Resea ch A icle
Tempe a u e Measu emen Using Op ical Fibe Me hods:
O e iew and E alua ion
Ma in Mikolajek ,
1
Radek Ma inek,
1
Ji i Kozio ek,
1
S anisla Hejduk,
2
Jan Vi asek,
2
Ales Vande ka,
2
Radek Pobo il,
2
Vladimi Vasinek,
2
and Radim He cik
1
1
Depa men o Cybe ne ics and Biomedical Enginee ing, Facul y o Elec ical Enginee ing and Compu e Science, VSB-
Technical Uni e si y o Os a a, 708 00 Po uba, Os a a, Czech Republic
2
Depa men o Telecommunica ions, Facul y o Elec ical Enginee ing and Compu e Science, VSB-Technical Uni e si y o Os a a,
708 00 Po uba, Os a a, Czech Republic
Co espondence should be add essed o Ma in Mikolajek; [email p o ec ed]
Recei ed 20 Ma ch 2020; Re ised 20 Augus 2020; Accep ed 28 Augus 2020; Published 12 Oc obe 2020
Academic Edi o : Qiang Wu
Copy igh © 2020 Ma in Mikolajek e al. This is an open access a icle dis ibu ed unde he C ea i e Commons A ibu ion
License, which pe mi s un es ic ed use, dis ibu ion, and ep oduc ion in any medium, p o ided he o iginal wo k is
p ope ly ci ed.
The pape deals wi h he o e iew o fibe op ic me hods sui able o empe a u e measu emen and moni o ing. The aim is o
e alua e he cu en esea ch o empe a u e measu emen s in he in e al om empe a u e close o 0 up o 1000
°
C. Since he
measu ing chain is a unc ional combina ion o op ical me hods, op ical fibe p ope ies, and o he pho onic elemen s oge he
wi h con ol elec onic ci cui s, i is necessa y o find a sui able comp omise be ween he chosen measu emen me hod,
measu ing ange, accu acy, and esolu ion. Op ical fibe senso s can be used in cases whe e s anda d elec ical measu emen
me hods canno be used. These may be a eas wi h high elec ical and magne ic in e e ence o c i ical a eas. The e o e, he e is
in ensi e de elopmen o op ical and fibe op ic me hods based on blackbody and g eybody adia ion, luminescence, fibe B agg
g a ings (FBGs), and in e e ome e s.
1. In oduc ion
A p esen , many undamen ally diffe en ways o measu ing
high empe a u es a e used. One o he p ima y use s o em-
pe a u e measu emen s is he au omo i e indus y. The e a e
a numbe o a eas whe e i is necessa y o measu e o a leas
moni o his high empe a u e di ec ly. An o e iew o some
key loca ions in which he empe a u e is measu ed is gi en
in Figu e 1. These a e, in pa icula , poin s in he exhaus
pipe, nea ca aly ic con e e s, o u bocha ge s. An example
o empe a u e measu emen using op ical me hods is deal
wi h in he a icle om Bock e al. [1]. Ano he a icle om
Jiang e al. is abou a empe a u e fibe senso o he a ia ion
indus y [2].
The e a e noncon ac measu emen me hods [3] using
emi ed adia ion o he obse ed body [4], known as empe -
a u e measu emen by he mal came as [5–8]. These
me hods equi e knowledge o he su ace emissi i y o he
measu ed bodies. By hese me hods, su ace empe a u es
o bodies can easily be de e mined [5, 6]. In a s anda d way,
howe e , me allically a ached he mocouple o esis i e sen-
so s, bime allic, ex ensible in olume, and o he p inciples
which a e connec ed o he measu ed objec so ha hey
ouch i o a e loca ed in a space whe e a gi en ambien
o gene al fluid empe a u e is measu ed, a e commonly
used in measu ing applica ions. Thei ypical p ope y is
low le els o elec ical cu en o ol age. When designing
he measu ing chain, accoun mus be aken o he ad e se
effec s o elec ical and magne ic dis u bances [9, 10] on
me allic conduc o s be ween he ac ual empe a u e senso
and he e alua ion uni ha con e s he signal om he
senso o he amplifie o analogue digi al con e e [10].
In applica ions whe e such elec ical in e e ence can be
expec ed, i is ad isable o use o he nonelec ical empe a-
u e measu emen me hods. I is possible o use op ical
me hods using op ical fibe s and he p inciples o
Hindawi
Jou nal o Senso s
Volume 2020, A icle ID 8831332, 25 pages
h ps://doi.o g/10.1155/2020/8831332
blackbody adia ion [11–14], luminescence in c ys als [15–
22], e ac i e index phenomenon in he fibe B agg
g a ings [23–28], o phase shi o wo cohe en beams in
in e e ome ic senso s [29–33].
The use o fibe senso s also offe s he possibili y o
gal anic isola ion, which b ings he possibili y o use in he
chemical indus y and a ious explosi e en i onmen s. The
use o fibe senso s o empe a u e measu emen is possible
using se e al physical p inciples which a e desc ibed in he
indi idual chap e s o his ex .
The indi idual p inciples o empe a u e measu emen
using op ical senso s also equi e hei own ways o con-
e ing he op ical signal o he measu ed empe a u e in
deg ees Celsius. One o he asks o he pape is o p esen
possible ways o e alua ing empe a u e da a. The pape
ocuses only on op ical fibe me hods o empe a u e mea-
su emen . The con ibu ion does no include compa ison
o o he op ical me hods o empe a u e measu emen ,
such as he mo ision.
2. Types o Tempe a u e Measu emen Using
Op ical Me hods
The me hod o measu emen using op ical fibe echniques
is based on se e al undamen al p inciples. Each measu e-
men me hod has i s specific uses in he ange o measu -
ing empe a u es, accu acy, e c. (see Table 1). The able
shows basic ad an ages and disad an ages o indi idual
fibe me hods.
This a icle goes on o desc ibe all o he abo e me hods.
The blackbody (g eybody) adia ion me hod uses op ical
fibe , one pa o which leads o a blackbody a he empe -
a u e measu emen poin . The ligh ou pu gene a ed by he
adia ion o he hea ed blackbody is led om he end o he
fibe o he pho ode ec o . Acco ding o Dona i [11], he
de ec ed op ical powe in ensi y o spec um o ligh
ecei ed by he pho ode ec o co esponds o he measu ed
empe a u e acco ding o he ecalcula ions below. This
measu emen p inciple is only sui able o high empe a-
u es o app oxima ely 500
°
C, o he use o special ypes
o pho ode ec o s and op ical fibe s, and e en o lowe em-
pe a u es o app oxima ely 300
°
C. This can be he case o
e alua ion using a spec ome e o a silicon, InGaAs diode,
o PbSe pho odiode [11].
The second way o measu ing he empe a u e men-
ioned in his a icle is he me hod using he p inciple o
c ys al luminescence [15–20]. The subjec deals wi h he
desc ip ion o indi idual c ys als and analysis o indi idual
esul s o he es ed ma e ials. The undamen al diffe ence
om he fi s me hod is ha his me hod is no passi e
bu equi es a ligh flux gene a o . The ligh om one
end o he fibe mus all on he selec ed luminescen c ys-
al. A he o he end o he fibe , he pho ode ec o
ecei es he ligh signal om he c ys al depending on
he ligh gene a ed and he empe a u e a he desi ed
poin o measu emen [18]. The dependence o he
ecei ed ligh ou pu on he empe a u e is gi en in he
chap e empe a u e-dependen c ys als. The me hod
desc ibed la e uses he fibe B agg g a ings [23–25]. The
ange o applica ions is conside able oday; some ex books
desc ibe he p inciples o FBG [34] and hei applica ions
[35]. The p inciple o his measu emen me hod lies in
he passage o ligh h ough he pe iodically modified op i-
cal fibe o p oduce a pe iodic o quasipe iodic change in
he e ac i e index o he op ical fibe . The las pa
desc ibes in e e ome ic measu emen s. These me hods
measu e phase shi be ween wo cohe en beams ha
ha e a elled he same pa h in one o wo op ical fibe s.
