength is observed in the optical spectrum. In practice, the effect of a
temperature change ΔT on the Bragg wavelength can approximately be described by the
linear relationship in Formula (8)

– 14 – IEC 61757-2-1:2021 © IEC 2021
Δλ (T)
B
αT+ξ⋅∆
( )
(8)
λ
B
where
λ , Δλ are the Bragg wavelength and wavelength shift;
B
T, ΔT are the temperature and temperature difference;
α is the coefficient of thermal expansion;
ξ is the thermo-optic coefficient.
4.3 Design features
Fibre Bragg gratings are usually supplied with a so-called primary coating, which protects the
sensor against damage or fibre breakage. If the fibre Bragg grating is applied without
mechanical stress to the measurement object, the fibre sensor usually includes this primary
coating. Since the thickness of the coating is in the range of several tens of micrometres, the
heat transfer from the object of measurement to the sensor is not impeded significantly.
However, the impact of thermally induced deformations of the temperature sensor caused by
the object of measurement shall be considered. The coating can also remain on the sensor
fibre in other installations of the FBG sensor. In certain cases, the coating can be removed
during the manufacturing process of encapsulated temperature sensors, in order to achieve
mechanical decoupling. In these cases, precautions should be taken not to expose the fibre to
corrosion, the so-called stress corrosion.
Depending on the measurement application, an FBG is pre-assembled in steel tubes, ceramic
tubes or other housings to form the complete FBG temperature sensor. The pre-assembly
usually includes gluing, screwing or fastening of the components. It is essential to achieve
optimal heat transfer (heat transition, heat conduction) to the temperature-sensing element,
which depends on both the mechanical and the thermal coupling.
Depending on the measurement task, the temperature sensor may contain a single FBG or a
chain of concatenated FBGs (sensor array). A single sensor arrangement allows local
(pointwise) measurement of temperature, whereas a chain of concatenated fibre Bragg
gratings (so-called quasi-distributed temperature sensors) allows measurement of
temperature profiles or temperatures at different locations on the object of measurement with
a single optical fibre sensor.
When several FBG sensors are concatenated with relatively long connecting fibre cables, the
transmission losses of the entire FBG temperature sensor shall be considered. When
specifying the transmission loss in the FBG temperature sensor, all loss contributions should
be determined, including attenuation in the optical fibre and in the FBG outside the FBG
spectrum as well as losses in connectors and splices. The signal amplitudes in the FBG
transmission spectrum can be reduced by application-specific effects impacting the sensor
quality. For example, transmission losses of serially operated wavelength-multiplexed FBG
temperature sensors should be considered separately.
5 Performance parameters
A single FBG temperature sensor shall be characterized by the following technical
performance parameters. Clause 7 describes the related test procedures to determine the
parameters:
– Bragg wavelength λ (7.2)
Bref
– FBG peak spectral width (7.3)
– FBG reflectivity (7.4)
=
IEC 61757-2-1:2021 © IEC 2021 – 15 –
– side-lobe suppression ratio (7.5)
– signal-to-noise ratio (7.6)
– characteristic curve (7.7)
– sensitivity (7.7.2)
– hysteresis (7.7.2)
– thermal time constant (7.8)
– response times (on request only) (7.8.2)
– sensor stability (7.9)
These performance parameters and additional technical information are needed for practical
applications and shall be provided by the manufacturer when referring to this document. A
blank detailed specification template is provided in Annex A.
6 Test apparatuses for performance parameter determination
6.1 Temperature calibration equipment
Suitable temperature calibration equipment shall be used for determining the performance
parameters and for calibrating an FBG temperature sensor. The principal options are
commercially available stirred liquid baths (up to approximately 550 °C) as well as block
calibrators or the specific temperature calibration equipment shown in Annex B. For higher
temperatures, tube furnaces are usually employed, whose limited thermal properties can be
appreciably improved through the use of so-called compensation bodies (metal inserts) or
heat pipes.
A temperature calibration bath is a uniform enclosure with a stirred fluid that can be adjusted
to various temperature test points. By using a stirred fluid (e.g. water, silicone oil, methanol,
ethanol), baths provide excellent thermal contact, uniformity, and stability for temperature
sensor calibration. They offer a large working volume and flexibility for calibrating temperature
sensors of different shapes and sizes. Typically, temperature calibration baths are equipped
with a reference thermometer for calibration traceability. Depending on the temperature
range, the fluids listed in Table 1 should be used for testing and calibration of FBG
temperature sensors.
Block calibrators use pre-drilled metal inserts for inserting the temperature sensor to be
calibrated. These block calibrators have a removable metal insert with pre-drilled holes where
sensors are inserted for measurement. The insert provides a stable temperature source,
which can be adjusted to different test points. In order to provide sufficiently good thermal
contact between the metal insert and the temperature sensor, inserts with different hole sizes
are available. Typically, block calibrators are equipped with a reference thermometer for
calibration traceability.
Table 1 – Calibration bath fluids
Temperature range Medium
−180 °C to −90 °C Refrigerant R13
−140 °C to 5 °C  Pentane
−110 °C to 5 °C Alcohol
−90 °C to 10 °C Methanol
−40 °C to 60 °C Water/glycol mixture
+5 °C to 95 °C Water
−30 °C to 300 °C Silicon or mineral oil

– 16 – IEC 61757-2-1:2021 © IEC 2021
6.2 Optical spectrum analyzer and interrogator
A commercial high resolution optical spectrum analyzer or an optical spectrum analyzer
system specifically designed for FBG spectral analysis shall be used to perform
comprehensive spectral analysis. The minimum requirements for such a device are:
– spectral resolution ≤ 2 pm for a single FBG under test (≤ 1 pm for a chain of concatenated
FBGs under test)
– wavelength deviation ≤ ±5 pm
– dynamic range ≥ 40 dB
If the entire fibre optic temperature measurement
...