This ype o senso can simul aneously measu e diffe en
physical quan i ies, including empe a u e, since he
mechanical pa ame e s o he op ical fibe changes wi h
he empe a u e.
The indi idual chap e s deal wi h he desc ip ion o
gene al p oblems o selec ed me hods and wi h indi idual
ea u es and possibili ies o used pa s employed in he
desc ibed manne . The a icle p esen s a comp ehensi e
o e iew o me hods o empe a u e measu emen by
op ical fibe .
High- empe a u e senso
(con ol o exhaus empe a u e)
Figu e 1: High- empe a u e measu emen poin s in ehicles.
2 Jou nal o Senso s
Table 1: The me hod o measu emen using op ical echniques.
Me hod Ad an ages Disad an ages Measu ing ange
Blackbody High- empe a u e measu emen Low empe a u es canno be measu ed 300
°
C and abo e
Simple design o he senso
C ys al luminescence Measu e empe a u es below 300
°
C Complica ed senso design -40 o 400
°
C
Two measu emen p inciples
Fibe B agg g a ing Low- empe a u e measu emen ,
mechanical base
Unsui able o high empe a u es in con en ional solu ions,
g a ing s uc u e dis o ion due o high empe a u e
No mally using 0-320
°
C, 1200
°
C
in case o sapphi e fibe s
In e e ome e High- empe a u e ange, simple and
ela i ely cheap p obe manu ac u ing
Spec ome e needed o measu emen O e 1200
°
C
Depends on mechanical cons uc ion o he senso
Possible in e e ences caused by mechanical s ess
3Jou nal o Senso s
3. Tempe a u e Measu emen Using Blackbody
Radia ion Me hod
This me hod is one o he easies ways o measu e empe a-
u e using op ical fibe s. Only an op ical fibe and a sui able
pho ode ec o a e essen ial o ealiza ion. Wi h minimal
financial cos s, we a e able o assemble a pho ode ec o wi h
a empe a u e ange o app oxima ely 500 o 1200
°
C.
Highe empe a u es can be measu ed using a sapphi e
fibe . Fo hese cases, empe a u es up o he mel ing poin
o he sapphi e fibe can be conside ed. This means he em-
pe a u e 2040
°
C [36, 37]. These bounda ies can hen be
expanded using special componen s up o he physical limi s
o fibe s and pho ode ec o s.
Each body wi h nonze o su ace empe a u e emi s a ce -
ain amoun o adian ene gy. Mos ene gy is adia ed by he
so-called black (blackbody adia ion, abb e ia ed as BBR)
[38]. The amoun o ene gy and he spec um o emi ed
adia ion depend only on he su ace empe a u e o he
body. Bo h o hese pa ame e s can also be desc ibed ma he-
ma ically by he Planck blackbody emission law, whe e we
can desc ibe he spec al densi y o he adia ion in ensi y
using he ollowing Equa ion (1), whe e “ ”is gi en by
Equa ion (2) [11], h=6:626 × 10−34 Js is he Planck cons an ,
k=1:38 × 10−23 J/K is he Bol zmann cons an , “λ”is he
wa eleng h, “T”is he empe a u e, and “c”is he speed o
ligh in acuum (c=3×10
8m/s).
λ
ðÞ
=h 2
λ3eh /kT−1
ðÞ
,ð1Þ
=c
λ:ð2Þ
This emi ed adia ion can easily be used o he op ical
measu emen o he body empe a u e, whe e he o al adi-
a ed ene gy acco ding o he S e an-Bol zmann law inc eases,
depending on he absolu e empe a u e wi h he ou h
powe (see Equa ion (3) [11]), whe e σis he S e an-
Bol zmann cons an acco ding o Equa ion (4).
E=σT4,ð3Þ
σ=2π5·k4
15c2·h4=5:670400 · 10−8Js−1m−2K−4
:ð4Þ
The Rayleigh-Jeans law can also be used o measu e
empe a u es i he sho wa e spec al egion is applied (see
Figu e 2), om which i is clea ha o a sui ably chosen
wa eleng h, he adia ed ene gy densi y a he wa eleng h
in e al will be dλas seen below:
λ
ðÞ
dλ=8πkTdλ
λ4:ð5Þ
The second ela ionship ha can be used o he e al-
ua ion is Wien’s displacemen law which says ha wi h
he inc easing empe a u e, maximum adia ion shi s o
sho e wa eleng hs. In ma hema ical w i ing, his ac is
exp essed by
λmaxT=hc
4:965k:ð6Þ
3.1. Measu emen P inciple Using Op ical Fibe . In o ma-
ion abou measu ed empe a u e migh be ans e ed
h ough diffe en en i onmen s. Vacuum, a mosphe e, o
o he gases a e sui able only o Line-O -Sigh (LOS) mea-
su emen s o blackbody, bu o mo e flexible access o he
measu ed empe a u e, we can use op ical fibe s. Howe e ,
he ange o he measu ed empe a u es depends on he
spec al sensi i i y o he used pho ode ec o and he
p ope ies o he ans e medium. When using he op ical
fibe , he ansmission medium is glass, which is qui e
es ic i e in he ansmission spec um. Con en ional
op ical fibe s (e en low-OH e sions) a e designed o
Wa eleng h 𝜆 (𝜇m)
101
100
10–1
10–8
10–6
10–4
10–2
100
102
T = 6000K
T = 4000K
T = 2000K
T = 1500K
T = 1000K
T = 500K
T = 273K
T = 77K
Spec al adiance (𝜆) (W.cm2s 𝜇m)
Figu e 2: Spec al adiance o he blackbody e sus λ.
4 Jou nal o Senso s
applica ions wi h wa eleng hs used o communica ion
(i.e., om he isible spec um o he nea in a ed ange
a ea as shown in Figu e 3) [39].
The spec al limi o low-OH op ical fibe s (2400 nm) can
be ex ended wi h special fluo ide-doped op ical fibe s (InF3)
[39] o chalcogenide glass. So he spec al limi can be shi ed
o wa eleng hs o e 5000 nm.
By compa ing Figu es 2 and 3, heo e ically, we can
ansmi in o ma ion abou empe a u es below 0
°
Cbyan
op ical fibe . Howe e , in he case o de ec ing such a signal
by a pho ode ec o , we will p obably obse e ew p oblems
in he o m o he spec al cha ac e is ics o he pho ode ec-
o (see Figu e 6 [11]) and possible ex e nal noise. Fo low
empe a u es, blackbody adia ion is applied no only o he
fibe op ic signal bu also o he op ical fibe i sel , case, and
icini y o he pho ode ec o . Fo he success ul blackbody
measu emen o low empe a u e, he empe a u e o he
measu emen op ical fibe and pho ode ec o case mus be
he mally s able; ideally i should be coole hen he mea-
su ed signal, so he measu ed ene gy could be easily de ec ed.
I he measu ed body does no beha e as an absolu ely
black adia o , i s adia ion densi y will be smalle . This
educed p opo ion is exp essed ei he by he emissi i y o
by he deg ee o g eyness o he pa icula body. The deg ee
o g eyness indica es how la ge he pe cen age o g eybody
adia ion is compa ed o an absolu ely blackbody a he same
empe a u e. Du ing he ac ual measu emen , i is necessa y
o ake in o accoun ha emissi i y is a wa eleng h-
dependen pa ame e .
The empe a u e measu emen sys em using he black-
body consis s o h ee pa s: op ical adia ion sou ce
app oaching he blackbody, op ical fibe o signal ansmis-
sion, and e alua ion elec onics, shown in Figu e 4. The basis
o he measu emen is o adjus he end o he op ical fibe so
ha i s end is as simila o he blackbody as possible. In mos
cases, his is achie ed by means o a me allic coa ing [13, 14]
o by inse ing a fibe in o he measu ing pi [12]. The fibe
hus cap u es he blackbody emissions and ansmi s hem
o he de ec o end.
3.2. Implemen a ion o High-Tempe a u e Measu emen
Poin (Coa ing o Ca i y). Two basic app oaches a e used
o cons uc he empe a u e senso . Using he coa ing,
he end o he fibe becomes a senso , and i is hus di ec ly
exposed o he measu ed empe a u es du ing measu e-
men . The coa ing ma e ials mus wi hs and high empe a-
u es and mus no change hei emissi i y alue du ing
epea ed cycles.
2.5
1.5
A enua ion (dB·m–1)
0.5
0
01234
Low
InF3
Z F4
56
1
2
Wa eleng h 𝜆 (𝜇m)
Figu e 3: Spec al cha ac e is ics o op ical fibe s.
Blackbody
coa ing/ca i y
Op ical
ibe s
Signal p ocessing
uni
Figu e 4: Op ical fibe deploymen o measu emen using he
blackbody p inciples.
ca
o
I2
I1
x
𝛷
Figu e 5: Configu a ion o he fibe and he blackbody elemen .
5Jou nal o Senso s

A highe empe a u es, howe e , we canno lea e p i-
ma y p o ec ion on he fibe as he fibe becomes b i le.
The senso p oduced his way ei he is no sui able o
demanding condi ions o mus be sui ably placed in ano he
empe a u e- esis an shield.
The ca i y p o ides he fibe wi h he possibili y o p o-
ec ion agains ad e se en i onmen al influences because
he fibe , in his case, does no ha e o be exposed di ec ly
o he measu ed empe a u es. Depending on he shape o
he ca i y, measu emen s con inue om he end o he fibe .
This is gi en by he alue o he accep ance angle a which he
fibe collec s ambien ligh and is defined by he nume ical
ape u e (NA) alue and he ambien e ac i e index; see
Equa ion (7) whe e o ai n=1, whe e Φis he maximal
hal -angle o he cone o ligh ha can en e o exi om
he op ical fibe .
NA = n· sin Φ:ð7Þ
Figu e 5 [12] shows he effec on he measu ed a ea whe e
he ene gy a he dis ances xand l1is no connec ed o he
fibe . The measu emen is hus mainly ocused on he l2a ea.
In p inciple, he op ical fibe in he ca i y unc ions as a non-
con ac he mome e and e ains his p ope y un il he fibe
op ic on is con amina ed. The e o e, i is essen ial ha he
ca i y does no cause e apo a ion o ma e ial due o high
empe a u es. The si ua ion whe e he end o he fibe
beha es like a g ey (black) emi e and i is a sou ce o adia-
ion i sel is mo e common. In his case, he fibe end
con amina ion is no a significan p oblem.
3.3. Op ical Fibe o Blackbody Radia ion Me hod. Fo mea-
su emen , we need o use he fibe o ans e as much op ical
signal as possible o he de ec o . The e o e, he op ical fibe
mus ha e he g ea es possible diame e o he co e. While
keeping he cos s low, we can use he 62.5 μmMMfibe ;
howe e , i is much be e o deploy fibe s wi h la ge co e
diame e s (e.g., 200 o 300 μm).
The fibe ma e ial also di ec ly de e mines he maximum
measu able empe a u e. As soon as we come close o he
mel ing poin , he fibe becomes de o med. Fibe de o ma-
ions esul in a change o he senso pa ame e s. The mel ing
poin o pu e SiO
2
glass is 1610
°
C. This empe a u e a ies
depending on he addi ion agen s used. Fo highe empe a-
u es, i is possible o use sapphi e [40] fibe s ha combine
he p ope ies o he sapphi e c ys al Al
2
O
3
and op ical fibe
flexibili y. Wi h he mel ing poin o 2045
°
C and chemical
p ope ies, hey a e sui able also o agg essi e en i onmen s
because hey a e almos chemically neu al.
3.4. Signal De ec ion o Blackbody Radia ion Me hod. To
e alua e he measu ed empe a u e, we can use he signal
ampli ude measu emen on he pho odiode [40] o e alua e
he spec um o he adia ion wi h a spec ome e [4].
The signal ampli ude measu emen is simple and inex-
pensi e; howe e , i p o ides oom o measu emen de ia-
ions caused, o example, by mechanical and he mal
s esses o he fibe .
The lowe pa o he empe a u e ange is de e mined in
his case by he ma e ial o he pho odiode used, as can be
seen in Figu e 6 [11].
When using a silicon (Si) pho odiode, he IR po ion o
he spec um is limi ed o abou 1000 nm, and a empe a-
u es below 500
°
C, he signal ampli ude is e y small.
Ano he op ion is he use o InGaAs-based pho odiodes,
capable o wo king up o 1800 nm. The applicable empe a-
u e ange is hus mo ed up o 300
°
C.
1.0
0.1
0.01
0.001
200 300 400 500 600 800
Wa eleng h (nm)
Spec al sensi i i y 𝜎 (A.W−1)
1000 1400 2000
Si-s anda d
Si-Mis
GaAs
GaAsP
Ge
InGaAs
GaP
Figu e 6: Spec al cha ac e is ics o pho ode ec o s depending on he composi ion.
6 Jou nal o Senso s
The heo e ical op ion is o use InAs [41], a PbSe pho o-
conduc i e pho ode ec o [42, 43] which is capable o de ec -
ing wa eleng hs a he 4800 nm line, which will allow o
u he educ ion o he wo king empe a u e.
Howe e , he measu emen p ocedu e diffe s in his case
because his ype o de ec o can p ocess di ec cu en (DC)
signals bu suffe s om a la ge da k cu en alue ha is
app oxima ely h ee o de s la ge han he InGaAs de ec o s.
To inc ease he esol ing powe o he de ec o , i is necessa y
o place a mechanical b eake (choppe ) as shown in Figu e 7
be ween he de ec o and he measu ed signal [44].
3.5. Measu emen De ia ions o Blackbody Radia ion
Me hod. BBR measu emen esul s in de ia ions due o i s
own emission o adia ion om he inside o he op ical fibe .
I he fibe hea s up, i s ma e ial becomes a sou ce o adia-
ion. The longe he hea ed po ion o he fibe , he g ea e
he mani es a ions. Fo signal ampli ude e alua ion, his
de ia ion can be elimina ed, o example, by using he wo-
fibe me hod shown in Figu e 8 [13].
Bo h fibe s a e exposed o he same empe a u e; one is
equipped wi h a blackbody ca i y and he o he one is co -
e ed wi h eflec i e coa ing. As a esul , we can elimina e
fibe -caused de ia ions by sub ac ing indi idual signals.
3.6. Pa ial Conclusion o he Blackbody Measu emen
Me hods. The blackbody (g eybody) measu emen me hod
is pa icula ly sui able because o i s simplici y since i
equi es only a fibe and a sui able pho ode ec o . Howe e ,
he pi all o his me hod lies in he measu ing ange and
accu acy ha can be measu ed by his me hod; i is pa icu-
la ly sui able o measu ing empe a u es om 300
°
C o em-
pe a u es which depend on he so ening o mel ing
empe a u e o he deployed op ical fibe , which may be up
o 2000
°
C. Fo lowe empe a u es, i is possible o use op ical
fibe s in p ima y o seconda y p o ec ion and only ba e fibe s
o high empe a u es. The disad an age o his me hod,
howe e , lies in he complica ed measu emen o empe a-
u es below he empe a u es lowe han 600
°
C. The measu -
ing a eas can be influenced by he ype o pho ode ec o used,
especially o he lowe empe a u e limi . In hese cases, i is
always necessa y o use special pho ode ec o s, hen he p ice
o he o e all measu ing chain inc eases. Fo s ill dec easing
empe a u e anges, he ins abili y and inaccu acy o he
measu ing chain a e inc easing. This is due o he ac ha
when he empe a u e a he measu ing poin app oaches
he empe a u e o he pho ode ec o , he ecei ed signal
begins o be “d owned”in noise.
4. Luminescen C ys als wi h
Tempe a u e Dependence
Some c ys al pa ame e s show dependence on ambien em-
pe a u e due o hei composi ion. The pa ame e showing
he empe a u e dependence is p ima ily luminescence. In
luminescence, we can measu e he exci ed spec um, exci ed
ligh in ensi y, o he li e ime o elec ons in he exci ed s a e.
Luminescence ime esponse measu emen is he mos com-
monly used me hod because i is ela i ely simple and
in ensely independen o he amoun o exci ed ligh [15,
45]. C ys als showing he empe a u e dependence o he li e-
ime o exci ed ligh -gene a ing elec ons a e uby (ch o-
mium-doped sapphi e) [15–20], alexand i e [15, 16, 19, 46,
47], C :LiSAF [15, 16, 19, 48, 49], C :YAG, o Nd
3+
:YAG
[15, 16, 19, 21, 50]. Ruby is a ch omium-doped aluminium
oxide (C
3+
:Al
2
O
3
). Ch omium a oms, due o he simila size,
can eplace he aluminium a oms in he Al
2
O
3
c ys al la ice.
Due o he p esence o ch omium, a phenomenon known as
fluo escence occu s a e abso bing he exci a ion ligh .
4.1. Desc ip ion o Fluo escence. A e deli e y o he exci a-
ion ligh , he elec ons mo e o he ene gy le el o 4T2, whe e
hey, howe e , do no hold and pass o he me as able le el o
2E. Thanks o spin-o bi coupling [51], he me as able le el o
2Eis di ided in o wo le els o −Eand 2−Awi h he ene gy
diffe ence ΔE. This di ision o he me as able le el causes
he o ma ion o wo emission spec al lines. The fi s emis-
sion spec al line R1is due o he ansi ion o −E≥4A2
(694.3 nm); he second emission spec al line R2is due o
De ec o
RD
R il e
R
C il e
Rload
R1
U
Op ical
choppe GND
RC il e
Ou
Feedback esis o
A−
+
+
B
Blackbody
adia ion
Figu e 7: PbSe pho oconduc o measu emen scheme.
Re lec i e
coa ing
T0
zL
Ie𝜆 (0)
Ie𝜆 (0) ≈ I~ 𝜆 (L)I~
𝜆 (𝜁)
𝜁 = L − z
Ie𝜆 (L)
Figu e 8: Two-fibe op ical he mome e .
7Jou nal o Senso s
he ansi ion o
2−
A≥
4
A
2
(692.9 nm). The second emission
spec al line R2is sligh ly weake han he fi s R1[18, 52].
A simplified diag am o he ene gy le els o uby is shown
in Figu e 9.
Fluo escence is ime-dependen , and a e swi ching off
he exci a ion ligh a e a ce ain pe iod o ime called he
li e ime, i disappea s.
Fo his eason, he exci a ion ligh sou ce mus be
modula ed by a ec angula pulse, hus al e na ely u ning
he exci a ion ligh on and off. The li e ime o he emission
spec al line R2is ~3-3.5 ms and R1is ~0.6 ms [15, 18].
Changing he empe a u e leads o a change in he cou-
pling ( ension) in he c ys al la ice and he dis ibu ion o
phonons and ene gy, and consequen ly, i influences he
fluo escence li e ime o uby. The ela i ely sho li e ime a
he 4T2le el causes he fluo escence o Rlines o domina e
due o he long-las ing 2Ele el a low empe a u es ( ansi-
ion 2E→4A2de e mines he empe a u e dependency o
he li e ime). A oom empe a u e (abou 300 K) he li e ime
is abou ~3.5 ms [15, 18]. Wi h inc easing empe a u e, some
C
3+
ions a e p opped up o e e o he 4T2le el om he
2Ele el apidly. Wi h u he inc eases in he empe a u e,
mo e and mo e ions a e p opped up o his e e sion, he eby
emp ying he 2Ele el and he non adian ansi ions 4T2
→4A2s a o domina e. Significan loss o adian ansi-
ions hus educes he li e ime o fluo escence o ~1μsa a
empe a u e o abou 600
°
C [15]. A he same ime, he
fluo escence-emi ed ligh in ensi y dec eases.
Figu e 10 desc ibes he adian ansi ions o C
3+
ions
om he 2E→4A2le el a low empe a u es whe e he non-
adian ansi ions om he 4T2le el a e negligible due o he
p e e ed occupancy o he 2Ele el. Wi h an inc easing em-
pe a u e, he non adian ansi ions 4T2→4A2s a o dom-
ina e, he le el 2Ebecomes a empo a y le el o he
exci a ion o ions a he 4T2le el. The ansi ion ime 2E→
4T2is a ound ~7 ps. The empe a u e dependence o he fluo-
escence li e ime can be exp essed by [53]
τ=τS
1+Cde−ΔE/kT
ðÞ
1+ τS/τi
ðÞ
e−ΔE/kT
ðÞ
=τS
1+3e−ΔE/kT
ðÞ
1+αe−ΔE/kT
ðÞ
,ð8Þ
whe e τis he fluo escence li e ime, ΔEis he ene gy diffe -
ence be ween he le els 4T2and 2E,τiand τsa e he li es
o ansi ions om he 4T2and 2Ele els, kis he Bol zmann
cons an , Tis he empe a u e, and Cdis he a io o degen-
e a ion 4T2/2E[54].
This model p o ides good esul s compa ed o measu e-
men s o empe a u es anging om 300 o 570 K [54].
F om he oom empe a u e o abou ~550 K, he li e ime
is dec easing. Beyond he ~600 K limi , he 4T2→4A2 ansi-
ions s a o domina e, esul ing in a apid d op in he li e-
ime wi h an inc easing empe a u e, which Equa ion (6)
al eady canno desc ibe accu a ely enough.
The selec ed a icle desc ibes a simplified model o uby
ha p edic s changes in li e ime wi h changing empe a u es,
as shown in Figu e 11 [18]. This model includes he adian
2E→4A2and non adian 4T2→4A2 ansi ions. The sche-
ma ic diag am o his model is shown in Figu e 10. The li e-
ime o fluo escence is desc ibed by
τ=τS
1+3e−ΔE/kT
ðÞ
1+αe−ΔE/kT
ðÞ
+βe−ΔEq+ΔE
ðÞ
,ð9Þ
whe e τqis he non adian ansi ion, ΔEqis he ene gy di -
e ence be ween 4T2and 4A2,α=τs/τi, and β=τs/τq. This
model is accu a e om 300 o 800 K wi h a ole ance o
~1% o e he gi en empe a u e ange.
4.2. Abso p ion Spec um. Al hough he abso p ion spec um
o uby is s ongly dependen on he pola iza ion o he exci-
a ion ligh [18, 55], he e a e ela i ely small diffe ences
be ween he in ensi ies R1and R2.
The abso p ion spec um o uby is spec ally b oad, as
shown in Figu e 12 [15, 18]. The e a e wo abso p ion peaks
a ound 410 and 550 nm. The emission spec um has he
main peaks, 694.3 nm (R1) and 692.9 nm (R2).
4.3. Dimensions o C ys als. Hu e al. [16] compa e he li e-
ime o uby luminescence o wo sizes, o a smalle and a
la ge c ys al. Measu emen s we e conduc ed a empe a-
u es anging om 77 o 800 K. A he ini ial empe a u e,
he li e ime o he small c ys al fluo escence was 4.2 ms, while
he li e ime o he la ge c ys al fluo escence was 7.1 ms. This
phenomenon occu s because he fluo escence ligh is eab-
so bed in he la ge c ys al and eexci a ion occu s, hus
ex ending he li e ime o fluo escence as he c ys al adia es
longe . This phenomenon, howe e , applies o low empe a-
u es; om he empe a u e o abou 0
°
C, he fluo escence
li es balance ou o small and la ge c ys als. Howe e , o
Ene gy
Pump
0
4T2
4A2
R2 (692.9) nm R1 (694.3) nm
𝛥E
2E
Figu e 9: Simplified diag am o uby ene gy le els.
R-line
ansi ion
Phonon-
e mina ed
ansi ion
Phonon
elaxa ion
Band o
ib a ional
le els
Ini ial le el
S o age le el
4T2
4A2
2E
𝛥E
Figu e 10: Two-le el uby model.
8 Jou nal o Senso s
p ac ical applica ions, i is ecommended ha he uby
c ys al size be as small as possible, jus o minimize he edis-
ibu ion o fluo escen ligh [16].
4.4. Concen a ion o Ch omium in he C ys al. The fluo es-
cence li e ime is independen o he concen a ion o ch o-
mium in uby up o he c i ical le el o abou ~0.3 w .%
[56]. Highe ch omium concen a ions lead o a dec ease in
he fluo escence in ensi y. Addi ionally, o e he concen a-
ion o ~1 w .%, he li e ime is no longe simply exponen ial,
as shown in Figu e 12 [19].
4.5. Luminescen Expe imen s. Sea e al. [15] exci ed uby
using a lase diode wi h a modula ed ec angula signal
wi h a wa eleng h o 635 nm ha flashed h ough he fibe
op ic (100/140 μm) h ough a 1×2fibe couple . The op i-
cal fibe was coupled o a uby c ys al using a silica glass
ubing, bo h ends o which we e mel ed, hus combining
he fibe wi h he c ys al. The senso hus c ea ed was
placed in a empe a u e-con olled u nace wi h a ype K
con ol he mocouple. The adia ed fluo escence ligh was
cap u ed wi h he same op ical fibe and passed h ough
he couple o he pho odiode (APD:Si). A band il e >
670 nm was placed be o e he pho ode ec o , which fil e ed
he exci a ion ligh . The phase-locked de ec ion (PLD) ech-
nique [57] was used o measu e he uby c ys al fluo es-
cence li e ime. The de ec ed signal was p ocessed o
p oduce a ecu ing signal whose pe iod was di ec ly
p opo ional o he ime o he li e ime. This educes he
effec o he exci a ion ligh , allowing high- esolu ion
𝛥Eq
𝛥E
Q
I
S
2E
4T2
4A2
Ene gy
Ruby
luo escence
B oadband
emission
Non adia i e
elaxa ion
Figu e 11: A simplified model o uby.
1
0.9
0.8
Abso p ion (a.u.)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
200 300 400 500 600 700 800 900
Bulk uby
Ruby ibe
Wa eleng h 𝜆 (nm)
Figu e 12: Ruby abso p ion spec um.
9Jou nal o Senso s
high eliabili y, and immuni y o elec omagne ic in e e -
ence [28, 65, 66, 68–72].
λb=2nΛ:ð10Þ
Changes in p essu e and empe a u e affec he e ac-
i e index o he dis ance o he indi idual g a ings, esul -
ing in a change in he eflec ed wa eleng h. This can be
oughly desc ibed by
Δλ
λ0
=1−pe·ε+αΛ+αn
ðÞ
∙ΔT,ð11Þ
whe e Δλ is a wa eleng h change, λ0is he ini ial wa e-
leng h, peis he op ical-s ain coefficien , and εis he
s ain ac ing on he FBG. The second pa desc ibes he
influence o empe a u e on he wa eleng h shi , whe e
αΛis he coefficien o he mal expansion, which desc ibes
he g id ex ension due o he empe a u e; αnis he he -
moop ical coefficien desc ibing he change o he e ac-
i e index; and ΔTis empe a u e change in K.
Fibe B agg g a ings a e highly sui able o accu a e
measu emen s o bo h low and high empe a u es. Fo e y
high- empe a u e anges, i is necessa y o use high-
empe a u e- esis an ma e ials o he p oduc ion o op i-
cal fibe s and o choose a du able me hod o pe manen
FBG egis a ion. The FBG empe a u e senso solu ion is
e y simple and compac . The connec ion consis s only o
an op ical sou ce, spec ome e , di ide , and he FBG—see
Figu e 26. Wi h he use o one op ical sou ce and spec-
ome e , a la ge numbe o he fibe B agg g a ings can
be moni o ed, bu he anges o indi idual g a ings canno
in e e e wi h each o he . The disad an age o his solu ion
is he he mal s abiliza ion o he ligh sou ce and he spec-
ome e [65–70, 73, 74].
The fibe B agg g a ing is e y sensi i e o empe a u e
changes. As he empe a u e inc eases, dis ances be ween
he fibe B agg g a ings a e widened, hus mo ing he cen al
wa eleng h o he FBG. I i is necessa y o inc ease he
sensi i i y, he FBG can be combined wi h a ma e ial ha
has a highe he mal expansion han glass.
5.1. Types o B agg G a ings. We dis inguish h ee basic ypes
o g a ings: Type I, Type II, and egene a ed ones. The com-
mon ype is Type I, which can be o med in many ypes o a
ge manium-doped fibe . They a e e y sensi i e o UV ligh .
Low-powe UV lamps a e used as sou ces which affec ge -
manium dioxide by side-ligh ing o he fibe . These me hods
include in e e ome ic, phase mask, and poin -by-poin p o-
duc ion. These g a ings a e capable o measu ing empe a-
u es up o 320
°
C wi h a esponse o 10 pm/
°
C; abo e his
empe a u e, hey a e no longe s able and deg ade. The adi-
a ed pa o he co e has a highe index han he non adia ed
co e. In addi ion, we can use he IH ype whe e ge manium is
250
200
150
100
50
0
0 200 400 600
Tempe a u e ( ∘C)
Fluo escence li e ime (𝜇s)
800 1000
Figu e 24: Nd
3+
:YAG fluo escence [19].
V
Bu e coa ing
Co e
Fibe B agg g a ing Cladding
Figu e 25: FBG in a fibe [71].
Fibe B agg g a in
g
B oadband
sou ce Couple
Dispe sion
elemen
CCD senso
Figu e 26: FBG connec ing he ligh sou ce and spec ome e o he
FBG [75].
16 Jou nal o Senso s

eplaced by hyd ogen ha abso bs UV pho ons o a combi-
na ion o hyd ogen dopan s and ge manium IHp, whe e
he empe a u e s abili y is shi ed up o 500
°
C wi h a
esponse o 7 pm/
°
C. The las ype is Id, whe e changes in
he densi y o he undoped qua z co e a e induced by he
lase (jus be o e damaging he qua z s uc u e) ha is c e-
a ed. This ype has highe dissipa ion losses han Type I bu
lowe han Type II. In his ype, he empe a u e s abili y is
in he ange o 500–800
°
C; he esponse is 11 pm/
°
C [76].
A he mal g a ings (Type II) can be c ea ed using powe -
ul lase s, chemical p oduc ion, and he mal egene a ion.
Using a high-powe lase , mic oscopic damage is p oduced
in he co e. Due o pe manen damage o he co e s uc u e
wi hou dopan s, hey ha e an inc eased empe a u e s abil-
i y o o e 1000
°
C [25].
The las ype is ep esen ed by he egene a ed fibe B agg
g a ings, which a e made by annealing o he fi s ype g a -
ings (a empe a u es abo e 1000
°
C). This makes i possible
o ob ain g a ings wi h ex eme s abili y wi hou u he deg-
ada ion bu a he expense o low eflec i i y.
The FBG can be c ea ed on a con en ional SMF using
a em osecond lase . Howe e , his s uc u e is s able up o
1050
°
C, a e exceeding his alue, he g a ing is i e e s-
ibly ansmi ed. In his me hod, he fibe is ma kedly
s essed by he empe a u e, and eliabili y and geome y
a e affec ed.
Ano he ype is a em osecond pulse o Type II whe e
esidual s ess is eleased due o long annealing, hus achie -
ing he s abili y up o 1200
°
C. Howe e , he fibe is agile
a e se e al hou s o annealing, which limi s he ange o
i s applica ion.
O he imp o emen s came wi h he use o apid ai cool-
ing when he FBG is c ea ed using a em osecond lase , and
consequen ly, he fibe is quickly cooled by cold ai . This will
s abilize he g a ing up o 1200
°
C while inc easing i s
mechanical esis ance. The g a ing emains unchanged o a
minimum o 26 hou s a 1200
°
C [23].
The fibe B agg g a ings ha a e esis an e en abo e
1500
°
C can be p oduced using sapphi e fibe s. The s uc-
u e is w i en in o hese fibe s by means o he em osec-
ond lase . The sapphi e fibe is mul imode; he e o e, i
has a la ge eflec ing bandwid h compa ed o he FBG
in SMF [77, 78].
Ano he op ion is he p oduc ion o he FBG using a
193 nm A F lase in o a p ehea ed ge manium-doped qua z
fibe [25].
5.2. Pa ial Conclusion o Measu emen Me hods Using he
Fibe B agg G a ings. The e a e se e al ways o measu ing
using he fibe B agg g a ings; he main ad an age is always
he esis ance o magne ic and elec ical in e e ence. Com-
pa ed o he op ical me hods desc ibed abo e, he FBG can
be used no only o empe a u e measu emen s bu also
o measu ing de o ma ions and o ques [28]. These
me hods can also be used in he biomedical a ea, o example,
[26, 27] p o ide possibili ies o moni o ing he basic human
unc ions. The a icle by Ma eenko e al. [28] deals wi h he
implemen a ion o measu emen s in plas ic ma e ials. The e
is a disad an age in some measu emen applica ions, because
o he na ow-spec um lase sou ce ha should be he mally
compensa ed. This disad an age can be o e come by using
wide spec um sou ces, whe e he magni ude o he spec al
shi o ligh eflec ed by he FBG is usually e alua ed.
Ano he disad an age o he FBG is he inabili y o use i
o high empe a u es, wi h a highe measu ing ange equi -
ing o he ypes o g a ings; lowe accu acy is achie ed, o he
cos o hese g a ings inc eases. As s anda d, he FBGs a e
used app oxima ely o measu e empe a u es up o 320
°
C.
In special cases, esis ance can be achie ed up o 1500
°
C.
6. In e e ome ic Tempe a u e Senso s
These in e e ome ic me hods measu e phase shi be ween
wo cohe en beams ha ha e a elled he same pa h in
one o wo op ical fibe s. In condi ions when he leng h o
bo h a ms and hei e ac i e index o he co e is he same
o in mul iples o wa eleng hs 2π ad, he esul ing in e e -
ence is cons uc i e and he ou pu o he in e e ome e is a
a maximum. I he diffe ence is in odd mul iples o he wa e-
leng hs ð2m−1Þπ ad, he ou pu is a a minimum. In he
case o a gene al double-a m in e e ome e , he phase
esponse inc eases linea ly wi h a m leng h.
In e e ome e s a e able o de ec 3 mechanisms ha
affec he op ical beam: change in pa h leng h, wa eleng h,
and change in he speed o ligh p opaga ion ( e ac i e
index).
A change in any o hese quan i ies will esul in a change
in he phase o he wa e. This depends on he pa h leng h L,
he e ac i e index n, and he wo king wa eleng h λ. The
phase delay o he ligh is gi en by
Φ=2πnL
λ

:ð12Þ
To measu e he empe a u e, i is necessa y o know
in o ma ion abou he cu en alue o he phase delay and
wa eleng h shi . So, we need in o ma ion abou he ampli-
ude and spec um.
The design o in e e ome e s o empe a u e measu e-
men can use diffe en ypes o in e e ome e s. A e iew
a icle on in e e ome e s is p esen ed by Lee e al. [30] whe e
he possibili ies and uses o diffe en ypes o fibe op ic
senso s we e summa ized.
6.1. Mach-Zehnde In e e ome e (MZI). Va ious ypes o
fibe in-line MZI s uc u es ha e been de eloped, such as
ape ed fibe s uc u es in single-mode fibe s [29, 79, 80].
The in-line Mach-Zehnde in e e ome e is shown in
Figu e 27.
O he MZI can be based on inne ai mic oca i y [81, 82]
and co e-offse s uc u es [83, 84]. The men ioned MZI a e
based on he diffe ence o he effec i e e ac i e index
be ween he co e and cladding, so he empe a u e sensi i i y
is ela i ely low. A solu ion can be he use o g aded index
fibe s as a collima o , which can achie e spec al empe a u e
sensi i i y a ound 12.37 nm/
°
C [85]. E en some hyb id solu-
ions can be ound [86, 87].
17Jou nal o Senso s
Wi h cascade connec ion o MZI, he empe a u e
esponse o each MZI was 0.063 nm/
°
C om 30
°
C o
1000
°
C, 0.071 nm/
°
C om 30
°
C o 500
°
C, and 0.059 nm/
°
C
om 30
°
C o 1000
°
C [31].
6.2. Michelson In e e ome e . The Michelson in e e ome e
comes up wi h he possibili y o a single-ended measu emen
p obe, so we can measu e empe a u e a he end o he fibe
[88–90]. O he configu a ions can employ a high bi e in-
gence fibe [91]. An ad an age is ha i is possible o make
his in e e ome e easily wi h an o dina y op ical fibe and
splice . And since silica fibe can sus ain high empe a u es,
he e is a possibili y o measu e in high empe a u es. The
maximum empe a u e sensi i i y o 115.34 pm/
°
C a 550
°
C
was achie ed [92].
6.3. Sagnac In e e ome e . This me hod o measu emen
compa es wo ligh wa es ha a e ansmi ed agains each
o he [93, 94]. I he equencies o elec omagne ic wa es
a e diffe en , he phase shi be ween hem is changing wi h
he ime. As a esul , we can de ec in e e ences (pe iodic
changes a minimum and maximum).
A schema ic o his ype o measu emen is shown in
Figu e 28.
The ligh is emi ed om SLED and goes h ough a
3 dB op ocouple . Single-mode op ical fibe s ans e ligh
o he measu emen pa wi h a pola iza ion main aining
fibe . As a esul , we can measu e he shi o he in e e -
ence spec um o he PMF. The sensi i i y o 1.38 nm/
°
C
was achie ed wi hin he empe a u e ange 21–50
°
C [93]
o e en 18.27 nm/
°
C wi hin he ange 0–40
°
C [95]. How-
e e , a wide spec um o esea ch a ound his ype o em-
pe a u e measu emen seems o be limi ed o empe a u es
o a ound 120
°
C [96]. The main eason o such limi a ion
is he pe iodic na u e o his measu emen , also men ioned
by Domínguez-C uz e al. [97] o Cui e al. [98]. Figu e 29
shows ha measu ed wa e o ms ha e mo e han one
spike, so we canno pe o m measu emen s ou side his
limi .
As a esul , he Sagnac in e e ome e is mo e sui able o
a close ange o empe a u es, while offe ing high sensi i i y.
The only possibili ies a e dec easing he sensi i i y o making
he pe iodic effec in he wa e o m wide .
6.4. Modal In e e ome e . This me hod is based on he
eflec i e single-mode–mul imode–single-mode fibe s uc-
u e. The measu emen is achie ed ia moni o ing he spec-
al shi o he modal in e e ence be ween he co e and
he cladding modes in he ended single-mode fibe . The
achie ed sensi i i y was −92.6 pm/
°
C in he ange o 28 o
51
°
C [99].
Ano he ype is a fibe op ic modal in e e ome e ab i-
ca ed by a segmen o a low ellip ical hollow-co e pho onic
bandgap fibe (EHC-PBGF), whe e 12.99 pm/
°
C was
achie ed wi hin he ange om 30 o 110
°
C [100].
Wi h ega d o high- empe a u e measu emen s, we can
men ion a hin-co e fibe (TCF) modal in e e ome e o
empe a u es up o 850
°
C wi h 18.3 pm/
°
C sensi i i y [101].
Figu e 30 shows a schema ic o a hin-co e modal in e -
e ome e , whe e he hin-co e fibe is placed be ween wo
s anda d single-mode fibe s (SMF).
Ano he high- empe a u e modal in e e ome e is based
on a hollow-co e fibe (HCF) whe e empe a u es up o
1000
°
C we e achie ed wi h sensi i i y up o 33.4 pm/
°
C [32].
6.5. Fab y-Pe o In e e ome e . Figu e 31 shows a hyb id-
s uc u ed Fab y-Pe o in e e ome e (HSFPI) based on
la ge la e al offse splicing o simul aneous measu emen
o he s ain and empe a u e. An ad an age o his s uc u e
is p ice and easy ab ica ion; howe e , spec um measu e-
men is no always con enien o eal low-cos deploymen .
In his case, sensi i i y o 12.71 pm/
°
C was achie ed wi hin
he empe a u e ange o 100–700
°
C [102].
In a icles om Wang e al. [103–105], he p oposals o
in insic and ex insic fibe op ic senso s based on he
Fab y-Pe o in e e ome e we e able o measu e up o
1600
°
C.
A p oposal o a fibe op ic Fab y-Pe o in e e ome ic
(FFPI) senso by using PFC (pho onic c ys al fibe ) as a
high- empe a u e senso was achie ed by Ding e al. [33].
The measu emen shows ha sensi i i y o 10 pm/
°
C can be
achie ed oge he wi h he empe a u e ange o e 1200
°
C.
Tempe a u e chambe
L
PMF
SMF SMF
Fusion
splice
3 dB
couple
SLED Spec um
analyze
Figu e 28: Sagnac in e e ome e o empe a u e measu emen [93].
Spec um
analyze
Lase
Tape ed
egion B
Tape ed
egion A
In e e ome e
egion
Figu e 27: In-line Mach-Zehnde in e e ome e [79].
18 Jou nal o Senso s
6.6. Pa ial Conclusion o he In e e ome ic Measu emen
Me hods. In e e ome ic senso s a e capable o measu ing
high empe a u es, especially wi h he Mach-Zehnde o
Fab y-Pe o configu a ions. Physical limi a ions a e gi en
by op ical fibe endu ance, so he e is also possibili y o mea-
su e wi h special fibe s up o 1600
°
C. The e a e also low-
empe a u e configu a ions like he Sagnac in e e ome e
which can offe much highe esolu ion. The esolu ion
eaches alues 18.27 nm/
°
C [95] in compa ison wi h only
10 pm/
°
C o he Fab y-Pe o in e e ome e [33]. So he
measu emen is mo e p ecise, bu he empe a u e ange is
low. A comp omise be ween ange and sensi i i y is offe ed
by MZI o Michelson.
Howe e , in e e ome ic measu emen s can also be
qui e sensi i e o ambien ib a ions o o he mechanical
changes.
7. Real Deploymen and he Fu u e
Nowadays, some manu ac u e s al eady s a ed o ocus on
fibe -based senso s. Op ical senso s can be specially
designed o allow measu emen s in ex eme en i onmen s
including au omo i e [106], whe e empe a u e o he
b ake and he clu ch can be measu ed in he 200–1200
°
C
ange. The ad an age o his measu emen is ha we do
no need in o ma ion abou low empe a u es, so he
blackbody measu emen and IR senso can be used.
Ano he ype o senso is specified by he manu ac u e
An on Paa [107]. This ype o op ical empe a u e senso
uses he p inciple o measu ing wi h a uby c ys al. The
senso uses he p inciple desc ibed in he chap e on lumi-
nescence measu emen . The p inciple o he measu emen
is based on measu ing he empe a u e-dependen decay
ime o he uby c ys al luminescence. The senso manu-
ac u e p o ides in o ma ion on empe a u e measu e-
men s up o 300
°
C. Ad an ages o his senso a e he
calib a ion- ee design and low- empe a u e measu emen ,
which is no possible wi h blackbody. And a nonme allic
design also mee s immuni y o use in elec omagne ic
in e e ence en i onmen s.
The u u e o e e y me hod is connec ed o i s low-cos
e sion. So he spec um-based senso s ha e a majo disad-
an age e en i hey co e he equi ed empe a u e ange.
Howe e , he e is possibili y o combine indi idual op ical
me hods ha a e cheape bu usually sui able only o
measu ing a na ow empe a u e ange. Figu e 32 shows
he possible combina ion o luminescence and blackbody
measu emen . A sui able luminescen ma e ial can su i e
high empe a u es du ing blackbody measu emen oge he
wi h he measu emen possibili y a low empe a u es.
Accu acy o he ampli ude-based measu emen could be
affec ed du ing he manu ac u ing, so any op ical senso
cons uc ed in his manne will equi e calib a ion be o e
being used o measu ing empe a u es. Bu he de ec ion
pa o he sys em could be a simple pho ode ec o , so i
is s ill an in e es ing way.
1500
−40
−30
Signal (dB)
Wa eleng h (nm)
−20
−10
1520 1540 1560 1580 1600 1620
20°C
24°C
28°C
32°C
36°C
40°C
Figu e 29: Tempe a u e and spec um measu emen o Sagnac in e e ome e [97].
S anda d SMF S anda d SMFThin-co e ibe
(leng h: L)
Figu e 30: Thin-co e fibe in e e ome e scheme [101].
B oadband
ligh sou ce
3 dB
couple
Glue Glue
Fixed s age T ansla ion s age
Fab y–Pe o
in e e ome e
Op ical
spec um
analyze
Figu e 31: Fab y-Pe o in e e ome e se up [102].
19Jou nal o Senso s
The p inciple o possible connec ion o his me hod by a
combina ion o measu ing me hods can be pe o med
acco ding o Figu e 33. This me hod o op ical connec ion
is also men ioned in he a icle “Tempe a u e Moni o ing
Sys em o Elec ic Appa a us Based On Op ical Fibe Fluo-
escence”[108]. I can be assumed ha such a senso allows
measu emen s om low empe a u es (use o luminescence)
o empe a u es a ound 1000
°
C (use o blackbody adia ion).
This ype o senso finds applica ion in many indus ial a eas
in which i is necessa y o esis elec omagne ic in e e ence
and gal anic sepa a ion o he he mome e om he mea-
su ed objec o measu ing high empe a u es, o example,
in he field o me al welding.
8. Discussion and Summa y o he Pi alls o he
Indi idual Op ical Measu ing
Me hods Desc ibed
Se e al me hods o empe a u e measu emen based on op i-
cal measu emen me hods we e p esen ed in his a icle. I
fibe B agg g a ings a e used, his me hod can also be applied
o measu e de o ma ions, bends, o ib a ions. Gene ally,
each me hod has i s own pi alls. A p esen , he p oblem o
empe a u e measu emen o mos applica ions can be
sol ed using he mas e ed and calib a ed me hodology using
esis ance o he mocouple me hods. The pu pose o his
a icle was no o de e mine a me hod ha would eplace a
specific empe a u e measu emen using a nonelec ic fibe
op ic p obe in cases whe e elec ical measu ing p obes can
be un eliable due o he effec s o elec ical and magne ic
in e e ence. The answe o his ques ion canno be easily
ob ained. This is mainly because each sys em equi es spe-
cific empe a u e anges, he dynamics o he measu ing sys-
em, he wo king en i onmen , he impac o ib a ions, and
so on. Howe e , he ollowing findings can be summa ized
om his a icle. The blackbody- (g eybody-) based me hod
is sui able o empe a u es ope a ing om 500
°
C o 2000
°
C
using commonly a ailable op ical fibe s. In e ms o mechan-
ical design, his me hod appea s o be he mos obus one,
mainly due o he ac ha only he end o he glass fibe wi h
he blackbody is b ough in o he measu ed a ea. This
me hod is sui able o he use in measu emen a eas o mea-
su ing empe a u es o 500
°
C and abo e, o simplici y; a
apid esponse o he measu ing sys em can be achie ed
because o he good dynamics o changing he empe a u e
o he blackbody negligible mass. Howe e , when designing
a measu ing sys em, i is always necessa y o conside he
ins abili y o his measu ing chain when changing he em-
pe a u e on he pa o he e alua ion side and he pho ode-
ec o . The e o e, i is necessa y o place a high emphasis on
he empe a u e s abili y o he e alua ion elec onics o o
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0 200 400 600 800 1000
Tempe a u e ( ∘C)
Vol age ampli ude (V)
Measu emen
by
luminescence
Measu emen by blackbody
adia ion
Possible
a ea o
me ging
me hods
Figu e 32: Expec ed signal om he pho ode ec o using a combina ion o op ical me hods.
Exci a ion signal
o luo escence
Fluo escence
li e ime
Fluo escence
signal
Ampli ie
Ampli ie Ligh sou ce
Beam spli e
Op ical ibe
Luminescen
ma e ial
De ec o
Signal e alua ion
uni
Figu e 33: A empe a u e measu emen sys em o combina ion o measu ing me hods [108].
20 Jou nal o Senso s
conside hese empe a u e changes o he pho ode ec o and
he e alua ion uni when e alua ing he ecei ed ligh powe
a he con e sion o he measu ed empe a u e. In cases
whe e we would like o apply he measu emen me hods
using c ys al luminescence, i is also necessa y o conside
he need o he mal compensa ion o he pho ode ec o
when e alua ing he ecei ed ligh ou pu . Mo eo e , in cases
whe e his me hod is used in a eal measu ing chain, i is also
necessa y o know i s empe a u e due o he nonlinea i y o
he exci a ion ligh sou ce and o include i in he o e all
esul e alua ion. Fo he luminescen measu emen me hod,
he co ec c ys al mus be selec ed o he measu ed empe -
a u e ange. The dynamics o he measu ing sys em will also
depend on he size o he c ys al used and he ype and size o
he p o ec i e shield. The me hod always equi es an ex e nal
exci a ion, o example, wi h a pho odiode. The indi idual
measu ing anges a e lis ed in he a icle, and hey always
depend on he exci a ion ligh and c ys al used. Using his
c ys al me hod, he basic ad an age is he possibili y o pe -
o m measu emen s in he empe a u e ange 0–400
°
C.
Howe e , he disad an age may be he need o use a c ys al
and o es ablish a mechanical and op ical connec ion
be ween he fibe and he c ys al; in his case, a nonlinea
ansmission o op ical powe be ween he indi idual pa s
may occu due o mechanical and he mal ins abili y. The
me hod o he fibe B agg g a ings also p o ides an al e na-
i e o measu ing empe a u es in lowe empe a u e anges.
When using his me hod, i is also necessa y o ake in o
accoun he empe a u e calib a ion and compensa ion o
he exci a ion sou ce and pho ode ec o . We ha e a choice
o se e al ypes o g a ings desc ibed in his a icle o ealize
he he mal o mechanical measu emen s using his me hod.
The me hod is pa icula ly sui able o lowe measu ing
anges o up o 320
°
C when we achie e he equi ed accu acy
o abou 10 pm/
°
C. Fo highe empe a u es, he special and
mo e expensi e g a ings men ioned in his a icle a e used.
The las men ioned me hods a e in e e ome ic measu e-
men s. The achie ed empe a u e ange depends on he cho-
sen configu a ion, and i is able o co e empe a u es o e
1200
°
C. Howe e , in e e ome ic measu emen is able o
in e ac wi h mechanical changes in he sys em, especially
ib a ions. Signal measu emen also equi es spec um anal-
ysis, which c ea es an expensi e pa o he sys em. As a
esul , we can achie e fibe op ic measu emen in wide ange
o empe a u es bu wi h he limi a ion o sensi i i y o
mechanical in e e ences (also depends on configu a ion). I
is possible o implemen his ype o measu emen in labo a-
o y condi ions, bu i s implemen a ion in ha sh indus ial
en i onmen s can be difficul .
9. Conclusion
This a icle con ains in o ma ion and pa ial e alua ion o
a ious op ical fibe me hods in empe a u e measu emen .
I is ad isable o conside measu emen s using op ical
me hods in cases whe e he elec ical and magne ic in e e -
ence can be aken in o accoun in he empe a u e measu ing
ange o in cases whe e we need o emo e he gal anic con-
nec ion be ween he measu ed objec and he e alua ion uni .
Fo example, op ical me hods can measu e he empe a u e
o elec ical wi es used o high- ol age and mic owa e appli-
ca ions. The a icle desc ibes indi idual op ical p inciples
and p esen s he benefi s and applicabili y o he me hods
men ioned. When designing he measu ing chain, i is always
necessa y o ake in o accoun he dynamic ange, he sensi-
i i y, and he esis ance o mechanical and elec ical influ-
ences. I is also impo an o ecall he necessa y he mal
compensa ion o op ical ecei e s and ansmi e s in cases
whe e i would be decided o eplace he exis ing pu ely elec-
ical modes o empe a u e measu emen by op ical
me hods. These op ical semiconduc o ansmi e s and
ecei e s a e usually mo e empe a u e dependen han hose
used o p ocess he signal om he mocouples and esis i e
empe a u e senso s. Howe e , in designing he empe a u e
measu ing chain using hese op ical me hods, in addi ion o
emo ing he effec o he abo e-men ioned magne ic in e -
e ence, a long nonelec ical connec ion be ween he mea-
su ed poin and he e alua ion uni is ob ained. Designing
such a measu emen sys em is always a comp omise be ween
he p ice and u ili y alue o such a sys em and choosing he
app op ia e me hod wi h he indi idual op ions desc ibed in
his a icle.
Da a A ailabili y
No da a used o suppo he findings o his s udy.
Con lic s o In e es
The e is no conflic o in e es .
Acknowledgmen s
This wo k was suppo ed by he MPO T io in he Resea ch
and De elopmen o Op ical and Fibe -Op ical Tempe a u e
Senso s o Au omo i e Applica ions P ojec (p ojec num-
be FV10422). This wo k was suppo ed by he Eu opean
Regional De elopmen Fund in he Resea ch Cen e o
Ad anced Mecha onic Sys ems p ojec , p ojec numbe
CZ.02.1.01/0.0/0.0/16_019/0000867 wi hin he Ope a ional
P og amme Resea ch, De elopmen and Educa ion.
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